HR: 0800h
AN: H41C-0663    [Abstracts]
TI: Isotopic Evidence of Nitrate Sources and its Relationship to Algae in the San Joaquin River, California
AU: * Silva, S R
EM: srsilva@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., M/S 434, Menlo Park, CA 94025, United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., M/S 434, Menlo Park, CA 94025, United States
AU: Young, M B
EM: mbyoung@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., M/S 434, Menlo Park, CA 94025, United States
AU: Stringfellow, W T
EM: wstringfellow@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States
AU: Borglin, S E
EM: seborglin@lbl.gov
AF: Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, United States
AU: Kratzer, C R
EM: ckratzer@usgs.gov
AF: U.S. Geological Survey, 6000 J street, Placer Hall, Sacramento, CA 95819, United States
AU: Dahlgren, R A
EM: radahlgren@ucdavis.edu
AF: U.C. Davis, 1 Sheilds ave., Davis, CA 95616, United States
AU: Schmidt, C
EM: cschmidt@ucsc.edu
AF: U.C. Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, United States
AU: Rollog, M E
EM: merollog@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., M/S 434, Menlo Park, CA 94025, United States
AB: Many competing demands have been placed on the San Joaquin River including deep water shipping, use as agricultural and drinking water, transport of agricultural and urban runoff, and recreation. These long-established demands limit the management options and increase the importance of understanding the river dynamics. The relationships among sources of water, nitrate, and algae in the San Joaquin River must be understood before management decisions can be made to optimize aquatic health. Isotopic analyses of water samples collected along the San Joaquin River in 2005-2007 have proven useful in assessing these relationships: sources of nitrate, the productivity of the San Joaquin River, and the relationship between nitrate and algae in the river. The San Joaquin River receives water locally from wetlands and agricultural return flow, and from three relatively large tributaries whose headwaters are in the Sierra Nevada. The lowest nitrate concentrations occur during periods of high flow when the proportion of water from the Sierra Nevada is relatively large, reflecting the effect of dilution from the big tributaries and indicating that a large fraction of the nitrate is of local origin. Nitrogen isotopes of nitrate in the San Joaquin River are relatively high (averaging about 12 per mil), suggesting a significant source from animal waste or sewage and/or the effects of denitrification. The d15N of nitrate varies inversely with concentration, indicating that these high isotopic values are also a local product. The d15N values of nitrate from most of the local tributaries is lower than that in the San Joaquin suggesting that nitrate from these tributaries does not account for a significant fraction of nitrate in the river. The source of the non-tributary nitrate must be either small unmeasured surface inputs or groundwater. To investigate whether groundwater might be a significant source of nitrate to the San Joaquin River, groundwater samples are being collected monthly from over 20 bank and in-stream wells. Preliminary data suggest that much of the groundwater nitrate has been variably denitrified thereby increasing its d15N values, but not by enough to account for the high d15N values in the river nitrate. The d15N of algae in the San Joaquin reflects the high values of the nitrate in the river indicating (1) that the San Joaquin is productive despite its relatively high opacity, (2) that the algae use the nitrate as a primary nutrient source, and (3) that the concentrations of algae in the San Joaquin are not principally dependent on algae from the tributaries being flushed into the river as has been suggested. The sources of nitrate to the San Joaquin River must be identified if algae production is to be controlled and hypoxic conditions in the downstream reaches eliminated.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0469 Nitrogen cycling
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1803 Anthropogenic effects (4802, 4902)
SC: Hydrology [H]
MN: 2007 Fall Meeting