Abstract
Mercury methylation, where inorganic mercury (Hg) is converted to methylmercury (MeHg), can increase in soils when flooded. While effects of the initial flooding of soils on MeHg production have been well studied, less is known about impacts of re-flooding on MeHg production. Lake Perez, an impounded recreational reservoir in the Appalachian Highlands, was completely drained then re-filled 7 years later. We use a combination of chemical, soil physical, and microbial data to quantify changes in MeHg before and after re-flooding of the lakebed. Portions that were transiently saturated due to pluvial flooding had the highest pre-flooded MeHg concentrations. When the lake was re-flooded, concentrations of MeHg in subaqueous soils increased by a factor of 2.74 (+174%) on average. Substantial variability was observed among the sampling sites, with smaller increases in MeHg at sites subjected to seasonal flooding during periods when the reservoir was drained. The increase of soil MeHg after re-flooding was lower in this study compared to studies that evaluated soil MeHg after initial flooding, indicating that re-flooding of a former lake bed caused a smaller response in MeHg production compared to initial flooding of terrestrial land. This study advances understanding of the environmental impact of impounded reservoirs.
Highlights
Artificial impoundments creating lakes or reservoirs are developed for many environmental or socio-economic purposes, including hydropower, wastewater treatment, flood control, and recreation
We focus in this study on the latter, evaluating how re-flooding of a drained lake bed influences the production of MeHg in lake bed soils
THg and MeHg in soils prior to flooding Topsoil (O horizon) mean THg concentrations in the L, CV, lake channel bank (LCB) and R areas during pre-flooding conditions ranged between 78.7–95.0 ng g−1, and mean carbon content ranged between 2.1%–4.3%C
Summary
Artificial impoundments creating lakes or reservoirs are developed for many environmental or socio-economic purposes, including hydropower, wastewater treatment, flood control, and recreation. Flooding of terrestrial land might, in turn, influence the biogeochemical status of the soils and can alter the cycling of certain elements, including mercury (Hg). Because MeHg is toxic to the nervous system, it can pose a health risk for humans and wildlife, and it accumulates and biomagnifies in the food web. Mailman et al (2006) have articulated several potential strategies to mitigate high levels of Hg in fish in hydroelectric reservoirs and lakes, such as burning before flooding, removing vegetation, capping bottom sediment, phosphorus addition, and site selection. We focus in this study on the latter, evaluating how re-flooding of a drained lake bed influences the production of MeHg in lake bed soils
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