Relation of Groundwater Age to Herbicide Concentration Trends in Fresno and Tulare Counties, California

Science Center Objects

The California Department of Pesticide Regulation (DPR) has been monitoring pesticide concentrations in about 70 domestic drinking water wells located in vulnerable areas of Fresno and Tulare Counties for approximately 15 years. A recent analysis of the monitoring data indicated that there were decreases in simazine, diuron, and bromacil concentrations in many wells during 2000-2010.

Map of study area showing classified crops

Map of study area showing classified crops, circa 2000, assigned to Central Valley Hydrologic Model cells (Faunt et al., 2009), counties, largest cities, and extent of Central Valley.

(Public domain.)

To better understand the contribution of land-use regulations implemented during 1999-2002 to decreases in herbicide concentrations compared with other factors such as changes in hydrology, it is desirable to determine improved estimates of the mean age and age distribution of groundwater samples in the monitoring network. Use of multiple age-dating tracers and models for interpreting the age distributions of groundwater samples provide the opportunity to determine well-constrained estimates for groundwater samples in the Fresno and Tulare County networks. The study area has been designated as a Groundwater Protection Area by DPR based on historical detections of pesticide residues in groundwater and on the vulnerability of groundwater to land-use impacts due to coarse-textured soils, relatively shallow depth to water, and irrigated land use. The study area is a high priority setting to better understand the relation between groundwater-age distribution and trends in pesticide concentrations.

The objective of this study is (1) Estimate the recharge age of water sampled from a subset of domestic wells (~30) in the DPR monitoring network in Fresno and Tulare Counties. (2) Relate the groundwater ages to the observed changes in herbicide concentrations measured in the wells by DPR during the last decade.

Science Plan: About 30 wells will be selected for sampling in consultation with DPR. Environmental tracers, including CFCs (CFC-12, CFC-11, CFC-113), SF6, and 3H-3He (including 4He and neon), will be used to estimate mean recharge ages and evaluate mixing of waters of different ages in samples. Dissolved gas samples, including nitrogen (N2) and argon (Ar), and additional noble gases (krypton, xenon) will be used to estimate recharge temperatures and excess air, parameters necessary for determining groundwater age. Age-tracer data will be interpreted on a well-by-well basis and as an ensemble data set by comparing the different tracers and lumped parameter models (LPMs). USGS and DPR project personnel will compare age-dating results, herbicide concentration trend and use data, and ancillary hydrogeochemical data and prepare a journal article reporting relations of changes in concentrations of regulated pesticides to groundwater age and changes in land management practices.

The proposed study fits within the USGS science strategy direction of conducting a water census of the United States by quantifying and forecasting freshwater resources. The proposed study addresses Water Resources Mission Area and USGS priorities concerning water availability, by contributing to better understanding of geochemical constraints on water availability, and environmental health, by documenting the occurrence of pesticides, a group of contaminants that can have adverse