Nonpoint source flux impact studies for wellhead treatment costs

Historical and ongoing regional use of pesticides, fertilizers, herbicides, and other agricultural chemicals is a major non-point source of harmful contaminants affecting groundwater quality in rural areas. Public water systems in these rural areas, whose ratepayers typically cannot afford the expensive treatment facilities necessary to comply with safe drinking water standards, are increasingly turning to the courts in order to recover the costs of groundwater remediation from responsible parties.

Remediation cost awards in toxic torts are based on estimates of the length of time that municipal well contamination persists above regulatory limits. This requires consideration of application period, infiltration below the root zone by crop and irrigation efficiency, travel time in the vadose zone, solute-specific retardation and half-life, persistence and travel time in the saturated zone, as well as incorporation of uncertainty in all of these variables. Yet high-resolution 3D models are often impractical tools for these cases due to the large numbers of assumptions required to develop simulations for data-sparse areas. Further, wellhead contaminant concentrations are more strongly correlated with well screen depths and well-field operations than any other measurable hydrologic variable. Conveying this information for litigation purposes requires conservative calculations, minimal assumptions, and reliance on local hydrologic data developed for other purposes. We have found that the most useful data and methods for mass flux and persistence analyses come from extension and government reports as well as regional groundwater management studies.