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

Hammond, PatrickA.

Reliable yields of public water-supply wells in the fractured-rock aquifers of central Maryland, USA | Des rendements fiables des puits publics d’approvisionnement en eau dans les aquifères de roche fracturée du centre du Maryland, Etats-Unis d’Amérique Rendimientos confiables de los pozos públicos de suministro de agua en los acuíferos de roca fracturada del centro de Maryland, EEUU 美国马里兰州中部断裂岩含水层中公共供水井可靠的出水量 Produções confiáveis de poços de água para abastecimento público em aquíferos de rocha fraturada do centro de Maryland, EUA

2018

Hammond, PatrickA.

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.

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