Reliable yields of public water-supply wells in the fractured-rock aquifers of central Maryland, USA

Patrick A Hammond

doi:10.1007/s10040-017-1639-4

Abstract

Most studies of fractured-rock aquifers are about analytical models used for evaluating aquifer tests or numerical methods for describing groundwater flow, but there have been few investigations on how to estimate the reliable long-term drought yields of individual hard-rock wells. During the drought period of 1998 to 2002, many municipal water suppliers in the Piedmont/Blue Ridge areas of central Maryland (USA) had to institute water restrictions due to declining well yields. Previous estimates of the yields of those wells were commonly based on extrapolating drawdowns, measured during short-term single-well hydraulic pumping tests, to the first primary water-bearing fracture in a well. The extrapolations were often made from pseudo-equilibrium phases, frequently resulting in substantially over-estimated well yields. The methods developed in the present study to predict yields consist of extrapolating drawdown data from infinite acting radial flow periods or by fitting type curves of other conceptual models to the data, using diagnostic plots, inverse analysis and derivative analysis. Available drawdowns were determined by the positions of transition zones in crystalline rocks or thin-bedded consolidated sandstone/limestone layers (reservoir rocks). Aquifer dewatering effects were detected by type-curve matching of step-test data or by breaks in the drawdown curves constructed from hydraulic tests. Operational data were then used to confirm the predicted yields and compared to regional groundwater levels to determine seasonal variations in well yields. Such well yield estimates are needed by hydrogeologists and water engineers for the engineering design of water systems, but should be verified by the collection of long-term monitoring data.

Highlights

The state of Maryland is located in the Mid-Atlantic region of the eastern United States, and has a wide range of geology and aquifer types
Most of the published work consists of programs designed to match test data to one or a few analytical models; while not endorsing the product, the commercially available AQTESOLV program (Duffield 2007), used in the present study, includes 35 different analytical models, many of which can be applied to fractured rock aquifers
These transition zones or permeable units act as high porosity reservoir rocks, but may not allow direct sufficient flow of groundwater to a well because the permeability of the units and the areas exposed to wellbores are small

Summary

Introduction

The state of Maryland is located in the Mid-Atlantic region of the eastern United States, and has a wide range of geology and aquifer types. Wells in the aquifers typically vary from high yielding ones (commonly more than 1,500 L/min) in confined and unconfined, unconsolidated sandstone layers on the eastern shore and southern Maryland to relatively low yielding ones (generally less than 300 L/min) in the fractured rock areas of the Piedmont, Blue Ridge, Valley and Ridge, and Appalachian Plateau provinces of central and western Maryland. Most of the metropolitan area is served by surface water from the Potomac River and the Baltimore City reservoir system. Some of the fastest growing suburban areas, are located in the Piedmont and Blue Ridge areas, and many are supplied by wells in fractured rock aquifers or small reservoirs. The major surface-water suppliers in the metropolitan area had little difficulty meeting customer demand, due to substantial reservoir storage facilities.

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Conclusion