HYPORHEIC AND WETLAND IMPROVEMENTS TO WATER QUALITY FOR A PUBLIC GROUNDWATER SUPPLY PRODUCING WATER BY INDUCED RECHARGE FROM BAZILE CREEK, KNOX COUNTY, NEBRASKA

Modeled water budgets indicate that 99% of the water produced by two public water supply wells pumping at 200 gpm 900 feet north of Bazile Creek, Knox County, Nebraska comes from induced recharge. Two transects of multi-level wells installed between Bazile Creek and the public water supply wells include a shallow and deep well at 4 evenly spaced locations along each transect. The sand and gravel alluvial aquifer adjacent to Bazile Creek is 40 feet thick and is bounded on the north, south and at the base by Cretaceous Pierre Shale, which is black marine shale with significant sulfate, manganese and iron bearing minerals. The bed of Bazile Creek is mainly sand and fine gravel. An extensive groundwater supported wetland defines the surface between the creek and the uplands where the public supply wells are located. Water levels in the wetland are at or near ground surface. Vegetation in the wetland is dominated by cattails. Particle tracking shows that conservative tracers placed in Bazile Creek reach the wells in as a little as 31 days. Nitrate-nitrogen in Bazile Creek ranges from 3-6 mg/L throughout the year, while nitrate-nitrogen ranges from non-detect to 0.7 mg/L in water from the wells along the transect and in water from the public supply wells. There is also an annual flush of pesticides observed in the creek during the period May to July each year, dominated by atrazine and alachlor (Davis et al., 1996; 2001), yet there were no detections of the 35 pesticides analyzed from water from the wells along each transect or in water from the public supply wells. The water in Bazile Creek is aerobic and supports a diverse biota, however the water beneath the wetland is highly anaerobic as indicted by shifting water quality parameters such as SO₄^2- (106 mg/L in Bazile Creek to a high of 445 mg/L along well transect) and Mn (0.02 mg/L in Bazile Creek to high of 6.63 mg/L along well transect). Rapid denitrification and degradation of pesticides in the hyporheic zone and the anaerobic alluvial aquifer beneath the wetland is occurring resulting in significant reductions in nitrate-nitrogen and pesticides. However, as a result of mineralization from the Pierre Shale there are significant increases of nuisance water quality parameters including S0₄^2-, Mn, and Fe that render the public supply aesthetically unpalatable, necessitating additional treatment.