Natural stressors in uncontaminated sediments of shallow freshwaters: the prevalence of sulfide, ammonia, and reduced iron

Bioelectrochemical reactor to manage anthropogenic sulfate pollution for freshwater ecosystems: Mathematical modeling and experimental validation

Anthropogenic sulfate loading into otherwise low-sulfate freshwater systems can cause significant ecological consequences as a biogeochemical stressor. To address this challenge, in situ bioremediation technologies have been developed to leverage naturally occurring microorganisms that transform sulfate into sulfide rather than implementing resource-intensive physio-chemical processes. However, bioremediation technologies often require the supply of electron donors to facilitate biological sulfate reduction. Bioelectrochemical systems (BES) can be an alternative approach for supplying molecular hydrogen as an electron donor for sulfate-reducing bacteria through water electrolysis. Although the fundamental mechanisms behind BESs have been studied, limited research has evaluated the design and operational parameters of treatment systems when developing BESs on a scale relevant to environmental systems. This study aimed to develop an application-based mathematical model to evaluate the performance of BESs across a range of reactor configurations and operational modes. The model was based on sulfate transformation by hydrogenotrophic sulfate-reducing bacteria coupled with the recovery of solid iron sulfide species formed by the oxidative dissolution of dissolved ferrous iron from a stainless steel anode. Sulfate removal closely corresponded to the rate of electrolytic hydrogen production and hydraulic residence time but was less sensitive to specific microbial rate constants. The mathematical model results were compared to experimental data from a pilot-scale BES tested with nonacidic mine drainage as a case study. The close agreement between the mathematical model and the pilot-scale BES experiment highlights the efficacy of using a mathematical model as a tool to develop a conceptual design of a scaled-up treatment system.

A Case Study of the Effects of Management Interventions on the Phosphorus Dynamics at a Coastal, Eutrophic, Caribbean lagoon (Laguna Cartagena, Puerto Rico)

Laguna Cartagena is a coastal, eutrophic, shallow lake and freshwater wetland in southwestern Puerto Rico, managed by the US Fish and Wildlife Service. This ecosystem has been impacted by phosphorus loading from adjacent agricultural areas since the 1950s, causing eutrophication and deteriorating wildlife habitats. Herein, we describe phosphorus input and export during September 2010–September 2011 (Phase One) and October 2013–November 2014 (Phase Two). These two phases bracket a period of intensified management interventions including excavation and removal of sediment and vegetation, draining, and burning during the summers of 2012 and 2013. Results indicate that Laguna Cartagena retains a phosphorus (sink) in its sediments, and exhibits nutrient-releasing events (source, mainly total phosphorus) to the lagoon water column, which are associated with rainfall and rising water levels. External factors including water level fluctuations and rainfall influenced phosphorus export during Phase One, but after management interventions (Phase Two), internal processes influenced sink/source dynamics, releasing elevated phosphorus concentrations to the water column. When exposed sediments were re-flooded, phosphorus concentrations to the water column increased, releasing elevated P concentrations downstream to an estuarine wetlands area and the Caribbean Sea. Herein we offer management recommendations to optimize wildlife habitat without elevating phosphorus concentrations.

Legacy and Current-Use Contaminants in Sediments Alter Macroinvertebrate Communities in Southeastern US Streams

Sediment contamination of freshwater streams in urban areas is a recognized and growing concern. As a part of a comprehensive regional stream-quality assessment, stream-bed sediment was sampled from streams spanning a gradient of urban intensity in the Piedmont ecoregion of the southeastern United States. We evaluated relations between a broad suite of sediment contaminants (metals, current-use pesticides, organochlorine pesticides, polychlorinated biphenyls, brominated diphenyl ethers, and polycyclic aromatic hydrocarbons), ambient sediment toxicity, and macroinvertebrate communities from 76 sites. Sediment toxicity was evaluated by conducting whole-sediment laboratory toxicity testing with the amphipod Hyalella azteca (for 28 d) and the midge Chironomus dilutus (for 10 d). Approximately one-third of the sediment samples were identified as toxic for at least one test species endpoint, although concentrations of contaminants infrequently exceeded toxicity benchmarks. Ratios of contaminant concentrations relative to their benchmarks, both individually and as summed benchmark quotients, were explored on a carbon-normalized and a dry-weight basis. Invertebrate taxa measures from ecological surveys tended to decline with increasing urbanization and with sediment contamination. Toxicity test endpoints were more strongly related to sediment contamination than invertebrate community measures were. Sediment chemistry and sediment toxicity provided moderate and weak, respectively, explanatory power for the similarity/dissimilarity of invertebrate communities. The results indicate that current single-chemical sediment benchmarks may underestimate the effects from mixtures of sediment contaminants experienced by lotic invertebrates. Environ Toxicol Chem 2020;39:1219-1232. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Cumulative Sulfate Loads Shift Porewater to Sulfidic Conditions in Freshwater Wetland Sediment

It is well established that sulfide can be toxic to rooted aquatic plants. However, a detailed description of the effects of cumulative sulfate loads on sulfide and iron (Fe) porewater geochemistry, plant exposure, and ecological response is lacking. Over 4 yr, we experimentally manipulated sulfate loads to self-perpetuating wild rice (Zizania palustris) populations and monitored increases in the ratio of sulfur (S) to Fe in sediment across a range of sulfide loading rates driven by overlying water sulfate. Because natural settings are complicated by ongoing Fe and S loads from surface and groundwater, this experimental setting provides a tractable system to describe the impacts of increased S loading on Fe-S porewater geochemistry. In the experimental mesocosms, the rate of sulfide accumulation in bulk sediment increased linearly with overlying water sulfate concentration up to 300 µg-SO4 cm-3 . Seedling survival at the beginning of the annual life cycle and seed mass and maturation at the end of the annual life cycle all decreased at porewater sulfide concentrations between 0.4 and 0.7 µg cm-3 . Changes to porewater sulfide, plant emergence, and plant nutrient uptake during seed production were closely related to the ratio of S to Fe in sediment. A mass balance analysis showed that porewater sulfide remained a small and relatively transient phase compared to sulfate in the overlying water and Fe in the sediment solid phase. The results illuminate the evolution of the geochemical setting and timescales over which 4 yr of cumulative sulfate loading resulted in a wholesale shift from Fe-dominated to sulfide-dominated porewater chemistry. This shift was accompanied by detrimental effects to, and eventual extirpation of, self-perpetuating wild rice populations. Environ Toxicol Chem 2019;38:1231-1244. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Influence of sediment chemistry and sediment toxicity on macroinvertebrate communities across 99 wadable streams of the Midwestern USA

Simultaneous assessment of sediment chemistry, sediment toxicity, and macroinvertebrate communities can provide multiple lines of evidence when investigating relations between sediment contaminants and ecological degradation. These three measures were evaluated at 99 wadable stream sites across 11 states in the Midwestern United States during the summer of 2013 to assess sediment pollution across a large agricultural landscape. This evaluation considers an extensive suite of sediment chemistry totaling 274 analytes (polycyclic aromatic hydrocarbons, organochlorine compounds, polychlorinated biphenyls, polybrominated diphenyl ethers, trace elements, and current-use pesticides) and a mixture assessment based on the ratios of detected compounds to available effects-based benchmarks. The sediments were tested for toxicity with the amphipod Hyalella azteca (28-d exposure), the midge Chironomus dilutus (10-d), and, at a few sites, with the freshwater mussel Lampsilis siliquoidea (28-d). Sediment concentrations, normalized to organic carbon content, infrequently exceeded benchmarks for aquatic health, which was generally consistent with low rates of observed toxicity. However, the benchmark-based mixture score and the pyrethroid insecticide bifenthrin were significantly related to observed sediment toxicity. The sediment mixture score and bifenthrin were also significant predictors of the upper limits of several univariate measures of the macroinvertebrate community (EPT percent, MMI (Macroinvertebrate Multimetric Index) Score, Ephemeroptera and Trichoptera richness) using quantile regression. Multivariate pattern matching (Mantel-like tests) of macroinvertebrate species per site to identified contaminant metrics and sediment toxicity also indicate that the sediment mixture score and bifenthrin have weak, albeit significant, influence on the observed invertebrate community composition. Together, these three lines of evidence (toxicity tests, univariate metrics, and multivariate community analysis) suggest that elevated contaminant concentrations in sediments, in particular bifenthrin, is limiting macroinvertebrate communities in several of these Midwest streams.

Effects-based spatial assessment of contaminated estuarine sediments from Bear Creek, Baltimore Harbor, MD, USA

Estuarine sediments in regions with prolonged histories of industrial activity are often laden to significant depths with complex contaminant mixtures, including trace metals and persistent organic pollutants. Given the complexity of assessing risks from multi-contaminant exposures, the direct measurement of impacts to biological receptors is central to characterizing contaminated sediment sites. Though biological consequences are less commonly assessed at depth, laboratory-based toxicity testing of subsurface sediments can be used to delineate the scope of contamination at impacted sites. The extent and depth of sediment toxicity in Bear Creek, near Baltimore, Maryland, USA, was delineated using 10-day acute toxicity tests with the estuarine amphipod Leptocheirus plumulosus, and chemical analysis of trace metals and persistent organic pollutants. A gradient of toxicity was demonstrated in surface sediments with 21 of 22 tested sites differing significantly from controls. Effects were most pronounced (100% lethality) at sites proximate to a historic industrial complex. Sediments from eight of nine core samples to depths of 80 cm were particularly impacted (i.e., caused significant lethality to L. plumulosus) even in locations overlain with relatively non-toxic surface sediments, supporting a conclusion that toxicity observed at the surface (top 2 cm) does not adequately predict toxicity at depth. In seven of nine sites, toxicity of surface sediments differed from toxicity at levels beneath by 28 to 69%, in five instances underestimating toxicity (28 to 69%), and in two instances overestimating toxicity (44 to 56%). Multiple contaminants exceeded sediment quality guidelines and correlated positively with toxic responses within surface sediments (e.g., chromium, nickel, polycyclic aromatic hydrocarbon (PAH), total petroleum hydrocarbon). Use of an antibody-based PAH biosensor revealed that porewater PAH concentrations also increased with depth at most sites. This study informs future management decisions concerning the extent of impact to Bear Creek sediments, and demonstrates the benefits of a spatial approach, relying primarily on toxicity testing to assess sediment quality in a system with complex contaminant mixtures.

Groundwater shapes sediment biogeochemistry and microbial diversity in a submerged Great Lake sinkhole

For a large part of earth's history, cyanobacterial mats thrived in low-oxygen conditions, yet our understanding of their ecological functioning is limited. Extant cyanobacterial mats provide windows into the putative functioning of ancient ecosystems, and they continue to mediate biogeochemical transformations and nutrient transport across the sediment-water interface in modern ecosystems. The structure and function of benthic mats are shaped by biogeochemical processes in underlying sediments. A modern cyanobacterial mat system in a submerged sinkhole of Lake Huron (LH) provides a unique opportunity to explore such sediment-mat interactions. In the Middle Island Sinkhole (MIS), seeping groundwater establishes a low-oxygen, sulfidic environment in which a microbial mat dominated by Phormidium and Planktothrix that is capable of both anoxygenic and oxygenic photosynthesis, as well as chemosynthesis, thrives. We explored the coupled microbial community composition and biogeochemical functioning of organic-rich, sulfidic sediments underlying the surface mat. Microbial communities were diverse and vertically stratified to 12 cm sediment depth. In contrast to previous studies, which used low-throughput or shotgun metagenomic approaches, our high-throughput 16S rRNA gene sequencing approach revealed extensive diversity. This diversity was present within microbial groups, including putative sulfate-reducing taxa of Deltaproteobacteria, some of which exhibited differential abundance patterns in the mats and with depth in the underlying sediments. The biological and geochemical conditions in the MIS were distinctly different from those in typical LH sediments of comparable depth. We found evidence for active cycling of sulfur, methane, and nutrients leading to high concentrations of sulfide, ammonium, and phosphorus in sediments underlying cyanobacterial mats. Indicators of nutrient availability were significantly related to MIS microbial community composition, while LH communities were also shaped by indicators of subsurface groundwater influence. These results show that interactions between the mats and sediments are crucial for sustaining this hot spot of biological diversity and biogeochemical cycling.

Assessing toxicity of contaminants in riverine suspended sediments to freshwater mussels

The Clinch River in Virginia and Tennessee, USA, is well known for its diverse native freshwater mussel assemblages; however, notable declines in mussel populations in recent decades have prompted much concern and subsequent research. The authors examined the toxicity of recently deposited sediments on juveniles of the freshwater mussel Epioblasma brevidens by collecting time-integrated sediment samples from the water column with sediment traps from 11 sites in the Clinch River basin, including 6 sites within an 88-km reach deemed a "mussel zone of decline." Mussels were exposed to the riverine sediments and to 3 control sediments for 28 d; survival, shell length, and biomass were then assessed. Sediment treatment (i.e., river location) had a significant effect on mussel survival (p < 0.01) and biomass (p = 0.02) but did not affect length (p = 0.37), and sediments from 2 of the tributaries were the most toxic. Inorganic and organic analyses of sediments indicated the presence of metals and polycyclic aromatic hydrocarbons at all sites. Manganese was negatively correlated with mussel survival and biomass, as was ammonia with survival and total organic carbon with biomass. Current land uses in the watershed indicate that fossil fuel mining and agriculture may be associated with elevated manganese and ammonia, respectively. The authors found that sediments collected with sediment traps over relatively short deployment durations can help elucidate recent contaminant influx and its potential for inducing toxicity in benthic organisms. Environ Toxicol Chem 2017;36:395-407. © 2016 SETAC.