Methylmercury in Flood-Control Impoundments and Natural Waters of Northwestern Minnesota, 1997–99

Abstract

We studied methylmercury (MeHg) and totalmercury (HgT) in impounded and natural surface waters innorthwestern Minnesota, in settings ranging from agriculturalto undeveloped. In a recently constructed (1995) permanent-pool impoundment, MeHg levels typically increased from inflowto outflow during 1997; this trend broke down from late 1998 toearly 1999. MeHg levels in the outflow reached seasonal maximain mid-summer (maximum of 1.0 ng L-1 in July 1997) andlate-winter (maximum of 6.6 ng L-1 in February 1999), andare comparable to high levels observed in new hydroelectricreservoirs in Canada. Spring and autumn MeHg levels weretypically about 0.1–0.2 ng L-1. Overall, MeHg levels inboth the inflow (a ditch that drains peatlands) and outflowwere significantly higher than in three nearby referencenatural lakes. Eleven older permanent-pool impoundments andsix natural lakes in northwestern Minnesota were sampled fivetimes. The impoundments typically had higher MeHg levels(0.071–8.36 ng L-1) than natural lakes. Five of six lakesMeHg levels typical of uncontaminated lakes (0.014–1.04 ngL-1) with highest levels in late winter, whereas ahypereutrophic lake had high levels (0.37–3.67 ng L-1)with highest levels in mid-summer. Seven temporary-poolimpoundments were sampled during summer high-flow events. Temporary-pool impoundments that retained water for about 10–15days after innundation yielded pronounced increases in MeHgfrom inflow to outflow, in one case reaching 4.6 ng L-1,which was about 2 ng L-1 greater than the mean inflowconcentration during the runoff event.