Ground-Water Quality in Agricultural Areas, Anoka Sand Plain Aquifer, East-Central Minnesota, 1984-90

Ground-water quality in the Anoka Sand Plain aquifer was studied as part of the multiscale Management Systems Evaluation Area (MSEA) study by collecting water samples from shallow wells during August through November 1990. The sampling was conducted to: (1) aid in selection of the MSEA research area; (2) facilitate comparison of results at the MSEA research area to the regional scale; and (3) evaluate changes in ground-water quality in the Anoka Sand Plain aquifer since a previous study during 1984 through 1987. Samples were collected from 34 wells screened in the upper 6 meters of the surficial aquifer and located in cultivated agricultural areas. Water temperature, pH, specific conductance, and presence or absence of triazine herbicides were determined at all sites and samples from selected wells were analyzed for concentrations of dissolved oxygen, alkalinity, major cations and anions, nutrients, and selected herbicides and herbicide metabolites. The results of the study indicate that the water-quality of some shallow ground water in areas of predominantly agricultural land use has been affected by applications of nitrogen fertilizers and the herbicide atrazine. Concentrations of nitrite plus nitrate nitrogen (nitrate-N) in 19 samples ranged from less than the detection limit of 0.1 to 21 milligrams per liter (mg/L) with a median of 10 mg/L. Concentrations of nitrate-N were not significantly correlated with depth below the water table or dissolved oxygen but were significantly correlated with concentrations of chloride. Concentrations of nitrate-N during 1990 were generally similar to concentrations measured during 1984 through 1987. Changes in concentrations of nitrate-N during 1984 through 1990 were likely related to variations in recharge or other site-specific factors. Immunoassay analyses were used as screening tools to detect the presence of triazine herbicides. All samples in which triazine herbicides were detected by immunoassay, and selected samples in which triazine herbicides were not detected by immunoassay, were analyzed for selected herbicides and herbicide metabolites by gas chromatography/mass spectroscopy (GC/MS). There was an excellent correspondence between concentrations of triazine herbicides indicated by immunoassay and concentrations of atrazine measured by GC/MS, indicating that the immunoassay method was a valuable qualitative to semi-quantitative tool for evaluating the presence and approximate concentration of atrazine. The combined results of immunoassay and GC/MS analyses indicate that atrazine was detected in 11 of the 34 wells with a median concentration less than the immunoassay detection limit of 0.1 micrograms per liter (u,g/L). Atrazine was detected in 11 of the 17 samples analyzed by GC/MS at concentrations ranging from the detection limit of 0.05 to 0.81