Simulation of Coupled Leaky Aquifers and Surface-Water System

Two types of models of groundwater flow in multiaqutfer systems are presented in this report. In the first part, a numerical modek for simulstion of groundwater flow in multiaqulfer systems is developed. A system of nonlinear partial differential equations, isomorphic to groundwater flow in the multiaquffer, are formulated in a finite difference form suitable for the line successive overrelaxation technique. The resulting system of difference equations is solved using an efficient algorithm. The algorithm serves as a means for solving the bitridiagonal system of difference equations representing flow ln two aquifers simultaneously during only one iteration cycle. Storativity of the semiconfining layer can be considered indirectly or can be neglected. The results derived from the numerical simulator for the flow to wells in two coupled aquffers are in good agreement with analytical results for the system.

In the second part of this report, analytical models of steady groundwater flow in a three-aquifer system are derived. The three-aquifer system consists of a valley area containing a shallow aquffer and a deep aquifer coupled through an aquitard, and a highland area containing an intake-area aquifer. Four cases of the system are analyzed: in two of the cases, the vertical variations of the hydraulic head in the shallow unconfined aquifer are assumed small; and for the other two cases, an approximate differential equation to describe the flow in the unconfined aquifer is presented. The analytical solutions were programmed for evaluation by a digital computer. The results show that a variation in the hydraulic characteristics of any one of the three aquifers affected the flow in all three aquifers and the leakage coefficient of the aquitard plays a key role in governing the flow in the system. The use of a weighted average depth of the flow profile facilitates the derivation of analytical solutions but its use yields results that are significantly different from the case where vertical variations can be neglected.

Project No. A-028-MMEX