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TECHNICAL REPORTS

Ground Water Quality

Hydrologic and Biogeochemical Controls of River Subsurface Solutes under Agriculturally Enhanced Ground Water Flow

^a Environmental Science and Engineering, California Institute of Technology, Caltech MC 138-78, Pasadena, CA 91125
^b present address: Dep. of Oceanography, Florida State Univ., 117 North Woodward Ave., Tallahassee, FL 32306-4320
^c United States Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819
^d Environmental Science and Engineering, California Institute of Technology, Caltech MC 138-78, Pasadena, CA 91125. Present address: Eawag, Ueberlandstrasse 133, 8600 Duebendorf, Switzerland

^* Corresponding author (janet.hering{at}eawag.ch).

Received for publication October 17, 2008. The relative influences of hydrologic processes and biogeochemistry on the transport and retention of minor solutes were compared in the riverbed of the lower Merced River (California, USA). The subsurface of this reach receives ground water discharge and surface water infiltration due to an altered hydraulic setting resulting from agricultural irrigation. Filtered ground water samples were collected from 30 drive point locations in March, June, and October 2004. Hydrologic processes, described previously, were verified by observations of bromine concentrations; manganese was used to indicate redox conditions. The separate responses of the minor solutes strontium, barium, uranium, and phosphorus to these influences were examined. Correlation and principal component analyses indicate that hydrologic processes dominate the distribution of trace elements in the ground water. Redox conditions appear to be independent of hydrologic processes and account for most of the remaining data variability. With some variability, major processes are consistent in two sampling transects separated by 100 m.

Abbreviations: CV, coefficient of variation • DOC, dissolved organic carbon • ICP–MS, inductively coupled plasma–mass spectrometry • PC, principal component • PCA, principal component analysis • SC, specific conductance • SD, standard deviation