HR: 0800h
AN: H41C-0652    [Abstracts]
TI: Tracing seasonal nitrate sources and loads in the San Joaquin River using nitrogen and oxygen stable isotopes
AU: * Young, M B
EM: mbyoung@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025, United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025, United States
AU: Silva, S
EM: srsilva@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 434, Menlo Park, CA 94025, United States
AU: Stringfellow, W T
EM: wstringfellow@lbl.gov
AF: Environmental Engineering Research Program, University of the Pacific, Sears Hall 3601 Pacific Avenue, Stockton, CA 95211, United States
AU: Dahlgren, R A
EM: radahlgren@ucdavis.edu
AF: Dept. of Land, Air and Water Resources, University of California, Davis, One Shields Avenue, Davis, CA 95616,
AB: The San Joaquin River (SJR) is a heavily impacted river draining a major agricultural basin in central California. This river receives nitrate inputs from multiple point and non-point sources including agriculture, livestock, waste water treatment plants, septic systems, urban run-off, and natural soil leaching. Nitrate inputs to the SJR may play a significant role in driving algal blooms and reducing overall water quality. The San Joaquin River discharges into the San Francisco Bay-Delta ecosystem, and reduced water quality and large algal blooms in the SJR may play a significant role in driving critically low oxygen levels in the Stockton Deep Water Shipping Channel. Correct identification of the major nitrate sources to the SJR is important for coordinating mitigation efforts throughout the SJR-Delta-San Francisco Bay region. Measurements of the nitrogen and oxygen isotopic composition of nitrate were made monthly to bimonthly from 2005 through 2007 within the Lower SJR, major tributaries, and various other water input sources in order to assess spatial and temporal variations in nitrate inputs and cycling in this heavily impacted watershed. The oxygen and hydrogen isotopic composition of water was also measured to better distinguish water sources and identify changes in water inputs. A very wide range of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ values were observed in the main stem SJR and tributaries. The δ¹⁵N values ranged from +2 to +17 ‰, and the δ¹⁸O values ranged from -1 to +18 ‰. Except for a major agricultural drain site (San Luis Drain), all the sites showed temporal changes in both δ¹⁵N-NO₃ and δ¹⁸O-NO₃ much greater than the differences seen between individual sites. In general, the δ¹⁵N values of nitrate in the larger tributary rivers (Merced, Tuolumne and Stanislaus) were much lower than those of the main stem SJR from April to May; however, after June the tributary values began to rise toward the values in the main stem river. Some of the highest δ¹⁵N-NO₃ values observed occurred in the Merced River during the latter half of the year. The general increase in δ¹⁵N with nitrate concentration, both downstream and during the low flow period, is consistent with increasing amounts of nitrate derived from waste in the downstream section of the SJR and increased agricultural inputs during the summer. Additionally, the influence of denitrification on the δ¹⁵N-NO₃ values in the SJR is still under investigation.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 1803 Anthropogenic effects (4802, 4902)
DE: 1834 Human impacts
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2007 Fall Meeting