Bias in Groundwater Samples Caused by Wellbore Flow
Publication: Journal of Hydraulic Engineering
Volume 115, Issue 2
Abstract
Design of physical installations and sampling procedures for ground-water monitoring networks, particularly for detection and analysis of possible contaminants, is a topic of great scientific and practical interest at the present time. Recent practice in the design of monitoring networks associated with known contaminant sources sometimes includes an array of monitoring wells with long well screens (up to 50 feet or more). Numerical experiments with a detailed three-dimensional ground-water flow model indicate that significant wellbore flow can occur in contaminant monitoring wells with long well screens that are embedded in homogeneous aquifers with very small vertical head differences in the aquifer. This ``short circuiting'' of flow through boreholes should exist at some level on all scales. Consideration of the general flow pattern within the borehole, the flow pattern in the aquifer adjacent to the borehole, and the process of obtaining water samples from the borehole suggests that in many situations the practice of installing long well screens in contaminant monitoring wells should be abandoned.
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References
1.
Bennett, G. D., and Patten, E. P. (1962). “Constant‐head pumping test of a multiaquifer well to determine characteristics of individual aquifers.” U.S. Geolog. Survey Water‐Supply Paper 1536‐G, 181–203.
2.
Giddings, T. (1987). “What is an adequate screen length for monitoring wells? Opinion 1.” Ground‐Water Monit. Rev., 7(2), 96–97.
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Copyright © 1989 ASCE.
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Published online: Feb 1, 1989
Published in print: Feb 1989
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