California GAMA Special Study: An isotopic and dissolved gas investigation of nitrate source and transport to a public supply well in California's Central Valley

Abstract

This study investigates nitrate contamination of a deep municipal drinking water production well in Ripon, CA to demonstrate the utility of natural groundwater tracers in constraining the sources and transport of nitrate to deep aquifers in the Central Valley. The goal of the study was to investigate the origin (source) of elevated nitrate and the potential for the deep aquifer to attenuate anthropogenic nitrate. The site is ideal for such an investigation. The production well is screened from 165-325 feet below ground surface and a number of nearby shallow and deep monitoring wells were available for sampling. Furthermore, potential sources of nitrate contamination to the well had been identified, including a fertilizer supply plant located approximately 1000 feet to the east and local almond groves. A variety of natural isotopic and dissolved gas tracers including {sup 3}H-{sup 3}He groundwater age and the isotopic composition of nitrate are applied to identify nitrate sources and to characterize nitrate transport. An advanced method for sampling production wells is employed to help identify contaminant contributions from specific screen intervals. Nitrate transport: Groundwater nitrate at this field site is not being actively denitrified. Groundwater parameters indicate oxic conditions, the dissolved gas data shows no evidence for excess nitrogen as the result of denitrification, and nitrate-N and -O isotope compositions do not display patterns typical of denitrification. Contaminant nitrate source: The ambient nitrate concentration in shallow groundwater at the Ripon site ({approx}12 mg/L as nitrate) is typical of shallow groundwaters affected by recharge from agricultural and urban areas. Nitrate concentrations in Ripon City Well 12 (50-58 mg/L as nitrate) are significantly higher than these ambient concentrations, indicating an additional source of anthropogenic nitrate is affecting groundwater in the capture zone of this municipal drinking water well. This study provides two new pieces of evidence that the Ripon Farm Services Plant is the source of elevated nitrate in Ripon City Well 12. (1) Chemical mass balance calculations using nitrate concentration, nitrate isotopic composition, and initial tritium activity all indicate that that the source water for elevated nitrate to Ripon City Well 12 is a very small component of the water produced by City Well 12 and thus must have extremely high nitrate concentration. The high source water nitrate concentration ({approx}1500 mg/L as nitrate) required by these mass balance calculations precludes common sources of nitrate such as irrigated agriculture, dairy wastewater, and septic discharge. Shallow groundwater under the Ripon Farm Services RFS plant does contain extremely high concentrations of nitrate (>1700 mg/L as nitrate). (2) Nitrogen and oxygen isotope compositions of nitrate indicate that the additional anthropogenic nitrate source to Ripon City Well 12 is significantly enriched in {delta}{sup 18}O-NO{sub 3}, an isotopic signature consistent with synthetic nitrate fertilizer, and not with human or animal wastewater discharge (i.e. dairy operations, septic system discharge, or municipal wastewater discharge), or with organic fertilizer. Monitoring wells on and near the RFS plant also have high {delta}{sup 18}O-NO{sub 3}, and the plant has handled and stored synthetic nitrate fertilizer that will have this isotopic signature. The results described here highlight the complexity of attributing nitrate found in long screened, high capacity wells to specific sources. In this case, the presence of a very high concentration source near the well site combined with sampling using multiple isotopic tracer techniques and specialized depth-specific techniques allowed fingerprinting of the source in the mixed-age samples drawn from the production well.