ALWI News

New Application of an Old Technique to Estimate Sustainable Well Yield From an Anisotropic And Heterogeneous Bedrock Aquifer

Presented at a 1999 Sectional Meeting, National Ground Water Association

by Mark W. Eisner, P.G.,
Advanced Land and Water, Inc.

Major ground water supply development projects in consolidated and fractured bedrock can cause anisotropic aquifer responses that are difficult to predict. Conventional pumping tests and the associated, classic evaluation techniques are predicated on assumed homogeneous and isotropic conditions. When applied to an anisotropic and heterogeneous aquifer, these may over-estimate well yields, under-estimate adjacent user impacts, and incur considerably more time, cost, and regulatory scrutiny.

Background

During the early 1990s, two municipal production wells were developed in the Triassic bedrock of south central Pennsylvania. The sustainable yields of both were evaluated using 72-hour pumping tests that exceeded state regulations for 48-hour pumping tests of new public supply wells. One of these wells was brought on line and, in spite of the conservatism inherent in the length of the 72-hour test, it proved incapable of providing even half of its test-based sustainable yield estimate. Planned housing projects could not be supported and a local building permit moratorium was imposed that covered the service area of both wells. As an added precaution, additional testing and predictive analyses were required as a precondition of use of the other well.

Technical Approach

The difference between the predicted and observed time-drawdown response of the under-performing well proved invaluable in developing a means for calibrating new predictive models of well yield and aquifer behavior. Using these calibration data and after considerable hydrogeologic research, field testing, and predictive impact analyses of locally unprecedented complexity, additional supplies were developed, tested, interpreted empirically, and conditionally approved for use.

It was theorized that the radial flow conditions associated with modest pumping rates and drawdown gave way to linear flow conditions at higher flow rates and drawdowns. The linear flow conditions were believed associated with transient turbidity increases in neighboring domestic wells. This theory was tested empirically during a 120-hour aquifer pumping test.

Results

The new well was brought on line November 2, 1998. The state required close monitoring of areal drawdown as a trough-of-depression developed and stabilized in the highly anisotropic rock. By regulatory requirement, the production wells´┐Ż owner is responsible for adverse water quality impacts that may develop within the trough. To date, the trough has developed in accordance with pre-pumping predictions and without adverse off-site impact and the building moratorium has been lifted.

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