Developing a Bacteria TMDL in Engineered Channels: The Los Angeles River, A Case Study
Keywords: Engineered Channels, Bacteria, TMDL, Los Angeles River
Problem Statement and Objectives: Many rivers in the US are channelized to control stormwater runoff and reduce the impacts of major flood events in the region. The characteristics of channelized rivers may vary from a completely concrete-box or trapezoidal channel to those with “soft-bottom” streambeds. These complex yet structurally simplified channels suffer from the problems of altered habitats and degraded water quality resulting from point and non-point discharges, homeless populations, and other factors. Despite limited public access, these channels are subject to water quality regulations. The Los Angeles River (LAR) that flows through Los Angeles County, California, is one of such engineered channels with a number of water quality impairments. The LAR and its tributaries are on the US EPA 303(d) list as impaired for recreational beneficial uses due to fecal coliform bacteria (Figure 1). Several publicly-owned treatment plants (POTWs) discharge to the LAR; what is notable is at these locations, the LAR attains bacteria standards. The focus of this paper is on the development of the LAR Bacteria TMDL in the context of stakeholder participation and planning to revitalize the river through increased habitat, recreation and development, against the reality of a river that is officially off-limits to public access.
Methodology and Findings: A conceptual model was developed to summarize some of the most likely sources of bacteria: tributaries, storm drains, re-growth, and bottom sediments. The conceptual model and data analysis for bacteria were conducted on the reaches of the river (see Figure 1 for reaches). Reaches 2 and 4 of the river were selected for further investigation and monitoring because these two reaches had shown an increase in bacteria concentrations. Those results and discussion among stakeholders pointed to further studies to result in a more
scientifically-sound TMDL. Building off these data, stakeholders wanted clarification of what kind of bacteria is in the river - whether from humans, wildlife, or possible re-growth, and where people are currently using the river.
Accordingly, two studies have been designed: 1) bacteria source identification study; 2) a water body survey of the LAR mainstem to determine where human use is occurring (even if officially prohibited) based upon field surveys and channel structure, and with implementation of the revitalization plan, where access is likely to occur in the future. With the results of these studies, it is anticipated that by overlaying the locations of the highest bacteria counts; differences in channel structure; and current and anticipated human use; priorities and implementation strategies will emerge that result in a TMDL that focuses first on those areas with the greatest potential to negatively impact human health and will have a high degree of consensus from stakeholders. The bacteria source identification study has been designed to utilize source tracking methods to quantify the relative impacts of bacteria sources on the LAR and its tributaries, by discriminating amongst different source types to evaluate if sources are human or non-human. The water body survey methodology will include field work, and interviews of key agency personnel to document homeless populations; an analysis of the channel morphology to determine where individuals can physically reach the water; a survey of access points (there are maintenance and park sites along the river); and finally the results of the master planning that is targeting locations for improvement.
Significance of the study: Unlike other TMDLs that conduct Use Attainability Analysis to determine whether beneficial use designations are actually occurring and should be maintained, these studies and this approach looks to predict increased opportunities for Recreation-1 use in the future as well as current use. A more complete scientific understanding of where high bacteria counts occur in a channelized river system impacted by diverse inputs and physical changes will provide more targeted water quality improvement opportunities. The results of these studies will
inform TMDL development, guide BMP implementation efforts and improve water quality in a highly challenged environment.