indices. For example, the screened intervals for the wells in the studies are at a wide range of depths from the water table, and many of the wells, particularly the community water system wells in the National Pesticide Survey, are from deeper, confined aquifers. Note, however, that in the National Alachlor Well Water Survey, a simple measure of vulnerability (based on the most likely aquifer tapped) is associated with pesticide contamination despite a less than clear relationship between the well-specific DRASTIC score and pesticide occurrence.
In addition to evidence from ground water observations of chemicals introduced by humans, various types of geochemical data may be useful in evaluating a vulnerability assessment, particularly the intrinsic vulnerability of an aquifer system. The types of ions in solution and their concentrations result from chemical processes responding to the lithology and hydrologic flow pattern of a particular hydrologic system (Freeze and Cherry 1979). Thus, the ionic composition of water in different locations may be an important indicator of flow paths of water through the subsurface and, in some instances, of the sources of water. For example, in a study of part of the coastal plain in Maryland and Delaware, Hamilton and Denver (1990) found that areas affected by agricultural chemicals could be identified by a distinct chemical signature of major inorganic constituents. Furthermore, measurements of isotopic data may be useful in the evaluation process. For example, elevated levels of tritium (a hydrogen isotope associated with the atmospheric testing of nuclear weapons) indicate that at least part of the ground water withdrawn from a well originally recharged the system after the early 1950s and hence helps distinguish young water from older water. The use of environmental isotopes and selected other chemicals as indicators of young ground water is reviewed in Plummer et al. (1993).
Vulnerability approaches are calibrated and validated using measured concentrations of contaminants in samples of ground water. In addition to analytical errors, the accuracy of the water quality data is constrained by how ground water samples are taken (Nelson and Dowdy 1990). The methods for obtaining representative ground water samples are relatively controversial, and errors can occur when: (1) samples are inappropriately handled, preserved, or stored, (2) ground water chemistry is stratified with depth below the water table, (3) and different pumping and purging methods are used. In addition, errors related to inappropriate sample processing occur when air is accidentally introduced to the sample (changing the redox status, which affects solubility of dissolved metals) and when samples are not preserved for later analysis (bioactivity may affect nutrients, organic compounds,