MERCURY CYCLING IN THE ALLEQUASH CREEK WATERSHED, NORTHERN WISCONSIN

Although there have been recent significant gains in our understanding of mercury (Hg) cycling in aquatic environments, few studies have add ressed Hg cycling on a watershed scale. In particular, attention to Hg species transfer between watershed components (upland soils, groundwa ter, wetlands, streams, and lakes) has been lacking. This study descri bes spatial and temporal distributions of total Hg and MeHg among wate rshed components of the Allequash Creek watershed (northern Wisconsin, USA). Substantial increases in total Hg and MeHg were observed as gro undwater discharged through peat to form springs that flow into the st ream, or rivulets that drain across the surface of the wetland. This i ncrease was concomitant with increases in DOC. During fall, when the A llequash Creek wetland released a substantial amount of DOC to the str eam, a 2-3 fold increase in total Hg concentrations was observed along the entire length of the stream. Methylmercury, however, did not show a similar response. Substantial variability was observed in total Hg (0.9 to 6.3) and MeHg (<0.02 to 0.33) concentrations during synoptic s urveys of the entire creek. For the Allequash Creek watershed, the con tributing groundwater basin is about 50% larger than the topographic d rainage basin. Total Hg concentrations in groundwater, the area of the groundwater basin, and annual stream flow data give a watershed-yield rate of 1.2 mg/km(2)/d, which equates to a retention rate of 96%. The calculated MeHg yield rate for the wetland area is 0.6 to 1.5 mg/km(2 )/d, a value that is 3-6 fold greater than the atmospheric deposition rate.