Nitrate Mixing Zone and Dilution Credit Assessment for the Regional Wastewater Control Facility

The City of Stockton’s Regional Wastewater Control Facility (RWCF) had previously been granted a dilution credit and associated mixing zone for nitrate plus nitrite as a means of obtaining NPDES permit effluent limitations greater than the MCL (10 mg/L-N). In support of the City’s request that the RWCF be granted a nitrate plus nitrite dilution credit and extended boundaries for the mixing zone in their renewed NPDES permit, RBI developed and conducted a study to determine whether current RWCF discharge levels of nitrate plus nitrite would be consistent with state mixing zone policies. This study not only addressed impacts within the vicinity of the RWCF’s discharge, but determined the impact on downstream beneficial uses of the Sacramento-San Joaquin Delta (Delta). 

To accomplish this task for their client, RBI’s staff of engineers, water quality specialists and biologists developed a study and work plan to guide data collection, analyses, and reporting. They conducted a comprehensive literature review of potential impacts to the San Joaquin River, Delta and drinking water exports from nutrients levels and stoichiometry, phytoplankton quantity/composition, and Microcystis aeruginosa. Then, using the Delta Simulation Model II (DSM2), RBI determined the proportion of water at drinking water intake locations that is constituted by RWCF effluent. DSM2 modeling of RWCF effluent mixing and river velocities in the study reach was coupled with a statistical assessment to determine the impact of the RWCF discharge on water quality and algal communities. Field monitoring of existing and proposed mixing zone for submerged/emergent vegetation, benthic macroinvertebrate community, water quality parameters (including nutrients), and algae community composition and biomass was also performed by RBI staff. Then an assessment was made of the impact of the RWCF discharge on the benthic macroinvertebrate community and submerged/emergent vegetation within the study reach. RBI prepared a final report and presented a comprehensive mixing zone assessment to Regional Water Board staff.

RBI concluded that granting dilution credit and a mixing zone for nitrate plus nitrite to the City of Stockton’s Regional Wastewater Control Facility, with its current operational procedures, would be protective of beneficial uses and consistent with state mixing zone policies.