Nonequilibrium type curves modified for two-well systems

Abstract

The Theis nonequilibrium formula is now used generally by the Ground Water Branch in analyzing ground-water movement in the vicinity of a pumping well. However, development of the Theis formula postulates homogeneous formations of wide areal extent and few formations fulfill these assumptions. In order to broaden the scope of its application, the Theis formula can be modified for use in situations where the pumped formation is adjacent to another formation having a considerably different transmissibility. The modification is made by an application of the image well theory, which can be explained rather briefly. Consider a pumping well in formation A. If it is assumed th-t this formation is abutted by a relatively impermeable formation B, located at distance a from the pumping well, and the boundary between the formations approximates a straight line, water cannot flow fram the impermeable zone to the well. This condition can be simulate by the supposition that formation A is infinite in extent and that an image well is located at distance a from the contact between the t::o formations end on the opposite side from the pumping well. If the image well begins pumping at the same time and at the same rate as the pumping well, a theoretical ground-water divide is developed along the line of contact between the two formations. Thus, the situation 12 Theis, C. V., The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using 1round-