U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Preliminary Type-Curve Analysis of an Aquifer Test in an Unconfined Sand and Gravel Aquifer, Cape Cod, Massachusetts

Abstract

An aquifer test was conducted in a sand and gravel, glacial-outwash deposit on Cape Cod, Massachusetts. A partially penetrating well was pumped at a constant rate of 320 gallons per minute for 72 hours. Changes in hydraulic head were observed at 20 locations through observation wells and piezometers distributed both radially and vertically within the saturated zone of the aquifer. The test was analyzed using Neuman's analytical model for flow to a partially penetrating well in a water-table aquifer. On the basis of geophysical well logs, cores, and other investigations conducted in the local area, the aquifer was initially assumed to have a saturated thickness of 80 feet for the purpose of analysis. However, drawdown observed in a piezometer located 108 feet below the water table, and an initial analysis, revealed a complete lack of conformity between theoretical and observed drawdown in piezometers located near the pumped well. Also, the computed horizontal hydraulic conductivity was twice that obtained by independent studies. Ongoing studies of the regional stratigraphy indicate that the saturated thickness of the aquifer is actually closer to 160 feet than to 80 feet. By increasing the assumed saturated thickness to 160 feet, the theoretical drawdown was found to match observed drawdown for most of the test period at nearly all points of observation. Results indicate that there is a remarkable degree of aquifer homogeneity with respect to flow at the scale of the test. The aquifer transmissivity and specific yield were determined to be 38 feet squared per minute and 0.23, respectively, and the ratio of vertical to horizontal hydraulic conductivity was estimated to be about 1:2.