FINE-SCALE TEMPORAL TRENDS IN WATER-LEVEL AND WATER-QUALITY IN TWO MONITORING WELLS IN THE CENTRAL SANDS REGION OF WISCONSIN

As part of the U.S. Geological Survey National Water-Quality Assessment Program’s Enhanced Trend Network (ETN), high-frequency water-level and water-quality data were collected in one shallow and one deep well in the glacial aquifer of the Central Sands region of Wisconsin. Water withdrawn from the shallow well represents very recent recharge into the aquifer, while the deep well represents water that recharged the aquifer more than 30 years ago. The data from these wells are being used to further understand the behavior and conditions of the aquifer relative to changes in land use and climate on a finer temporal scale than previous data collection efforts. Land use in the recharge area for this study includes forest, grassland and agricultural (both irrigated and non-irrigated). Past water-quality samples taken from the glacial aquifer in this area, at near decadal intervals, have consistently shown elevated and increasing concentrations of nitrate and elevated but decreasing concentrations of atrazine in groundwater. The high-frequency data from the ETN wells will provide a more thorough understanding of how conditions change over much smaller time scales in this groundwater system.

Comparisons of fine-scale data from the two ETN wells show similarities in water-level response but variable response of water-quality parameters. Dissolved oxygen showed minimal fluctuation over time in both wells, though the average value in the shallow well was twice that of the deep well. Both wells showed a comparable pattern of annual fluctuation in groundwater temperature but with distinct timing. Water temperature reached a maximum in the deep well on December 12^th (9.2 ^oC) and a minimum on June 14^th (8.5 ^oC). The maximum water temperature in the shallow well was on November 13^th (9.6 ^oC) and the minimum on June 5^th (8.0 ^oC). Specific conductance, which can be correlated to nitrate concentrations in groundwater, showed large seasonal variations in the shallow well and minimal variation in the deep well. Discrete data that was collected from the wells have also shown variability but were too infrequent to determine trends or a climatic response of these parameters.