Evaluating Alternative Hydraulic Solutions to Limit Nutrient Contamination of an Aquifer in Southern California

Many small communities depend on groundwater sources for drinking water and they often use septic tanks for wastewater treatment and disposal. Nitrate and other pollutants leaking from poorly designed septic tank systems can percolate to the aquifers and alter quality of the groundwater. This study describes a groundwater model developed using Visual MODFLOW for an aquifer that is used as a water supply source for the communities of Beaumont and Cherry Valley, CA. The aquifer has been contaminated by nitrates leaking from septic tank systems. The model will assist in clarifying the extent of interactions between nitrate pollutants, percolation from a recently established series of artificial recharge ponds, natural groundwater recharge, and production wells. The primary objective of the study is to evaluate alternative hydraulic solutions that would limit the movement of contaminants and minimize the risk of polluting production wells. The study will identify artificial recharge scenarios that would limit movement of the nitrates so that polluted waters may be remediated in the future, rather than allowed to encroach on critical production wells or forced away from production wells to become a problem for future generations or neighboring areas. The data needed to build the model including geological logs, aquifer properties, hydrologic data, well locations, pumping schedules, water levels and septic tank density have been collected from various sources. The groundwater model is calibrated to accurately simulate observed groundwater levels and the extent of pollution corresponding to historical pumping rates, recharge rates and climate. The calibrated model is used to evaluate alternative hydraulic solutions that would localize the nitrate pollutions thus limiting impact on public welfare.