HR: 16:45h
AN: H54D-04    [Abstracts]
TI: Nitrate and Dissolved Organic Carbon Concentrations in Riparian-Zone Ground Water of the Lower San Joaquin River, California
AU: * Kratzer, C R
EM: ckratzer@usgs.gov
AF: US Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819, United States
AU: Bergamaschi, B A
EM: bbergama@usgs.gov
AF: US Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819, United States
AU: Dahlgren, R A
EM: radahlgren@ucdavis.edu
AF: UC Davis, LAWR Dept, 3134 PES Bldg, Davis, CA 95616, United States
AU: Dileanis, P D
EM: dileanis@usgs.gov
AF: US Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819, United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: US Geological Survey, McKelvey Bldg, Menlo Park, CA 94025, United States
AU: Phillips, S P
EM: sphillip@usgs.gov
AF: US Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819, United States
AU: Russell, A D
EM: russell@geology.ucdavis.edu
AF: UC Davis, Geology Dept, One Shields Ave, Davis, CA 95616, United States
AU: Schmidt, C
EM: cschmidt@es.ucsc.edu
AF: UC Santa Cruz, Earth and Planetary Sciences Dept, Earth and Marine Sci., Santa Cruz, CA 95064, United States
AU: Young, M
EM: mbyoung@usgs.gov
AF: US Geological Survey, McKelvey Bldg, Menlo Park, CA 94025, United States
AB: Previous studies have estimated ground water inputs to the SJR on the order of 0.1 cms per river km, which could contribute up to 15 percent of downstream flow during the summer. However, there is a paucity of information concerning the chemical composition of ground water accretions in the lower SJR. The objective of this study was to quantify the amount of ground water accretions to the lower SJR and its nitrate and DOC contributions to the river. The study area is a 106 km reach of the SJR from the confluence with Salt Slough to Vernalis. Sampling of nested monitoring wells (3-30 m) on both banks at three sites and at nested (1-5 m) in-stream wells at six sites was initiated to obtain temporal data (monthly) for the modeling of ground water inputs and their associated nitrate and DOC loads. A synoptic study making measurements at 30 sites using temporary drive-points (0.3 and 0.9 m depths) was conducted by boat to provide enhanced spatial coverage. The sampling at each site included measurements of hydraulic and temperature gradients, general water quality characterization and nitrate and DOC concentrations. Based on the first year of monitoring the permanent wells (since September 2006), the specific conductance ranged from 0.3 to 8.3 dS per m (median = 2.6 dS per m) and pH values were near neutral (range = 6.4-7.6; median = 7.2). Nitrate was only present in detectable concentrations (greater than 0.01 mg N per L) in 8 of the 26 wells. Nitrate concentrations ranged from 0.8 to 13.1 mg N per L (median = 2.0 mg N per L) for those wells containing nitrate. Ground water collected from the wells with no detectable nitrate was anoxic with the majority of these wells displaying the presence of sulfide. This suggests that denitrification is a prevalent process leading to the loss of nitrate from many of the ground water sources. Some of the anoxic wells displayed high ammonium concentrations (greater than 2 mg N per L) that could be oxidized to nitrate upon entering the river. DOC concentrations ranged from 0.1 to 3.6 mg per L with a median value of 1.5 mg per L. Preliminary data suggest that many of the in-stream wells (0.3 and 0.9 m depth) are also anoxic. This suggests that nitrate concentrations from in-stream wells are also attenuated by denitrification within the riparian zone and under the river bed. Future aspects of the study include comparison of geochemical, isotopic, and optical characteristics of the ground water with various end-members in an attempt to identify the sources of nitrate and DOC in ground water accretions. Hydraulic gradient and temperature data will be used to model rates of ground water flow into the river. These flux rates will be combined with water quality data to estimate the amount of nitrate and organic carbon contributed to the river from ground water.
DE: 1830 Groundwater/surface water interaction
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1860 Streamflow
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Fall Meeting