EFFECTS OF SLUG SIZE ON CALCULATED HYDRAULIC CONDUCTIVITY VALUES USING FOUR ANALYSIS METHODS

Slug tests are used to determine in situ aquifer hydraulic conductivity (K) quickly and economically. Many slug tests were conducted in two separate unconfined aquifers, a man-made aquifer and a natural aquifer. The closely-spaced wells in these aquifers have different screen lengths, with some screens entirely submerged and others partially above the water table. The results of Bouwer and Rice (1976), Kansas Geological Survey (1994) (KGS), Hvorslev (1951) and Dagan (1978) methods used to calculate K values from slug test data were compared.

The main purpose of this study is to answer questions such as: How well does the Bouwer and Rice method work? Does a big slug yield better results than a small slug? Does the KGS method option of assuming a well skin make a significant difference in the slug test results?

The man-made aquifer is a 7-foot diameter culvert installed vertically in a 7.5-foot deep hole and then backfilled with uniform medium sand. It has 6 PVC wells, all 2 inches in diameter. The slug tests were conducted using two different-sized slugs. The bigger rod is 6.95 feet long and 1.5 inches in diameter; the smaller rod is 5 feet long and 1 inch in diameter. The bigger slug has a slot cut along one side of the rod to make room for the transducer’s vented cable. The same slugs were used to conduct tests on five other 2-inch diameter wells installed in a natural unconfined sandy aquifer. These wells have different screen lengths and depths relative to the water table.

Comparing the K values calculated using four different methods in AQTESOLV, the Bouwer and Rice method produced smaller K values than the KGS or Hvorslev methods; the difference is statistically significant. Also, Bouwer and Rice K values have a smaller standard deviation than the KGS or Hvorslev K values. Finally, hydraulic conductivity values obtained using the bigger slug are significantly bigger than the K’s obtained using a smaller slug, regardless of analysis method chosen. Since pump test K values are generally larger than slug test K values, and pump test results are considered more reliable, the larger K’s obtained using the big slug are believed to be more accurate than the K results from the smaller slug. For that reason the bigger slug appears to be better than the smaller one.