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

Abstract

Glacial 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.

A vadose zone Transport Processes Investigation within the glacial till at the Fernald Environmental Management Project.

“…These processes may produce fast transport pathways to underlying aquifers. Diffusive flow through massive clays in glacial tills may also occur (Jones et al, 1992).…”

Transport and fate of a LNAPL in fractured clayey till

“…The differences in sample size and clay content may account for the differences in magnitude of hydraulic conductivity values. The high value of hydraulic conductivity measured at this site with in situ pumping tests relative to that measured by Hendry, 1988 with pumping tests may be a result of differences in parent material (Jones et al, 1992). The Canadian till has a grain size distribution of 10% sand, 33% silt, and 57% clay with clay texture (Fernandez and Quigley, 1991).…”

“…It has been shown that due to the interconnectivity of the fractures and macropores contaminants may travel preferentially through a natural clayey till . Preferential transport that occurs in these natural clayey tills indicates that soils of this type cannot be thought of as a banier for contaminants (Jones, 1992). Thus, an understanding on the genesis of fractures and macropores is essential for understanding this preferential flow occunring in glacial tills and other engineering properties.…”

“…Previous drilling at the site indicated that the weathered zone extends to a depth of approximately 4 m (Jones et al, 1992). Grass covers the site with grass roots and remnant root channels present to depths of approximately 2.0 m (6 ft).…”

“…Other hydraulic conductivity tests on fractured till have produced similar results with values ranging from 10"^ cm/s to 80 10"® cm/s for tests perfomied by Middleton et al, 1990 and 10"® cm/s to 10"® cm/s for tests performed by . For comparison, the hydraulic conductivity measured during in situ pumping tests on the site was on the order of 3x10"^ cm/s to 5 x 10"^ cm/s (Jones, et al 1992). …”