DEVELOPMENT OF A BENTHIC-FLUX CHAMBER FOR MEASUREMENT OF GROUND-WATER SEEPAGE RATES AND WATER SAMPLING FOR MERCURY ANALYSIS AT THE SEDIMENT-WATER INTERFACE

The U.S. Geological Survey, in cooperation with the U.S. Environmental Protection Agency and the Minnesota Department of Health, designed and constructed a benthic-flux chamber as a tool to evaluate ground-water and surface-water interaction and mercury concentrations in water at sediment-water interfaces. The benthic-flux chamber was used to measure ground-water seepage rates with an ultrasonic-flow meter and to collect water-quality samples for chemical analysis. We tested the flux chamber in a laboratory calibration tank and by deploying it in two lakes alongside two 55-gallon drum seepage meters. The flux chamber seepage rates were comparable to those measured concurrently by an inline flow meter in a 152-cm diameter by 152-cm high laboratory calibration tank. In addition, benthic-flux chamber seepage rates to rates measured with drum seepage meters from two lakes also were comparable.

The minimum, maximum, and average flow rates for the benthic-flux chamber at Long Lake in Ramsey County were 1.6, 39.5 and 18.1 cm/day respectively, compared to 1.2, 26.0, and 11.2 cm/day for the 55-gallon drum seepage meters. At Square Lake in Washington County, the minimum, maximum, and average flow rates for the benthic-flux chamber were 5.9, 18.3, and 13.1 cm/day, respectively compared to 10.9, 16.8, and 11.2 cm/day for the 55-gallon drum seepage meters. The flux-chamber samples yielded total mercury concentrations of 0.47 and 0.53 nanograms per liter (ng/L), while the lake concentration was 0.27 ng/L. Methyl mercury concentration for all three of the samples was below the analytical detection limit of 0.04 ng/L. It was not possible to collect mercury samples using the 55-gallon drum seepage meters.

The flux chamber was useful for measuring slow ground-water seepage rates and for collecting water samples for analysis of mercury concentrations at the nanogram per liter level.