Effects of vegetation on methylmercury concentrations and loads in a mercury contaminated floodplain.

The Yolo Bypass (YB) is a large flood conveyance system designed to protect the city of Sacramento, California, USA from flooding when the Sacramento River approaches flood stage. The Sacramento River watershed and YB are a source of methylmercury (MeHg) to downstream habitat as a result of historic mercury (Hg) and gold mining practices. In the dry season, the YB is extensively farmed and grazed. However, depending on the water year, the floodplain may remain inundated for months. Our experiments focused on the role of pasture land and decomposing vegetation as a source of MeHg during extensive periods of floodplain flooding. Decomposing vegetation, rather than sediment, was identified as the principal source of filter passing MeHg (fMeHg) within the floodplain. The decomposing vegetation provided a substrate for microbial methylation of inorganic Hg contained within the plants. In replicated flooded mesocosm experiments, MeHg concentrations increased from 2.78 to 31.0 ng g-1 dw and 3.41 to 56.8 ng g-1 dw in decomposing vegetation. In field collections, the concentrations of MeHg in vegetation increased from preflood levels of 2.78 to 45.4 ng g-1 dw after 17 weeks of flooding. The importance of vegetation was shown in laboratory experiments where there was a positive correlation between the amount of fMeHg in water and the amount of vegetation added. These results also provide Hg concentration data for an important functional type of vegetation, grasses, and fill a data gap that contributed to uncertainties with regards to the role of vegetation in Hg cycling.

Regional assessment of contaminant bioaccumulation in sport fish tissue in the Southern California Bight, USA.

Marine recreational fisheries in California are economically and culturally important; however, consumption of contaminated seafood may be a human health risk. The California Environmental Protection Agency Office of Environmental Health Hazard Assessment developed Advisory Tissue Levels (ATLs) to be used in developing consumption recommendations protecting the health of fish consumers. This study characterizes extent and magnitude of bioaccumulation of contaminants in sport fish in Southern California relative to ATLs. Most zones exceeded the ATL for mercury corresponding to consumption of not more than 3 servings per week in one or more target species. A third of zones exceeded the ATL for total polychlorinated biphenyls (PCBs) corresponding to consumption of not more than 7 servings per week. However, neither mercury nor total PCBs exceeded the most restrictive, "do not consume", thresholds in measured tissues. Contaminant concentrations in fish tissues have remained the same or decreased since a similar survey in 2009.

Pollutant bioaccumulation in the California spiny lobster (Panulirus interruptus) in San Diego Bay, California, and potential human health implications.

While the California spiny lobster (Panulirus interruptus) is an important commercial and recreational fishery species in California, there is a lack of data on bioaccumulation for the species. This study examined pollutant tissue concentrations in lobsters from San Diego Bay, California. Observed lobster pollutant tissue concentrations in tail muscle were compared to State of California pollutant advisory levels. Concentrations were then used to conduct risk assessment using catch data from the California Department of Fish and Wildlife. Study results found little bioaccumulation of organic pollutants in tail tissue, likely due to low observed lipids. Mercury was present, predominantly in methyl form, at concentrations above advisory levels. Recreational catch data for San Diego Bay showed increased non-cancer risk for fishers at the 90th percentile or greater of reported annual catch. Further studies should focus on non-tail tissues, as exploratory whole lobster samples (n = 2) showed elevated organic pollutants and metals.

Aircraft Measurements of Total Mercury and Monomethyl Mercury in Summertime Marine Stratus Cloudwater from Coastal California, USA.

Water samples from marine stratus clouds were collected during 16 aircraft flights above the Pacific Ocean near the Central California coast during the summer of 2016. These samples were analyzed for total mercury (THg), monomethyl mercury (MMHg), and 32 other chemical species in addition to aerosol physical parameters. The mean concentrations of THg and MMHg in the cloudwater samples were 9.2 ± 6.0 ng L-1 (2.3-33.8 ng L-1) ( N = 97) and 0.87 ± 0.66 ng L-1 (0.17-2.9 ng L-1) ( N = 22), respectively. This corresponds to 9.5% (3-21%) MMHg as a fraction of THg. Low and high nonsea salt calcium ion (nss-Ca2+) concentrations in cloudwater were used to classify flights as "marine" and "continental", respectively. Mean [MMHg]marine was significantly higher ( p < 0.05) than [MMHg]continental consistent with an ocean source of dimethyl Hg (DMHg) to the atmosphere. Mean THg in cloudwater was not significantly different between the two categories, indicating multiple emissions sources. Mean [THg]continental was correlated with pH, CO, NO3-, NH4+, and other trace metals, whereas [THg]marine was correlated with MMHg and Na+. THg concentrations were negatively correlated with altitude, consistent with ocean and land emissions, coupled with removal at the cloud-top due to drizzle formation.

Long-term variation in concentrations and mass loads in a semi-arid watershed influenced by historic mercury mining and urban pollutant sources.

Urban watersheds are significantly anthropogenically-altered landscapes. Most previous studies cover relatively short periods, without addressing concentrations, loads, and yields in relation to annual climate fluctuations, and datasets on Ag, Se, PBDEs, and PCDD/Fs are rare. Intensive storm-focused sampling and continuous turbidity monitoring were employed to quantify pollution at two locations in the Guadalupe River (California, USA). At a downstream location, we determined loads of suspended sediment (SS) for 14yrs., mercury (HgT), PCBs, and total organic carbon (TOC) (8yrs), total methylmercury (MeHgT) (6yrs), nutrients, and trace elements including Ag and Se (3yrs), DDTs, chlordanes, dieldrin, and PBDEs (2yrs), and PCDD/Fs (1yr). At an upstream location, we determined loads of SS for 4yrs. and HgT, MeHgT, PCBs and PCDD/Fs for 1yr. These data were compared to previous studies, climatically adjusted, and used to critically assess the use of small datasets for estimating annual average conditions. Concentrations and yields in the Guadalupe River appear to be atypical for total phosphorus, DDTs, dieldrin, HgT, MeHgT, Cr, Ni, and possibly Se due to local conditions. Other pollutants appear to be similar to other urban systems. On average, wet season flow varied by 6.5-fold and flow-weighted mean (FWM) concentrations varied 4.4-fold, with an average 7.1-fold difference between minimum and maximum annual loads. Loads for an average runoff year for each pollutant were usually less than the best estimate of long-term average. The arithmetic average of multiple years of load data or a FWM concentration combined with mean annual flow was also usually below the best estimate of long-term average load. Mean annual loads using sampled years were also less than the best estimate of long-term average by a mean of 2.2-fold. Climatic adjustment techniques are needed for computing estimates of long-term average annual loads.

Identification and prioritization of management practices to reduce methylmercury exports from wetlands and irrigated agricultural lands.

The Sacramento-San Joaquin Delta's (Delta) beneficial uses for humans and wildlife are impaired by elevated methylmercury (MeHg) concentrations in fish. MeHg is a neurotoxin that bioaccumulates in aquatic food webs. The total maximum daily load (TMDL) implementation plan aimed at reducing MeHg in Delta fish obligates dischargers to conduct MeHg control studies. Over 150 stakeholders collaborated to identify 24 management practices (MPs) addressing MeHg nonpoint sources (NPS) in three categories: biogeochemistry (6), hydrology (14), and soil/vegetation (4). Land uses were divided into six categories: permanently and seasonally flooded wetlands, flooded and irrigated agricultural lands, floodplains, and brackish-fresh tidal marshes. Stakeholders scored MPs based on seven criteria: scientific certainty, costs, MeHg reduction potential, spatial applicability, technical capacity to implement, negative impacts to beneficial uses, and conflicting requirements. Semi-quantitative scoring for MPs applicable to each land use (totaling >400 individual scores) led to consensus-based prioritization. This process relied on practical experience from diverse and accomplished NPS stakeholders and synthesis of 17 previous studies. Results provide a comprehensive, stakeholder-driven prioritization of MPs for wetland and irrigated agricultural land managers. Final prioritization highlights the most promising MPs for practical application and control study, and a secondary set of MPs warranting further evaluation. MPs that address hydrology and soil/vegetation were prioritized because experiences were positive and implementation appeared more feasible. MeHg control studies will need to address the TMDL conundrum that MPs effective at reducing MeHg exports could both exacerbate MeHg exposure and contend with other management objectives on site.

Mercury cycling in agricultural and managed wetlands: a synthesis of methylmercury production, hydrologic export, and bioaccumulation from an integrated field study.

With seasonal wetting and drying, and high biological productivity, agricultural wetlands (rice paddies) may enhance the conversion of inorganic mercury (Hg(II)) to methylmercury (MeHg), the more toxic, organic form that biomagnifies through food webs. Yet, the net balance of MeHg sources and sinks in seasonal wetland environments is poorly understood because it requires an annual, integrated assessment across biota, sediment, and water components. We examined a suite of wetlands managed for rice crops or wildlife during 2007-2008 in California's Central Valley, in an area affected by Hg contamination from historic mining practices. Hydrologic management of agricultural wetlands for rice, wild rice, or fallowed - drying for field preparation and harvest, and flooding for crop growth and post-harvest rice straw decay - led to pronounced seasonality in sediment and aqueous MeHg concentrations that were up to 95-fold higher than those measured concurrently in adjacent, non-agricultural permanently-flooded and seasonally-flooded wetlands. Flooding promoted microbial MeHg production in surface sediment of all wetlands, but extended water residence time appeared to preferentially enhance MeHg degradation and storage. When incoming MeHg loads were elevated, individual fields often served as a MeHg sink, rather than a source. Slow, horizontal flow of shallow water in the agricultural wetlands led to increased importance of vertical hydrologic fluxes, including evapoconcentration of surface water MeHg and transpiration-driven advection into the root zone, promoting temporary soil storage of MeHg. Although this hydrology limited MeHg export from wetlands, it also increased MeHg exposure to resident fish via greater in situ aqueous MeHg concentrations. Our results suggest that the combined traits of agricultural wetlands - slow-moving shallow water, manipulated flooding and drying, abundant labile plant matter, and management for wildlife - may enhance microbial methylation of Hg(II) and MeHg exposure to local biota, as well as export to downstream habitats during uncontrolled winter-flow events.

Multidrug efflux transporters limit accumulation of inorganic, but not organic, mercury in sea urchin embryos.

Mercuric compounds are persistent global pollutants that accumulate in marine organisms and in humans who consume them. While the chemical cycles and speciation of mercury in the oceans are relatively well described, the cellular mechanisms that govern which forms of mercury accumulate in cells and why they persist are less understood. In this study we examined the role of multidrug efflux transport in the differential accumulation of inorganic (HgCl(2)) and organic (CH(3)HgCl) mercury in sea urchin (Strongylocentrotus purpuratus) embryos. We found that inhibition of MRP/ABCC-type transporters increases intracellular accumulation of inorganic mercury but had no effect on accumulation of organic mercury. Similarly, pharmacological inhibition of metal conjugating enzymes by ligands GST/GSH significantly increases this antimitotic potency of inorganic mercury, but had no effect on the potency of organic mercury. Our results point to MRP-mediated elimination of inorganic mercury conjugates as a cellular basis for differences in the accumulation and potency of the two major forms of mercury found in marine environments.

Spatial and habitat-based variations in total and methyl mercury concentrations in surficial sediments in the San Francisco Bay-Delta.

Recent studies indicate significant amounts of mercury (Hg) are annually transported into the San Francisco Bay-Delta (Bay-Delta) as a result of historic gold and Hg mining activities. We examined temporal and spatial variation in concentrations of total Hg (Hg(T)) and monomethylmercury (MMHg) in surficial sediments of various ecosystem types in the Bay-Delta. We sampled surficial sediments across the Bay-Delta system and found Hg(T) sediment concentrations in the central Delta were generally 100-200 ng g(-1) and increased westward through Suisun Bay to 250-350 ng g(-1). MMHg concentrations in the central Delta were between 1 and 3 ng g(-1), while those in sediments in the perimeter waterways and adjacent bays were less than 1 ng g(-1). Six sites were monitored monthly for over a year to identify seasonal changes in Hg sediment concentrations. Hgtau sediment concentrations ranged from 48 to 382 ng g(-1) and varied as a function of location not season. However, MMHg concentrations varied seasonally, increasing from 1 ng g(-1) during winter months to 6 ng g(-1) during spring and summer. Transects conducted at three marshes in the central Delta revealed MMHg sediment concentrations of 4-8 ng g(-1) at the interior and 2 ng g(-1) at the exterior of the marshes. Habitat type was a major factor controlling MMHg concentration and the MMHg to Hg(T) ratio in sediments of the Bay-Delta. MMHg was significantly correlated to Hgt (r2 = 0.49) in marsh sediments.