Hydrogeochemical characterization in relation to nitrate concentrations in Central Valley (California, USA) domestic wells
Lawrence Livermore National Laboratory
Agriculture and groundwater are both of vital socio-economic importance to the California Central Valley (USA). High nitrate concentrations from agricultural practices threaten the quality of domestic well drinking water. Hydrogeochemical characteristics of the aquifer can reduce or enhance the risk that nitrate or naturally occurring contaminants like arsenic or uranium exceed the Maximum Contaminant Level (MCL) at the domestic wells.We analyzed a data set of nitrate concentrations and isotopes, 3H/3He groundwater ages, major ion chemistry (including arsenic and uranium) and hydrological setting, collected from 200 domestic wells in the San Joaquin Valley. Nitrate exceeds the MCL in 44% of the wells, uranium in 17% of the wells and arsenic in 11% of the wells. 13% of the wells produce pre-modern groundwater (recharged prior to 1950) with less than 1 pCi/L tritium and 6% of the wells produce anoxic water (Fe > 0.05 mg/L). Nitrate concentrations correlate strongest with area of manure lagoons (R=0.45), corrals (0.41) or dairy manure application areas (0.26) within a 1.5 mile radius (Groundwater Nitrogen Loading Model input) but nitrate concentrations are difficult to predict reliably based on land use data alone. Nitrate negatively correlates with younger groundwater ages (R=-0.42) and with water table depth (R=-0.31),These findings are supported by kmeans cluster analysis performed on nitrate, mean groundwater age, and average depth to groundwater near each well. Wells with the greatest mean age (52.3 years) and relatively deep mean depth to groundwater (37.8 m) have the lowest mean nitrate concentrations (2.3 mg/L). Wells with relatively young mean age (9.6 years) and relatively shallow mean depth to groundwater (16.7 m) have the highest mean nitrate measurements (40.8 mg/L). Kmeans cluster analysis performed with groundwater age, nitrate, arsenic, and uranium reveals four groundwater contaminant groupings. One group represents groundwater with a mean age 51.1 years with low concentrations of all three contaminants. A second group represents old groundwater (mean age 46.6 years) with the lowest mean nitrate concentrations (1.65 mg/L) but with the greatest amount of arsenic (mean of 44.7 µg/L). Two groups contained mean nitrate concentrations above the MCL. The group with the greatest mean nitrate concentration (29.1 mg/L) also has the greatest mean uranium concentration (95.2 µg/L). In conclusion, predictions of nitrate, uranium and arsenic concentrations improve when hydrogeochemical parameters are included and are suited to predict the risk of wells to future contamination. The hydrogeochemical characterization also revealed risk factors for nitrate co-contaminants like uranium and natural contaminants like arsenic.