PUMPING TEST METHODOLOGY FOR IGNEOUS AND METAMORPHIC ROCK AQUIFER/WELL SYSTEMS: TIME FOR A PARADIGM SHIFT?

A paradigm shift is needed in the evaluation of igneous and metamorphic rock aquifer systems for water-supply purposes. The performance of these aquifer systems is typically described with pumping tests evaluated using one of the variety of methods derived from the Theis equation. These methods may not characterize the anisotropy of heterogeneous discontinuities in igneous and metamorphic rock aquifers. As a result, the pumping tests themselves are designed to provide the input data required for the analytical method to be employed. Except in rare instances, however, the drawdown/recovery characteristics in wells completed in these aquifers cannot be accurately estimated using these methods; consequently, their use typically has little practical value. Indeed, these systems are better described as aquifer/well systems because wells constructed adjacent to each other will commonly have very different performance characteristics.

Water supply system design requires knowledge of the maximum sustainable yield and maximum drawdown for an aquifer/well system. Because of limited time and limited funds, many groundwater users need to evaluate these parameters within a short pumping period, without installation of observation wells and monitoring systems. Because of this, we propose a form of severely modified step-drawdown pumping tests or constant-head drawdown tests as the most cost effective methods to determine the aquifer/well parameters.

The modified step-drawdown stabilizes the pumping water level at a predetermined level early in the test, and holds that pumping level for 72 hours. A reverse-step test is performed by initiating and sustaining a pumping rate well above the anticipated capacity of the well until such time as a designed pumping water level is achieved. Once this pumping water level is achieved, changes in the pumping rates are used, as necessary, to maintain the predetermined pumping water level. In either method, after the flow rate has stabilized and been maintained for the term of the pumping test, the pump is turned off and the recovery is monitored until the water level has returned to its pre-pumping-test level. The recovery curve can then be used to estimate the maximum sustainable yield of the aquifer/well system being tested.