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Mercury (Hg) pollution is a concern to human and wildlife health worldwide, and management strategies that reduce Hg inputs to aquatic systems are of broad interest. Using a replicated field-scale study in California's Sacramento-San Joaquin Delta, we tested the effectiveness of chemically enhanced treatment wetlands (CETWs) under two coagulation treatments, polyaluminum chloride (Al treatment) and ferric sulfate (Fe treatment), in their initial removal and longer-term sequestration of Hg compared to untreated control wetlands. The primary mechanism for Hg removal by CETWs was the transfer of Hg from filtered forms to insoluble particulate forms and enhanced settling of particles. CETWs resulted in total Hg annual load removals of 63 ng m -2 yr -1 (71%) and 54 ng m -2 yr -1 (54%) for the Al and Fe treatments, respectively. Control wetlands removed significantly less at 13 ng m -2 yr -1 (14%). Load removals indicate that Fe treatment wetlands more effectively reduced filtered and total methylmercury (MeHg) exports, while Al treatment wetlands more effectively reduced particulate MeHg and total Hg exports. These differences in Hg species load reductions possibly indicate different mechanisms of Hg sequestration; current data suggest more effective floc formation and particle settling was likely responsible for the Al treatment behavior, while either preferential MeHg sequestration or methylation suppression was potentially responsible for Fe treatment behavior. Differences in Hg sequestration behavior post-coagulation between the flocs formed by different coagulants indicate the importance of in-situ studies and the need for careful selection of coagulant treatment depending on the Hg species requiring remediation., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Land surface subsidence is a concern in many deltas worldwide as it contributes to water quality degradation, loss of fertile land and increased potential for levee failure. As a possible solution to these concerns, on-site coagulation enhanced treatment wetlands (CETWs), coagulation water treatment followed by wetland passage serving as a settling basin, were implemented in a field-scale study located on a subsided island of the Sacramento-San Joaquin Delta in northern California under three treatments; coagulation with polyaluminum chloride (PAC), coagulation with ferric sulfate and an untreated control. Because CETWs offer a relatively novel solution for water quality improvement and subsidence reversal due to its low-infrastructure requirements and in-situ nature, effects from these systems remain uncharted and they may have adverse effects on plant biomass production that also contribute to sediment accretion. This study focuses on the effect CETWs had on the growth of Typha spp.; the dominant vegetation in the wetlands. Plant growth parameters and nutrient content were measured in conjunction with soil, pore water and surface water chemistry. Soil analysis indicated there was no intermixing of newly formed flocs and original soil material. Where there was significant deposition of floc, PAC treatment reduced phosphate concentrations and ferric sulfate treatment increased total Fe concentrations in surrounding water compared to the control. Results indicated coagulation treatments had no negative effects on Typha leaf nutrient content, Typha growth or allometric parameters. Additionally, no signs of plant toxicity such as necrosis, wilting or chlorosis were observed in any of the treatments. Overall, this study suggests that CETWs are viable treatment option for water quality improvement and sediment accretion while having no negative impact on the growth of Typha plants., (Copyright © 2018. Published by Elsevier B.V.)

Many deltas worldwide face subsidence issues due to increased anthropogenic activity. The Sacramento-San Joaquin delta similarly faces ongoing subsidence, more than 8 m in some areas, that increases levee failure risks and threatens the security of this water source for 25 million California residents and 1.2 million ha of agriculture. Rice ( L.) fields are an integral part of a proposed new strategy for managing subsidence because they have been shown to stop subsidence and provide an alternative crop for growers. Two important considerations for implementing rice fields are additional water requirement and the effect on water quality from mobilized dissolved organic carbon (DOC) and disinfection byproduct precursors. To understand constituent transport and potential management opportunities for rice farming, a plug flow reactor mass balance model was used to quantify surface and subsurface hydrologic pathways. Management of adjacent drainage ditch water levels under low and high scenarios were tested as a strategy to reduce seepage and water quality loads. Under high drains, groundwater met 10% of evapotranspiration (ET). Low drains resulted in a 100% increase in ET demand, which was met by surface water applied for irrigation. High drains reduced subsurface seepage by 95%. Subsurface DOC, trihalomethane, and total dissolved nitrogen loads were reduced 10-fold in high drains compared with low drains. Flow rate accounted for 74 to 90% of load variance and was the primary determinant of constituent loads. Thoughtful implementation of rice cultivation, with high water levels in adjacent drains, can be leveraged to reduce irrigation water demand and constituent load outputs., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

Constructed wetlands are used worldwide to improve water quality while also providing critical wetland habitat. However, wetlands have the potential to negatively impact drinking water quality by exporting dissolved organic carbon (DOC) that upon disinfection can form disinfection byproducts (DBPs) like trihalomethanes (THMs) and haloacetic acids (HAAs). We used a replicated field-scale study located on organic rich soils in California's Sacramento-San Joaquin Delta to test whether constructed flow-through wetlands which receive water high in DOC that is treated with either iron- or aluminum-based coagulants can improve water quality with respect to DBP formation. Coagulation alone removed DOC (66-77%) and THM (67-70%) precursors, and was even more effective at removing HAA precursors (77-90%). Passage of water through the wetlands increased DOC concentrations (1.5-7.5mgL -1 ), particularly during the warmer summer months, thereby reversing some of the benefits from coagulant addition. Despite this addition, water exiting the wetlands treated with coagulants had lower DOC and DBP precursor concentrations relative to untreated source water. Benefits of the coagulation-wetland systems were greatest during the winter months (approx. 50-70% reduction in DOC and DBP precursor concentrations) when inflow water DOC concentrations were higher and wetland DOC production was lower. Optical properties suggest DOC in this system is predominantly comprised of high molecular weight, aromatic compounds, likely derived from degraded peat soils., (Published by Elsevier B.V.)

Mercury pollution is widespread globally, and strategies for managing mercury contamination in aquatic environments are necessary. We tested whether coagulation with metal-based salts could remove mercury from wetland surface waters and decrease mercury bioaccumulation in fish. In a complete randomized block design, we constructed nine experimental wetlands in California's Sacramento-San Joaquin Delta, stocked them with mosquitofish (Gambusia affinis), and then continuously applied agricultural drainage water that was either untreated (control), or treated with polyaluminum chloride or ferric sulfate coagulants. Total mercury and methylmercury concentrations in surface waters were decreased by 62% and 63% in polyaluminum chloride treated wetlands and 50% and 76% in ferric sulfate treated wetlands compared to control wetlands. Specifically, following coagulation, mercury was transferred from the filtered fraction of water into the particulate fraction of water which then settled within the wetland. Mosquitofish mercury concentrations were decreased by 35% in ferric sulfate treated wetlands compared to control wetlands. There was no reduction in mosquitofish mercury concentrations within the polyaluminum chloride treated wetlands, which may have been caused by production of bioavailable methylmercury within those wetlands. Coagulation may be an effective management strategy for reducing mercury contamination within wetlands, but further studies should explore potential effects on wetland ecosystems.

Concentration and mass balance analyses were used to quantify methylmercury (MeHg) loads from conventional (white) rice, wild rice, and fallowed fields in northern California's Yolo Bypass. These analyses were standardized against chloride to distinguish transport pathways and net ecosystem production (NEP). During summer, chloride loads were both exported with surface water and moved into the root zone at a 2:1 ratio. MeHg and dissolved organic carbon (DOC) behaved similarly with surface water and root zone exports at ~3:1 ratio. These trends reversed in winter with DOC, MeHg, and chloride moving from the root zone to surface waters at rates opposite and exceeding summertime root zone fluxes. These trends suggest that summer transpiration advectively moves constituents from surface water into the root zone, and winter diffusion, driven by concentration gradients, subsequently releases those constituents into surface waters. The results challenge a number of paradigms regarding MeHg. Specifically, biogeochemical conditions favoring microbial MeHg production do not necessarily translate to synchronous surface water exports; MeHg may be preserved in the soils allowing for release at a later time; and plants play a role in both biogeochemistry and transport. Our calculations show that NEP of MeHg occurred during both summer irrigation and winter flooding. Wild rice wet harvesting and winter flooding of white rice fields were specific practices that increased MeHg export, both presumably related to increased labile organic carbon and disturbance. Outflow management during these times could reduce MeHg exports. Standardizing MeHg outflow:inflow concentration ratios against natural tracers (e.g. chloride, EC) provides a simple tool to identify NEP periods. Summer MeHg exports averaged 0.2 to 1μgm(-2) for the different agricultural wetland fields, depending upon flood duration. Average winter MeHg exports were estimated at 0.3μgm(-2). These exports are within the range reported for other shallow aquatic systems., (Copyright © 2014. Published by Elsevier B.V.)

Concentration and mass balance analyses were used to quantify methylmercury (MeHg) loads from conventional (white) rice, wild rice, and fallowed fields in northern California's Yolo Bypass. These analyses were standardized against chloride to distinguish transport pathways and net ecosystem production (NEP). During summer, chloride loads were both exported with surface water and moved into the root zone at a 2:1 ratio. MeHg and dissolved organic carbon (DOC) behaved similarly with surface water and root zone exports at ~3:1 ratio. These trends reversed in winter with DOC, MeHg, and chloride moving from the root zone to surface waters at rates opposite and exceeding summertime root zone fluxes. These trends suggest that summer transpiration advectively moves constituents from surface water into the root zone, and winter diffusion, driven by concentration gradients, subsequently releases those constituents into surface waters. The results challenge a number of paradigms regarding MeHg. Specifically, biogeochemical conditions favoring microbial MeHg production do not necessarily translate to synchronous surface water exports; MeHg may be preserved in the soils allowing for release at a later time; and plants play a role in both biogeochemistry and transport. Our calculations show that NEP of MeHg occurred during both summer irrigation and winter flooding. Wild rice wet harvesting and winter flooding of white rice fields were specific practices that increased MeHg export, both presumably related to increased labile organic carbon and disturbance. Outflow management during these times could reduce MeHg exports. Standardizing MeHg outflow:inflow concentration ratios against natural tracers (e.g. chloride, EC) provides a simple tool to identify NEP periods. Summer MeHg exports averaged 0.2 to 1 μg m(-2) for the different agricultural wetland fields, depending upon flood duration. Average winter MeHg exports were estimated at 0.3 μg m(-2). These exports are within the range reported for other shallow aquatic systems., (Copyright © 2013 Elsevier B.V. All rights reserved.)

In recent decades, the transport of stormwater-associated fine particles and phosphorus into Lake Tahoe has led to decreased water clarity and related ecological changes. Polyaluminum chloride coagulants (PACs) have shown great promise in removing these constituents from stormwater before it enters the lake. However, the potential risks of coagulant treatment to aquatic organisms are not well understood. This study investigated stormwater and coagulant toxicity under non-dosed, optimally-dosed, and over-dosed conditions using the US EPA 3-species test through growth of green algae (Selenastrum capricornutum), zooplankton (waterflea, Ceriodaphnia dubia) mortality and reproduction, and larval fish (fathead minnow, Pimephales promelas) mortality and biomass. Stormwater samples were collected during a 2005 spring snowmelt runoff event from three sites representing various forms of developed regions around Lake Tahoe. Samples were dosed with two different coagulants (a chitosan and a PAC) at levels optimized with a streaming current detector (SCD). Non-treated highway runoff was toxic to zooplankton and fish. Optimal coagulant dosing increased algal growth and reduced zooplankton toxicity. Overdosing at two and three times the optimal level of a PAC decreased zooplankton reproduction and increased fish mortality. PAC-related toxicity was correlated with increasing total unfiltered aluminum and decreasing alkalinity, pH, and DOC. Because of toxicity risks, we recommend keeping PAC coagulant dosing at or below optimal levels in practice.