fluctuations between oxidizing and reducing conditions. Casing materials should be chosen

with a knowledge of existing and anticipated ground water chemistry. Because subsurface

conditions cannot be predicted without some preliminary sampling and analysis, the choice of

appropriate well casing materials should be contingent upon preliminary water quality

analyses, which will be critical to the success of a ground water monitoring program.

Information collected during interim status (see  265.92(b)) can assist in assessing ground 

water quality. When anticipated water quality is unknown, it is prudent initially to use

conservative materials (i.e., the most chemically inert). The "Chemical Resistance Chart"

presented in the 1991 1992 catalog of the Cole Parmer Instrument Company of Chicago

(Appendix 3) may provide general information regarding the resistance of various well

materials to degradation, although this chart is presumably reporting the effects of reagent

grade chemicals on the various materials. General recommendations regarding the selection

of well casing materials to minimize chemical interactions are presented in Table 10.

Chemical Interference Characteristics

Monitoring well casing and screen materials should not chemically alter ground water

samples as a result of their sorbing, desorbing, or leaching analytes, especially with respect to

the analytes of concern. If a casing material sorbs selected constituents from the ground

water, those constituents either will not be present in any water quality sample or the

concentration of constituents will be reduced. Additionally, if ground water chemistry

changes over time, the chemical constituents that were previously sorbed onto the casing may

begin to desorb and/or leach into the ground water. In either situation, the water quality

samples are not representative.

Sorptive solute removal processes by interaction with casing materials or filter packs

may reduce actual constituent concentrations below quantitation limits or regulatory

thresholds, resulting in biased contaminant plume delineations, reduced sensitivity of

detection, or false negative assessments of ground water contamination (Palmer et al., 1987).

Proper well purging may minimize the impact of sorption or leaching effects; however,

purging efficiency is difficult to document. Effective purging may rarely be achieved if

bailers are used. The effectiveness of purging in minimizing sorption or leaching effects of

well materials will be dependent on the relative rates and magnitudes of these processes in the

borehole, filter pack, wells, and the actual time of sample exposure to the materials.

In the presence of chemically reactive aqueous solutions, certain chemical constituents

can be leached from casing materials. If this occurs, chemical constituents that are not

indicative of formation water quality may be detected in samples collected from the well.

This phenomenon might be considered an indication of possible contamination when the

constituents do not relate to ground water contamination per se, but rather to water sample

contamination contributed by the well casing material. The selection of a casing material

should therefore consider potential interactions between the casing material and the natural

and human induced geochemical environment.

November 1992

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