6.10

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.

6.11

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