Numerical evaluation of analytical solution for a 5-layer aquifer -aquitard system with application to the Oxnard Basin in California

Abstract

In nature, a pumping aquifer is not an independent hydrological unit. The confining beds typically have some permeability rendering the aquifer leaky or slight leaky. Together they make up a multi-aquifer system. This work is an attempt to investigate transient fluid flow to a well in a 5-layer system based on former work of Neumann (1968). The latter author developed an analytical solution for drawdown in a five-layer system with three aquifers separated by two aquitards, and a well of infinitesimal radius completely penetrates one aquifer and discharges at a constant rate. Numerical evaluation has been limited to a two-aquifer system (Neuman, 1968). In this work, numerical Laplace transform inversion is utilized to evaluate the five-layer analytical solutions in terms of dimensionless variables. The numerical results are presented in graphical form and compared to those of the two-aquifer solution (Neuman, 1968). The five-layer solution is then used to reinterpret two long-term pumping tests conducted near Oxnard, California. The hydraulic properties of each aquifer and aquitard are estimated through optimization using the inverse code PEST (Doherty, 2001). The results confirm the accuracy of the ratio method of Neuman and Witherspoon (1972), which can calculate the permeability of aquitard if the specific storage of aquitard is known.