Results of Two Pumping Tests in Wisconsin Age Weathered Till in Iowa

Abstract

AbstractGlacial till overlies many bedrock aquifers in Iowa, and a number of landfills and leaking underground storage tank sites are located in weathered and unweathered till. To investigate the potential for significant lateral ground‐water flow in Wisconsin age weathered loam till in Iowa, two 24‐hour pumping tests were performed in the shallow unconfined ground water present in the weathered till, a constant flowrate pumping test and a constant drawdown pumping test. The saturated thickness in the weathered till at the research site is less than 3 m, and choosing a constant flowrate sufficient for significant drawdown in the monitoring wells without dewatering the pumping well was difficult. In a low conductivity medium with a thin saturated thickness, the constant drawdown pumping test was found to be a more practical field technique. In the constant drawdown test, a chosen level of constant drawdown is maintained in the pumping well for the duration of pumping, and the flowrate decreases with time. The changing flowrate of the constant drawdown pumping test was approximated as a series of step decreases, and superposition of the Theis solution was used for simulation. Both types of pumping tests yielded similar parameter estimates for the weathered loam till region in Iowa, with an average bulk horizontal hydraulic conductivity of around 5×10‐4 cm/s and an average specific yield of about 0.032. Single well response (bail) tests produced comparable estimates for hydraulic conductivity, with a mean of 3×10‐4 cm/s. The results imply significant lateral ground‐water flow is possible, and the weathered till should not be considered a significant barrier to the lateral spread of shallow ground‐water contamination.