Research Areas > Stormwater > Characterization of Dry-Weather Metals and Bacteria in Ballona Creek

Project: Dry Weather Water Quality in Ballona Creek

Background and Objectives

Increased urbanization has been shown to result in increased runoff and pollutant loading to receiving waters. Over the past 10 years, management of urban runoff has focused primarily on the evaluation and control of stormwater. However, recent studies have shown that dry-season runoff from streams and rivers can be a significant contributor to total annual pollutant loads, particularly in arid environments where dry season stream flow comes mainly from urban runoff and other urban effluents. Making a distinction between wet- and dry-season pollutant loading characteristics is important because management strategies differ for these two types of sources. For example, stormwater management typically focuses on retention or detention, whereas dry-season runoff control focuses on treatment, diversion, infiltration, and source control. Management strategies for dry-weather pollutant loading also rely on an understanding of the specific sources or locations that contribute the most to loading.

The Ballona Creek watershed in the greater Los Angeles, California, area provides an optimum environment for conducting a source identification study. The watershed is approximately 80% urbanized, but there are no permitted wastewater or consistent industrial discharges to Ballona Creek, excepting discharges associated with construction, cleanup, and dewatering activities. Routine monitoring of dry-weather flow has shown concentrations of cadmium, copper, nickel, zinc, and lead that exceed State water quality criteria at least occasionally. However, monitoring has not been done to investigate pollutant contributions from storm drains.

The primary goal of this study was to investigate spatial patterns of dry-season metal concentrations, mass emissions, and bacterial counts in Ballona Creek, and then to compare these patterns with the locations of storm drain inputs. The secondary goal of this project was to characterize temporal variability in water quality of sources and receiving water quality. The results of this investigation may be used by watershed managers to determine appropriate strategies to control dry season water quality, such as clean up or diversion of specific storm drains, identification of constituents of concern, and identification of key watershed sources of the constituents of concern.

Status

This study was completed in 2004.

Methods

The study evaluated spatial and temporal trends in metals and bacteria levels by sampling storm drains and in river sites throughout the Ballona Creek watershed on three separate occasions from 2002-2003. Approximately 90 storm drains and 12 in-river sites were identified for potential sampling. Of these, all 12 in-river sites and a subset of 35-40 storm drains were sampled during each sampling event. (The remaining drains lacked sufficient flow to sample or were inaccessible.) The sampled drains spanned the entire above-ground portion of Ballona Creek. At each sampling location, discharge was measured and water quality samples were collected. Samples were analyzed for traditional fecal indicator bacteria, as well as total and dissolved metals. These data were used to investigate spatial and temporal patterns in constituent concentrations and loads.

Findings

Ballona Creek experienced storm flows during approximately 15% of the year. During the remaining 85% of the year, urban runoff was the predominant source of in-river flow. However, flow volume from this dry season urban runoff represented only about 30% of the total annual volume. Dry season flow constituted an appreciable portion of the total annual metal load. In years with low rainfall, dry-season metal loads were found to comprise 10-25% of the total annual load based on empirical data. Similarly, the relative annual contribution of dry weather flows varied from <1% (during wet years) to >10% (during dry years) according to modeling results. The majority of the remaining metal loading to the creek, around 90%, came from nine storm drains.

Spatially, the two locations of elevated metal and bacteria loads in Ballona Creek corresponded to two of the largest four storm drains. These locations were (a) immediately upstream of the tidal portion of the creek, and (b) below the portion of the watershed where Ballona Creek opens from an underground tunnel to an exposed channel. Temporally, the magnitude of concentrations and loads varied by more than five-fold over the course of a year.

Concentrations of copper, lead, and zinc in Ballona Creek were generally below the freshwater and saltwater chronic toxicity standards established under the California Toxics Rule (CTR). (Although the CTR standards are typically not enforced for storm drain discharges, comparison of storm drain samples to standards can be instructive.) However, storm drain copper concentrations exceeded CTR standards in 22-26% of the drains, while lead concentrations exceeded standards in 10-35% of drains. No storm drain samples exceeded the CTR standards for zinc. Metals in dry season urban runoff may have had a greater influence on toxicity because, in contrast to storm water runoff, dry season metals occur predominantly in the more bioavailable dissolved phase.

Relatively high bacteria concentrations were observed throughout both Ballona Creek and the storm drains draining to the creek, where the majority of samples (72-95%) along the entire length of Ballona Creek exceeded the AB411 freshwater standards for bacteria. Of the 35-40 storm drains sampled, nine had consistently high concentrations of E. Coli and/or Enterococcus. Temporally, bacterial counts varied by up to five orders of magnitude on daily, seasonal, and inter-annual scales.

Partners

This project was designed and implemented in cooperation with the Los Angeles Regional Water Quality Control Board, U.S. Environmental Protection Agency Region IX, Los Angeles County Department of Public Works, City of Los Angeles Department of Sanitation, and Santa Monica Baykeeper.