First reports of low-level mercury and methylmercury in Iowa surface waters, agricultural tile drainage channels, and nutrient reduction wetlands receiving elevated nitrate loads
Date
2022-05
Authors
Judge, Casey Jo
Major Professor
Advisor
Crumpton, William G
Krabbenhoft, David P
Hall, Steven J
Helmers, Matthew J
van der Valk, Arnold G
Committee Member
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Abstract
The Iowa Nutrient Reduction Strategy has identified wetlands and other anaerobic edge-of-field practices as essential for reducing excess nitrate loads to downstream waters. The anaerobic environments provided by these practices promote the activity of denitrifying bacteria, but may also provide ideal environments for other anaerobic microbial communities that methylate mercury. Methylmercury (MeHg) is an environmental contaminant and developmental neurotoxicant of human health concern in Iowa as well as globally. Wetlands have been identified as important environments for the methylation of mercury, but elevated nitrate concentrations may inhibit the activity of microbial communities involved with mercury methylation. This study provides the first known reports of the total mercury (HgT) and MeHg received by and exported from nutrient reduction wetlands receiving elevated nitrate loads from agricultural tile drainage systems. It also provides insights regarding transport of HgT and MeHg in agricultural tile outlet channels, and potential impacts to larger perennial 4th and 5th order streams. Results show that temporal variations in HgT, MeHg, and the methylated fractions were similar for three 4th and 5th order stream locations, but appeared to be influenced by other site characteristics in tile outlet channels and wetland outlets. Results indicate that both tile outlet channels and nutrient reduction wetlands can exhibit periods of increased MeHg concentrations during low flow and low nitrate conditions, however low flow conditions generally limit associated increases in MeHg yields. Methylmercury yields received from the tile outlet channels accounted for 17-87% of the median MeHg yield exported from the wetland outlets and MeHg yields from the wetland outlets were not substantially different than those transported in the 4th and 5th order streams.
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dissertation