Retrospective Groundwater Monitoring

There are two distinct types of retrospective groundwater (RGW) studies. Tapwater studies address potential human exposure to pesticides, assessing the dietary contribution of tapwater to the pesticide exposure risk cup. For tapwater studies, samples should rightfully be collected from a house tap used as a drinking/cooking water source. Conversely, studies of groundwater using monitoring wells assess contamination of the groundwater resource. While this assessment may have definite implications for existing potable wells and construction of new water wells in monitored areas, the link between the groundwater resource and drinking water supplies is not explicit.

In some cases, a monitoring well RGW study may be a precursor to a tapwater study. With some study designs, the monitoring wells (which are referred to as ‘sentinel wells’ in the protocol) are installed at the edge of treated fields; although no definitive conclusions can be reached regarding drinking water exposure, significant detections at the edge of the field in any well would trigger a subsequent tapwater study.

Stone has successfully implemented a large number of retrospective groundwater monitoring (RGW) studies in more than a dozen states. We work closely with agrochemical sales representatives, local pesticide applicators, chemical distributors, and crop consultants to implement studies that are technically rigorous while maintaining sensitivity to the local agricultural community.

For our largest study, we identified, characterized, and monitored more than 500 potable wells in five regions. The wells were hydraulically down-gradient of treated fields in areas with vulnerable groundwater conditions. We have also performed a similar tap water study, selecting 150 potable wells in five regions.

When an investigation demands the use of monitoring wells, we work with federal, state, and local governments to identify existing monitoring wells where possible. If monitoring well installation is warranted, we either contract with local drillers or install them ourselves. In one study, we installed 92 monitoring wells in six states. Stone worked closely with lead state agencies to ensure that the protocol and well locations were acceptable to the specific requirements of each state. We sampled the wells quarterly for three years using dedicated pumps, and finally decommissioned them per state standards.

Stone has the resources to conduct an RGW of any scope in any region of the United States or Canada.