GROUNDWATER AGE AND RECHARGE DISTRIBUTIONS BENEATH INCISED GLACIAL TILL IN EASTERN NEBRASKA

Eastern Nebraska’s glaciated region provides an attractive natural laboratory to investigate the effects of spatially-variable lithology on groundwater recharge patterns. This study examines recharge to aquifers underlying river-incised glaciated terrain where the distribution of clay-rich till is largely intact in upland locations but has been removed by alluvial erosion in stream valleys. Methods included a diverse range of physical and geochemical methodologies in order to bridge spatial scales from point-scale to regional-scale (matric potential monitoring, trace gases, radioisotopes, solute mass balance, remote sensing and water balance modeling). Results show a robust pattern of lower diffuse recharge rates in upland locations than lowland locations, and lower diffuse recharge rates that total recharge rates on the regional scale. Total recharge (diffuse plus non-diffuse) was estimated to be 58 mm/yr based on groundwater tracers and 56 mm/yr based on a remote sensing-assisted water-balance model. Diffuse recharge rate estimates ranged from ~0-34 mm/yr across three unsaturated zone monitoring locations. The results highlight the relevance of lithologic controls on groundwater renewability in glacial terrain and provide new recharge rate estimates for groundwater management purposes in the region.