PUMPING TEST METHODOLOGY FOR ESTIMATING MAXIMUM SUSTAINABLE YIELD IN IGNEOUS AND METAMORPHIC ROCK AQUIFER/WELL SYSTEMS

Kath, Crawford, and Brackett (2004) suggested that a paradigm shift is needed in the evaluation of igneous and metamorphic rock aquifer systems for water-supply purposes. This change in mind-set is needed because the performance of these aquifer systems is typically described with pumping tests designed and evaluated using one of the variety of methods derived from the Theis Equation. Theis-based methods do not characterize the anisotropy of heterogeneous discontinuities in igneous and metamorphic rock aquifers. Except in rare instances, however, the drawdown/recovery characteristics in wells completed in these aquifers cannot be accurately estimated using the Theis methods; consequently, their use typically has little practical value. Based on over 30 years of data collection and interpretation, these systems are better described as aquifer/well systems, because wells constructed in close proximity one another will commonly have 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 funds, many groundwater users need to evaluate these parameters within a short pumping period, without the expense of installing observation wells and monitoring systems. Because of this, we propose a form of severely modified step-drawdown pumping test or constant-head drawdown test as the most cost-effective methods to determine the aquifer/well characteristics.

The modified step-drawdown test stabilizes the pumping water level at a predetermined level early in the test, and holds that pumping level for 72 hours. A constant-head test is performed by initiating and sustaining a pumping rate well above the anticipated capacity of the well until such time as a designated 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 pumping 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.