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Published July 23, 2009 | public
Journal Article

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


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.

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© 2009 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. Received for publication October 17, 2008; published online 23 July 2009. The authors gratefully acknowledge Steven Phillips and Celia Zamora (USGS) for fi eld assistance. Laboratory analysis would have been impossible without Nathan Dalleska, Megan Ferguson, Kate Campbell, and Giehyeon Lee (Caltech). Tapio Schneider and Drew Keppel (Caltech) gave advice at key points during statistical analysis. The manuscript was improved with the help of Claire Farnsworth (Caltech), Peter Reichert (Eawag), Laura Sigg (Eawag), and two anonymous reviewers. This work was partially supported by the U.S. Geological Survey National Water Quality Assessment (NAWQA) Program Cycle II (ACT study); for more information on NAWQA and the Agricultural Chemicals Transport Study, see Capel et al. (2008) or http://in.water.usgs.gov/NAWQA_ACT. NSF SGER grant EAR- 0408329 funded sample analysis and part of Rich Wildman's graduate study. Use of trade names is for identification purposes only and does not imply endorsement by the USGS.

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