Abstract

Dissolved arsenic (As) in groundwater has reached concentrations as high as 0.211 mg/L in the Kern Fan Element of the Kern Water Bank, a proposed groundwater recharge project. General constituent analyses and stable isotopes were utilized to identify four distinct groundwater types: 1) a Sierra Nevada–sourced, Na-HCO₃ water (eastside groundwater); 2) a Coast Range-sourced, SO₄-rich water (westside groundwater); 3) an eastern-sourced anthropogenically altered, mixed-ion water (anthro-east groundwater); and 4) a western-sourced anthropogenically altered Na-Cl water (anthro-west groundwater). Oil field and agricultural activities such as brine disposal and pesticide application account for anthropogenically altered waters.

The highest concentrations of dissolved As are closely associated with the Na-HCO₃ water. Low As concentrations (<0.005 mg/L) in Sierran recharge waters at the head of the flow path, combined with minimal increases in total dissolved solids along the groundwater flow path, suggest dissolution of aquifer framework materials are the likely source of As in the main aquifer. The western anthropogenically altered waters have slightly elevated As concentrations and show evidence of possible alteration by brine disposal, indicating human activities may be a potential source of As. However, the western-altered waters are not well connected hydraulically to the main aquifer; therefore human activities appear to be relatively insignificant as a source of dissolved As.