Investigating the Source of Nitrate in a Water Supply Well in the Salinas Valley with Isotopic Tracers

Nitrate loading is a pervasive water quality problem in the Salinas Valley due to its rich agricultural history. Row crops, including strawberries and lettuce, are grown in the area immediately surrounding the drinking water supply well that is the focus of this study. The application of fertilizers to these crops is compounded because the crops are irrigated with nutrient-rich agricultural return fed groundwater. A small-scale wastewater treatment plant is another potential source of nitrogen in the immediate vicinity of the well. The nitrate impacted drinking water supply well in this small agricultural labor cooperative community had been identified as producing water unsafe for human consumption because nitrate concentrations were well above the regulatory limit for drinking water (Maximum Contaminant Level; MCL). The nitrate impacted drinking water supply well, located in the Salinas Valley Groundwater Basin - Eastside Subarea (SVGWB-ES), was completed at a depth of 137 m under semi-confined conditions of the alluvial Paso Robles Formation. One of the goals of the study was to unravel the complex dynamics associated with local and regional source loading, recharge, and discharge, to define the source of the nitrate contamination in order to recommend best management practices into the future. The study applied an interdisciplinary approach to investigate the source of nitrate. Stable isotopes of nitrate (δ15N, δ18O-NO3) and very high nitrate concentrations indicate that inorganic fertilizer is the dominant source. The groundwater age suggests that nitrate has a ʻlegacyʼ source that began infiltrating decades ago when intensive agriculture began. Nitrate concentrations fluctuate with seasonal pumping and are strongly, inversely correlated with precipitation levels. Stable isotope ratios of water (δ18O-H2O, δ2HH2O) show two different water masses; one with an evaporative signal likely related to inflow of excess irrigation water or possibly originating from the wastewater ponds, and a second that plots along the Global Meteoric Water Line (GMWL). High dissolved oxygen concentrations and a lack of dissolved excess nitrogen indicate that denitrification does not occur in the saturated zone. Data were gathered from various existing sources to assess nitrogen mass balance: agricultural statistics databases, climate models, climatological databases, grower reports, research studies, water quality reports, well data and farm extension publications. In the immediate area of the San Jerardo well, the nitrogen contents of residential wastewater, synthetic fertilizer, and irrigation return water are very high and could account for the high concentrations observed in the well. The isotopic methods used here allow identification of the most likely source – synthetic fertilizers.