The relative merits of monitoring and domestic wells for ground water quality investigations

The results of two studies of the effect of agricultural land use on shallow ground water quality indicate that monitoring wells may be a better choice than domestic wells for studies of pesticide occurrence or transport, or for use as early-warning indicators of potential drinking water contamination. Because domestic wells represent the used resource, and because domestic well water may be affected by historical rather than current pesticide and landuse practices, domestic wells would be the best choice for an investigation of drinking water quality. The key difference between the domestic and monitoring wells appears to be that the monitoring wells in this study were installed exclusively to sample the shallowest possible ground water. For these studies, 48 shallow domestic wells and 41 monitoring wells were located randomly within two land-use settings (row crops and orchards) in an irrigated agricultural region of eastern Washington and sampled for 145 pesticides (including nine pesticide degradates) and common water quality indicators. Constructing and sampling monitoring wells required approximately four times the resources (including manpower and materials) as locating and sampling domestic wells. Sample collection and quality assurance procedures and analytical techniques were identical except that a portable submersible pump was required for monitoring wells. In both land-use settings, no significant difference in nitrate concentration was found between well types; however, the average number of pesticides detected per well was significantly higher (p<0.05) in the monitoring wells. A greater variety of pesticides was detected in monitoring wells; many were detected only in monitoring wells. More than 60% of detections of pesticides that were found only in domestic wells were of compounds that are no longer in use. These differences in ground water quality found in this study relate to the depth of the well and are apparently related to the age of ground water in the two types of wells and the greater effects of sorption, degradation, dilution, and dispersion that accompany longer ground water residence times. The decision to invest resources in monitoring wells should be made in light of the study objective and should consider these differences in results from the two types of wells as well as the relative costs.