Los Angeles River study looks at wastewater reuse impact on receiving waters

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In 2020, Los Angeles County released an updated 2020 LA River Master Plan detailing plans for the river over the next 25 years. The current plan addresses goals related to flood risk, parks, ecosystems, access, arts and culture, housing, engagement and education, water supply, and water quality. Photo credit: trekandphoto, stock.adobe.com

The Los Angeles River consists mainly of treated wastewater effluent during periods of drought, so local officials are looking to expand their options for reusing that water, and are learning from a new study’s simulations that show how varying degrees of wastewater reuse could affect quantity and quality.

The water reuse simulations are part of a recent study by the Colorado School of Mines and the Southern California Coastal Water Research Project. They used nine separate models to quantify the effects that reuse could have on the loadings and concentrations of certain pollutants in the LA River, which receives discharge from three local wastewater treatment plants.

The models are based on the Storm Water Management Model developed by the U.S. Environmental Protection Agency. Researchers analyzed the resulting loads and concentrations of heavy metals, copper, zinc and lead, as well as the total suspended solids (TSS) and total dissolved solids (TDS) in the river, as those heavy metals in the river have already been extensively monitored, and those older findings could calibrate their model.

To simulate varying degrees of wastewater reuse, the researchers modelled water quality in the river under the conditions of 0%, 50% and 100% reuse. In other words, zero water reuse up to full water reuse. Non-storm storm drain flows were modelled under the same three conditions.

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As flow volumes decreased under all scenarios, so did the loads of all pollutants; however, the researchers found a change in pollutant concentrations. The concentration of certain pollutants increased in excess of water quality objectives, despite a reduction in flow to the river, the study found. This is due to dilution capacity and the sources of the pollutants, said researchers.

In simulations where non-storm storm drain flows were held constant under baseline conditions, increased reuse led to higher median concentrations of nearly all pollutants monitored. For example, the study revealed that increasing reuse by 50% resulted in a nearly 33% rise in TSS and a nearly 26% increase in lead concentrations. Furthermore, when all of the wastewater was modelled for reuse, TSS jumped to 101% and lead increased by more than 80%.

Conversely, when reuse was held constant under baseline conditions, reductions in non-storm storm drain flows led to decreases in the median concentrations of the pollutants. For example, TSS and lead decreased by about 33% and 27%, respectively, when dry weather runoff was reduced by half.

In 2020, Los Angeles County released an updated 2020 LA River Master Plan detailing plans for the river over the next 25 years. The current plan addresses goals related to flood risk, parks, ecosystems, access, arts and culture, housing, engagement and education, water supply, and water quality.

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