Presenter: Minna Ho (Southern California Coastal Water Research Project)

Description:
In the Southern California Bight (SCB), twenty-three publicly owned treatment works (POTWs), or wastewater treatment plants, discharge approximately 130,000 kg dissolved inorganic nitrogen (DIN) per day  through ocean outfalls into the ocean from a coastal population of 22 million. This effluent discharge accounts for about 90% of the anthropogenic nitrogen inputs into the SCB and, through numerical modeling, has been found to drive coastal eutrophication and exacerbate acidification and oxygen loss in this region. Water quality managers are considering whether nitrogen management and/or wastewater recycling represents cost-effective local solutions to mitigate climate change effects on California’s coastal ocean. Potable use recycling, which decreases wastewater volume but typically results in a partial reduction of nitrogen loading to outfalls, is expanding among POTWs to increase water security in this arid region. The question is whether additional nitrogen management is necessary, which would require plant upgrades in the billions of dollars. This talk will present results from a numerical modeling study to investigate the effect of varying levels of wastewater recycling and nutrient management on coastal eutrophication with potential implications on some keystone species, such as zooplankton calcifiers (pteropods). These scenarios are simulated using the physical Regional Ocean Modeling System (ROMS) coupled to the Biogeochemical Elemental Cycling (BEC), which is forced with land-based carbon and nutrients. The scenarios simulate a range of 50% and 90% potable recycling of POTW effluent and reduction of DIN loads via partial and full nitrification-denitrification. In this presentation, we will present the results from the analysis to illustrate how these potential management actions could influence changes in nitrogen, oxygen and carbon budgets and biologically relevant thresholds of pH and oxygen in the coastal ocean.