Criteria and methods for delineation of groundwater source protection areas


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July 2016


Only one of the five standard criteria for delineation of a groundwater-source protection area (GSPA), identification of flow boundaries, is valid in unconfined carbonate aquifers. The most powerful method for reliable delineation of flow boundaries of a GSPA in such aquifers- indeed, the essential and only method for reliably doing so at other than faults, lithologic contacts, and water bodies - is groundwater tracing, usually with fluorescent dyes, and preferably with hydrogeologic mapping and contouring of a sufficient amount of relevant potentiometric data. Hydrogeologic mapping alone and discharge balancing, the latter using base-flow discharge, normalized base-flow (discharge per unit area), and hydrogeologic mapping, can each be used to estimate a GSPA, but such mapping or balancing is not a replacement for the judicious use of tracer tests. The most dangerous, misapplied criterion for GSPA delineation in any unconfined carbonate aquifer is time of travel (TOT) that is not based on tracer tests. Its use, when based on calculations from aquifer tests or on computer modeling, each of which assumes porous-medium equivalency and Darcian flow, is self-delusory and gives a false sense of security. In consideration of what is now known about flow velocities from more than 2250 tracer tests from 25 countries, use of non-tractt-based 10T in unconfined carbonate aquifezs is not defensible. Hydrogeologic mapping, combined with tracer-test and potentiometric data, has been successfully used to delineate a GSPA for a group of springs in a groundwater basin in Mississippian limestones of west-central Kentucky that is a water supply for approximately 36,000 people. No computer model could have predicted the tracer-test results or the basin boundaries - and had them recognizable as valid - unless confirmatory tracer tests had been conducted. The results of tracer testing are moderately predictable, but they can only be detennined empirically.

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International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, Vol. 33 (2016-07-01).