Microbial and Dissolved Organic Carbon Characterization of Stormflow in the Santa Ana River at Imperial Highway, Southern California, 1999-2002

By John A. Izbicki, M. Isabel Pimentel, Menu Leddy, and Brian Bergamaschi



Scientific Investigations Report 2004-5116—ONLINE ONLY

Sacramento, California 2004

In cooperation with the
Orange County Water District

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     The Santa Ana River drains about 2,670 square miles of densely populated coastal southern California, near Los Angeles. Almost all the flow in the river, more than 200,000 acre-feet annually, is diverted to ponds where it infiltrates and recharges underlying aquifers pumped to supply water for more than 2 million people. Base flow in the river is almost entirely treated municipal wastewater discharged from upstream treatment plants and, in the past, stormflow was considered a source of high-quality water suitable for use as a source of ground-water recharge that would dilute poorer quality water recharged during base flow.

     Stormflow in the Santa Ana River at the Imperial Highway diversion contains total coliform bacteria concentrations as high as 3,400,000 colonies per 100 mL (milliliters). Fecal indicator bacteria concentrations, including fecal coliforms, Escherichia coli, and enterococci, were as high as 310,000, 84,000, and 102,000 colonies per 100 mL, respectively. Although concentrations were high owing to urban runoff during the first stormflow of the rainy season, the highest concentrations occurred during the recessional flows of the first stormflow of the rainy season after streamflow returned to pre-storm conditions. Molecular indicators of microbiological organisms in stormflow, including phospholipid fatty acid (PLFA) and genetic data, show that the diversity of the total microbial population decreases during stormflow while fecal indicator bacteria concentrations increase. This suggests that the source of the bacteria must be poorly diverse and dominated by only a few types of bacteria. Although direct runoff of fecal indicator bacteria from urban areas occurs, this process cannot explain the very high concentrations of fecal indicator bacteria in runoff from upstream parts of the basin characterized by urban, agricultural (including more than 300,000 head of dairy cattle), and other land uses. Although other explanations are possible, fecal indicator bacteria concentrations and molecular microbiological data indicate accumulation and extended survival of bacteria in streambed sediments, and subsequent mobilization of those sediments and associated bacteria during stormflow. Both PLFA and genetic data indicate that water from dairy-waste storage ponds was not present during sampled stormflows. This is consistent with the relatively dry conditions and the absence of large stormflows during the study.

     Dissolved organic carbon (DOC) concentrations in stormflow ranged from 3 to 15.3 mg/L. In general, concentrations increased during stormflow and were distributed across the stormflow hydrograph in a manner similar to that of fecal indicator bacteria. DOC concentrations typically remained high for several days after flow returned to pre-storm conditions. Ultraviolet absorbance, excitation emission spectroscopy, and sequential fractionation of DOC using XAD-8 and XAD-4 resins showed that the composition of DOC changed rapidly during stormflow. Hydrophobic and hydrophilic acids were the largest fraction of DOC composing between 27 and 45 percent and between 24 and 37 percent of the DOC, respectively.

     The fraction of DOC composed of hydrophobic acids decreased due to urban runoff and increased during the recession of the first stormflow of the rainy season; the hydrophilic-acid fraction generally decreased throughout the stormflow hydrograph; the transhydrophilic-acid fraction did not vary greatly during stormflow; and the hydrophobic-neutral fraction increased from low values in base flow to almost 30 percent of the DOC after more soluble and more mobile hydrophobic and hydrophilic acids were washed from urban areas. Comparison of ultraviolet absorbance data with data collected during previous studies shows that the optical properties and, presumably, the composition of the DOC were different in this study than DOC collected during wetter periods.

     Samples of shallow ground water collected during stormflow from beneath recharge facilities along the Santa Ana River show that most fecal indicator bacteria and DOC were removed as water infiltrated into the streambed.





Purpose and Scope

Previous Studies

Description of the Santa Ana River Basin



Sampled Stormflows

Analytical Methods

Total Coliforms and Fecal Indicator Bacteria

Historical Data

Total Coliforms and Fecal Indicator Bacteria Concentrations during Stormflow

Phospholipid Fatty Acids

Phospholipid Fatty Acid Concentrations and Compositions during Stormflow

Comparison with Phospholipid Fatty Acids from Other Sources

16S rRNA Gene

Amplicon Size and Diversity during Stormflow

Comparison with Amplicons from Other Sources

Dissolved Organc Carbon

Dissolved Organic Carbon Concentrations during Stormflow

Dissolved Organic Carbon Compositions during Stormflow

Ultraviolet Absorbance

Excitation/Emission Spectroscopy

Dissolved Organic Carbon Fractionation

Total Coliform Bacteria, Fecal Coliform Bacteria, and Dissolved Organic Carbon Concentrations in Shallow Ground Water during Stormflow


References Cited


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