Solutions for Progress 

Sacramento Regional Wastewater Treatment Plant Diffuser Modification Project: Assessment of Effects on Surface Water Quality and Beneficial Uses of the Sacramento River and Delta

RBI was the lead technical consultant for the Sacramento Regional County Sanitation District (SRCSD) to prepare an assessment of the near-field (zone of initial mixing) and far-field (fully mixed) water quality conditions that would result from implementing a proposed diffuser modification under both existing and 2020 conditions for the Sacramento Regional Wastewater Treatment Plant (SRWTP), and determine the effects that such water quality conditions would have on designated beneficial uses for the Sacramento River and Sacramento-San Joaquin Delta. The diffuser modification consisted of closing between 10 and 25 of the diffuser’s 99 ports to prevent diluted effluent discharged from the SRWTP from surfacing along the east bank of the Sacramento River downstream of the diffuser under low river flow conditions. Working cooperatively with Flow Science, Inc., RBI refined and interpreted the scaled output of the modeling and assessed the scaled modeling output regarding changes in water quality and temperature in both the far-field and near-field. RBI provided an assessment of potential water quality and temperature impacts under modeled conditions to fisheries and aquatic resources of the Sacramento River and Delta, with emphasis on anadromous salmonids. The results of the assessment were presented by RBI to NOAA Fisheries biologists who also gave their technical concurrence with the findings.