USING TRACER TESTS IN IN SITU MESOCOSMS: DENITRIFICATION IN THE ELK VALLEY AQUIFER

Large (1.5 m long, 0.4 m diameter) stainless steel chambers have been installed in undisturbed sediments below the water table at nine sites in Minnesota and North Dakota. These chambers form in situ mesocosms (ISMs) of aquifer sediments. At our first site in the Elk Valley aquifer near Larimore, in eastern North Dakota, multiple tracer tests with bromide- and nitrate-amended groundwater have been performed in the same pair of ISMs. Nitrate and bromide are added to the nitrate ISM; only bromide is added to the control ISM. Loss of nitrate beyond that explained by dilution of the bromide tracer is attributed to denitrification. Zero-order denitrification rates ranged from 0.1 mg/L/day to 0.2 mg/L/day. Based on sulfate increases, sulfide in pyrite appears to be the primary electron donor for denitrification, explaining 50% to 80% of the denitrification measured. Insights into other electron donors participating in denitrification are facilitated by the large size of the ISMs, which allows relatively large samples to be collected and comprehensive analyses of major ions to be performed. Mineral saturation indices for the groundwater in the ISMs may be calculated with these data. These indices and other geochemical data suggest that ferrous iron and organic carbon also serve as electron donors for denitrification.