USGS Groundwater Information: Branch of Geophysics
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Carole D. Johnson, U.S. Geological Survey, Storrs, Conn., USA
F.P. Haeni, U.S. Geological Survey, Storrs, Conn., USA
John W. Lane, Jr., U.S. Geological Survey, Storrs, Conn., USA
Monitoring of hydraulic head is an important part of many hydrogeologic investigations because it is used to determine the rates and directions of ground-water flow. In unconfined aquifers, open-hole water levels can be used to establish meaningful conceptual models of ground-water flow. In heterogeneous fractured-rock aquifers, however, where the hydraulic head can be different for each fracture zone, open-hole water levels should not be used to determine the rates and directions of ground-water flow or to establish the conceptual model of ground-water flow.
In heterogeneous fractured rock aquifers, a discrete-zone monitoring (DZM) system is necessary to determine the spatial distribution and temporal variation of hydraulic head. The information provided by DZM systems can be used to determine vertical and horizontal hydraulic gradients, to identify connections between transmissive zones within the aquifer, and to establish the extent of connection between the fractured-rock and surficial aquifers.
Discrete-zone monitoring systems perform the equally important role of preventing cross contamination between fractures. When hydraulic heads associated with individual fracture zones vary within a single well, there is potential for vertical flow within the well. At contaminated sites, vertical flow can spread contamination through the open hole from zones of higher head to zones of lower head, which may have been previously uncontaminated (Williams and Conger, 1990). Results of analyses of water samples obtained from open holes where vertical flow has resulted in cross contamination can be ambiguous, misleading, or completely erroneous. In the presence of vertical flow, sampling at a low flow rate adjacent to a fracture does not insure that the water sample will come only from that fracture (Allen Shapiro, written commun.). A water sample drawn from an isolated fracture that was receiving water from a fracture with a higher head will be representative of the water in the fracture with the higher head. Installation of a DZM system soon after drilling minimizes cross contamination and ensures a water-quality sample from a discrete isolated zone.
Final copy as submitted to Fractured Rock 2001 for publication as: Johnson, C.D., Haeni, F.P., and Lane, J.W., Jr., 2001, Importance of discrete-zone monitoring systems in fractured-bedrock wells - a case study for the University of Connecticut landfill, Storrs, Connecticut: in Fractured Rock 2001 Conference, Proceedings, Toronto, Ontario, March 26-28, 2001, CD-ROM.
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