In the City of Beaumont and surrounding unincorporated areas, the only source of drinking water is groundwater supplied by Beaumont Cherry Valley Water District (BCVWD), from the Beaumont Management Zone. Rising nitrate levels have been observed in wells underlying the Cherry Valley Community of Interest (CVCOI), and recently nitrate-nitrogen (NO3-N) concentrations in groundwater production wells owned by BCVWD have approached the maximum contaminant level of 10 mg/L. Onsite Waste Disposal Systems (OWDS) currently used due to unavailable sewer service, have been identified as a possible source of nitrates to groundwater in the CVCOI because of the density of households with OSWDS.
WEI conducted field studies to determine the source of nitrate to groundwater. Several production wells were sampled and groundwater samples were analyzed for δ15N, pharmaceuticals, and personal care products (PPCPs). All groundwater samples were within the range of δ15N values of nitrate from septic tank effluent. Six wells had detectable levels of PPCPs, which included acetaminophen, ibuprofen, sulfamethoxazole, estradiol, progesterone, and testosterone. The simultaneous occurrence of high NO3-N concentrations, PPCPs, and the nitrogen isotopes that are associated with OSWDS discharge indicates that OSWDS are the source of nitrate contamination in the Beaumont Management Zone.
At build out, the future discharge from OSWDS is projected to be about 13 to 21 percent of the total recharge in the Beaumont Management Zone. This equates to about 185,000 to 500,000 pounds of nitrogen entering the groundwater from OWDS per year. This means that if the only source of nitrate is OSWDS, they will contribute enough nitrate to groundwater that in the fullness of time, the entire Beaumont Management Zone will be rendered non-potable.
- San Timoteo Watershed Management Authority