Abstract: The Relative Contributions of Human Activities and Nature to Elevated Dissolved Arsenic in Ground Water in the Kern Water Bank, Southern San Joaquin Valley, CA

Robert J. Swartz, Geoffrey D. Thyne

Dissolved arsenic (As) in ground water has reached concentrations as high as 0.211 mg/l in the Kern Water Bank, a proposed ground water recharge project. Stiff diagrams, Piper plots, ^dgrD and ^dgr¹⁸O analyses were utilized to identify four ground water types: (1) a Sierra Nevada-sourced, Na-HCO₃ water (avg. TDS 128 mg/l); (2) a Coast Range-sourced, SO₄-rich water (avg. TDS 810 mg/l); (3) an eastern-sourced, anthropogenically altered, mixed ion water (avg. TDS 348 mg/l); (4) a western-sourced anthropogenically altered Na-Cl water (avg. TDS 1373 mg/l). Oil field and agricultural activities, such as brine disposal and pesticide applications, account for anthropogenically altered waters.

The eastern-altered waters have very low As concentrations (avg. <0.001 mg/l). The western-altered waters have an average As concentration of 0.019 mg/l. The western-altered waters show evidence of possible alteration by brine disposal, indicating human activities may be a potential source of arsenic. However, the western-altered waters are not well-connected hydraulically to the main aquifer, so human activities are relatively insignificant.

High concentrations of dissolved As (avg. 0.036 mg/l) are closely associated with the Na-HCO₃ water. Low As content (<0.005 mg/l) in Sierran recharge waters at the head of the flow path, combined with minimal increases in TDS along the ground water flow path, suggest dissolution of aquifer framework materials such as volcanic grains is the likely source of arsenic in the main aquifer.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California