An essential component of a detection ground-water monitoring plan is the establishment of a program for the statistical evaluation of ground-water chemistry data. The objective of such a data evaluation program is to assess whether there is statistically significant evidence of ground-water contamination, based on the indicator parameters for the site being monitored. The occurrence of false conclusions of evidence of contamination (Type I errors) or failures to detect contamination when it actually is present (Type II errors) is minimized by the use of appropriate indicator parameters.

To optimize the effectiveness of a statistical evaluation program, a limited number of indicator parameters should be selected on a site-specific basis, consistent with the following factors: (1) Concentrations of the parameter in contaminant source and background ground-water samples; (2) Hydrogeologic conditions of the site being monitored, particularly as they may affect spatial and temporal variability of parameter concentrations in ground water; (3) Chemical fate and mobility characteristics of the constituents used asindicator parameters; (4) Precision and limitations of the laboratory analytical procedure specified for the parameter; and (5) Characteristics of the existing dataset (if any) for the parameter. This paper explain the importance of the above factors and provides examples from a number of case studies to illustrate their use.