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

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 singlewell 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 longterm monitoring data.

Hydraulic characteristics of a fractured crystalline basement aquifer in Nzhelele area, Limpopo Province, South Africa

“…The latter study noted that caution should be placed when assigning sustainable yields to boreholes in aquifers characterised by fracture dewatering since it is dependent on the abstraction rate. Clogging of fractures due to calcite precipitation and fracture compaction were also associated with fracture dewatering in a study by Hammond (2018), and this resulted to reduction of yield by 40-50%. Khan et al (2020) indicated that dewatering influences total dissolved solids interactions through rock-water interaction especially in aquitard and aquifer rich fine sediments (clay, carbonate and hard rocks).…”

“…Leaky aquifer models provided best matches of estimated and recorded drawdown data than any other solutions in the basement aquifers in Ethiopia in a study by Daddi (2015). Leakage conditions was also identified by Hammond (2018) as one of the dominant flow regimes based on analysis of hydraulic tests data in the fractured-rock aquifers of central Maryland, United States of America. Holland (2011) also noted that crystalline basement rocks are usually semiconfined (fractured bedrock).…”

Litho-structural conditioning in the exploration of fractured aquifers: a case study in the Crystalline Basement Aquifer System of Brazil

“…More robust techniques, such as geophysical profiling, for example, thermal, sonic, optical profiling, etc. (Danielsen et al 2007;Fiume et al 2020), as well as more traditional techniques, such as field mapping (Gleeson and Novakowski 2009;Fernandes et al 2016) and well monitoring (Comte et al 2012;Hammond 2018) are important to improve the conceptual model of the aquifer, however this study aims to make an initial contribution to the region's crystalline aquifer, in a regional and remote way, using, therefore, investigative techniques that will serve as a basis in the future for further studies in greater detail such as outcrop analysis, geophysical profiling and hydraulic well monitoring. In this sense, we address in this study the location of the wells and their production values, and the remote sensing mapping of outcropping structures in the relief and magnetized structures, associating these mapped structures and the wells with the spatial distribution of the lithologies and the tectonic context past and current of Paraná's crystalline basement.…”

Analysis of the impact of hydraulic properties and climate change on estimations of borehole yields

“…Published approaches to quantifying individual borehole yields in fractured rocks during drought periods are scant. Individual case studies have been reported in the USA (Hammond, 2018), UK (Beeson et al, 1997), South Africa (van Tonder et al, 2001), Italy (Piscopo and Summa, 2007) and…”

“…Even in countries with well-developed water resources management practices, estimates of borehole yields during droughts have been shown to be substantially overestimated (Hammond, 2018). Methods for estimating groundwater yields are often not reported and in areas of conjunctive water distribution borehole yields are combined with surface water yields to derive a total yield for the water resource system (UK Water Industry Research Ltd, 2014).…”