HR: 0800h
AN: H41D-0331    [Abstracts]
TI: Investigation of Nitrate Loading in Groundwater from Eastern San Joaquin Valley, California Using \delta$^{15}$N Isotopic Ratios
AU: * Marrero-Cuebas, R
EM: rosenelsy@csufresno.edu
AF: California State University, Fresno, Dept of Earth and Environmental Sciences, M/S MH24, Fresno, CA 93740 United States
AU: Suen, C J
EM: john_suen@csufresno.edu
AF: California State University, Fresno, Dept of Earth and Environmental Sciences, M/S MH24, Fresno, CA 93740 United States
AU: Suen, C J
EM: john_suen@csufresno.edu
AF: California Water Institute, 6014 North Cedar Avenue, Fresno, CA 93710 United States
AB: Agricultural activities in the Eastern San Joaquin Valley have affected groundwater quality in the area. The dependence of its population on groundwater for drinking has triggered several water quality studies in the past two decades. A total of 20 wells were sampled during the summer of 2003. These wells are installed to different depths at six clustered locations along the flow-path southeast of Fresno in a 4.6 Km transect agricultural area. The depth of these wells ranges from 70 to 270 feet. Vineyards and orchards are the predominant land use in the area but other crops are also cultivated in a minor scale. On a regional scale the groundwater flows towards southwest but on a local scale the water flows towards WSW because of local pumping. The United States Geological Survey has been monitoring the groundwater in this area as part of their National Water Quality Assessment Program. They have established a series of monitoring wells to detect the concentration of an agricultural fumigant DBCP banned from use since the 1970's and to evaluate trends in nitrate concentrations. Our main purpose in this study is to study the \delta$^{15}$N ratios along with \delta$^{18}$O ratios of dissolved nitrate to investigate the possible denitrification process occurring in the subsurface along the flow-path. The measured \delta$^{15}$N isotopic ratios of dissolved nitrate in groundwater samples from different depths and locations range from 4 to 9.5. The data show that there are strong negative correlations between depths and nitrate concentrations and also depths and TDS concentrations. On the contrary, we found no significant correlation between depths and \delta$^{15}$N or between nitrate concentrations and \delta$^{15}$N. These observations suggest the absence of significant denitrification process. However, the decrease in nitrate concentrations with increasing depths could be caused simply by the process of dilution by fresh ground water as the contaminated irrigation water travels downwards and along the flow-path. This interpretation is supported by Piper diagram plots. Distinct mixing trends are observed pointing towards the HCO$_{3}$+CO$_{3}$ corner as depth increases. This observation is consistent with the fact that the surface infiltration is mixed and diluted with the carbonate-type ground water originated from the Sierra Nevada mountains. Alternatively, these trends may indicate an increase in nitrate loading with time, as the age of water increases with depth. [This study was performed with the collaboration of Karen Burow of USGS, Sacramento.]
DE: 1806 Chemistry of fresh water
DE: 1831 Groundwater quality
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting