Evaluation of Existing Wells
Existing monitoring wells should meet the performance standards presented in 40 CFR
Part 264 Subpart F, as determined by the Regional Administrator. There are two situations in
particular where wells may fail to meet the performance standards: (1) where existing wells
are physically damaged; and (2) where the owner/operator can produce little or no
documentation of how existing wells were designed and installed.
Wells that are physically damaged, or wells for which there is not sufficient
documentation of design and construction, may need to be replaced. In addition, wells that
produce consistently turbid samples (
5 NTUs) and were not properly designed or constructed
also may warrant replacement. In such cases, knowledge of site hydrogeology and
professional judgment should be used in deciding when to replace wells.
When there is a question regarding whether or not the well casing material is
negatively affecting the chemical quality of the ground water samples, a side by side
comparison at selected wells should be undertaken using the well construction materials in
question. If analytical results are comparable, then it is likely that chemical bias is not a
major issue at the time of the test.
When existing wells do not meet the performance standards, the wells should be
properly decommissioned and, if required by the Regional Administrator, replaced. Pursuant
to  264.97(c)(1), the design, installation, development, and decommissioning of any
monitoring wells, piezometers and other measurement, sampling, and analytical devices must
be documented in the operating record.
Decommissioning Ground Water Monitoring Wells and Boreholes
Ground water contamination resulting from improperly decommissioned wells and
boreholes is a serious concern. Any borehole that will not be completed as a monitoring well
should be properly decommissioned. The USEPA (1975) and the American Water Works
Association (1985) provide the following reasons, summarized by Aller et al. (1989), as to
why improperly constructed or unused wells should be properly decommissioned:
To eliminate physical hazards;
To prevent ground water contamination;
To conserve aquifer yield and hydrostatic head; and
To prevent intermixing of subsurface water.
November 1992
6 53






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