Analysis Of The Distribution Of Pump Discharge In A Two Aquifer Groundwater System Via Mathematical Models With Application To A Study Area In Pennsylvania

Abstract

The determination of the aquifer parameters of transmissivity and storativity from pump test data of a two layer confined aquifer system is not possible due to the indeterminacy of the discharge from each aquifer to the pumping well bore. To resolve this theoretical problem and analyze pump test data from a groundwater system in the Triassic Brunswick Formation of northeastern Montgomery County, Pennsylvania, the Darcy Weisbach model of turbulent flow in pipes or conduits was coupled with the Theis model of confined aquifer flow to produce a mathematical model that simulates the distribution of discharge within a pumping well bore and the accompanying distribution of hydraulic head throughout the two aquifer layers. Two analytic models and one finite difference model were developed, tested and applied to the analysis of the study area pump test data. Analysis of the models showed that approximations of the transmissivities of the two aquifer layers could be determined from pump test data via a method similar to the Jacob straight line method. The models demonstrated that the distribution of discharge between the two aquifer layers is a variable function of time, that the change in the distribution of discharge over time is small in reference to the initial distribution and decreases in magnitude over time. Model simulation runs based on the transmissivity (T) calculation and a range of storativities (S) and boundary conditions yielded a good fit to pump test drawdown data for lower aquifer layer values of 4900 gal/ft/day (T) and 5.SE-6 (S) and upper aquifer layer values of 400 gal/ft/day (T) and lE-4 (S), plus a single barrier boundary in the lower aquifer located 2500 feet updip from the pumping well. The simulation results provide theoretical evidence in support of the interpretation of the study area groundwater system as a two layer system. The models demonstrated theoretically that two layer aquifer systems can produce distributions of drawdown data in observation wells that appear to evidence non-Theisian conditions, though in fact the individual layers are Theisian aquifers. As such, pump test data from Triassic Brunswick aquifers that exhibit non-Theisian distributions of drawdown may result from the distribution of pump discharge within the pumping well bore to vertically separated Theisian aquifer layers.