INVESTIGATING THE SOURCE OF NITRATE IN A SALINAS VALLEY DRINKING WATER SUPPLY WELL 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 in this study. The application of fertilizers to these crops is compounded because the crops are irrigated with nutrient-rich agricultural return fed groundwater. The nitrate impacted drinking water supply well in this small agricultural labor co-operative community has been warranted unsafe for human consumption because nitrate concentrations were well above the MCL. A new water supply has recently replaced the former well. The former drinking water supply well, located in the Salinas Valley Groundwater Basin - Eastside Sub-basin, was completed at a depth of 450 ft. under semi-confined conditions of the alluvial Paso Robles Formation.

One of the goals of the study is to unravel the complex dynamics associated with local and regional source loading, recharge, and discharge, and make a definitive statement about the source of the nitrate contamination in order to recommend best management practices into the future. This study implements an interdisciplinary approach to investigate the source of nitrate. Stable isotope ratios of water (δ²H,δ¹⁸O-H₂O) and nitrate (δ¹⁵N, δ¹⁸O-NO₃) were analyzed as source indicators. Additionally the study utilized dissolved oxygen content, dissolved nitrogen and argon gas concentrations, selected anions (F^-, Cl^-, NO₂^-, Br^-, NO₃^-, SO₄^2-), and tritium-helium groundwater age-dating to examine the processes taking place in the subsurface. A high-resolution time-series dataset of pumping volumes and nitrate concentrations also provided invaluable insight into the changes in nitrate concentration that are observed seasonally.