Evaluation of Nitrate Concentration in Tulare Aquifer, California and Finding its Sources using Stable Isotopes
The Tulare Shallow Aquifer consists of the Tulare, Kaweah, and Tule groundwater basins located in the southern part of San Joaquin Valley, California. In this study, based on The U.S. Geological Survey data in 2014-2015, the nitrate concentration and its sources have been identified. Accordingly, the data are divided into four categories: The data relating to the studied wells in the west part, central-north part, central-south part, and East part. The nitrogen and oxygen isotopes of nitrate in the groundwater show the source of nitrate in the Tulare aquifer is manure and septic waste, soil nitrogen, and fertilizer. The average value of nitrate concentration for the Tulare aquifer is about 9 mg/l, showing the sources could be anthropogenic rather than natural. The measured range of δ15N-NO3− in the east part is 1.66‰ and 14.32‰ with an average value of 6.59‰, a value range which is characteristic of nitrate sources from manure and septic waste, and fertilizers. The average concentration of nitrate in the central-north and south part is 13.71 and 10.28mg/l respectively, showing the central part of Tulare aquifer has the highest nitrate concentration. Moreover, the range of δ15N-NO3 indicates the source of nitrate in the groundwater could be manure and septic waste, soil nitrogen, and fertilizer. The west part has less nitrate concentration about 5.72 mg/l, and plotting δ15N -NO3 versus nitrate, natural log of nitrate, and 1/NO3 show denitrification have an important role in nitrate concentration in this part. So, less nitrate in this part might be due to denitrification and reduction of nitrate in the groundwater than the other two-part.
Keywords - Tulare Aquifer, Groundwater, Nitrate Concentration, Stable Isotopes