Effects of an Extreme Flood on Trace Elements in River Water-From Urban Stream to Major River Basin

Contaminant mobilization from the vadose zone to groundwater during experimental river flooding events

Natural river flooding events can mobilize contaminants from the vadose zone and lead to increased concentrations in groundwater. Characterizing the mass and transport mechanisms of contaminants released from the vadose zone to groundwater during these recharge events is particularly challenging. Therefore, conducting highly-controlled in-situ experiments that simulate natural flooding events can help increase the knowledge of where contaminants can be stored and how they can move between hydrological compartments. This study specifically targets uranium pollution, which is accompanied by high sulfate levels in the vadose zone and groundwater. Two novel experimental river flooding events were conducted that utilized added non-reactive halides (bromide and iodide) and 2,6-difluorobenzoate tracers. In both experiments, about 8 m3 of traced water from a nearby contaminant-poor river was flooded in a 3-m diameter basin and infiltrated through the vadose zone and into a contaminant-rich unconfined aquifer for an average of 10 days. The aquifer contained 13 temporary wells that were monitored for solute concentration for up to 40 days. The groundwater analysis was conducted for changes in contaminant mass using the Theissen polygon method and for transport mechanisms using temporal moments. The results indicated an increase in uranium (21 and 24%), and sulfate (24 and 25%) contaminant mass transport to groundwater from the vadose zone during both experiments. These findings confirmed that the vadose zone can store and release substantial amounts of contaminants to groundwater during flooding events. Additionally, contaminants were detected earlier than the added tracers, along with higher concentrations. These results suggested that contaminant-rich pore water in the vadose zone was transported ahead of the traced flood waters and into groundwater. During the first flooding event, elevated concentrations of contaminants were sustained, and that chloride behaved similarly. The findings implied that contaminant- and chloride-rich evaporites in the vadose zone were dissolved during the first flooding event. For the second flooding event, the data suggested that the contaminant-rich evaporites continued to dissolve whereas chloride-rich evaporites were previously flushed. Overall, these findings indicated that contaminant-rich pore water and evaporites in the vadose zone can play a significant role in contaminant transport during flooding events.

Geochemical characteristics and environmental implication of rare earth elements in sediments from the Three Gorges Reservoir, China

The construction and operation of the Three Gorges Dam occluded sediment transportation in the Yangtze River. However, the sources, transport processes, and environmental impacts of these sediments on the Three Gorges Reservoir (TGR) remain unclear. Here, we used rare earth elements (REEs) to trace the transport pathways of sediments in the TGR, China. Geochemical characteristics including the chemical composition and fractionation, temporal and spatial distribution, and potential sources of REEs were also evaluated in this study. The individual REEs concentration in the TGR sediments followed the Oddo-Harkins rule, with the mean REEs value of 207.33 μg/g. REEs concentrations in the midstream were higher than those in the upstream and downstream of the TGR. Statistical analysis showed that water impoundment phase had no significant influence on the distribution of REEs. TGR sediments are mainly derived from terrigenous detrital particulates, characterized by a distinctive enrichment in light REEs, with its percentage higher than 90 % of the total REEs. The significant positive correlation among the REEs confirmed that they are co-existed and shared the similar sources. Multiple provenance analysis approaches using discriminant function analyses, provenance indices, and La/Yb-La/Sm-Gd/Yb ternary diagrams further indicated that the REEs in sediments originated from the weathering of mudstone in the basin of TGR. After periodic water level fluctuation for more than six years, the chemical compositions of REEs in TGR sediments slightly differed from those of the Yangtze River sediments before TGR construction, but were similar to those of the downstream of the Yangtze River. Therefore, this study indicated that the construction and operation of the TGR changed the chemical compositions and the origin of the sediments in the Yangtze River, which can provide useful insights into the transport pathways of TGR sediments and their impacts on the fluvial environment.

The concentration of Lithium in water resources: A systematic review, meta-analysis and health risk assessment

The presence of trace elements such as lithium (Li) in water resources in the long term can endanger consumers' health. Several studies have been conducted on Li concentration in water sources; hence, this study attempted to retrieve studies using a systematic search. The search was conducted in Web of Sciences, Embase, PubMed, and Scopus databases from 1 January 2010 to 15 January 2023. Li concentration was meta-analyzed based on the type of water resources and countries subgroups in the random effects model (REM) statistical analysis. In addition, health risk assessment in different age groups was calculated using the target hazard quotient (THQ). This study included 76 papers with 157 data reports in our meta-analysis. The overall pooled concentration of Li was 5.374 (95 % CI: 5.261-5.487 μg/L). The pooled concentration of Li in groundwater (40.407 μg/L) was 14.53 times surface water (2.785 μg/L). The highest water Li content was attributed to Mexico (2,209.05 μg/L), Bolivia (1,444.05 μg/L), Iraq (1,350 μg/L), and Argentina (516.39 μg/L). At the same time, the lowest water Li content was associated with Morocco (1.20 μg/L), Spain (0.46 μg/L), and India (0.13 μg/L). THQ due to Li in water resources in consumers of Iraq, Mexico, South Africa, Afghanistan, Bolivia, Portugal, Malawi, South Korea, Nepal, South Korea, Argentina, and the USA was higher than 1 value. Therefore, continuous monitoring of Li concentration in water sources and reducing Li concentration, especially in groundwater water, using new water treatment processes in these countries are recommended.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Contaminant Exposure and Transport from Three Potential Reuse Waters within a Single Watershed

Global demand for safe and sustainable water supplies necessitates a better understanding of contaminant exposures in potential reuse waters. In this study, we compared exposures and load contributions to surface water from the discharge of three reuse waters (wastewater effluent, urban stormwater, and agricultural runoff). Results document substantial and varying organic-chemical contribution to surface water from effluent discharges (e.g., disinfection byproducts [DBP], prescription pharmaceuticals, industrial/household chemicals), urban stormwater (e.g., polycyclic aromatic hydrocarbons, pesticides, nonprescription pharmaceuticals), and agricultural runoff (e.g., pesticides). Excluding DBPs, episodic storm-event organic concentrations and loads from urban stormwater were comparable to and often exceeded those of daily wastewater-effluent discharges. We also assessed if wastewater-effluent irrigation to corn resulted in measurable effects on organic-chemical concentrations in rain-induced agricultural runoff and harvested feedstock. Overall, the target-organic load of 491 g from wastewater-effluent irrigation to the study corn field during the 2019 growing season did not produce substantial dissolved organic-contaminant contributions in subsequent rain-induced runoff events. Out of the 140 detected organics in source wastewater-effluent irrigation, only imidacloprid and estrone had concentrations that resulted in observable differences between rain-induced agricultural runoff from the effluent-irrigated and nonirrigated corn fields. Analyses of pharmaceuticals and per-/polyfluoroalkyl substances in at-harvest corn-plant samples detected two prescription antibiotics, norfloxacin and ciprofloxacin, at concentrations of 36 and 70 ng/g, respectively, in effluent-irrigated corn-plant samples; no contaminants were detected in noneffluent irrigated corn-plant samples.

New insights into the pollutant composition of stormwater treating wetlands

With intensified climate change and urbanisation, constructed wetland (CW) serves as an alternative to conventional wastewater treatment plants. In Australia, the primary function of CW is to reduce sediments, nutrients from runoffs and attenuate floods. However, water quality analysis after construction is limited, hence, pollutant composition in established CWs and target pollutants in many guidelines remain outdated. To refresh the understanding of pollutants in urban discharges, this study reviewed two CWs in industrialised regions of Victoria, Australia. A total number of 26 pollutants were analysed in the collected water and sediment samples from both CW. The findings highlighted excessive concentrations of Zinc, Aluminium, Iron and Copper in one wetland and less commonly found pollutants like Barium, Titanium and Strontium are also detected. While Arsenic, Zinc, Copper, Nickel and hydrocarbons' accumulations are particularly significant in the other wetland. This study also reviews the pollutants discovered in 136 stormwater wetlands and covers the sources and impacts of various metal pollutants in stormwater runoffs. Overall, it is found that the concentrations of Zinc, Aluminium and Iron are particularly high in the CWs reviewed. This study brings attention to the pollutants profile of established CWs and the impact of heavy metals on the aquatic environment. The findings from this research revealed that the existing design and management guidelines for constructed wetlands in urban catchments are lacking in reduction targets for metal pollutants, thus improvements are essential to safeguard the water quality and performance of CWs.

Soil Nematodes as the Silent Sufferers of Climate-Induced Toxicity: Analysing the Outcomes of Their Interactions with Climatic Stress Factors on Land Cover and Agricultural Production

Unsustainable anthropogenic activities over the last few decades have resulted in alterations of the global climate. It can be perceived through changes in the rainfall patterns and rise in mean annual temperatures. Climatic stress factors exert their effects on soil health mainly by modifying the soil microenvironments where the soil fauna reside. Among the members of soil fauna, the soil nematodes have been found to be sensitive to these stress factors primarily because of their low tolerance limits. Additionally, because of their higher and diverse trophic positions in the soil food web they can integrate the effects of many stress factors acting together. This is important because under natural conditions the climatic stress factors do not exert their effect individually. Rather, they interact amongst themselves and other abiotic stress factors in the soil to generate their impacts. Some of these interactions may be synergistic while others may be antagonistic. As such, it becomes very difficult to assess their impacts on soil health by simply analysing the physicochemical properties of soil. This makes soil nematodes outstanding candidates for studying the effects of climatic stress factors on soil biology. The knowledge obtained therefrom can be used to design sustainable agricultural practices because most of the conventional techniques aim at short-term benefits with complete disregard of soil biology. This can partly ensure food security in the coming decades for the expanding population. Moreover, understanding soil biology can help to preserve landscapes that have developed over long periods of climatic stability and belowground soil biota interactions.

Watershed-Scale Risk to Aquatic Organisms from Complex Chemical Mixtures in the Shenandoah River

River waters contain complex chemical mixtures derived from natural and anthropogenic sources. Aquatic organisms are exposed to the entire chemical composition of the water, resulting in potential effects at the organismal through ecosystem level. This study applied a holistic approach to assess landscape, hydrological, chemical, and biological variables. On-site mobile laboratory experiments were conducted to evaluate biological effects of exposure to chemical mixtures in the Shenandoah River Watershed. A suite of 534 inorganic and organic constituents were analyzed, of which 273 were detected. A watershed-scale accumulated wastewater model was developed to predict environmental concentrations of chemicals derived from wastewater treatment plants (WWTPs) to assess potential aquatic organism exposure for all stream reaches in the watershed. Measured and modeled concentrations generally were within a factor of 2. Ecotoxicological effects from exposure to individual components of the chemical mixture were evaluated using risk quotients (RQs) based on measured or predicted environmental concentrations and no effect concentrations or chronic toxicity threshold values. Seventy-two percent of the compounds had RQ values <0.1, indicating limited risk from individual chemicals. However, when individual RQs were aggregated into a risk index, most stream reaches receiving WWTP effluent posed potential risk to aquatic organisms from exposure to complex chemical mixtures.

Water quality variability affected by landscape patterns and the associated temporal observation scales in the rapidly urbanizing watershed

Landscape patterns are a major factor affecting river water quality variations. However, the related temporal scaling effects of water quality variations and associated responses to landscape have not been fully explored in rapidly urbanizing watersheds. In this study, we explored the event and seasonal water quality variations by multi-section monitoring, and annual water quality variations using model simulation. Results showed that spatial heterogeneity of river water quality varied from event to annual scales. Rainfall showed greater impacts on event water quality variations (changed by 155.20%-183.70% after the rainfall) than seasonal variations (changed by 35.32%-92.25% from dry to wet season). Nutrients varied more significantly than other pollutants, such as chemical oxygen demand and suspended solids. Compared to annual scale, landscape pattern showed a more significant correlation with event and seasonal variations of water quality. Besides, grassland and forest might change from sink to source landscapes as observation scales changed from season and annum to event. Fragmentation (diversity) metrics showed negative (positive) relationships with event and seasonal water quality variations. However, the impacts of these landscape metrics would become heterogeneous for annual water quality variations. These results highlighted the consideration of multi-temporal studies and provided useful suggestions for urban water quality protection.

Contrasting distribution of REE and yttrium among particulate, colloidal and dissolved fractions during low and high flows in peri-urban and agricultural river systems

Understanding the possible consequences of anthropogenic activities on REY environmental fate and adverse effects on biota requires a detailed knowledge of their distribution between the particulate, colloidal and dissolved fractions. Such information is practically non-existent for peri-urban rivers having heavily populated basins and suffering from direct impacts from various human activities. The present study compared the distribution of REY among the particulate (>1000 nm), coarse colloidal (1000 nm - 220 nm), small colloidal (220 nm - 10 kDa) and dissolved (<10 kDa) water fractions in two peri-urban river basins having contrasted land uses (agricultural vs urban/industrial) under low and high flow conditions. Regardless of hydrological conditions, most of the REY were in the particulate fraction for both catchments. These results suggest erosion of soils as the main source of particulate REY in the two rivers, although a Nd anomaly of industrial origin occurred in the particulate and coarse colloidal fractions of the industrialized river basin. During low flow, the REY patterns of the dissolved fraction displayed marked Gd and Eu anomalies and a fractionation between Light REY and Heavy REY. Both characteristics reflect the influence of wastewater treatment plant effluents on the dissolved REY patterns in the two rivers. During high flow, the dissolved fraction acquired a less fractionated, more natural Light REY and Middle REY pattern, including much lower Gd and Eu positive anomalies. The REY fractionation of the coarse colloidal fraction was close to the particulate, while small colloids were depleted in Light REY and more similar to the dissolved fraction. These different patterns suggest a difference in the nature of REY bearing phases between the two colloidal fractions. The available results collectively show that a complete understanding of REY environmental fate and anomalies cannot be achieved from the sole study of filterable water fractions (typically <0.45 μm).

Understanding the spatiotemporal pollution dynamics of highly fragile montane watersheds of Kashmir Himalaya, India

Pollution of riverine ecosystems through the multidimensional impact of human footprints around the world poses a serious challenge. Research studies that communicate potential repercussions of landscape structure metrics on snowmelt riverine water quality particularly, in climatically fragile Himalayan watersheds are very scarce. Though, worldwide, grasping the influence of land-use practices on water quality (WQ) has received renewed attention yet, the relevance of spatial scale linked to landscape pattern is still elusive due to its heterogenic nature across diverse geomorphic regions. In this work, therefore, we tried to capture the insights on landscape-aquascape interface by juxtapositioning the impacts of landscape structure pattern on snowmelt stream WQ of the whole Jhelum River Basin (JRB) under three varying spatial scales viz., watershed scale, riparian corridor (1000 m wide) and reach buffer (500 m wide). The percentage of landscape pattern composition and configuration metrics in the JRB were computed in GIS utilizing Landsat-8 OLI/TIRS satellite image having 30 m resolution. To better explicate the influence of land-use metrics on riverine WQ with space and time, we used Redundancy analysis (RDA) and multilinear regression (MLR) modeling. MLR selected land-use structure metrics revealed the varied response of WQ parameters to multi-scale factors except for total faecal coliform bacteria (TC) which showed perpetual presence. The reach-scale explained slightly better (76%) variations in WQ than riparian (75%) and watershed (70%) scales. Likewise, across seasonal scale, autumn (75%), winter (83%), and summer (77%) captured the most WQ variation at catchment, riparian, and reach scales respectively. We observed impairing WQ linkages with agriculture, built-up and barren rocky areas across watersheds, besides, pastures in riparian buffer areas, and fragmentation of landscape patches at the reach scale. Due to little appearance of spatial scale differences, a multi scale perspective landscape planning is emphasized to ensure future sustainability of Kashmir Himalayan water resources.

Baseflow and Stormflow Zinc Loads in a Small Agricultural River Catchment Influenced by an Industrial Area

A stringent environmental quality standard for zinc (Zn) has been enacted by regulators because of its toxicity to aquatic life. This study’s objective was to evaluate the variability of Zn concentrations and fluxes in the baseflow and stormflow and to estimate the contribution of Zn from point and non-point sources. By using high-resolution temporal sampling, the suspended solids (SS), iron (Fe), and Zn concentrations were measured in a small agricultural river catchment. Fe, as the natural non-point source, and Zn were evaluated using the end member mixing analysis (EMMA) to identify the source apportionment (point and non-point). The results indicate that in the baseflow, Zn mainly originated from point sources and was possibly discharged by manufacturing industries. By contrast, the non-point sources (diffuse sources) were responsible for extremely high SS, Fe, and Zn levels in the stormflow. In addition, Zn discharge during the stormflow was 93 times higher than that during the baseflow. According to the EMMA, approximately 74% of the Zn was from point sources. River management can be improved if Zn point sources are adequately treated. During a storm event, it is also important to control the particulate Zn released into the river.

Pollution Source Apportionment and Water Quality Risk Evaluation of a Drinking Water Reservoir during Flood Seasons

Reservoirs play an important role in the urban water supply, yet reservoirs receive an influx of large amounts of pollutants from the upper watershed during flood seasons, causing a decline in water quality and threatening the water supply. Identifying major pollution sources and assessing water quality risks are important for the environmental protection of reservoirs. In this paper, the principal component/factor analysis-multiple linear regression (PCA/FA-MLR) model and Bayesian networks (BNs) are integrated to identify water pollution sources and assess the water quality risk in different precipitation conditions, which provides an effective framework for water quality management during flood seasons. The deterioration of the water quality of rivers in the flood season is found to be the main reason for the deterioration in the reservoir water quality. The nonpoint source pollution is the major pollution source of the reservoir, which contributes 53.20%, 48.41%, 72.69%, and 68.06% of the total nitrogen (TN), phosphorus (TP), fecal coliforms (F.coli), and turbidity (TUB), respectively. The risk of the water quality parameters exceeding the surface water standard under different hydrological conditions is assessed. The results show that the probability of the exceedance rate of TN, TP, and F.coli increases from 91.13%, 3.40%, and 3.34%, to 95.75%, 25.77%, and 12.76% as the monthly rainfall increases from ≤68.25 mm to >190.18 mm. The risk to the water quality of the Biliuhe River reservoir is found to increase with the rising rainfall intensity, the water quality risk at the inlet during the flood season is found to be much greater than that at the dam site, and the increasing trend of TP and turbidity is greater than that of TN and F.coli. The risk of five-day biochemical oxygen demand (BOD₅) does not increase with increasing precipitation, indicating that it is less affected by nonpoint source pollution. The results of this study can provide a research basis for water environment management during flood seasons.

The impact of increased flooding occurrence on the mobility of potentially toxic elements in floodplain soil - A review

The frequency and duration of flooding events is increasing due to land-use changes increasing run-off of precipitation, and climate change causing more intense rainfall events. Floodplain soils situated downstream of urban or industrial catchments, which were traditionally considered a sink of potentially toxic elements (PTEs) arriving from the river reach, may now become a source of legacy pollution to the surrounding environment, if PTEs are mobilised by unprecedented flooding events. When a soil floods, the mobility of PTEs can increase or decrease due to the net effect of five key processes; (i) the soil redox potential decreases which can directly alter the speciation, and hence mobility, of redox sensitive PTEs (e.g. Cr, As), (ii) pH increases which usually decreases the mobility of metal cations (e.g. Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Zn²⁺), (iii) dissolved organic matter (DOM) increases, which chelates and mobilises PTEs, (iv) Fe and Mn hydroxides undergo reductive dissolution, releasing adsorbed and co-precipitated PTEs, and (v) sulphate is reduced and PTEs are immobilised due to precipitation of metal sulphides. These factors may be independent mechanisms, but they interact with one another to affect the mobility of PTEs, meaning the effect of flooding on PTE mobility is not easy to predict. Many of the processes involved in mobilising PTEs are microbially mediated, temperature dependent and the kinetics are poorly understood. Soil mineralogy and texture are properties that change spatially and will affect how the mobility of PTEs in a specific soil may be impacted by flooding. As a result, knowledge based on one river catchment may not be particularly useful for predicting the impacts of flooding at another site. This review provides a critical discussion of the mechanisms controlling the mobility of PTEs in floodplain soils. It summarises current understanding, identifies limitations to existing knowledge, and highlights requirements for further research.

River Water Quality of the Selenga-Baikal Basin: Part II—Metal Partitioning under Different Hydroclimatic Conditions

The partitioning of metals and metalloids between their dissolved and suspended forms in river systems largely governs their mobility and bioavailability. However, most of the existing knowledge about catchment-scale metal partitioning in river systems is based on a limited number of observation points, which is not sufficient to characterize the complexity of large river systems. Here we present an extensive field-based dataset, composed of multi-year data from over 100 monitoring locations distributed over the large, transboundary Selenga River basin (of Russia and Mongolia), sampled during different hydrological seasons. The aim is to investigate on the basin scale, the influence of different hydroclimatic conditions on metal partitioning and transport. Our results showed that the investigated metals exhibited a wide range of different behaviors. Some metals were mostly found in the dissolved form (84–96% of Mo, U, B, and Sb on an average), whereas many others predominantly existed in suspension (66–87% of Al, Fe, Mn, Pb, Co, and Bi). Nevertheless, our results also showed a consistently increasing share of metals in dissolved form as the metals were transported to the downstream parts of the basin, closer to the Lake Baikal. Under high discharge conditions (including floods), metal transport by suspended particulate matter was significantly greater (about 2–6 times). However, since high and low water conditions could prevail simultaneously at a given point of time within the large river basin, e.g., as a result of on-going flood propagation, snap-shot observations of metal partitioning demonstrated contrasting patterns with domination of both particulate and dissolved phases in different parts of the basin. Such heterogeneity of metal partitioning is likely to be found in many large river systems. These results point out the importance of looking into different hydroclimatic conditions across space and time, both for management purposes and contaminant modeling efforts at the basin scale.

Urban Stormwater: An Overlooked Pathway of Extensive Mixed Contaminants to Surface and Groundwaters in the United States

Increasing global reliance on stormwater control measures to reduce discharge to surface water, increase groundwater recharge, and minimize contaminant delivery to receiving waterbodies necessitates improved understanding of stormwater-contaminant profiles. A multiagency study of organic and inorganic chemicals in urban stormwater from 50 runoff events at 21 sites across the United States demonstrated that stormwater transports substantial mixtures of polycyclic aromatic hydrocarbons, bioactive contaminants (pesticides and pharmaceuticals), and other organic chemicals known or suspected to pose environmental health concern. Numerous organic-chemical detections per site (median number of chemicals detected = 73), individual concentrations exceeding 10 000 ng/L, and cumulative concentrations up to 263 000 ng/L suggested concern for potential environmental effects during runoff events. Organic concentrations, loads, and yields were positively correlated with impervious surfaces and highly developed urban catchments. Episodic storm-event organic concentrations and loads were comparable to and often exceeded those of daily wastewater plant discharges. Inorganic chemical concentrations were generally dilute in concentration and did not exceed chronic aquatic life criteria. Methylmercury was measured in 90% of samples with concentrations that ranged from 0.05 to 1.0 ng/L.

Release of Heavy Metals and Metalloids from Two Contaminated Soils to Surface Runoff in Southern China: A Simulated-Rainfall Experiment

The release of heavy metals and metalloids (HMs), including Pb, Zn, Cd, As, and Cu, from two typical contaminated soils with different properties, namely red soil and limestone-dominated soil, was characterized through simulated-rainfall experiments in order to investigate the effects of soil properties on HM release. Significant differences in the HM concentrations between the two soils resulted in various concentrations of dissolved and particulate HMs in the runoff. Differences in the dissolved HM concentrations in the runoff were inconsistent with the HM concentrations in the soils, which is attributed to the variable solubilities of HMs in the two soils. However, the HM enrichment ratios were not significantly different. The strong correlation between dissolved organic carbon and dissolved HMs in the runoff, and between the total organic carbon and particulate HMs in sediments, were observed, especially in the limestone-dominated soil. The specific surface area and HM concentrations in sediments were weakly correlated. Acid-rainfall experiments showed that only the limestone-dominated soil buffered the effects of acid rain on the runoff; the concentrations of dissolved Pb, Zn, Cd, and Cu increased in the red soil under acid rainfall and were 60, 29, 25, and 19 times higher, respectively, than under the neutral conditions. The results contribute to the understanding of HM behavior in the two typical soils in southern China, exposed to frequent storms that are often dominated by acid rainfall.

Behavior of major and trace elements in a transient surface water/groundwater system following removal of a long-term wastewater treatment facility source

In many aquatic environments, municipal wastewater treatment facility (WWTF) effluent discharges influence local hydrologic and chemical connectivity between the surface-water and adjacent alluvial shallow-groundwater systems. Fourmile Creek located in Polk County, Iowa received effluent from the Ankeny WWTF for nearly forty years before it was shut down in November 2013. The decommissioning of the municipal WWTF provided a unique opportunity to characterize the recovery from impacts of treated wastewater discharge on water quality at the surface-water/groundwater interface in a shallow, unconfined alluvial aquifer. Dissolved major element and trace element concentrations in Fourmile Creek surface water, hyporheic-zone water, and shallow, unconfined groundwater were monitored upstream and downstream from the WWTF discharge before and after the shutdown. Multivariate statistical techniques including principal component analysis (PCA) and agglomerative hierarchical clustering (AHC) were used to differentiate source-water contributions, characterize elemental components, and describe surface-water/groundwater interaction dynamics. During the post-closure assessment, there was subsurface attenuation of wastewater constituents including Al, B, Cu, Gd, K, Mo, Na, P, Pb, Sb, and Zn. During the same time, groundwater concentrations increased for As, Ba, Ca, Fe, Mg, Mn, SiO₂, Sr, and U and represented a profile characteristic of the shallow alluvial aquifer. Hydrologic conditions transitioned from predominantly wastewater infiltration and hyporheic exchange before the WWTF shutdown, to predominantly discharge of native groundwater. Precipitation-driven streamflow events created fluctuations in the groundwater water-table elevations, resulting in variable contact between the saturated and unsaturated zones within the unconfined, alluvial aquifer and intermittent exposure to constituents stored in the sediments. The inorganic fingerprint of municipal wastewater was flushed relatively quickly (≤19 weeks) from the hyporheic zone indicating that processes like diffusion or sorption/desorption that might extend recovery may not be important for many trace elements in this system.