Mercury exposure in humans through food consumption from the Everglades of Florida

In March 1989, The Florida Department of Health and Rehabilitative Services (HRS) issued a Health Advisory recommending the limited consumption of several fish species caught from the Everglades region of South Florida due to elevated methylmercury (MeHg) levels (average 2–3 ppm in fish meat). There were no reports of clinical MeHg poisoning in humans in Florida, although deaths of Florida panthers were attributed to mercury (Hg). poisoning. This study evaluated the extent of MeHg exposure in persons eating contaminated fish in the Everglades region. Populations at risk were identified including sport fishermen, Everglades Residents and subsistence fishermen. Over 1700 individuals were approached; those who had eaten fish or wildlife from the contaminated areas at least once/month for the prior three months were asked to participate. Three hundred and fifty (350) participants completed a brief questionnaire and provided a hair sample for Hg analysis. In 119 (36%) of individuals with levels above the limits of detection, the mean total Hg in hair was 3.62±3.0μg/g [± standard deviation] with a range of 1.28 – 15.57. The most at risk populations identified with respect to Hg levels were Blacks and men. Although the majority of the participants had fished in the Everglades for many years (>15 years), they reported relatively low intake of fish and had low hair Hg levels compared with similar populations in prior studies of other populations at risk. Although 71% of participants knew of the State Health Advisories concerning ingestion of Hg contaminated fish from the Everglades, this did not change their consumption habits. In addition, Blacks, individuals of lower income and of lower education levels were less likely to know about the Health Advisories. Given recent studies of neurologic effects from relatively low in utero MeHg exposure, the continuation of the Mercury Health Advisories and wildlife monitoring in the Everglades are warranted, especially for women of childbearing age and children. However, public education must be targeted for the populations at risk identified in this study in order to reach these populations more effectively.     

Application of the Regional Mercury Cycling Model (RMCM) to Predict the Fate and Remediation of Mercury in Onondaga Lake,New York

Onondaga Lake exhibits elevated concentrations of total mercury (HgT) and methyl-mercury (MeHg) in the water column, sediments and fish tissue due to industrial inputs, wastewater discharge and urban runoff. The steady-state Regional Mercury Cycling Model (RMCM) was calibrated to Onondaga Lake and applied to evaluate various remediation scenarios. Because of detailed data available for Onondaga Lake, the RMCM was effectively calibrated. Model predictions of water column and fish concentrations of Hg generally agreed with measured values. The model underestimated concentrations of Hg in sediments. Mass balance calculations show that inputs of HgT largely originate from tributary and wastewater inflows to the lake. In contrast, MeHg is largely derived from internal production. Model calculations suggest that elimination of Hg inputs from wastewater effluent and of drainage from a former chlor-alkali facility could greatly decrease Hg concentrations in fish tissue.     

Chemical correlates of Hg and methyl-Hg in northern Wisconsin lake waters under ice-cover

Total and dissolved concentrations of Hg and methyl-Hg (MeHg) were determined in the surface waters of 19 northern Wisconsin lakes under ice-cover when differences due to temperature, hydrology, productivity, and atmospheric exchange were minimal. Measured concentrations ranged from 0.3 to 5.3 ng/L for HgT and from 0.01 to 2.8 ng/L for MeHgT. Dissolved species comprised 30% to 95% of the HgT and MeHgT. MeHg was strongly correlated with Hg for both total and dissolved fractions. Thirteen ancillary constituents were measured in conjunction with the Hg determinations (pH, DOC, DIC, DO₂, conductivity, suspended particulate matter (SPM), Ca, Mg, Mn, Fe, Na, SO₄, Cl). Simple linear regressions indicated that DOC explained 87% of the variability in HgT and 79% of the variability in MeHgT. Of the other measured variables, pH, DO₂, Fe and Mn showed weak but significant simple correlations with Hg and MeHg (@ p < 0.05). Multiple regression models containing two independent variables, (DOC and pH), explained 92% of the variability in HgT and 83% of the variability in MeHgT. Models containing DOC alone fit the dissolved Hg data well. We conclude that organic carbon concentrations have a strong effect on the concentrations of Hg and MeHg in these lakewaters.     

Toxicological studies in tropical ecosystems: an ecotoxicological risk assessment of pesticide runoff in South Florida estuarine ecosystems

A multiyear study in the C-111 canal system and associated sites in Florida Bay was undertaken to determine the potential pesticide risk that exists in South Florida. After the examination of extensive pesticide concentration data in surface water, tissues, and semipermeable membrane devices (SPMDs), canal contamination seems to be derived from the extensive agricultural production that drains into the C-111 canal. The results of this study indicate that runoff from agricultural processes led to quantifiable pesticide residues in both canal and bay surface water, which occasionally exceeded current water quality criteria. The major pesticide of concern was endosulfan, which was detected at 100% of the sites sampled. Endosulfan exposure did not cause any acute effects in fish and crustaceans deployed in field bioassays. Chronic effects were observed in copepods, clams, and oysters but could not be attributed to endosulfan exposure. The decision to alter the C-111 canal flow and allow increased freshwater flow into the adjacent Everglades National Park may result in discharges of pesticides into the Everglades. Continued monitoring in this area is needed during this change in flow regime.     

Physicochemical assessment and phosphorus storage of canal sediments within the Everglades Agricultural Area, Florida

Purpose

Excess nutrients such as phosphorus (P) transported from the Everglades Agricultural Area (EAA) in South Florida, USA, to downstream water bodies have been identified as contributing to trophic imbalances within the Florida Everglades. Decades of farming drainage from the EAA has led to accumulation of sediments in regional canals which may be transported to downstream ecosystems and act as potential internal source of P.

Materials and methods

Intact sediment cores were collected from three main conveyance and three farm canals within the EAA. Physicochemical assessment, mineralogy, P speciation, and storage were determined for surface and subsurface layers.

Results and discussion

The main conveyance canal sediments had higher total P (TP) concentrations (1,280?±?360?mg?kg^-1) than the farm canals (960?±?540?mg?kg^-1), while farm canal sediments showed higher organic matter content (28?C53?%) compared to the main canal (24?C27?%) sediments. The minerals found in main conveyance canal sediments were similar to those found in Lake Okeechobee. The labile KCl?CP fraction comprised <2?% of TP from all canal sediments, while NaOH?CPi (i?=?inorganic) fraction consisted of 1?C14?% of TP. The majority of P in the canal sediments was contained in the HCl?CP fraction (Ca?CP and Mg?CP), comprising >50?% of TP in the main and farm canal sediments. An estimated 73 metric tons (mt) of P was stored in the 0?C10-cm layer of the three main conveyance canals within the EAA boundary of which 57-mt P is reactive and potentially available for release at different time scales.

Conclusions

The EAA canal sediments are highly organic with low bulk density and susceptible of being transported to downstream ecosystems. Many factors can impact the potential release of the reactive stored P, including redox potential of sediments as well as the overlying water column P concentration. Further investigation of potential P release from these sediments is warranted.     

Comparison of Two Digestion Methods for Determining Total Phosphorus in Farm Canal Water

Abstract

Rapid and accurate determination of low‐level (0.01 to 1.0 mg L^?1) phosphorus (P) concentrations in farm canal water is important in evaluating water quality in the Everglades Agricultural Area (EAA) canals in south Florida. Two U.S. Environmental Protection Agency methods, persulfate digestion (365.1) and Kjeldahl digestion with mercury oxide (365.4), were used to analyze total P (TP) and total dissolved P (TDP) in two sets of representative canal water samples collected at low‐flow conditions in 2003 and high‐flow conditions in 2004. Quality assurance samples (blanks, duplicates, and spikes) were included to evaluate differences between the two digestion methods. Precision analysis had a mean of less than 5% for both TP and TDP using both methods. The high coefficient of correlations (r>0.98) indicated that the two methods were significantly correlated in determining TP and TDP of the samples. Low detection limits (0.004 mg L^?1) were achieved by the persulfate method. This method offers many other advantages over the mercury digestion: it produces no toxic mercury waste, uses less time, and uses a lower temperature. High suspended solids in canal water samples were not proven to be a problem when using the persulfate digestion, though lower spike recoveries were observed than those when using the mercury digestion. We conclude that persulfate digestion is a more sensitive and environmentally responsible alternative to and is, as precise as, the mercury method for routine determination of TP and TDP in water samples. This information is useful to environmental laboratories in monitoring P concentrations in surface and groundwater.     

The role of dissolved organic carbon in the chemistry and bioavailability of mercury in remote Adirondack lakes

A number of recent studies have documented elevated concentrations of mercury (Hg) in fish caught in remote lakes and a pattern of increased concentrations of Hg in fish tissue with decreasing water column pH. Because of the potential linkage between fish Hg and surface water acidification, factors regulating water column concentrations and bioavailability of Hg were investigated in Adirondack lakes through a field study and application of the Mercury Cycling Model (MCM). Concentrations of total Hg and total MeHg were highly variable, with concentrations of total MeHg about 10% of total Hg in lakes which did not show anoxic conditions. In lakes exhibiting anoxic conditions in the hypolimnion during summer stratification, concentrations of total MeHg were elevated. Concentrations of total Hg and total MeHg increased with decreasing pH in remote Adirondack lakes. However, more importantly, concentrations of total Hg and total MeHg increased with increasing concentrations of dissolved organic carbon (DOC) and percent near-shore wetlands in the drainage basin. Mercury concentrations in muscle tissue of yellow perch from Adirondack lakes were elevated above the U.S. FDA action level (1 μg/g Hg) in 7% of the fish sampled or in one or more individual fish from 9 of the 16 lakes sampled. Fish Hg concentrations generally increased with increasing fish length, weight and age. Patterns of increasing Hg concentration with age likely reflect shifts in prey of yellow perch and the bioconcentration of Hg along the food chain. For age 3 to 5 perch, concentrations of Hg increased with increasing concentrations of DOC and percent near-shore wetlands in the drainage basin. However, for a lake with very high DOC concentrations, fish concentrations of Hg declined. Calculations with the MCM also show that concentrations of Hg species increase with increasing DOC due to complexation reactions. Increases in DOC result in increasing concentrations of Hg in biota but decreases in the bioconcentration factor of Hg in fish tissue. This research suggests that DOC is important in the transport of Hg to lake systems. High concentrations of DOC may complex MeHg, diminishing its bioavailability. At high concentrations of monomeric Al, the complexation of MeHg with DOC apparently decreases, enhancing the bioavailability of MeHg.     

Total gaseous mercury measurements in Florida: The FAMS project (1992–1994)

Measurements of Total Gaseous Mercury (TGM) in the atmosphere are being conducted as part of the Florida Atmospheric Mercury Study (FAMS). FAMS is a multi-year study focusing on the atmospheric transport and deposition of Hg and other trace metals at several locations in central and south Florida. A major component of this study, which this paper addresses, involves determining regional TGM concentrations and seasonal variability patterns at the various collection sites. Occasional problems were encountered in the collection efficiency of TGM on Au. The reason for this interference was not identified, but it was dramatically reduced by heating the collection columns to 80 °C. Atmospheric Hg samples are currently being collected using Au-coated silica sand traps from atop 48 ft. aluminum sampling towers using an unattended and automated sampling and data recording system. We currently have approximately a two year record of TGM, beginning in the summer of 1992, for the Lake Barco Site, located in central Florida approximately 30 miles east of Gainesville, FL. The records at the south Florida sites are shorter, ranging from one month at Andytown to 16 months at both Fort Myers and Fakahatchee Strand. Average TGM concentrations (ng/m³) for 3 to 6 day integrated samples at the various sites are: Lake Barco, 1.59±0.58 (n=78); Fort Myers, 1.59±0.40 (n=42), Fakahatchee Strand, 1.42±0.41 (n=30); Tamiami Trail 1.46±0.80 (n=27); Everglades National Park 3.11±1.52 (n=13); and Andy Town, 1.78±0.62 (n=3). The mean Hg concentration for all the sites is 1.64±0.76 ng/m³ (n=191).     

Distributions of Mercury and Phosphorous in Everglades Soils From Water Conservation Area 3A,Florida, U.S.A.

Soils in the southern half of Water Conservation Area3A are mostly peats with some organic-rich marls. Mercury contents of 64 surface samples over a500 km² area average 28.7 ng cc^-1 (209 ppb drysediment), which is typical of organic-rich soils. High Hg contents in Everglades fish are therefore notcaused by anomalously high soil Hg. Hg contents showno systematic lateral variation, consistent withdeposition from well-mixed atmospheric sources ratherthan nearby point sources or runoff from canals.Cores from 9 sites contain more Hg and P at or nearthe surface than at 20–30 cm depth. Hg and P contentsof individual cores correlate well and define separatebackground and anomalous populations. The subsurfacedistribution of P is determined largely by uptake bysawgrass and other plants. The correlation between Pand Hg suggests that, although atmospheric Hgdeposition has undoubtedly increased in recentdecades, postdepositional mobilization of Hg may beimportant in Everglades soils. This finding, togetherwith recent direct measurements of atmospheric Hgdeposition, indicates that previous estimates of Hgdeposition rates based on Everglades peat cores, whichassumed that Hg is immobile in peat after deposition,have yielded large overestimates.     

Phosphorus Release and Equilibrium Dynamics of Canal Sediments within the Everglades Agricultural Area, Florida

High phosphorus (P) in surface drainage water from agricultural and urban runoff is the main cause of eutrophication within aquatic systems in South Florida, including the Everglades. While primary sources of P in drainage canals in the Everglades Agricultural Area (EAA) are from land use application of agricultural chemicals and oxidation of the organic soils, internal sources from canal sediments can also affect overall P status in the water column. In this paper, we evaluate P release and equilibrium dynamics from three conveyance canals within the EAA. Incubation and flux experiments were conducted on intact sediment cores collected from four locations within the Miami, West Palm Beach (WPB), and Ocean canal. After three continuous exchanges, Miami canal sediments reported the highest P release (66?±?37 mg m^?2) compared to WPB (13?±?10 mg m^?2) and Ocean (17?±?11 mg m^?2) canal over 84 days. Overall, the P flux from all three canal sediments was highest during the first exchange. Miami canal sediments showed the highest P flux (2.4?±?1.3 mg m^?2 day^?1) compared to WPB (0.83?±?0.39 mg m^?2?d^?1) and Ocean canal sediments (0.98?±?0.38 mg m^?2 day^?1). Low P release from WPB canal sediments despite having high TP content could be due to carbonate layers distributed throughout the sediment column inhibiting P release. Equilibrium P concentrations estimated from the sediment core experiment corresponded to 0.12?±?0.04 mg L^?1, 0.06?±?0.03 mg L^?1, and 0.08?±?0.03 mg L^?1 for Miami, WPB, and Ocean canal sediments, respectively, indicating Miami canal sediments behave as a source of P, while Ocean and WPB canal sediments are in equilibrium with the water column. Overall, the sediments showed a significant positive correlation between P release and total P (r?=?0.42), Fe_ox (r?=?0.65), and Al_ox (r?=?0.64) content of sediments. The contribution of P from the three main canals sediments within the EAA boundary corresponded to a very small portion of the total P load exiting the EAA. These estimates, however, only take into consideration diffusive fluxes from sediments and no other factors such as canal flow, bioturbation, resuspension, and anaerobic conditions.     

An intensive multi-site pilot study investigating atmospheric mercury in Broward County,Florida

An intensive multi-site pilot study of atmospheric Hg was conducted in Broward County, Florida in August and September of 1993. Broward County, which contains the city of Fort Lauderdale, is located in southeastern Florida. The county borders the Florida Everglades on the west and the Atlantic Ocean on the east. A network of four sampling sites was set up for 20 days throughout Broward County to measure Hg in both the vapor phase and the particle phase as well as Hg in precipitation. The mean concentrations of total vapor phase Hg measured at two inland sites were found to be significantly higher (3.3 and 2.8 ng/m³) than that measured at a site located on the Atlantic shore (1.8 ng/m³). The mean concentrations of particle phase Hg collected at the two inland sites (51 and 49 pg/m³) were found to be 50% greater than that measured at the coastal site (34 pg/m³). In addition, event precipitation samples were collected at four sampling sites over the 20 day study period and were analyzed for both reactive and total Hg. The mean concentration of total Hg in the precipitation samples was found to be 44 ng/L, with a range of 14 to 130 ng/L. It was determined that further meteorological analysis and a more complete characterization of the aerosol and precipitation composition are needed to identify the probable source(s) contributing to the increased deposition of Hg.     

Pesticide occurrence in selected South Florida canals and Biscayne Bay during high agricultural activity

Climate and soil conditions in South Florida along with an extensive canal system facilitate movement of agricultural pesticides into surface waters. In a two-year study (2002-2004) of the currently used pesticides in South Florida, atrazine, endosulfan, metolachlor, chlorpyrifos, and chlorothalonil were the most frequently detected in the canals and in Biscayne Bay, with average concentrations of 16, 11, 9.0, 2.6, and 6.0 ng/L, respectively. Concentrations of atrazine and chlorpyrifos were highest near corn production. Chlorothalonil and endosulfan concentrations were highest near vegetable production, with no clear trend for metolachlor, which is used on multiple crops. Concentration data were used to calculate an aquatic life hazard potential for the planting period (November) versus the harvest period (March). This analysis indicated that a higher hazard potential occurs during harvest, primarily from the use of endosulfan. These data will also serve to document canal conditions prior to implementation of the Comprehensive Everglades Restoration Plan (CERP).     

Evaluating the mercury distribution and release risks in sediments using high-resolution technology in Nansi Lake,China

Purpose

Mercury (Hg) and methylmercury (MeHg) are easily released from sediments to overlying water and cause secondary contamination. In general, Hg concentrations are low in natural aquatic environments, but Hg toxicity is high. Therefore, it is important to assess the mobility and release risks of Hg and MeHg from surface sediment using in situ high-resolution sampling techniques.

Methods

The profile distribution of Hg and MeHg was obtained for samples from Weishan sub-lake (WL) and Dushan sub-lake (DL) of Nansi Lake, China, by high-resolution dialysis (HR-Peeper probes) and the diffusive gradients in thin films (DGT) technique at mm-resolution. Furthermore, Hg mobility and release risks in sediments were evaluated by combining BCR (European Community Reference Bureau) extraction and DGT-measured data.

Results

The soluble concentrations of Hg in surface sediments in WL and DL were 21.70 and 19.38 ng L^?1 and the DGT-labile concentration of Hg were 8.21 and 10.30 ng L^?1, respectively. The soluble and labile Hg and MeHg concentrations were higher in the surface sediments (from??40 to 0 mm) than in deep sediments. The distribution of the labile-Hg was controlled by the ferrimanganic (hydr)oxide and total nitrogen rather than organic carbon content. The non-residual components accounted for a greater proportion of the interface, which further confirmed Hg was more active on the surface layer of the sediment. The resupply ability indicated that the release of Hg from sediment was insufficient to maintain the initial concentration in the porewater before consumption. The MeHg fluxes in WL (6.18 ng m^?2 day^?1) were twice those in DL (2.89 ng m^?2 day^?1), and the risk assessment code revealed a higher risk in the surface layer (25.21–61.88%) than in the deep layer (0–27.75%).

Conclusions

Dissolved Hg and MeHg accumulated on the surface of the sediments and were more active than in the deeper sediments. The DGT-labile state can be used for a better understanding of the bioavailability and mobility of Hg. The diffusion direction of Hg and MeHg was from sediment to the overlying water. The release risks of Hg and MeHg from surface sediments (especially in WL) were found to be worthy of concern.

     

Cycling of methyl mercury and mercury — Responses in the forest roof catchment to three years of decreased atmospheric deposition

Studies of the biogeochemistry of total mercury (Hg) and methyl mercury (MeHg) in the Lake Gårdsjön watershed have shown that the atmosphere is the most important source of Hg and MeHg in the ecosystem. Soils are accumulating most of the deposited Hg and MeHg, but transport of Hg and MeHg from the forested catchments into the lake ecosystems is enough to explain elevated concentrations of MeHg in fish in more than 10 000 Swedish lakes. An experimental roof was constructed to study effects of decreased atmospheric input on an entire forested catchment. The experiment started in April 1991, and decreases in the output of both MeHg and Hg occurred during 1991, 1992 and 1993. Runoff fluxes from the control catchment during the pre-treatment period were related to the experimental catchment using regression analyses. Since April 1991, after three year experiment, predicted compared to measured fluxes showed that Hg output decreased by 32% and MeHg by 28%. The decrease in Hg was most obvious during high water flows in winter/spring while MeHg decreased during all seasons of the year. The decreased input of Hg and MeHg to the Forest Roof Catchment is the most probable explanation to the rapid decrease in output of Hg and MeHg by runoff from the catchment basin.     

Food as the Dominant Pathway of Methylmercury Uptake by Fish

A field experiment was conducted to determine the degree to which fish accumulated methylmercury (MeHg) via their food or via passive uptake from water through the gills. Finescale dace (Phoxinus neogaeus) were held in 2000 L enclosed pens floating in an undisturbed, oligotrophic lake in northwestern Ontario. Fish were exposed to water containing either low (0.10–0.40 ng L^-1), intermediate (0.45–1.30 ng L^-1), or high (0.80–2.1 ng L^-1) concentrations of MeHg. Zooplankton with either low (0.16–0.18 µg g^-1 d.w.) or high (0.28–0.76 µg g^-1 d.w.) concentrations of MeHg were added daily to each pen. Fish fed zooplankton with high concentrations of MeHg had significantly higher concentrations of mercury in muscle after 32 days than fish fed zooplankton with low concentrations of MeHg (ANCOVA, P<0.0001). Fish feeding on zooplankton with low concentrations of MeHg had the same amount of Hg in their tissues as fish at the start of the experiment. Uptake from water was at most 15%. This is the first experiment to confirm that food is the dominant pathway of MeHg bioaccumulation in fish at natural levels of MeHg.     

Mercury speciation in Lake Baikal

Research on mercury (Hg) distribution and speciation was carried out in Lake Baikal, a large, strong-oligotrophic freshwater reservoir in Siberia, Russia, during June 1992 and march 1993. In summer, total Hg in the water column ranged from 0.14 to 0.77 ng Hg/L, with the highest concentrations observed in the central basin of the lake in surface water samples. Labile inorganic Hg was found to be 7 to 20 % of the total Hg content. Highest total Hg concentrations were found in river waters: up to 2 ng Hg/L. Labile methylmercury (MeHg) concentrations ranged from 2 to 38 pg Hg/L in the water column, with the higher concentrations in the central part of the lake, and showing a slight increase in near bottom waters. Labile MeHg makes up 1 to 15 % of the total Hg content in the water column, with larger fractions in deep waters. The slight increase of the MeHg gradient with depth corresponds with the O₂ minimum region. Highest MeHg concentrations were observed in river waters (up to 145 pg Hg/L) and in some bays of the lake (up to 160 pg Hg/L). In these high temperature- and phytoplankton-rich water masses, the MeHg-fraction increased up to 35 % of total Hg. Labile MeHg concentrations in water samples taken in winter in the southern basin (under the ice cover), showed slightly higher concentrations than in summer, possibly due to an early spring bloom. In rainwater, total Hg ranged from 3 to 20 ng Hg/L and MeHg from 0.1 to 0.25 ng Hg/L. In snow, a large fraction of total Hg is bound to particulate matter; concentrations of total Hg ranged from 8 to 60 ng Hg/L and labile MeHg from 0.1 to 0.25 ng Hg/L. Atmospheric Hg was found to be 0.73 to 2.31 ng/m³ as gaseous Hg and 0.005 to 0.02 ng/m³ in its particulate form. Spatial distribution patterns of atmospheric Hg show slightly higher concentrations over the central part of the lake and the Selenga river delta. In winter, atmospheric Hg values (measured in the southern region), ranged from 1.2 to 6.1 ng/m³ as total gaseous Hg and 0.02 to 0.09 ng/m³ as total particulate Hg, and are higher than in summer, probably influenced by coal burning and traffic by the local population. MeHg contents in fish ranged from 20 ng Hg/g dry weight in small Cottocomephorus to 300 ng Hg/g dry weight in pike and trout species, which were caught in organic-rich waters.     

Use of the mercury cycling model (MCM) to predict the fate of mercury in the Great Lakes

In response to U.S. EPA's proposed Great Lakes water quality criteria for mercury (Hg), a fieldvalidated Hg cycling model (MCM) was used to predict Hg levels in the abiotic and biotic components of Lake Superior and Lake Erie. The U.S. EPA criteria are based on water column Hg concentrations and simple trophic level transfer and, thus, do not consider sediment interactions and water chemistry factors. The model, using data from published reports, was run to simulate a 25 year steady state period. For these simulations, methylmercury (MeHg) represented 5% of total Hg in Lake Erie and 8% of total Hg in Lake Superior. These proportions are roughly 3–5 times lower than U.S. EPA's estimate that MeHg contributes about 25% of total Hg in the water column of the Great Lakes. The predicted median concentrations of total Hg in top-carnivore fish were 0.13 mg/kg in Lake Superior and 0.16 mg/kg in Lake Erie. Predicted median MeHg concentrations in Lake Superior and Lake Erie (water column) were 0.019 and 0.075 ng/L, respectively. For both lakes, most (>55%) of the Hg was partitioned to sediments. Although the MCM simulation does have practical limitations (e.g., lakes are treated as fully-mixed open systems), the results demonstrate that generic assumptions of Hg behavior in all Great Lakes waterbodies are too simplistic.     

Mercury accumulation trends in Florida Everglades and Savannas Marsh flooded soils

Global and regional increases in atmospheric mercury (Hg) concentrations have previously been identified as the cause of increased mercury accumulation rates in north temperate lakes in Sweden, Wisconsin, and Minnesota. Atmospheric deposition can often account for elevated Hg concentrations in fish from these systems. Mercury levels in sportfish collected from some areas of the Florida Everglades and Savannas Marsh exceed limits that are acceptable for human consumption. Forty five soil cores and soil grab samples were retrieved from the Everglades and Savannas Marsh wetlands. Eighteen sediment cores were dated radiochemically with²¹⁰Pb and¹³⁷Cs using γ-ray spectroscopy to determine modern and historic mercury accumulation rates for these subtropical wetland systems. Recent (“post-1985”) Hg accumulation rates averaged 53 μg m^?2 y^?1 (23 to 141, n=18) corresponding to an average rate increase of 4.9 times (1.6 to 19.1) over those observed around the turn of the century. This accumulation seems to result more from either global or regional atmospheric deposition rather than from lateral transport via overlying surface water. The trends for mercury accumulation match those reported for lakes in Sweden and the northern United States, even though these systems are distinctly different in their climate, vegetational composition, and location. We provide the first data on accumulation of mercury in subtropical wetland systems, and demonstrate the feasibility of radiochemical dating of wetland sediment.     

Measurement of methylmercury and mercury in run-off,lake and rain waters

During one year, samples from eight drainage lakes, seven run-off stations and three deposition sites from various geographical areas in Sweden were collected and analyzed for methyl Hg (MeHg) and total Hg (Hg-tot). The MeHg concentrations ranged from 0.04 to 0.64 ng L^?1, 0.04 to 0.8 ng L^?1, and <0.05 to 0.6 ng L^?1 in run-off, lake water and rain water, respectively. The corresponding Hg-tot concentrations were found in the range 2 to 12 ng L^?1, 1.35 to 15 ng L^?1, and 7 to 90 ng L^?1, respectively. A Hg-tot level of about 60 ng Hg L^?1 was found in throughfall water. The MeHg and Hg-tot concentrations are positively correlated in both run-off and lake water, but not in rain and throughfall water. A strong positive correlation between the MeHg, as well as the Hg-tot concentration, and the water color is observed in both run-off and lake waters, which suggests that the transport of MeHg and other Hg fractions from soil via run-off water to the lake is closely related to the transport of organic substances; and is a consequence of the biogeochemical processes and the water flow pathway. The ratio between the mean values of MeHg and Hg-tot seems to be an important parameter, with an indicated negative coupling to the mean value of pH for run-off water, but a strong positive correlation to Hg-content in fish, the ratio between the area of the catchment and the lake, as well as to the retention time of lake.     

Influence of suspended particulates on phosphorus loading exported from farm drainage during a storm event in the Everglades Agricultural Area

Purpose

The purpose of this study was to evaluate the influence of suspended particulates on P loading captured during a single storm event. The Everglades Agricultural Area of Florida comprises 280,000 ha of organic soil farmland artificially drained by ditches, canals, and pumps. Phosphorus (P)-enriched suspended particulates in canals are susceptible to transport and can contribute significantly to the overall P loads in drainage water.

Materials and methods

A settling tank experiment was conducted to capture suspended particulates during tropical storm Isaac in 2012 from three farms approximately 2.4 to 3.6 km² in size. Farm canal discharge water was collected in a series of two 200-L settling tanks over a 7-day drainage period, during tropical storm Isaac. Water from the settling tanks was siphoned through Imhoff settling cones, where the suspended particulates were allowed to settle and collected for P fractionation analyses, and compared to intact sediment cores collected from the canals.

Results and discussion

The discharged suspended particulates contained higher organic matter content, total P, and labile P fractions compared to the drainage canal sediments. Based on the equilibrium P concentrations, drainage canal sediments behave as a source of P to the water column. A 7-day continuous drainage event exported 4.7 to 11.1 t of suspended solids per farm, corresponding to 32 to 63 kg of particulate P being lost to downstream ecosystems. Drainage associated to this single 7-day storm event exported up to 61 % of the total annual farm P load.

Conclusions

It is evident from this study that short-term, high-intensity storm events can skew annual P loads due to the export of significantly higher suspended particulate matter from farm canals. Exported particulates rich in P, if captured and replenished back on farmlands, would be a sustainable farming practice that can provide a supplemental source of nutrients.