SOLUTE AGE AS A TOOL FOR ASSESSING AQUIFER VULNERABILITY

Groundwater age is a measure of the amount of time that water particles have been in the aquifer. In the context of aquifer vulnerability, an older groundwater can lead to increased vulnerability. For example, if a well field is contaminated and the source of contamination is removed to prevent further contamination, the well field will remain contaminated until all of the contaminated water that recharged the aquifer prior to source removal has traveled past the well field. For non-sorbing contaminants, the distribution of travel times of the contamination from the source to the well field is identical to the distribution of groundwater ages of the water particles at the well field. Thus, the distribution of groundwater ages characterizes the amount of time after source removal that the well field will remain contaminated. Since sorbing contaminants travel through the aquifer at a slower rate than groundwater, the distribution of travel time of sorbing contaminants has larger values than the distribution of groundwater ages. Consequently, groundwater age is not an appropriate tool for assessing the vulnerability of aquifers to contamination by sorbing solutes. Instead, we use a new concept, called solute age, that represents the amount of time that sorbing contaminant particles have been in the aquifer. We present approaches for simulating mean solute age and the distribution of solute ages, which build off of approaches for simulating groundwater age. We illustrate the application of solute age in assessing aquifer vulnerability.