Pacific Ocean Aquafarms propose to construct and operate cage finfish (Seriola dorsalis) aquaculture facilities offshore of Long Beach and San Diego. An analysis of the impacts to water quality from the release of residual solids (i.e., residual feed and fecal matter) is required to inform environmental review and issuance of the EPA National Pollutant Discharge Elimination System (NPDES) permit. DHI Water & Environment Inc. was commissioned to carry out appropriately designed biogeochemical models to examine water quality impacts. The presented study applied an internationally recognized - offshore aquaculture tailored nutrient, phytoplankton, zooplankton and detritus (NPZD) modeling approach that relied on the integrated modeling capabilities of MIKE Powered by DHI, i.e. MIKE 3 FM (hydrodynamics), MIKE PT / MIKE MT (deposition of solids), and MIKE ECO Lab (Water Quality).
As apparent, hydrodynamic modeling forms the basis of the applied model complex as it provides the necessary underlying oceanic conditions that drive subsequent sediment dispersal and ECO Lab modeling. This is evident (see diagram) for the operational releases of residual solids via the solid orange line from the ‘Hydrodynamics’ box to the ‘MIKE MT Module box’, ending with, the ‘Solids Deposition and Dispersion’ results box. The full integration of all model components is apparent for the ECO Lab Model modeling of nutrients by the orange line from the ‘Hydrodynamics’ and ‘Solids Disposition / Dispersion’ result boxes to the ‘ECO Lab’ Template box. These model outputs, along with various baseline data, ultimately allow for the biological algorithms of ECO Lab to generate the required water quality results.
The modeling study, which consisted of an initial phase involving 1 year of hindcast modeling and a second phase involving 5 years of hindcast modeling, produced net difference output for both predetermined compliance water quality parameters (i.e., Ammonia, Dissolved Oxygen, pH, and TSS), and a number of additional general water quality indicators (i.e., Dissolved Inorganic Nitrogen, Chlorophyll-a, Particulate Organic Carbon, Total Ammoniacal Nitrogen, and Total Phosphorous.