An Assessment of Optical Properties of Dissolved Organic Material as Quantitative Source Indicators in the Santa Ana River Basin, Southern California

By Brian A. Bergamaschi, Erica Kalve, Larry Guenther, Gregory O. Mendez, and Kenneth Belitz



Water–Scientific Investigations Report 2005-5152 (ONLINE ONLY)

Sacramento, California 2005

Prepared in cooperation with the
Orange County Water District

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    The ability to rapidly, reliably, and inexpensively characterize sources of dissolved organic material (DOM) in watersheds would allow water management agencies to more quickly identify problems in water sources, and to more efficiently allocate water resources by, for example, permitting real-time identification of high-quality water suitable for ground-water recharge, or poor-quality water in need of mitigation. This study examined the feasibility of using easily measurable intrinsic optical properties—absorbance and fluorescence spectra—as quantitative indicators of DOM sources and, thus, a predictor of water quality. The study focused on the Santa Ana River Basin, in southern California, USA, which comprises an area of dense urban development and an area of intense dairy production. Base flow in the Santa Ana Basin is primarily tertiary treated wastewater discharge. Available hydrologic data indicate that urban and agricultural runoff degrades water quality during storm events by introducing pathogens, nutrients, and other contaminants, including significant amounts of DOM. These conditions provide the basis for evaluating the use of DOM optical properties as a tracer of DOM from different sources.

Sample spectra representing four principal DOM sources were identified among all samples collected in 1999 on the basis of basin hydrology, and the distribution of spectral variability within all the sample data. A linear mixing model provided quantitative estimates of relative endmember contribution to sample spectra for monthly, storm, and diurnal samples. The spectral properties of the four sources (endmembers), Pristine Water, Wastewater, Urban Water, and Dairy Water, accounted for 94 percent of the variability in optical properties observed in the study, suggesting that all important DOM sources were represented. The scale and distribution of the residual spectra—that not explained by the endmembers— suggested that the endmember spectra selected did not adequately represent Urban Water base flow. However, model assignments of sources generally agreed well with those expected, based on sampling location and hydrology. The results suggest that with a fuller characterization of the endmember spectra, analysis of optical properties will provide rapid quantitative estimates of the relative contribution of DOM sources in the Santa Ana Basin.




Purpose and Scope



Study Area Description

Sampling and Analytical Methods

Dissolved Organic Material

Ultraviolet Absorbance


Data Analysis


Excitation/Emission Pairs

Specific Ultraviolet Absorbance (SUVA)

Specific Fluorescence Intensity


Results and Discussion

Model Endmember Selection

Model Response to Blank and Replicate Samples

Diurnal Variation

Large-Scale Trends

Results from Monthly Sampling

South Fork of the Santa Ana River


Warm Creek

Metropolitan Water District Crossing

Cucamonga Creek at Highway 60

Inland Empire Utilities Agency Reclamation Plant 1

Cucamonga Creek near Mira Loma


Santa Ana River at Imperial Highway

Results from Storm Sampling

Results from Comparison to Common Materials

Results from Data Reduction

Evaluation of Results


References Cited

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