TEMPORAL VARIATION IN RECHARGE AND WATER QUALITY IN A FRACTURED DOLOMITE AQUIFER WITH MODERATE SOIL COVER

The fractured Silurian-dolomite aquifer of northeastern Wisconsin is an important, but vulnerable, source of drinking water. Previous work has shown that recharge to the dolomite aquifer is extremely rapid in areas of thin soils (<5 ft) and water quality is also highly variable. It has been assumed that recharge and water-quality variations in the dolomite aquifer would be dampened in areas of thicker soil cover and that the aquifer is less vulnerable to contamination there. Recent efforts to develop best management practices (BMPs) designed to minimize groundwater contamination caused by the application of animal wastes has highlighted a lack of detailed water-level and water-quality data in areas of thicker soil cover.

The goal of this project was to gain an understanding of seasonal variations in recharge and the resulting water-quality variations in the dolomite aquifer in areas where the dolomite was the uppermost bedrock aquifer, soil was 10 to 20 feet thick, and manure or sewage sludge was being applied. At each of four field sites we installed a shallow bedrock well and monitored variations in water levels and water quality for one year. Water levels and fluid temperature and conductivity were recorded every 30 minutes. Monthly water-quality samples were analyzed for nitrate-nitrogen, chloride and phosphorus.

Water levels in all wells responded rapidly (within 24 to 48 hrs) to episodic recharge events during the winter, spring and summer of 2008. The overall range in water level fluctuation varied from 2.4 ft to 8.97 ft. Interestingly, depth to water table did not seem to be an important control on well response to recharge. Rather, the thickness and texture of the unlithified material were the important factors.

All of the wells showed rapid variations in temperature and conductivity in response to recharge events. Water-quality results were also variable over time. All wells exhibited elevated nitrate-nitrogen and chloride values and variations in these parameters were similar over time. Three of the wells had average NO3-N values greater than the drinking water standard of 10 mg/L. Generally the lowest nitrate values followed recharge events.

These results show that, even in areas of up to 20 feet of unlithified materials, recharge to the carbonate bedrock aquifer is rapid and contaminant attenuation is minimal.