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Project: Dry Weather Water Quality in the San Gabriel River

Background and Objectives

Urbanization has had many consequences on the hydrology of southern California’s coastal watersheds. Because it is an arid region, southern California streams typically convey seasonal storm flows, but remain dry for large portions of the year. Today, larger engineered channels have replaced most major streams and rivers to accommodate sudden increases in storm water runoff. In addition, the historical intermittent hydroperiod has been replaced by year-round baseflow from supplemental sources. In the lower San Gabriel River watershed, large engineered channels predominate. These receive dry-weather flow from a variety of “introduced” sources, such as water reclamation plant (WRP) discharges, other National Pollutant Discharge Elimination System (NPDES) discharges, storm drain discharges, and imported water that is sent seasonally to spreading grounds located throughout the watershed. These dry-season water sources may contain a suite of pollutants generated in urban areas. In the past, high concentrations of nutrients, metals, and bacteria in the lower San Gabriel watershed have resulted in water quality impairment listings under Section 303(d) of the Clean Water Act.

Managing dry-season water quality requires an understanding of the water quality associated with primary sources of dry-weather flow, as well as the relationship between these sources and in-stream water quality. Although hundreds of storm drains discharge to the 303(d) listed portions of the San Gabriel River and its tributaries, the spatial and temporal patterns of these inputs have not been quantified. Furthermore, the relative mass contribution of pollutants from storm drains and WRP discharges has not been investigated, nor has the response of in-stream water quality to these mass loadings.

The goal of this study was to characterize storm drain and WRP inputs to the San Gabriel River system, and the associated in-stream response. The relative contribution of non-point source (i.e., storm drain) and point source (i.e., WRP) inputs were quantified for bacteria, metals, and nutrients. The observed concentrations were also compared to existing water quality standards, in order to provide managers with information needed to address water quality impairments.


This study was completed in 2004.


This study was comprised of two parts. The first part consisted of identifying and sampling the major inputs to the San Gabriel River and its major tributaries. The second part was comprised of sampling in-stream water quality to assess spatial water quality patterns, and the relationship between sources and in-stream water quality. Two synoptic surveys were conducted approximately one year apart, during which approximately 85 storm drains and 16 in-river sites were sampled twice over a 2-day period each year. At all locations, flow was also measured and water samples were collected for analysis of bacteria, nutrients and metals.


This study resulted in the following major findings:

1.  Nearly 80% of measured flow in the San Gabriel River watershed was from the WRPs during both surveys. Over 80% of the storm drains discharged at rates less than 28 liters/second (1 cubic foot per second), with approximately 5 storm drains accounting for the majority of storm drain discharge.

2.  The WRPs contributed approximately 90% (or more) of the nutrient mass loadings to the system.

3.  Almost all bacteria loading was contributed by storm drains.

4.  Metals loading varied by constituent. Stormdrains were the main sources of copper and lead, while the WRPs were the main sources of zinc.

5.  The spatial distribution of pollutants throughout the four reaches of the San Gabriel River watershed did reflect the influence of major mass emission sources.

• For nutrients, in-stream ammonia levels were markedly higher downstream of the WRP discharge locations. (Note that this study occurred prior to the WRPs instituting a nitrification/denitrification process into their treatment; this change should shift the dominant form of nitrogen from ammonia to nitrate.)

• Where storm drains were the only inputs; i.e., in upper Coyote Creek and Walnut Creek, nutrient concentrations were consistently low.

• Bacteria concentrations were generally high throughout all stream reaches, with no apparent spatial pattern.

• In some cases, metals concentrations appeared to reflect the locations of large inputs. For example, the in-stream copper concentrations were higher near the locations where large storm drains discharged.

6.  Water quality from the storm drains did frequently exceed water quality standards for bacteria, in 98% of samples. However, in-river metals concentrations never exceeded hardness-adjusted acute water quality standards under the California Toxics Rule (CTR). Storm drain and large NPS discharges were also generally below CTR standards. (Note that although the CTR generally applies only to receiving waters, such a comparison is instructive for identifying potentially problematic discharges.)

Map showing locations of highest zinc loading in the San Gabriel River watershed during dry weather


This study was conducted in partnership with the Los Angeles Regional Water Quality Control Board, the Los Angeles County Sanitation Districts, the Los Angeles County Department of Public Works, the City of Downey, the Los Angeles/San Gabriel Rivers Watershed Council, the Friends of the San Gabriel River, and U.S. EPA Region IX.


Download data from this project
For more information on Dry-Weather Water Quality in the San Gabriel River, contact Eric Stein at (714) 755-3233.
This page was last updated on: 3/1/2012