Assessing the efficacy of BMPs to reduce metal loads in the Los Angeles River Basin at the watershed scale

Abstract

The Los Angeles River Basin is a large (825 mi2) and diverse watershed containing highly developed urban areas, as well as expansive chaparral landscapes. Municipalities within this watershed, and around the country, are required to meet water quality standards for pollutant loads in their receiving waterbodies. The current research quantifies the ability of BMPs to improve water quality in the Los Angeles River Basin as well as ancillary benefits (e.g. groundwater recharge) at the watershed scale. The EPA-developed System for Urban Stormwater Treatment and Analysis INtegration (SUSTAIN) model is used to simulate flow, load, and five BMPs. Two regional BMPs are modeled (infiltration trenches & dry ponds) and three distributed BMPs (vegetated swales, bioretention cells, porous pavement). Each BMP type provides a unique optimal benefit, infiltration trenches: infiltration rates, vegetated swales: water quality performance, dry ponds: lowest cost, porous pavement: minimal footprint (i.e. replaces existing infrastructure). The modeled BMPs are combined in various ways to produce six unique Compliance Options. Each Compliance Option equally satisfies water quality criteria, but consist of a unique composition of BMP types and quantity, therefore each Option offers a distinct blend of ancillary benefits and associated costs. Of the six compliance options crafted, none are optimal across all criteria, indicating stakeholders need to consider their particular near-term and long-term needs, and balance them with the various cost and ancillary benefits each Compliance Option can offer. The Compliance Option identified from the perspective of this research as meeting the region’s most pressing needs (reducing metal load, high stormwater infiltration & low cost of construction) contains a mix of vegetated swales and infiltration trenches (Option 3a). Option 3a significantly reduces peak flow (58%), infiltrates stormwater (172,000 AFY), and with the lowest construction cost ($3.8 billion) of all options considered. Although this research informs policymakers and stakeholders about the capacity of these six Compliance Options to provide a range of ancillary benefits, there remains tremendous opportunity to further develop the capability of BMPs and their understanding. More research is needed to spatially optimize the best locations for BMPs in a watershed, better quantify the infiltration of stormwater to recharge groundwater aquifers, as well as how to further improve the functionality of BMPs (e.g. optimizing geomedia or vegetation for particular pollutants or climates). Lastly there is a need for further data integration and monitoring to better inform the current state of water quality throughout watersheds and track its progress as BMPs are implemented.