COMPARING SLUG TEST RESULTS IN UNCONFINED AQUIFERS ANALYZED USING DIFFERENT METHODS

Slug test methods are used to determine aquifer hydraulic conductivity (K) in situ more quickly and economically than with a pump test. This study compares slug test methods for unconfined aquifers, including Bouwer and Rice (1976), Kansas Geological Survey (1994), Hvorslev (1951) and Dagan (1978). Slug test data from several wells in two separate unconfined aquifers is analyzed to examine the effects of having well screens either entirely submerged or crossing the water table. This study attempts to answer questions such as: Does the Bouwer and Rice method work? Does a big slug yield better results than a small slug? Does the option of assuming a well skin make a significant difference in the slug test results?

To address these questions, experiments were conducted in a 7-foot diameter culvert installed vertically in a 7.5-foot deep hole and then backfilled with uniform sand. Six monitoring wells were installed in this uniform man-made unconfined aquifer, and two sizes of slug rods were used in testing these wells. One rod was five feet long and 1 inch in diameter. The larger rod was almost 7 feet long and 1.5 inches in diameter, with an embayment in the bottom and a slot in one side to make room for a pressure transducer and its cable. Tests also were conducted on five closely-spaced wells installed in a natural unconfined aquifer with different screen lengths and depths relative to the water table. All wells tested at both sites were two inches in diameter.

Hydraulic conductivity (K) values calculated from tests in the culvert and the natural aquifer show that the Bouwer and Rice method results are closer to the KGS results than to the Hvorslev values. Bouwer and Rice K’s are noticeably smaller than KGS or Hvorslev K’s; the difference is statistically significant. Bouwer and Rice K values have a smaller standard deviation than the KGS or Hvorslev values. Hydraulic conductivity values obtained from tests using the big slug rod are significantly bigger than K’s obtained using a smaller slug rod, regardless of analysis method chosen. Since slug test K’s are invariably smaller than pump test K’s, the larger K’s obtained using the big slug are believed to be more accurate. This means that better results are obtained when slug size is maximized.