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 (μg/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 μg/L with a median of 0.09 μg/L. Atrazine metabolite de-ethylatrazine (DEA) was the most frequently detected herbicide or herbicide metabolite and generally was present at the greatest concentrations. There were detections of DEA in 13 of the 17 samples analyzed at concentrations ranging from the detection limit of 0.05 to 1.12 μg/L with a median of 0.15 μg/L. Concentrations of DEA were significantly correlated with concentrations of calcium, magnesium, and specific conductance. The atrazine metabolite de-isopropylatrazine and the herbicide metolachlor both were detected in 1 of the 17 wells. The frequency of detection and the concentrations of atrazine in shallow ground water during 1990 were very similar to results of sampling conducted in 1984.

Concentrations of atrazine and DEA generally were greater near the water table and decreased or were not detected in deeper wells. All of the samples in which atrazine and DEA were detected also had increased (greater than 3 mg/L) nitrate-N concentrations. However, not all samples with increased concentrations of nitrate-N had detections of atrazine or DEA. This likely indicates either that there were sources of nitrate-N other than cultivated fields on which both atrazine and nitrogen were applied or that nitrate-N reached ground water more readily than atrazine or DEA.