SAND PACK DRAINAGE DURING SLUG TESTING AND ITS EFFECT ON SITE ASSESSMENTS IN LOW-K TERRAINS OF IOWA

The petroleum underground storage tank (UST) program in Iowa has on file thousands of hydraulic conductivity (K) determinations from slug tests done in monitor wells that were constructed with the well screen crossing the water table. Many of these are from low-K, clay-rich formations. Comparing K determinations of the clay-rich formations obtained by routine environmental assessment methods with published K values obtained under research conditions shows a tendency for the former group to cluster near the high end of the K range defined by the latter group. This tendency might result from methodology that allowed sand pack drainage to influence the slug test strongly.

An experimental well was built that closely resembles the typical UST program monitor well. The well was in an 8 inch diameter pipe with a 2 inch diameter screen centered in a medium grain sand pack. This well was slug tested in a manner routinely employed for UST site assessments, and the sand pack drainage data was analyzed by the Bouwer and Rice method. Analyzed as slug tests, the drainage curves achieve only 67%-76% recovery; they resemble some recovery curves for slug tests done at UST sites in Iowa. "K" values calculated from sand pack drainage fall near the high end of the range expected for low-K formations.

Erroneously high K values obtained for sites in low-K terrain result in unrealistically long contaminant plumes when modeled according to the method prescribed by the Iowa Administrative Code. This sometimes results in the expenditure of funds for further assessment of erroneously designated high risk sites. To assess the quality of slug test data from monitor wells in low-K terrains, it is necessary to know the slug volume, well dimensions, and to have a complete record of water level from pre-initiation until 90%-100% recovery. This information is compared to expected recovery times estimated with the Bouwer and Rice formula.