Determination of quantity and quality of groundwater discharge to a section of Bear Creek in central Iowa

Abstract

Groundwater's contribution to surface water contamination from agricultural activities is poorly understood, particularly in till-dominated watersheds. The purpose of this study was to estimate the quantity of groundwater discharge to a 737-meter long section of Bear Creek in central Iowa. The site is located on an 18 hectare section of a private farm 2.5 km north of Roland, Iowa, Holocene alluvium and late Wisconsin till and supraglacial .sediment of the Des Moines Lobe (Dows Formation) overlie limestone, shale, and sandstone of Mississippian age (St. Louis Formation). Groundwater discharge to the creek was estimated using: 1) Darcy's Law, using hydraulic gradient and K data from minipiezometers in the creek, 2) seepage meter data, and 3) mass balance. Calculated groundwater discharges exhibited good general agreement between methods during April and May, 1994, and were 1.0 Lis (Darcy's Law), 0.7 -1.0 Lis (seepage meters), and less than 2 Lis (mass balance), Seepage meters indicated variability of groundwater discharge rates with sediment type. Mass balance revealed that upstream water and drainage tile flow within the site typically contribute over 95% of stream discharge, and groundwater samples from directly beneath the stream channel contain much less N03-N and atrazine than ambient stream water. The effect of low groundwater discharge rates coupled with the water quality data suggest that groundwater may not be a significant contributor to agrichemical contamination of surface water in this till-dominated watershed.