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