SEPTIC NITRATE CONTAMINATION OF MUNICIPAL WATER SUPPLY WELLS: TRENDS AND REMEDIAL EFFORTS

Fifteen years of groundwater quality monitoring in Spanish Springs Valley, Nevada, have shown increasing levels of nitrate in municipal wells. Three of the six municipal wells have nitrate concentrations just under the U.S. EPA maximum contaminant level (MCL) of 10 mg/L. More than 2,000 homes on septic sewage disposal systems are located within a four square-mile area – with almost half of these systems within 2,000 feet of one or more of the municipal wells. A 1999 USGS study concluded that increasing nitrate levels can be linked to local septic systems. An investigation to characterize the extent of nitrate contamination was completed in 2003. Concentrations of nitrate in groundwater collected from shallow monitor wells were above the MCL, with a maximum value greater than three times the MCL. The preliminary conceptual model of contaminant migration was confirmed by the study; with high concentrations of nitrate at the water table decreasing with depth. Although a discontinuous clay aquitard may be acting to slow vertical contaminant migration, the combination of numerous septic systems in proximity to shallow-screened, high-volume municipal wells, has lead to the present situation.

The current project is examining trends in nitrate and estimating the nitrate load being contributed to the aquifer by septic systems. In 2004, concentrations of nitrate in monitoring wells appear to be increasing, with a maximum value of nearly four times the MCL. To determine the nitrate load to the aquifer from individual septic tanks in Spanish Springs Valley, four study sites have been equipped with lysimeters, neutron probes, flux meters, and monitor wells to provide an estimate of contributions from a range of septic systems. This study will: (1) measure nitrate concentrations that pass through the soil zone around septic tanks, (2) determine if nitrate is lost in the soil zone by chemical and/or biological reactions in the soil, (3) generalize this nitrate contamination and apply this as a source term for nitrate from septic tanks in Spanish Springs Valley, and (4) use the derived source term as part of the nitrogen budget for the Spanish Springs ground-water system. The assessment of the nitrate source term is critical for modeling nitrate contaminant transport and protecting municipal wells.