in situ degradation rates (e.g., hydrolysis, methylation, biodegradation), and develop methods for characterizing changes in degradation rate as a function of other physical parameters (e.g., depth in soil). Develop methods for scaling measurements that sample different volumes of porous material to provide equivalent measures. This information is of primary importance in determining contaminant fate and transport in the soil. Emphasis should be placed on developing methods that are relatively inexpensive.
Develop improved approaches to obtaining information on the residence time of water along flow paths and identifying recharge and discharge areas. It is important to protect recharge zones from contamination. Additional research into methods that provide the necessary information should be encouraged. For example, methods that use environmental isotopes may be useful and, therefore, should be developed further and evaluated in this context.
Develop unified ways to combine soils and geologic information in vulnerability assessments. A tendency exists to consider only soil or only geologic information in vulnerability assessments. Both are important and need to be integrated in assessing vulnerability.
Improve the chemical databases which are currently the source of much uncertainty in vulnerability assessments. It has been shown that for some measures of ground water vulnerability, the largest component of uncertainty involves the chemical aspects of transport. For example, the sorption process (expressed by "chemical" as Koc and "soil" as foc) has been found to produce large uncertainty in vulnerability assessments using the Attenuation or Retardation Factor approaches. The uncertainty in foc could be reduced by incorporating this parameter more systematically into current soil survey sampling.
Determine the circumstances in which the properties of the intermediate vadose zone are critical to vulnerability assessments and develop methods for characterizing the zone for assessments. Research is needed to identify environmental situations where the reference or compliance surface must be below the root zone and where the base of the root zone is adequate. At present, soil surveys contain large amounts of information that can be used in vulnerability assessments, but very few data exist on the hydrologic, geochemical, and microbial properties of the intermediate vadose zone (the unsaturated zone below the root zone). Criteria need to be developed that can help to establish when the properties of the intermediate vadose zone will have little effect on vulnerability. For situations where the intermediate vadose zone cannot be ignored, methods should