Specific Capacities of Wells in Crystalline Rocks

Abstract

ABSTRACTIn the Rothschild area, Wisconsin, most water wells are completed in glacial deposits. When glacial deposits are unsaturated or thin, wells are completed in fractured crystalline rocks of Pre‐Cambrian age. Specific capacity data from 56 wells tapping these fractured rocks reveal: Yields and specific capacity are generally small, with a median specific capacity of log 0.17 gpm/ft and a mean value of log 0.10 gpm/ft. Wells which are close together may have substantially different specific capacities. The occurrence of saturated sand and gravel above the crystalline rocks has no obvious influence on yield or specific capacity. Specific capacities of wells seem to be inversely proportional to (a) thickness of rocks penetrated below the water table, (b) depth of the wells below the water table, and (c) depth of the wells below the land surface. Assuming that specific capacities of wells are related to the number of contributing fractures penetrated and that the number of fractures diminishes linearly with depth, the following relation is derived: image where Sc= specific capacity, Q = discharge, s = drawdown, B = a constant, b = fracture frequency at the land surface, z = depth, and E = maximum depth of fracturing. In the Rothschild area, E = 100 feet.In exploring for ground water in a fractured rock, the problem is to find the area of maximum b. If sound scientific methods are not available, then exploration will consist of drilling wells at random until a satisfactory well is completed. However, because wells achieve 60 to 80 percent of their yield in the first 20 feet below the water table, test wells should be pump tested when they have reached a depth of about 20 feet below the water table. Unsuccessful wells need not be drilled to completion.