Long-term sodium chloride retention in a rural watershed: legacy effects of road salt on streamwater concentration

A regional examination of the footprint of agriculture and urban cover on stream water quality

Freshwater systems in cold regions, including the Laurentian Great Lakes, are threatened by both eutrophication and salinization, due to excess nitrogen (N), phosphorus (P) and chloride (Cl-) delivered in agricultural and urban runoff. However, identifying the relative contribution of urban vs. agricultural development to water quality impairment is challenging in watersheds with mixed land cover, which typify most developed regions. In this study, a self-organizing map (SOM) analysis was used to evaluate the contributions of various forms of land cover to water quality impairment in southern Ontario, a population-dense, yet highly agricultural region in the Laurentian Great Lakes basin where urban expansion and agricultural intensification have been associated with continued water quality impairment. Watersheds were classified into eight spatial clusters, representing four categories of agriculture, one urban, one natural, and two mixed land use clusters. All four agricultural clusters had high nitrate-N concentrations, but levels were especially high in watersheds with extensive corn and soybean cultivation, where exceedances of the 3 mg L-1 water quality objective dramatically increased above a threshold of ‍∼30 % watershed row crop cover. Maximum P concentrations also occurred in the most heavily tile-drained cash crop watersheds, but associations between P and land use were not as clear as for N. The most urbanized watersheds had the highest Cl- concentrations and expansions in urban area were mostly at the expense of surrounding agricultural land cover, which may drive intensification of remaining agricultural lands. Expansions in tile-drained corn and soybean area, often at the expense of mixed, lower intensity agriculture are not unique to this area and suggest that river nitrate-N levels will continue to increase in the future. The SOM approach provides a powerful means of simplifying heterogeneous land cover characteristics that can be associated with water quality patterns and identify problem areas to target management.

Evaluating the Effects of Diet on the Sensitivity of Hyalella azteca to an "Eco-friendly" Deicing Agent

Salting of roadways contaminates local waterways via snowmelt and precipitation runoff, eliciting various toxicological impacts on aquatic ecosystems. Recently, "eco-friendly" deicing alternatives have been introduced in hopes of mitigating environmental impacts of deicing agents, while maintaining human safety. These "eco-friendly" alternatives may pose their own set of environmental concerns that require further study. While the potential toxicity of road salts has been evaluated for various aquatic species, the environmental factors that may influence this toxicity are less understood; and for emerging deicing alternatives, there is a lack of literature documenting these potential implications. For aquatic organisms, the highest exposure to road salts may coincide with reduced food availability, namely during the winter months. The present study evaluates the effect of a conditioning diet on the sensitivity of adult Hyalella azteca to an "eco-friendly"-labeled beet deicer (Snow Joe MELT Beet-IT). Various conditioning diets were examined, including TetraMinTM, TetraMin and diatom (Thalassiosira weissflogii) combinations, and TetraMin and conditioned Acer sacharum leaves. For each diet type, 48- and 96-h water-only toxicity bioassays were conducted with adult H. azteca. These results were compared to organisms which experienced a 96-h starvation period prior to exposure and culture organisms. Diet types representing excess quality and quantity of food significantly decreased the toxicity of beet deicer to the organisms. However, starvation likely increases the toxicity of road salts to H. azteca. Therefore, the quantity and quality of food available to H. azteca may influence their sensitivity to deicing agents. Environ Toxicol Chem 2024;00:1-8. © 2024 SETAC.

Exploring spatial and temporal symptoms of the freshwater salinization syndrome in a rural to urban watershed

The freshwater salinization syndrome (FSS), a concomitant watershed-scale increase in salinity, alkalinity, and major-cation and trace-metal concentrations, over recent decades, has been described for major rivers draining extensive urban areas, yet few studies have evaluated temporal and spatial FSS variations, or causal factors, at the subwatershed scale in mixed-use landscapes. This study examines the potential influence of land-use practices and wastewater treatment plant (WWTP) effluent on the export of major ions and trace metals from the mixed-use East Branch Brandywine Creek watershed in southeastern Pennsylvania, during the 2019 water year. Separate analysis of baseflow and stormflow subsets revealed similar correlations among land-use characteristics and streamwater chemistry. Positive associations between percent impervious surface cover, which ranged from 1.26 % to 21.9 % for the 13 sites sampled, and concentrations of Ca2+, Mg2+, Na+, and Cl- are consistent with road-salt driven reverse cation exchange and weathering of the built environment. The relative volume of upstream WWTP was correlated with Cu and Zn, which may be derived in part from corroded water-conveyance infrastructure; chloride to sulfate mass ratios (CSMR) ranged from ~6.3 to ~7.7× the 0.5 threshold indicating serious corrosivity potential. Observed exceedances of U.S. Environmental Protection Agency Na+ and Cl- drinking water and aquatic life criteria occurred in winter months. Finally, correlations between percent cultivated cropland and As and Pb concentrations may be explained by the persistence of agricultural pesticides that had been used historically. Study results contribute to the understanding of FSS solute origin, fate, and transport in mixed-use watersheds, particularly those in road salt-affected regions. Study results also emphasize the complexity of trace-metal source attribution and explore the potential for FSS solutes to affect human health, aquatic life, and infrastructure.

Combined effect of freshwater salinization and harmful algae on the benthic invertebrate Chironomus pallidivittatus

Global climate change as well as human activities have been reported to increase the frequency and severity of both salinization and harmful algal blooms (HABs) in many freshwater systems, but their co-effect on benthic invertebrates has rarely been studied. This study simultaneously examined the joint toxicity of salinity and different cyanobacterial diets on the behavior, development, select biomarkers, and partial life cycle of Chironomus pallidivittatus (Diptera). High concentrations of salts (e.g., 1 g/L Ca2+ and Mg2+) and toxic Microcystis had synergistic toxicity, inhibiting development, burrowing ability and causing high mortality of C. pallidivittatus, especially for the Mg2+ treatment, which caused around 90% death. Low Ca2+ concentration (e.g., 0.01 g/L) promoted larval burrowing ability and inhibited toxin accumulation, which increased the tolerance of Chironomus to toxic Microcystis. However, low Mg2+ concentration (e.g., 0.01 g/L) was shown to inhibit the behavior, development and increase algal toxicity to Chironomus. Toxic Microcystis resulted in microcystin (MC) accumulation, inhibited the burrowing ability of larvae, and increased the proportion of male adults (>50%). The combined toxicity level from low to high was verified by the weight of evidence and the grey TOPSIS model, which integrated five lines of evidence to increase the risk assessment accuracy and efficiency. This is the first study that provided insights into ecological risk arising from the joint effect of salinity and harmful algae on benthic organisms. We suggest that freshwater salinization and HABs should be considered together when assessing ecological threats that arise from external stress.

Characterization of subsurface pathways contributing to freshwater salinization of urban streams using electrical and electromagnetic imaging techniques

Salinization of inland fresh surface waters in temperate climates is a growing concern due to increasing salt inputs from sources including chloride (Cl)-containing road salt de-icers, industrial waste, and landfill leachate. Groundwater pathways play an important role in the year-round delivery of Cl to streams, but quantifying this pathway, including spatiotemporal variability and amount of Cl mass stored in the subsurface, is challenging. The objective of this study was to demonstrate, evaluate, and compare the potential applications of the geoelectrical techniques - electromagnetics (EM) and direct current (DC) resistivity - for mapping salt contamination in shallow urban groundwater and characterizing the groundwater pathways delivering Cl to urban streams. EM and DC surveys were conducted (3D mapping and 2D time-lapse) across a 20 m salt-impacted stream section and surrounding riparian zone that is located near an arterial road and parking lot. Groundwater samples and soil cores were also collected to validate the geoelectrical results. Both the EM and DC surveys detected high salt concentrations in the shallow subsurface (up to 3 m depth) near the road, parking lot, and stream; however, DC more accurately represented groundwater Cl concentrations. DC results were used to calculate the total Cl mass in the subsurface, with the spatial mass distribution used to infer the temporal variability in the subsurface salt plume. Finally, time-lapse DC showed that the highest groundwater salt concentrations existed near the stream between June and October - this is expected to contribute to the elevated salt concentrations in the stream during summer months. This study has shown that EM and DC can be useful for identifying groundwater salt concentration, storage, and transport in a non-intrusive and efficient manner, making them valuable field tools for characterizing and quantifying groundwater salt pathways to urban streams.

Urban lake water quality responses to elevated road salt concentrations

Road salt runoff from de-icing applications is increasingly impacting water quality around the globe. Excess salt (especially chloride) concentrations can negatively impact the biological, chemical, and physical properties of freshwater ecosystems. Though road salt pollution is a prevalent issue affecting many northern temperate lakes, there are few studies on how freshwater salinization interacts with other ecological stressors such as eutrophication. We investigated how chloride from road deicers influences water quality in an urban lake. We sampled a tributary and lake receiving large amounts of road salt runoff from a nearby highway in Grand Rapids, Michigan over a 20-month period. Chloride concentrations in the deepest part of the lake consistently exceeded the US EPA chloride chronic toxicity threshold of 230 mg/L, at times reaching up to 331 mg/L. These high chloride concentrations appear to be responsible for preventing part of the lake from complete mixing, and causing hypoxia in the deepest regions of the lake. Total phosphorus concentrations near the surface averaged 35 μg/L but exceeded 7500 μg/L in the deepest part of the lake, which occupies 3-5 % of total lake volume. Phosphorus release rates from the sediments were low and unlikely to be a current source of the high phosphorus concentrations. Rather, both phosphorus and chloride likely have been accumulating in the hypolimnion over a relatively long period of time. Lake management actions will require control of both internal and external phosphorus and chloride sources in the future. We recommend that phosphorus be addressed first to avoid the extremely high phosphorus concentrations from reaching the photic zone and stimulating algal blooms, which would occur if salt was removed first and the halocline broke down. Our findings and recommendations are applicable to other lakes facing similar issues.

Freshwater salinization syndrome limits management efforts to improve water quality

Freshwater Salinization Syndrome (FSS) refers to groups of biological, physical, and chemical impacts which commonly occur together in response to salinization. FSS can be assessed by the mobilization of chemical mixtures, termed "chemical cocktails", in watersheds. Currently, we do not know if salinization and mobilization of chemical cocktails along streams can be mitigated or reversed using restoration and conservation strategies. We investigated 1) the formation of chemical cocktails temporally and spatially along streams experiencing different levels of restoration and riparian forest conservation and 2) the potential for attenuation of chemical cocktails and salt ions along flowpaths through conservation and restoration areas. We monitored high-frequency temporal and longitudinal changes in streamwater chemistry in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management or conservation efforts in six urban watersheds in the Chesapeake Bay watershed. Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths (i.e., permanent reaches of a stream) varied due to pollution events. In response to winter road salt applications, the chemical cocktails were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails were less enriched in salt ions and trace metals. Downstream attenuation of salt ions occurred during baseflow and stormflow conditions along flowpaths through regional parks, stream-floodplain restorations, and a national park. Conversely, chemical mixtures of salt ions and metals, which formed in response to multiple road salt applications or prolonged road salt exposure, did not show patterns of rapid attenuation downstream. Multiple linear regression was used to investigate variables that influence changes in chemical cocktails along flowpaths. Attenuation and dilution of salt ions and chemical cocktails along stream flowpaths was significantly related to riparian forest buffer width, types of salt pollution, and distance downstream. Although salt ions and chemical cocktails can be attenuated and diluted in response to conservation and restoration efforts at lower concentration ranges, there can be limitations in attenuation during road salt events, particularly if storm drains bypass riparian buffers.

Spatial distribution of major and trace elements in artificial lakes in Serbia: health risk indices and suitability of water for drinking and irrigation purposes

Lakes and reservoirs are the main water resources for household, irrigation, and industrial use. Hence, it is necessary to control water pollution and have reliable information on water quality, which refers to its suitability for a particular purpose. The main objective of this study was to evaluate the suitability of water for drinking and irrigation purposes from ten artificial lakes on the territory of the Republic of Serbia. The physicochemical parameters were determined using standard analytical methods, whereas the optical emission spectrometry with inductively coupled plasma, ICP-OES, was used to determine the concentrations of 23 macro-, micro-, and trace elements in water. Health risk indices of drinking water and its suitability for irrigation purposes were evaluated. The results of physicochemical parameters, except for total organic matter, indicated that most samples were within the recommended values. Among analyzed elements, the concentration of Mg was elevated in the water of the lakes Srebrno and Prvonek. The results of health risk assessment clearly showed that there was no acute health risk (HQA < 1), whereas long-term risk (HQL) in the studied lakes was observed with respect to As (lakes Grlište and Srebrno), Mn (lakes Srebrno and Prvonek), Ni (Lake Vrutci), and Sr (Lake Grlište). The irrigation criteria indicated that the water from the examined lakes had satisfactory quality for this purpose, except for residual sodium carbonates (RSC). This criterion indicated low quality or unsuitability of the water in 7 lakes. The main outcomes provided valuable information on the suitability of water from the artificial lakes for intended uses.

Responses to saltwater exposure vary across species, populations and life stages in anuran amphibians

To predict the impacts of environmental change on species, we must first understand the factors that limit the present-day ranges of species. Most anuran amphibians cannot survive at elevated salinities, which may drive their distribution in coastal locations. Previous research showed that coastal Hyla cinerea are locally adapted to brackish habitats in North Carolina, USA. Although Hyla squirella and Hyla chrysoscelis both inhabit coastal wetlands nearby, they have not been observed in saline habitats. We take advantage of naturally occurring microgeographic variation in coastal wetland occupancy exhibited by these congeneric tree frog species to explore how salt exposure affects oviposition site choice, hatching success, early tadpole survival, plasma osmolality and tadpole body condition across coastal and inland locations. We observed higher survival among coastal H. cinerea tadpoles than inland H. cinerea, which corroborates previous findings. But contrary to expectations, coastal H. cinerea had lower survival than H. squirella and H. chrysoscelis, indicating that all three species may be able to persist in saline wetlands. We also observed differences in tadpole plasma osmolality across species, locations and salinities, but these differences were not associated with survival rates in salt water. Instead, coastal occupancy may be affected by stage-specific processes like higher probability of total clutch loss as shown by inland H. chrysoscelis or maladaptive egg deposition patterns as shown by inland H. squirella. Although we expected salt water to be the primary filter driving species distributions along a coastal salinity gradient, it is likely that the factors dictating anuran ranges along the coast involve stage-, species- and location-specific processes that are mediated by ecological processes and life history traits.

Lethal and Sub-lethal Implications of Sodium Chloride Exposure for Adult Unionid Mussel Species: Eurynia dilatata and Lasmigona costata

The elevated use of salt as a de-icing agent on roads in Canada is causing an increase in the chloride concentration of freshwater ecosystems. Freshwater Unionid mussels are a group of organisms that are sensitive to increases in chloride levels. Unionids have greater diversity in North America than anywhere else on Earth, but they are also one of the most imperiled groups of organisms. This underscores the importance of understanding the effect that increasing salt exposure has on these threatened species. There are more data on the acute toxicity of chloride to Unionids than on chronic toxicity. This study investigated the effect of chronic sodium chloride exposure on the survival and filtering activity of two Unionid species (Eurynia dilatata, and Lasmigona costata) and assessed the effect on the metabolome in L. costata hemolymph. The concentration causing mortality after 28 days of exposure was similar for E. dilatata (1893 mg Cl^-/L) and L. costata (1903 mg Cl^-/L). Significant changes in the metabolome of the L. costata hemolymph were observed for mussels exposed to non-lethal concentrations. For example, several phosphatidylethanolamines, several hydroxyeicosatetraenoic acids, pyropheophorbide-a, and alpha-linolenic acid were significantly upregulated in the hemolymph of mussels exposed to 1000 mg Cl^-/L for 28 days. While no mortality occurred in the treatment, elevated metabolites in the hemolymph are an indicator of stress.

Impacts of water hardness and road deicing salt on zooplankton survival and reproduction

Rising salinity from road deicing salts threatens the survival and reproduction of freshwater organisms. We conducted two experiments to address how Daphnia pulex survival and reproduction were affected by road salt concentration (control, 120, 640 and 1200 mg Cl-/L) crossed with three concentrations of water hardness (20, 97, 185 mg CaCO3 /L). D. pulex survival was poor in our hard water treatment in both experiments (185 mg CaCO3 /L), potentially indicating a low tolerance to hard water for the strain used in our experiments. With the remaining two hardness treatments (20 and 97 mg CaCO3 /L), we found no evidence of an interactive effect between salt concentration and water hardness on D. pulex survival. In our population-level experiment, D. pulex survival was reduced by > 60% at 120 mg Cl-/L compared to the control. In the individual experiment, survival was similar between the control and 120 mg Cl-/L, but ≤ 40% of individuals survived in 640 and 1200 mg Cl-/L. For the surviving individuals across all treatments, the number of offspring produced per individual declined with increasing Cl- concentration and in hard water. Our results indicate that current Cl- thresholds may not protect some zooplankton and reduced food availability per capita may enhance the negative impacts of road salt.

Spatial patterns and seasonal timing of increasing riverine specific conductance from 1998 to 2018 suggest legacy contamination in the Delaware River Basin

Increasing salinization of freshwater threatens water supplies that support a range of human and ecological uses. The latest assessments of Delaware River Basin (DRB) surface-water-quality changes indicate widespread salinization has occurred in recent decades, which may lead to meaningful degradation in water quality. To better understand how and when salinity transport occurs and implications for DRB streams, this study: 1) explores the variability of specific conductance (SC) trends spatially and seasonally from 1998 to 2018, and 2) investigates how trends relate to streamflow, land disturbance, and impervious surface area to better understand regional salinization drivers. We find widespread increases in SC across the DRB, with several sites in the lower basin exceeding thresholds for aquatic life and experiencing increasing frequencies of exceedance over time. In general, the greatest basin wide increases in SC occurred during low flow conditions, indicating that a legacy component resulting from subsurface retention and transport processes has driven observed changes in riverine SC. For a subset of sites in the lower basin, where impervious area and cumulative land disturbance are higher, the greatest SC increases occurred during high flow conditions in winter months. Given the patterns of SC and watershed changes across the basin, as well as strong relationships between SC trends and sodium and chloride trends, deicing salt appears to be a likely driver of observed SC change. Even if deicing salt application plateaus or declines in coming years, the continued release and transport of the legacy subsurface component may still contribute to elevated DRB riverine SC.

Asphalt Mixtures Fatigue Life Considering Various Environmental Impacts

The pavement structure during the colder seasons (winter) or in regions located above sea level is commonly affected and deteriorated by many environmental factors. Two prominent factors are water and frost (weather) or road salt (maintenance). According to the article's literature review, there are only a few studies related to water and frost or road salt impact on mineral asphalt mixes considering fatigue. Most of the tests were performed on mixes containing common road asphalt or only one binder content level was investigated. There are no articles that investigate this problem comprehensively including new asphalt, its content levels, or production technology. Based on the literature review, the main problem regarding degradation impact on mixtures fatigue life was stated. The investigation was performed using two proprietary experimental methods allowing approximates in situ conditions regarding environmental impacts. A dynamic four-point bending fatigue test was applied to evaluate degradation considering fatigue. The investigation was performed using four coarse-graded asphalt mixtures (asphalt concrete AC 22) which differed in binder type (35/50 WMA, 35/50, 25/55-60, and 25/55-80 HIMA), content level (4.24%, 4.03%, 3.82%), and production technology (hot and warm). Regarding the results obtained, the authors proposed a degradation ratio regarding fatigue life variability. Based on the obtained results and ratio used, it was found that both interactions caused a significant fatigue life decrease-in the worst case, over tens of percent. Furthermore, it was found that asphalt mixture resistance to environmental factors depends on binder type, its content level, air void content, and discussed impact. Moreover, asphalt mixtures' susceptibility to degradation (fatigue) is extreme at lower binder content levels and accelerates due to air void content increase. In the article, it was also stated that the highest resistance was reached by a mixture with highly modified asphalt (25/55-80 HIMA). It was also found that the SBS polymer dosage increase in the asphalt matrix enhances asphalt mixture resistance to environmental impacts. The least resistant to the environmental degradation mixture was WMA (35/50 WMA).

Long-term impacts of impervious surface cover change and roadway deicing agent application on chloride concentrations in exurban and suburban watersheds

Roadway deicing agents, including rock salt and brine containing NaCl, have had a profound impact on the water quality and aquatic health of rivers and streams in urbanized areas with temperate climates. Yet, few studies evaluate impacts to watersheds characterized by relatively low impervious surface cover (ISC; < 15 %). Here, we use long-term (1997-2019), monthly streamwater quality data combined with daily streamflow for six exurban and suburban watersheds in southeastern Pennsylvania to examine the relations among chloride (Cl^-) concentrations and ISC. Both flow-normalized Cl^- concentrations and ISC increased over time in each of the six watersheds, consistent with changes in watershed management (e.g., ISC, road salt application, etc.). The watersheds that experienced the greatest changes in percent ISC (e.g., agriculture replaced by residential and commercial development) experienced the greatest changes in flow-normalized Cl^- concentrations. We also utilized a comprehensive mass-balance model (2011-2018) that indicated Cl^- inputs exceeded the outputs for the study watersheds. Road salt applied to state roads, non-state roads, and other impervious surfaces accounted for the majority of Cl^- inputs to the six watersheds. Furthermore, increasing Cl^- concentrations during baseflow conditions confirm impacts to shallow groundwater. Although flow-normalized Cl^- concentrations are below the U.S. Environmental Protection Agency's chronic threshold value for impacts to aquatic organisms, year-round exceedances may result before the end of this century based on current trends. Though reduced Cl^- loading to streams may be achieved by limiting the expansion of impervious surfaces in exurban and suburban watersheds, changes in baseflow concentrations are likely to be gradual because of the accumulated Cl^- in groundwater.

Freshwater Salinization Syndrome Alters Retention and Release of 'Chemical Cocktails' along Flowpaths: from Stormwater Management to Urban Streams

We investigate impacts of Freshwater Salinization Syndrome (FSS) on mobilization of salts, nutrients, and metals in urban streams and stormwater BMPs by analyzing original data on concentrations and fluxes of salts, nutrients, and metals from 7 urban watersheds in the Mid-Atlantic U.S. and synthesizing literature data. We also explore future critical research needs through a survey of practitioners and scientists. Our original data show: (1) sharp pulses in concentrations of salt ions and metals in urban streams directly following both road salt events and stream restoration construction (e.g., similar to the way concentrations increase during other soil disturbance activities); (2) sharp declines in pH (acidification) in response to road salt applications due to mobilization of H+ from soil exchange sites by Na+; (3) sharp increases in organic matter from microbial and algal sources (based on fluorescence spectroscopy) in response to road salt applications likely due to lysing cells and/or changes in solubility; (4) significant retention (~30-40%) of Na+ in stormwater BMP sediments and floodplains in response to salinization; (5) increased ion exchange and mobilization of diverse salt ions (Na+, Ca2+, K+, Mg2+), nutrients (N, P), and trace metals (Cu, Sr) from stormwater BMPs and restored streams in response to FSS; (6) downstream increasing loads of Cl-, SO4 2-, Br-, F-, and I- along flowpaths through urban streams, and P release from urban stormwater BMPs in response to salinization, and (7) a significant annual reduction (> 50%) in Na+ concentrations in an urban stream when road salt applications were dramatically reduced, which suggests potential for ecosystem recovery. We compared our original results to published metrics of contaminant retention and release across a broad range of stormwater management BMPs from North America and Europe. Overall, urban streams and stormwater management BMPs consistently retain Na+ and Cl- but mobilize multiple contaminants based on salt types and salinity levels. Finally, we present our top 10 research questions regarding FSS impacts on urban streams and stormwater management BMPs. Reducing diverse 'chemical cocktails' of contaminants mobilized by freshwater salinization is now a priority for effectively and holistically restoring urban waters.

The joint effects of salt and 6PPD contamination on a freshwater herbivore

Using sodium chloride (NaCl) for de-icing roads is known to have severe consequences on freshwater organisms when washed into water bodies. N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, also known as 6PPD, is an antiozonant mainly found in automobile tire rubber to prevent ozone mediated cracking or wear-out. Especially the ozonated derivate, 6PPD-quinone, which is washed into streams after storm events, has been found to be toxic for coho salmon. Studies on other freshwater organisms could not confirm those findings, pointing towards distinct species-specific differences. Storm events result in greater run-offs from all water-soluble contaminants into freshwater bodies, potentially enhancing the concentrations of both chloride and 6PPD during winter. Here we show that these two contaminants have synergistic negative effects on the population growth of the rotifer Brachionus calyciflorus, a common freshwater herbivore. Hence, while only high concentrations of 6PPD and even higher concentrations of 6PPD-quinone, beyond environmentally relevant concentrations, had lethal effects on rotifers, the addition of NaCl enhanced the sensitivity of the rotifers towards the application of 6PPD so that their negative effects were more pronounced at lower concentrations. Similarly, 6PPD increased the lethal effect of NaCl. Our results support the species-specific toxicity of 6PPD and demonstrate a synergistic effect of the antiozonant on the toxicity of other environmentally relevant stressors, such as road salt contamination.

Removal of chloride from water and wastewater: Removal mechanisms and recent trends

Increased chloride concentration can cause salinization, which has become a serious and widespread environmental problem nowadays. This review aims at providing comprehensive and state-of-the-art knowledge and insights of technologies for chloride removal. Mechanisms for chloride removal mainly include chemical precipitation, adsorption, oxidation and membrane separation. In chemical precipitation, chloride removal by forming CuCl, AgCl, BiOCl and Friedel's salt. Adsorbents used in chloride removal mainly include ion exchangers, bimetal oxides and carbon-based electrodes. Oxidation for chloride removal contains ozone-based, electrochemical and sulfate radical-based oxidation. Membrane separation for chloride removal consists of diffusion dialysis, nanofiltration, reverse osmosis and electrodialysis. In this review, we specifically proposed the factors that affect chloride removal process and the corresponding strategies for improving removal efficiency. In the last section, the remaining challenges of method explorations and material developments were stated to provide guidelines for future development of chloride removal technologies.

Chemical and Carbon Isotopic Characterization of a Karst-Dominated Urbanized Watershed: Case of the Upper San Antonio River

Urbanization and agriculture are two key factors that place demands on water resources and serve as sources of anthropogenic pollution into inland waterways. The San Antonio River, which is sourced from a karst aquifer, plays an important recreational and scenic role, yet effective management is often hampered by the lack of understanding of the chemical characterization of the water system. The karst-dominated Edwards Aquifer watershed in south-central Texas is an ideal watershed to understand water-rock interaction (carbonate dissolution) and anthropogenic impact on our water resources. In order to understand groundwater-surface water interactions, we made chemical and isotopic measurements over a 17-km stretch of the San Antonio River beginning at the headwater sanctuary and moving downstream. The chemistry of the headwaters and at along the longitudinal profile of the river showed that the Edwards Aquifer is dominated by Ca²⁺, Mg²⁺ and HCO₃^- ions resulting from carbonate dissolution. The carbon isotopic signature of dissolved inorganic carbon (δ¹³C_DIC) showed that the Edwards Aquifer is in chemical and isotopic equilibrium with soil CO_2(g). The relationships between δ¹³C_DIC and solutes (Cl^-, Na⁺, F^-, NO₃^-) showed that anthropogenic sources of these solutes are associated with low δ¹³C_DIC values, indicating that carbon isotopic composition of dissolved inorganic carbon can be a useful tracer for contaminants in the environment. The anthropogenic inputs into the San Antonio River were sourced mainly from effluents of the San Antonio Zoo, waste discharge from the River Walk in downtown San Antonio and from fertilizers and animal waste in the less urbanized section of the sampled area (Mission Concepcion to Mission Espada). To protect and sustain the water quality of urban waterways and karst aquifers, urban sewage and effluents must be treated and controlled.

Multi-class secondary metabolites in cyanobacterial blooms from a tropical water body: Distribution patterns and real-time prediction

Cyanobacterial blooms that produce toxins occur in freshwaters worldwide and yet, the occurrence and distribution patterns of many cyanobacterial secondary metabolites particularly in tropical regions are still not fully understood. Moreover, predictive models for these metabolites by using easily accessible water quality indicators are rarely discussed. In this study, we investigated the co-occurrence and spatiotemporal trends of 18 well-known and less-studied cyanobacterial metabolites (including [D-Asp3] microcystin-LR (DM-LR), [D-Asp3] microcystin-RR (DM-RR), microcystin-HilR (MC-HilR), microcystin-HtyR (MC-HtyR), microcystin-LA (MC-LA), microcystin-LF (MC-LF), microcystin-LR (MC-LR), microcystin-LW (MC-LW), microcystin-LY (MC-LY), microcystin-RR (MC-RR) and microcystin-WR (MC-WR), Anatoxin-a (ATX-a), homoanatoxin-a (HATX-a), cylindrospermospin (CYN), nodularin (NOD), anabaenopeptin A (AptA) and anabaenopeptin B (AptB)) in a tropical freshwater lake often plagued with blooms. Random forest (RF) models were developed to predict MCs and CYN and assess the relative importance of 22 potential predictors that determined their concentrations. The results showed that 11 MCs, CYN, ATX-a, HATX-a, AptA and AptB were found at least once in the studied water body, with MC-RR and CYN being the most frequently occurring, intracellularly and extracellularly. AptA and AptB were detected for the first time in tropical freshwaters at low concentrations. The metabolite profiles were highly variable at both temporal and spatial scales, in line with spatially different phytoplankton assemblages. Notably, MCs decreased with the increase of CYN, possibly revealing interspecific competition of cyanobacteria. The rapid RF prediction models for MCs and CYN were successfully developed using 4 identified drivers (i.e., chlorophyll-a, total carbon, rainfall and ammonium for MCs prediction; and chloride, total carbon, rainfall and nitrate for CYN prediction). The established models can help to better understand the potential relationships between cyanotoxins and environmental variables as well as provide useful information for making policy decisions.

The Healing Power of Clean Rivers: In Silico Evaluation of the Antipsoriatic Potential of Apiin and Hyperoside Plant Metabolites Contained in River Waters

Humanity may benefit greatly from intact riverine ecosystems not only because they supply water to be used in the most common human activities, but also for the effects that clean rivers can have on human health. Herein, we used a computational approach to show that some phytochemicals produced by riparian plants as secondary metabolites, which are naturally released into river waters, can have therapeutic properties. These include antipsoriatic activities which we demonstrated in silico by modelling the interaction of apiin, guanosine and hyperoside, a few main river plant metabolites, with NF-kB, IL-17 and IL-36, which are recognized targets involved in psoriasis disease. In particular, we found that apiin and hyperoside are endowed with docking energies and binding affinities which are more favorable than the known reference inhibitors of the three protein targets whilst, in silico, guanosine shows comparable activity with respect to the inhibitors of IL-36 and NF-kB. The low skin permeation (logKp < −8) we predicted for apiin and hyperoside led us to hypothesize their possible utilization as topic antipsoriatic therapeutics, and in particular after PAINS (pan-assay interference compounds) score evaluation, we reached the conclusion that apiin, with no predicted tendency to react nonspecifically with the numerous targets involved in the biological cellular pathways, is particularly interesting for the desired therapeutic application.

Impacts of Road Deicing Application on Sodium and Chloride Concentrations in Philadelphia Region Drinking Water

Historical application of roadway deicing agents (e.g., road salt and brines) has led to an increase in sodium and chloride concentrations in surface water over time. Numerous studies have explored the impacts of road salt on freshwater aquatic organisms such as amphibians and benthic macroinvertebrates; however, the public health risk associated with consuming drinking water with elevated sodium has been largely unexplored in the literature. Yet, sodium ingestion, primarily through diet, has been linked to adverse human health conditions, such as hypertension. This study documents weekly sodium and chloride concentrations in municipal tap water from three municipalities within the Philadelphia metropolitan area during winter 2018-2019 (November through March). A late winter peak in sodium and chloride concentrations was observed for all three municipalities immediately following successive snow events coupled with daily high temperatures above 0°C. Among municipalities, mean and peak sodium and chloride concentrations were associated with relatively higher development in upstream areas. Observed sodium concentrations ranged from 1 to 6.4x the USEPA recommended guideline of 20 mg/L for individuals restricted to a total sodium intake of 500 mg/day. Additionally, the contribution of sodium ingestion from water consumption to the recommended daily sodium intake limits for adults ranged from 3.5% to 18.8% for non-restricted and 4.2%-33.3% for "low salt" (i.e., <1,500 mg/day) diets, respectively. The study results coupled with a records review for 40 U.S. municipalities in snow affected regions indicate the need for real-time communication between water utilities and the general public regarding sodium exposure risk during winter months.

Stream Salamanders are Relatively Tolerant of Salty Streams

The application of road salt as a deicing agent is common, but investigations of potential negative effects of salt runoff on stream salamanders have been limited. Additionally, modern stormwater management practices should influence the delivery of salt ions to streams. We used data loggers in streams draining watersheds with and without stormwater management ponds and acute 96-h LC50 tests to investigate exposure of, and road salt toxicity among, two widespread salamanders in the Eastern USA, northern dusky salamanders (Desmognathus fuscus) and northern two-lined salamanders (Eurycea bislineata). In streams below stormwater ponds, base levels of conductivity were elevated throughout the year and elevated Cl^- levels led to more frequent acute and chronic exceedances of U.S. Environmental Protection Agency ambient water quality criteria for Cl^- when compared to streams draining watersheds with no stormwater management ponds. However, five of the six streams studied had exceedance frequencies suggesting Cl^- associated with road salt application represented a persistent threat to aquatic life. Larval stream salamanders were relatively tolerant of salt, not exhibiting any lethal effects over a 96-h period until chloride levels exceeded 5000 mg/L for both species, and concentrations in streams rarely exceeded these levels and only for very short periods of time. Our results suggest road salts are not having acute lethal effects on salamanders in the streams we studied, but exceedance of U.S. Environmental Protection Agency ambient water quality standards for Cl^- suggest the potential for sublethal and indirect effects of Cl^- on salamander populations that require further study.

The effects of road salt on freshwater ecosystems and solutions for mitigating chloride pollution - A review

Road salt (mainly NaCl) is commonly used during the winter to ensure road and pavement safety; however, the long-term application of NaCl has negative consequences on soil and the water environment. The aims of the present review were to evaluate the impact of road salt on catchment processes which accelerate the eutrophication of waters, and to identify a possible approach for reducing the impact of winter salt treatments of roads and sidewalks, on water body quality. The objectives were implemented in accordance with the ecohydrological approach, which recommends using hierarchical steps to solve problems. The first step was the monitoring of threats, in which the causes of high chloride (Cl) concentrations in groundwater and surface water were identified. The results indicate that long-term winter application of road salt increases the annual mean concentrations of Cl in rivers and lakes, due to Cl entering groundwater. The second step was a cause-effect analysis of the impact of NaCl on the abiotic processes in soil and water, and on the biotic response to chloride exposure. Chlorides appear to decrease the biodiversity of aquatic animals and plants but favour the growth of phytoplankton, especially cyanobacteria. Moreover, Cl reduces the self-purification processes of water by decreasing nutrient accumulation in macrophytes, decreasing the denitrification rate and reducing organic matter decomposition. The third step was to evaluate possible solutions for reducing the negative impact of NaCl on the environment, and to improve the effectiveness of alternative de-icing agents. An analysis of available literature indicates that a system-based approach integrating engineering knowledge with an understanding of biological and hydrological processes is necessary to indicate solutions for reducing environmental risks from road salt use.

Kinetics of Thallium(I) Oxidation by Free Chlorine in Bromide-Containing Waters: Insights into the Reactivity with Bromine Species

The oxidation of thallium [Tl(I)] to Tl(III) by chlorine (HOCl) is an important process changing its removal performance in water treatment. However, the role of bromide (Br^-), a common constituent in natural water, in the oxidation behavior of Tl(I) during chlorination remains unknown. Our results demonstrated that Br^- was cycled and acted as a catalyst to enhance the kinetics of Tl(I) oxidation by HOCl over the pH range of 5.0-9.5. Different Tl(I) species (i.e., Tl⁺ and TlOH(aq)) and reactive bromine species (i.e., HOBr/BrO^-, BrCl, Br₂O, and BrOCl) were kinetically relevant to the enhanced oxidation of Tl(I). The oxidation by free bromine species became the dominant pathway even at a low Br^- level of 50 μg/L for a chlorine dose of 2 mg of Cl₂/L. It was found that the reactions of Tl⁺/BrCl, Tl⁺/BrOCl, and TlOH(aq)/HOBr dominated the kinetics of Tl(I) oxidation at pH < 6.0, pH 6.0-8.0, and pH > 8.0, respectively. The species-specific rate constants for Tl⁺ reacting with individual bromine species were determined and decreased in the order: BrCl > Br₂ > BrOCl > Br₂O > HOBr. Overall, the presented results refine our knowledge regarding the species-specific reactivity of TI(I) with bromine species and will be useful for further prediction of thallium mobility in chlorinated waters containing bromide.

Groundwater Recharge Assessment Using Multi Component Analysis: Case Study at the NW Edge of the Varaždin Alluvial Aquifer, Croatia

Exploring the interaction between precipitation, surface water, and groundwater has been a key subject of many studies dealing with water quality management. The Varaždin aquifer is an example of an area where high nitrate content in groundwater raised public concern, so it is important to understand the aquifer recharge for proper management and preservation of groundwater quality. The NW part of the Varaždin aquifer has been selected for study area, as precipitation, Drava River, accumulation lake, and groundwater interact in this area. In this study, groundwater and surface water levels, water temperature, water isotopes (2H and 18O), and chloride (Cl−) were monitored in precipitation, surface water, and groundwater during the four-year period to estimate groundwater recharge. Head contour maps were constructed based on the groundwater and surface water levels. The results show that aquifer is recharged from both Drava River and accumulation lake for all hydrological conditions–low, mean, and high groundwater levels. The monitoring results of water temperature, chloride content, and stable water isotopes were used as tracers, i.e. as an input to the mixing model for estimation of the contribution ratio from each recharge source. The calculation of mixing proportions showed that surface water is a key mechanism of groundwater recharge in the study area, with a contribution ratio ranging from 55% to 100% depending on the proximity of the observation well to surface water.

Toxicological impacts of roadway deicers on aquatic resources and human health: A review

During winter, snow and ice on roads in regions with cold weather can increase traffic crashes and casualties, resulting in travel delays and financial burdens to society. Anti-icing or deicing the roads can serve a cost-effective method to significantly reduce such risks. Although traditionally the main priorities of winter road maintenance (WRM) have been level of service, cost-effectiveness, and corrosion reduction, it is increasingly clear that understanding the environmental impacts of deicers is vital. One of the most important problems in this regard is environmental contamination caused by cumulative use of deicers, which has many detrimental effects on the aquatic systems. Among the deicers, the chloride-based ones raise the most toxicological concerns because they are highly soluble, can migrate quickly in the environment and have cumulative effects over time. In this review, we summarize and organize existing data, including the latest findings about the adverse effects of deicers on surface water and groundwater, aquatic species, and human health, and identify future research priorities. In addition, the data provided can be used to develop a framework for quantifying some of the variables that stakeholders and agencies use when preparing guidelines and standards for WRM programs. PRACTITIONER POINTS: Pollution from the increasing use of roadway deicers may have detrimental effects on the environment. Of particular concern are the acute and cumulative risks that chloride salts pose to aquatic species. Chloride salts are water-soluble, very difficult to remove, highly mobile, and non-degradable. Deicers cause water stratification, change the chemicophysical properties of water, and affect aquatic species and human health. Current guidelines may not be appropriate for environmental protection and need to be revised.

Comparative assessment of fluorine, sodium, and lithium distributions in snow cover in Siberia

Based on field studies of the snow cover and systematization and analysis of scientific data and technical literature data, the distributions of fluorine, sodium, and lithium, as elements included in the raw materials used for aluminum production, in the snow cover in areas proximal to Siberian aluminum smelters were considered. The results showed that the changes in concentrations of fluorine, sodium, and lithium in the snow cover near various plants have the same dispersion pattern, which can be described by an exponential relationship. Exponential relationships of diminishing concentration with distance from the emission source had high correlation coefficients. From the examples established by these relationships, an assumption was made that the behavior of these aerosols in the atmosphere is determined by the general physical and chemical properties, irrespective of the technologies and natural climatic regions of the plant locations. It is suggested that deposition of aerosols from industrial aluminium production can be achieved at a minimum distance from the plants or within the plant area through particle enlargement by various technological methods in aluminium production or by changing the atmospheric scattering capacity.

Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit evolutionary change, especially in non-native species that may harbor substantial within-population variability. To test whether urban stressors drive phenotypic differentiation and influence local adaptation, we compared stress responses of populations of a ubiquitous invader, reed canary grass (Phalaris arundinacea). Specifically, we quantified responses to salt, copper, and zinc additions by reed canary grass collected from four populations spanning an urbanization gradient (natural, rural, moderate urban, and intense urban). We measured ten phenotypic traits and trait plasticities, because reed canary grass is known to be highly plastic and because plasticity may enhance invasion success. We tested the following hypotheses: (a) Source populations vary systematically in their stress response, with the intense urban population least sensitive and the natural population most sensitive, and (b) plastic responses are adaptive under stressful conditions. We found clear trait variation among populations, with the greatest divergence in traits and trait plasticities between the natural and intense urban populations. The intense urban population showed stress tolerator characteristics for resource acquisition traits including leaf dry matter content and specific root length. Trait plasticity varied among populations for over half the traits measured, highlighting that plasticity differences were as common as trait differences. Plasticity in root mass ratio and specific root length were adaptive in some contexts, suggesting that natural selection by anthropogenic stressors may have contributed to root trait differences. Reed canary grass populations in highly urbanized wetlands may therefore be evolving enhanced tolerance to urban stressors, suggesting a mechanism by which invasive species may proliferate across urban wetland systems generally.

Managing Stormwater by Accident: A Conceptual Study

Stormwater-driven road salt is a chronic and acute issue for streams in cold, urban environments. One promising approach for reducing the impact of road salt contamination in streams and adjacent aquifers is to allow “accidental wetlands” to flourish in urban areas. These wetlands form naturally as a byproduct of human activities. In this study, we quantified the ability of an accidental wetland in northwestern North Carolina, USA, to reduce the timing and peak concentration of road salt in a stream. Monitoring suggests that flow and transport processes through the wetland reduce peak concentrations and delay their arrival at the adjacent stream. We expand these findings with numerical simulations that model multiple meltwater and summer storm event scenarios. The model output demonstrates that small accidental wetland systems can reduce peak salinities by 94% and delay the arrival of saltwater pulses by 45 days. Our findings indicate that accidental wetlands improve stream water quality and they may also reduce peak temperatures during temperature surges in urban streams. Furthermore, because they find their own niche, accidental wetlands may be more effective than some intentionally constructed wetlands, and provide opportunities to explore managing stormwater by letting nature take its course.

Dietary lipid quality mediates salt tolerance of a freshwater keystone herbivore

Salinization of freshwater ecosystems is a growing hazard for organisms and ecosystem functioning worldwide. In northern latitudes, road salt that is being transported into water bodies can cause year-round increases in lake salinity levels. Exploring the environmental factors driving the susceptibility of freshwater zooplankton to road salt is crucial for assessing the impact of salinization on food web processes. We studied the role of essential lipids, i.e., sterols and long-chain polyunsaturated fatty acids (PUFAs), in mediating salt tolerance of the freshwater keystone herbivore Daphnia. Sterols and PUFAs are involved in regulating ion permeability of biological membranes and thus we hypothesized that the susceptibility to salt is affected by the dietary sterol and PUFA supply. Life history experiments revealed opposing effects of sterol and PUFA supplementation on salt tolerance, i.e., tolerance increased upon sterol supplementation but decreased upon PUFA supplementation, which is consistent with their proposed impact on membrane permeability. Our results suggest that the susceptibility of freshwater zooplankton to salinization strongly depends on the dietary lipid supply and thus the phytoplankton community composition. Hence, trophic state related differences in the phytoplankton community composition need to be considered when assessing the consequences of salinization for freshwater ecosystem functioning.

Effects of clay in a sandy soil on saturated/unsaturated pore water flow and dissolved chloride transport from road salt applications

Saturated/unsaturated pore water flow induced by rainwater infiltration in a soil column composed of a mixture of Toyoura sand and a small amount of clay (kaolin minerals) and the rinsing rate (mass transfer) of dissolved NaCl accumulated in the pore system from previous road salt application were investigated by experiments and simulations. Experiments were conducted with variable kaolin minerals mass contents (mixing ratios) in the soil columns. Measured saturated hydraulic conductivity (Ks) diminished with increased clay contents, i.e., Ks=0.00771, 0.00560, 0.00536, 0.00519, and 0.00314 cm s-1, for clay contents = 0.2, 0.5, 1, 2, and 5%, respectively. Experimental NaCl concentrations in the effluent from the bottom of the soil columns were about constant for times t ≈ 800, 1200, 1300, 1400, and 3400 s from the beginning of a rinsing experiment for the clay contents = 0.2, 0.5, 1, 2, and 5%, respectively. These NaCl concentrations then decreased with time quickly, and finally, approached zero. The presented model can reproduce experimental time variations of NaCl concentration in the effluent from the soil column reliably. Simulated salt mass left in the soil column with time also matches the experimental results for the clay contents = 0.2 and 0.5%. An inconsistency between simulated and experimental salt mass left in the soil columns becomes more significant as the clay content increases. These results suggest that the soil-water retention curve for the pure Toyoura sand can be applied to the soil column composed of kaolin minerals/Toyoura sand mixture when the clay content is small, i.e., less than 1%. Prediction of rinsing process becomes more difficult with increased clay content. However, the time required to remove saline water from the soil column to less than 1% of its initial value simulated by the model agrees closely with experimental results of 1000, 1500, 1700, 2100, and 5400 s, respectively.

Evaluation of chloride contributions from major point and nonpoint sources in a northern U.S. state

Chloride pollution of groundwater and surface water resources is an environmental concern in many regions. While use of road salt for winter road maintenance is known to be a major source of chloride in the environment, limited research has investigated the environmental impacts of chloride discharged from water softeners, particularly in areas with hard water. A chloride budget was developed for the state of Minnesota to estimate the amount of chloride discharged from household water softeners as well as other domestic, agricultural, commercial, and industrial sources. The analysis used multiple data sources, including salt sales records and wastewater monitoring data, and used statistical, spatial, and survey methods to estimate chloride loading from major sources statewide. Annual chloride mass contributions were estimated for the following sources: household water softener use; human excretions; household product use; chloride concentrations in drinking water; atmospheric deposition; road salt use; dust suppressant use; fertilizer application; industrial discharge; and livestock excretions. A mass balance for 96 wastewater treatment plants with effluent monitoring data showed that across these facilities, discharge from water softeners was the largest chloride source. A statewide chloride budget found that road salt was the largest source of chloride to the environment, but that WWTPs and fertilizer were also substantial sources, discharging 221,300 t and 209,900 t annually. Water softeners were estimated to contribute 65% of the chloride discharged to all 613 municipal WWTPs statewide. Methods used in this analysis could be applied to other communities, watersheds, or states with similar conditions. The results of the analyses indicate that water softening is an important chloride source in areas with hard water and underscore the importance of identifying and characterizing chloride sources in less urban areas, where deicing salt may be a less important contributor and receiving water bodies are often lakes, reservoirs, and streams.

The Impact of Water and Road Salt with Anti-Caking Agent on the Stiffness of Select Mixes Used for the Road Surface

An original experimental method was used to investigate the influence of water and road salt with anti-caking agent on the material used in pavement construction layers. This method allowed for monitoring material changes resulting from the influence of water and road salt with anti-caking agent over time. The experiment used five different mineral road mixes, which were soaked separately in water and brine for two time intervals (2 days and 21 days). Then, each sample of the mix was subjected to tests of the complex module using the four-point bending (4PB-PR) method. The increase in mass of the soaked samples and the change in value of the stiffness modulus were analyzed. Exemplary tomographic (X-ray) imaging was performed to confirm the reaction of the road salt and anti-caking agent (lead agent) with the material. Based on measurements of the stiffness modulus and absorption, the correlations of the mass change and the value of the stiffness modulus were determined, which may be useful in estimating the sensitivity of mixes to the use of winter maintenance agents—e.g., road salt with anti-caking agent (sodium chloride). It was found that the greatest changes occur for mixes intended for base course layers (mineral cement mix with foamed asphalt (MCAS) and mineral-cement-emulsion mixes (MCE)) and that the smallest changes occur for mixes containing highly modified asphalt (HIMA).

Road salt retention and transport through vadose zone soils to shallow groundwater

Increasing background salinity in watersheds has largely been attributed to road salt retention in groundwaters due to their long residence times. However, laboratory studies demonstrate that soils temporarily store salts, either in porewater or adsorbed onto particles. Field studies of road salt retention in soils are nevertheless rare, and mechanisms of salt transport across multiple hydrological reservoirs (e.g., from soil to groundwater) are unknown. Thus, we collected roadside soil porewater and karst spring water weekly for ~1.5 yr to determine salt transport through the vadose zone into the phreatic zone. We observed dual retention mechanisms of sodium (Na⁺) and chloride (Cl^-) in soils due to slow porewater movement, causing ion movement through the soil as slow as 1.3 cm/day, and cation exchange processes, leading to initial Na⁺ retention followed by later release months after application. Cation exchange processes also caused base cation loss from exchange sites into mobile porewater. Rapid Na⁺ and Cl^- delivery to groundwater occurred through karst conduits during the winter. However, elevated background levels of salt ions in groundwater during the non-salting months indicated accumulation in the catchment due to slower porewater flow in the soil and rock matrix and delayed Na⁺ release from soil exchange sites.

Chlorine cycling and the fate of Cl in terrestrial environments

Chlorine (Cl) in the terrestrial environment is of interest from multiple perspectives, including the use of chloride as a tracer for water flow and contaminant transport, organochlorine pollutants, Cl cycling, radioactive waste (radioecology; 36Cl is of large concern) and plant science (Cl as essential element for living plants). During the past decades, there has been a rapid development towards improved understanding of the terrestrial Cl cycle. There is a ubiquitous and extensive natural chlorination of organic matter in terrestrial ecosystems where naturally formed chlorinated organic compounds (Clorg) in soil frequently exceed the abundance of chloride. Chloride dominates import and export from terrestrial ecosystems while soil Clorg and biomass Cl can dominate the standing stock Cl. This has important implications for Cl transport, as chloride will enter the Cl pools resulting in prolonged residence times. Clearly, these pools must be considered separately in future monitoring programs addressing Cl cycling. Moreover, there are indications that (1) large amounts of Cl can accumulate in biomass, in some cases representing the main Cl pool; (2) emissions of volatile organic chlorines could be a significant export pathway of Cl and (3) that there is a production of Clorg in tissues of, e.g. plants and animals and that Cl can accumulate as, e.g. chlorinated fatty acids in organisms. Yet, data focusing on ecosystem perspectives and combined spatiotemporal variability regarding various Cl pools are still scarce, and the processes and ecological roles of the extensive biological Cl cycling are still poorly understood.

The Combined Effects of Road Salt and Biotic Stressors on Amphibian Sex Ratios

Aquatic systems worldwide are threatened by the anthropogenic use of synthetic chemicals, including pesticides, pharmaceuticals, and road de-icers. Exposure to contaminants can alter the behavior, morphology, and physiology of organisms if it occurs during sensitive life stages. For instance, past studies have documented feminization of male amphibians following herbicide exposure and skewed sex ratios among amphibian populations exposed to road salt. However, many of these studies lack the complexities found within natural environments, such as competition with conspecifics or threat of predation, which are also known to influence development. Thus, it is important to understand how anthropogenic and natural stressors interact to alter animal sex ratios. Given the growing concern of secondary salinization of freshwater systems, we exposed larval wood frogs (Rana sylvatica) to either road salt (sodium chloride [NaCl]) or an alternative salt mixture (NaCl, magnesium chloride [MgCl₂ ], and potassium chloride [KCl]) at 3 concentrations (200, 600, and 1000 mg Cl^- /L) crossed with 3 biotic stressors (no-stressor control, competition, or predator cues) to examine their potentially interactive effects on sex. Exposure to biotic stressors and NaCl did not influence wood frog sex ratios. In contrast, tadpole exposure to the intermediate salt mixture concentration significantly reduced the proportion of female frogs. Future studies are needed to determine whether such changes in sex are widespread among sensitive species with complex life cycles, and to assess the consequences of sex ratio changes on long-term population dynamics. Environ Toxicol Chem 2021;40:231-235. © 2020 SETAC.

Salty summertime streams—road salt contaminated watersheds and estimates of the proportion of impacted species

Road salt runoff is a leading cause of secondary freshwater salinization in north temperate climates. Increased chloride concentrations in freshwater can be toxic and lead to changes in organismal behavior, lethality, biotic homogenization, and altered food webs. High chloride concentrations have been reported for winter months in urban centers, as road density is highest in cities. However, summer chloride conditions are not typically studied as road salt is not actively applied outside of winter months, yet summer is when many taxa reproduce and are most sensitive to chloride. In our study, we test the spatial variability of summer chloride conditions across four watersheds in Toronto, Canada. We find 89% of 214 sampled sites exceeded the federal chronic exposure guidelines for chloride, and 13% exceeded the federal acute guidelines. Through a model linking concentration to cumulative proportion of impacted species, we estimate 34% of sites show in excess of one-quarter of all species may be impacted by their site-specific chloride concentrations, with up to two-thirds of species impacted at some sites. Our results suggest that even presumed low seasons for chloride show concentrations sufficient to cause significant negative impacts to aquatic communities.

Green stormwater infrastructure redirects deicing salt from surface water to groundwater

Winter deicing salt application has led to water quality impairment as stormwater carries salt ions (Na⁺ and Cl^-) through watersheds. Green infrastructure (GI) is a promising urban stormwater management practice, but its efficacy in managing salt is unknown. GI is not yet designed to remove salt, but may have potential to mitigate its loading to surface waters. Two roadside infiltration-based GI practices in Northern Virginia (bioretention and bioswale) were monitored year-round over 28 precipitation events to investigate the transport of salt through modern stormwater infrastructure. Stormwater runoff volumes and concentrations of salt ions entering and exiting each GI were monitored to determine reductions of salt ions. Both the bioretention and bioswale significantly reduced effluent surface loads of Cl^- and Na⁺ (76% to 82%), displaying ability to temporarily retain and infiltrate salts and delay their release to surface waters. Changes in bioretention soil chemistry revealed a small percentage of Na⁺ was stored long-term by ion exchange, but no long-term Cl^- storage was observed. Limited soil storage along with groundwater observations suggest the majority of salt removed from stormwater by the bioretention infiltrates into groundwater. Infiltration GI can buffer surface waters from salt, but are also an avenue for groundwater salt loading.

A large-scale assessment of lakes reveals a pervasive signal of land use on bacterial communities

Lakes play a pivotal role in ecological and biogeochemical processes and have been described as "sentinels" of environmental change. Assessing "lake health" across large geographic scales is critical to predict the stability of their ecosystem services and their vulnerability to anthropogenic disturbances. The LakePulse research network is tasked with the assessment of lake health across gradients of land use on a continental scale. Bacterial communities are an integral and rapidly responding component of lake ecosystems, yet large-scale responses to anthropogenic activity remain elusive. Here, we assess the ecological impact of land use on bacterial communities from over 200 lakes covering more than 660,000 km² across Eastern Canada. In addition to community variation between ecozones, land use across Eastern Canada also appeared to alter diversity, community composition, and network structure. Specifically, increasing anthropogenic impact within the watershed lowered diversity. Likewise, community composition was significantly correlated with agriculture and urban development within a watershed. Interaction networks showed decreasing complexity and fewer keystone taxa in impacted lakes. Moreover, we identified potential indicator taxa of high or low lake water quality. Together, these findings point to detectable bacterial community changes of largely unknown consequences induced by human activity within lake watersheds.

Road Salt Impacts Freshwater Zooplankton at Concentrations below Current Water Quality Guidelines

Widespread use of NaCl for road deicing has caused increased chloride concentrations in lakes near urban centers and areas of high road density. Chloride can be toxic, and water quality guidelines have been created to regulate it and protect aquatic life. However, these guidelines may not adequately protect organisms in low-nutrient, soft water lakes such as those underlain by the Precambrian Shield. We tested this hypothesis by conducting laboratory experiments on six Daphnia species using a soft water culture medium. We also examined temporal changes in cladoceran assemblages in the sediments of two small lakes on the Canadian Shield: one near a highway and the other >3 km from roads where salt is applied in the winter. Our results showed that Daphnia were sensitive to low chloride concentrations with decreased reproduction and increased mortality occurring between 5 and 40 mg Cl^-/L. Analysis of cladoceran remains in lake sediments revealed changes in assemblage composition that coincided with the initial application of road salt in this region. In contrast, there were no changes detected in the remote lake. We found that 22.7% of recreational lakes in Ontario have chloride concentrations between 5 and 40 mg/L suggesting that cladoceran zooplankton in these lakes may already be experiencing negative effects of chloride.

Long-term impacts of road salt application on the groundwater contamination in urban environments

This study explores the contamination potential of groundwater due to the use of sodium chloride (NaCl) in the wintertime. The research was conducted in two Iranian cities, Malayer and Hamedan, where groundwater is the major source of water for drinking and irrigating purposes. However, the amount of deicing salt used in the former is about 10 times less than that used in the latter. The assessment of geochemical dataset from 2004 to 2018 revealed no significant trend in the groundwater characteristics of Malayer where the water quality indices were in the range of WHO and USEPA permissible limits. In contrast, the indices had a continually increasing trend (~ 2.3% annually) in Hamedan's supply wells over the same period and particularly near the urban areas that showed higher levels (> 5 times on average) than those observed in Malayer. This could mainly be ascribed to the influx of halite. Based on the USSL diagram, the water samples retrieved from the latter system were mostly classified as C3-S1 (decreasing the soil fertility) and even as C4-S2 (harmful for agriculture activities). Chloride contamination rates also reached 250 mg/L, which could negatively affect the water potability and threaten the aquatics microorganisms. In this region, a rather similar distribution of NaCl and arsenic was observed, implying mobilization of toxic trace metals with the increased salt encroachment into the aquifer. Based on such findings, it is suggested that in snow-influenced cities (e.g., Hamedan), new approaches for winter maintenance be considered to prevent the gradual deterioration of water resources.

Trace element fractionation through halite crystallisation: Geochemical mechanisms and environmental implications

Halite is an important mineral for industry, agriculture and food production. It crystallises during water evaporation, and the progressive increase of dissolved metal ions in the brine occurs simultaneously. Thus, halite exploitation may deliver metal ions into the environment and the mechanism of this trace element accumulation has to be studied. In this work, we investigate the distribution of lanthanides and Y (hereafter called rare earth elements, REE), Zr and Hf between crystallising halite and brines in the Dead Sea as geochemical tools for recognising the mechanism of metal ion removal from brines and accumulation in halite. Halite forms cubic crystals where octahedral planes sometimes occur under particular thermal gradient conditions. Our findings indicate that crystal morphology influences the mechanism of metal ion removal from brines because octahedral surfaces are polar unlike those that are cubic. Accordingly, octahedra preferentially fractionate aqueous charged species such as [Hf(OH)₅]^-, compared to neutral species such as [Zr(OH)₄]⁰. Cubic surfaces do not fractionate aqueous species. In crystal cores, positive Eu anomalies occur suggesting Eu substitution for Na in the lattice. This substitution is energetically justified by ab initio calculations. Hf enrichment relative to Zr also occurs in primary halite-rich evaporites. It is not found in cubic halite from saltworks. The results of this study suggest that primary halite kinetically crystallised from brines can concentrate dissolved metal ions onto crystal surfaces where dissolved charged species are adsorbed. Accordingly, the dissolution of halite due to human activity can release these metal ions to the environment.

Observations of the seasonal buildup and washout of salts in urban bioswale soil

The objective of this study was to quantify the seasonal risk of salt damage to bioswale plants, soil, microbes, and downstream waterbodies. To do so, we measured sodium, chloride, and electrical conductivity levels at seven bioswales located in the Bronx, New York City, over 42 storm events during a three-year monitoring period. The bioswale with the greatest salt contamination (median 206 mg/L chloride) had a unique inlet design without any possibility of inlet bypass. The most severe effects at all sites were found during the winter season, as infiltrate concentrations frequently (40% of winter samples) exceeded 1000 mg/L chloride, a level lethal to aquatic plants and invertebrates, and electrical conductivity exceeded 1500 μS cm^-1 (50% of winter samples), a level that may displace bound metals from bioswale soils and into the subsurface. However, low levels of permanent salt contamination may be expected all year, as concentrations frequently (87% of all samples) exceeded the United States Environmental Protection Agency drinking water standard of 20 mg/L sodium. A regression of chloride washout over the year yielded concentrations greater than those damaging to soil structure and soil microbes (90 mg/L) until August 20th, and above those damaging to roadside vegetation (30 mg/L) for the entire year. Today, the vast majority of bioswales in cold climates are built with salt-tolerant vegetation, but prior to this study, it was unclear to what degree this was, in fact, necessary. Our findings confirm salt-tolerant vegetation to be optimal, as winter de-icing salts are not sufficiently flushed from soils by the spring growing season. Our findings also demonstrate how bioswale inlet design and site location can influence soil contamination.

Lakes at Risk of Chloride Contamination

Lakes in the Midwest and Northeast United States are at risk of anthropogenic chloride contamination, but there is little knowledge of the prevalence and spatial distribution of freshwater salinization. Here, we use a quantile regression forest (QRF) to leverage information from 2773 lakes to predict the chloride concentration of all 49 432 lakes greater than 4 ha in a 17-state area. The QRF incorporated 22 predictor variables, which included lake morphometry characteristics, watershed land use, and distance to the nearest road and interstate. Model predictions had an r² of 0.94 for all chloride observations, and an r² of 0.86 for predictions of the median chloride concentration observed at each lake. The four predictors with the largest influence on lake chloride concentrations were low and medium intensity development in the watershed, crop density in the watershed, and distance to the nearest interstate. Almost 2000 lakes are predicted to have chloride concentrations above 50 mg L^-1 and should be monitored. We encourage management and governing agencies to use lake-specific model predictions to assess salt contamination risk as well as to augment their monitoring strategies to more comprehensively protect freshwater ecosystems from salinization.

Modifying SWAT-CS for simulating chloride dynamics in a Boreal Shield headwater catchment in south-central Ontario, Canada

Rising chloride concentrations in surface water due to applications of deicing practices is proving detrimental to aquatic systems. In this study, a new chloride module is developed for a version of the Soil and Water Assessment Tool specially designed for Canadian Shield catchments (SWAT-CS) to model long-term chloride dynamics in a headwater catchment in south-central Ontario, Canada. In this modified model (SWAT-CS-CL; extended SWAT-CS model for chloride), chloride sources, sinks, internal storages or pools, and movement between these components are depicted. Performance of SWAT-CS-CL is assessed using a two-stage evaluation process based on the generalized likelihood uncertainty analysis (GLUE) framework. SWAT-CS-CL was found to perform moderately well, with simulated monthly chloride in streams and lake outflow following overall chloride trends and capturing regular chloride dynamics. However, simulations fail to consistently reproduce some instances of large or low chloride fluxes. Limitations in simulating large chloride fluxes may be attributed to the inadequate ability for SWAT-CS-CL to closely simulate snowpack and snowmelt processes. Parameter transferability among sub-catchments does suggest that there is a potential to extend SWAT-CS-CL to other Canadian Shield catchments for chloride modelling. Further improvements are needed through more trials to other catchments in a same or different landscape, and by modifying the simulation structure, especially representation of snow hydrology and chloride inputs.

Road salt impact on soil electrical conductivity across an urban landscape

Road salt application is a necessary component of winter road maintenance but comes with an environmental cost. Salts are transported via stormwater drainage or overland and soil throughflow to surface waterbodies, where excess ions create unfavorable or even uninhabitable conditions for freshwater organisms. Soils may retain salts during the process of overland and subsurface flow, thus acting as reservoirs that slow the transport of salt into freshwaters. Understanding the capacity and consistency of anthropogenic salt storage in urban soils may allow us to discover when and where deicing salt applications are most harmful. This article investigates the degree to which soils across a heterogeneous urban landscape retain salts. We measured the electrical conductivity (EC) of soils in an urban setting. Land covers included forests, grasslands, open spaces, low- and medium-density developments and along roadsides. We found that across land-cover types, soil carbon and porosity were correlated to EC in late summer, which suggests that pore space is an important and long-lasting reservoir for salt. In addition, more developed areas, had higher mean soil EC and greater EC variability within and between sites, with 75% of overall variance occurring within individual sites. We hypothesize that this within-site heterogeneity is driven by anthropogenic modifications to salt inputs and soil characteristics. The high EC variance in highly developed urban soils is a previously undiscussed phenomenon and highlights the fine-scale complexity of heterogeneous urban landscapes and the need for high-resolution sampling to accurately characterize urban ecosystems.

High-Frequency Data Reveal Deicing Salts Drive Elevated Specific Conductance and Chloride along with Pervasive and Frequent Exceedances of the U.S. Environmental Protection Agency Aquatic Life Criteria for Chloride in Urban Streams

Increasing specific conductance (SC) and chloride concentrations [Cl] negatively affect many stream ecosystems. We characterized spatial variability in SC, [Cl], and exceedances of Environmental Protection Agency [Cl] criteria using nearly 30 million high-frequency observations (2-15 min intervals) for SC and modeled [Cl] from 93 sites across three regions in the eastern United States: Southeast, Mid-Atlantic, and New England. SC and [Cl] increase substantially from south to north and within regions with impervious surface cover (ISC). In the Southeast, [Cl] weakly correlates with ISC, no [Cl] exceedances occur, and [Cl] concentrations are constant with time. In the Mid-Atlantic and New England, [Cl] and [Cl] exceedances strongly correlate with ISC. [Cl] criteria are frequently exceeded at sites with greater than 9-10% ISC and median [Cl] higher than 30-80 mg/L. Tens to hundreds of [Cl] exceedances observed annually at most of these sites help explain previous research where stream ecosystems showed changes at (primarily nonwinter) [Cl] as low as 30-40 mg/L. Mid-Atlantic chronic [Cl] exceedances occur primarily in December-March. In New England, exceedances are common in nonwinter months. [Cl] is increasing at nearly all Mid-Atlantic and New England sites with the largest increases at sites with higher [Cl].

Assessing the recycling of chlorine and its long-lived 36Cl isotope in terrestrial ecosystems through dynamic modeling

It is unclear to what extent chlorine (Cl) and its long-lived isotope ³⁶Cl are recycled in different terrestrial environments in response to time-variable inputs. A new version of a dynamic compartment model was developed to examine the transformation and transfer processes influencing the partitioning and persistence of both Cl and ³⁶Cl in forest ecosystems. The model's performance was evaluated by comparing simulations and field observations of scenarios of stable Cl atmospheric deposition and of global ³⁶Cl fallout. The model reproduced Cl storage in soil reasonably well, despite wide heterogeneity in environmental conditions and atmospheric deposits. Sensitivity analysis confirmed that the natural production of organochlorine in soil plays a major role in Cl build-up and affects long-term Cl dynamics. The timeframe required for the soil organochlorine pool to reach equilibrium in a steady-state system was several thousands of years. Interestingly, root uptake flux, a predominant pathway of the inorganic cycle, was found to affect both inorganic and organic pools in soil, highlighting the importance of plant-soil interactions in Cl dynamics. Model outputs agreed well with local ³⁶Cl measurements, and demonstrated that 90% of the ³⁶Cl found in soil may have come from bomb-test fallout. The pattern of estimated ³⁶Cl/Cl ratios showed that soil ³⁶Cl was not in equilibrium with ³⁶Cl levels in rain input in the post-bomb period. Complete recovery of a natural isotopic ratio in drainage water will need a time close to the residence time of organic ³⁶Cl in soil: i.e., 800 years. A simple dynamic model concept was found to be suitable to illustrate the plant-soil interactions combining both the inorganic and organic Cl cycles acting over different time scales.

Chloride inputs to the North Saskatchewan River watershed: the role of road salts as a potential driver of salinization downstream of North America's northern most major city (Edmonton, Canada)

Multiple anthropogenic activities are driving the salinization of freshwater environments threatening water resources worldwide. Accordingly, this research will first examine the spatial and temporal variability of major ions (i.e. Ca²⁺, Mg²⁺, Na⁺, K⁺, SO₄^2-, CO₃^2-, and HCO₃^-) upstream and downstream of the northernmost major city in North America (Edmonton, Canada). Second, this research will estimate the relative contributions of the major sources of chloride (Cl), the main constituent of road deicers, to the sub-basin around Edmonton. Monthly water quality data was for three sites on the North Saskatchewan River (NSR): Rocky Mountain House (RMH - downstream of the Rocky Mountain headwaters), Devon Bridge (upstream of Edmonton) and Pakan Bridge (downstream of Edmonton). Change ratios investigate the downstream alterations of major ions at Pakan and Devon, relative to RMH. Seasonal Kendall tests examine temporal trends (1987-2017). A mass-balance approach then quantifies the major sources of Cl in the sub-basin of the NSR between Devon and Pakan. Progressing from the Rocky Mountain headwaters (at RMH) to downstream of Edmonton (at Pakan), Cl^- increased by >5.5 times, Na⁺ by 4.8 times and K by 2.7 times. No significant temporal trends for Cl^-, Na⁺ and K⁺ were evident at Devon (upstream of Edmonton), whereas all three significantly increased at Pakan (downstream of Edmonton). Deicers (54%), agriculture (20%), along with waste water treatment plant and industrial effluent (15%) were the largest Cl sources in the NSR Devon-Pakan sub-basin. In total, 77 Gg yr^-1 of Cl (or 6 t km² yr^-1) is added to the Devon-Pakan sub-basin, of which, 43 Gg yr^-1 is retained. Understanding and managing the major drivers of freshwater salinization will be of increasing importance in the 21^st century owing to the potential salinization of freshwater resources in the context of a changing climate.