Mercury (Hg) and methylmercury (MeHg) are flushed from watersheds during hydrological events, contaminating downstream surface waters and resident fish populations. We monitored total mercury (THg), MeHg, and ancillary water chemistry parameters in two streams (Cedar Creek and Trott Brook) in east-central Minnesota on a weekly or semiweekly basis from April through October 2003. Heavy precipitation in late June resulted in discrete episodes of high concentrations (>1.2 ng/L) of MeHg in both streams in early July. The MeHg/THg ratio increased from 0.15 to 0.36 in Cedar Creek and from 0.13 to 0.46 in Trott Brook during the event. The high MeHg concentrations were accompanied by low dissolved oxygen concentrations and increased concentrations of dissolved organic carbon, Mn, Fe, and orthophosphate. A prolonged absence of precipitation during August and early September brought stream levels back to baseflow values, and MeHg concentrations decreased to less than 0.1 ng/L. These results suggest that warm-weather, high-discharge events are the primary route of export of MeHg from these watersheds, and baseflow contributes much less MeHg to downstream waters. The redox water chemistry during the,events sampled here suggests that MeHg in these streams is discharged from wetland areas where anoxic/anaerobic conditions prevail.
[Show abstract][Hide abstract] ABSTRACT: Total mercury (THg) and mono-methylmercury (MeHg) levels in water, sediment, and largemouth bass (LMB) (Micropterus salmoides) were investigated at 52 sites draining contrasting land use/land cover and habitat types within the Mobile Alabama River Basin (MARB). Aqueous THg was positively associated with iron-rich suspended particles and highest in catchments impacted by agriculture. Sediment THg was positively associated with sediment organic mater and iron content, with the highest levels observed in smaller catchments influenced by wetlands, followed by those impacted by agriculture or mixed forest, agriculture, and wetlands. The lowest sediment THg levels were observed in main river channels, except for reaches impacted by coal mining. Sediment MeHg levels were a positive function of sediment THg and organic matter and aqueous nutrient levels. The highest levels occurred in agricultural catchments and those impacted by elevated sulfate levels associated with coal mining. Aqueous MeHg concentrations in main river channels were as high as those in smaller catchments impacted by agriculture or wetlands, suggesting these areas were sources to rivers. Elevated Hg levels in some LMB were observed across all types of land use and land cover, but factors such as shallow water depth, larger wetland catchment surface area, low aqueous potassium levels, and higher Chl a concentrations were associated with higher Hg burdens, particularly in the Coastal Plain province. It is suggested that the observed large variability in LMB Hg burdens is linked to fish displacement by anglers, differences in food web structure, and sediment biogeochemistry, with surficial sediment iron oxides buffering the flux of MeHg from sediments to deeper water pelagic food webs.
Science of The Total Environment 08/2005; 347(1-3):187-207. DOI:10.1016/j.scitotenv.2004.12.011 · 4.10 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Previous studies have identified flooded landscapes (e.g., wetlands, impoundments) as sites of elevated methylmercury (MeHg) production. Here we report MeHg and total Hg (THg) concentrations and mass loadings in rivers in Minnesota during major flooding episodes in the summer of 2002. Frequent intense precipitation events throughout the summer resulted in extraordinarily wet conditions in east-central and northwestern Minnesota. Streamflow remained at record-setting high levels in many rivers and streams in these regions for several weeks. We observed high concentrations of MeHg (>1.4 ng/L) accompanied by high MeHg/THg ratios (0.39 to 0.50) in the Roseau River in northwestern Minnesota and in the Elk and Rum Rivers in east-central Minnesota. Very high MeHg mass loadings were observed in the Mississippi River just upstream of Minneapolis on July 17 (51 g MeHg/day) and July 23 (42 g MeHg/day), when MeHg concentrations at this site were 0.89 and 0.99 ng/L, respectively. The elevated MeHg concentrations in the Roseau River were associated with low dissolved oxygen and high dissolved reactive phosphorus concentrations, both of which are characteristic of anoxic waters. These rivers drain landscapes containing varying amounts of wetlands, and some of the MeHg discharged is thought to have been flushed from anoxic wetland soils. In addition, the flooding of vast areas of normally dry land surfaces probably also resulted in increased MeHg production, adding to the quantities of MeHg exported from these watersheds. Changing climate patterns are expected to result in more frequent heavy precipitation and flooding events in Minnesota. Our results suggest that as flooding and wet conditions in this region increase, the production of MeHg and its export from terrestrial areas to surface waters will increase also.
Science of The Total Environment 10/2006; 368(1):138-48. DOI:10.1016/j.scitotenv.2005.09.045 · 4.10 Impact Factor
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