Reports

Reliable Drought Yields of Public Supply Wells in the Fractured Rock Areas of Central Maryland

2020, Hammond, P.A.

Special Publication 2020 [in review]

Key Results

In Maryland, there have been relatively few investigations on estimating reliable, long-term drought yields of fractured rock wells. In this study, “reliable drought yield” is defined as the amount of water that a well can supply to meet peak summertime demand during a severe drought without having to impose water restrictions. During two droughts that occurred between 1998 and 2002, many municipal water suppliers in the Piedmont/Blue Ridge (fractured rock) areas of central Maryland had to institute water restrictions related to declining well yields. Estimates of the yields of those wells, made prior to placing them into service, were commonly based on extrapolating drawdowns, measured during short-term pumping tests to shallowest, primary or major, water-bearing fractures and often did not consider drought conditions. Because the extrapolations were often made from apparent pseudo-equilibrium phases of drawdown curves, this frequently resulted in substantially over-estimated well yields. The methods used to estimate reliable drought yields in this study consist of (1) extrapolating drawdown data from infinite acting radial flow (IARF) periods, which includes pseudo-radial flow in fracture-controlled aquifers, or (2) by fitting type curves of other conceptual models to the data, using combinations of diagnostic plots, and inverse analysis and derivative analysis techniques. The positions of transition zones in crystalline rocks or thin-bedded consolidated sandstone/limestone layers (reservoir rocks) were used to determine available drawdowns in the wells. Aquifer dewatering effects were detected by type-curve matching of step-test data or by breaks (sharp changes in slopes) in the drawdown curves constructed from aquifer pumping tests. Operational water use and water-level data collected by water system personnel were then used to confirm the predicted yields. Those data were also compared to changes in regional groundwater levels to determine seasonal variations in well yields. Estimates of reliable drought yields are critical for effective design of production wells in fractured rock aquifers in central Maryland. Additionally, long-term monitoring should be conducted to verify those estimates and provide evidence of any decline in yield due to changes in aquifer properties or mechanical malfunctions of the well.

Downloads and Data

Special Publication 2020 [in review] (pdf)