Comparison of the Effects of Rotary Wash and Air Rotary Drilling Techniques on Pumping Test Results

摘要

Pumping tests were performed on monitor wells distributed in a 1-mile sup 2 area. Thirty-five of the 41 wells were screened in fractured dolomite (bedrock), and 6 wells were screened in an overlying layer of sand and gravel. The results of 63 sets of pumping test data revealed four trends: (1) wells screened and sand packed in the sand or gravel layer have consistently higher transmissivities than those screened and sand packed in the fractured bedrock, regardless of the drilling method used; (2) bedrock wells drilled by rotary wash methods have transmissivities averaging about 1 order of magnitude lower than bedrock wells drilled by air rotary methods, regardless of the type of screen or sand pack used; (3) four-inch-diameter monitor wells (all drilled by air rotary method) almost always have transmissivities 1 to 2 orders of magnitude higher than 2-in.-dia wells drilled by air rotary methods; and (4) transmissivities of 6-in.-dia wells are about 1 order of magnitude less than 4-in.-dia wells. Based on these trends, the drilling method (i.e., air versus rotary wash), the geologic medium, and the diameter of the monitor well or borehole significantly influence whether pumping test data are representative of aquifer conditions before drilling. The drilling method has significant influence, but the diameter of the monitor well appears to have the most influence. (ERA citation 11:041345)