Scripps Institution of Oceanography

California experiences multi-decadal climate variability in rainfall leading to alternating periods of dry and wet climate, each lasting 20-30 years. A dry period extended from about 1945-1977, followed by an episodically wet period from 1978-1998, that included the occurrence of six strong El Niño events. Because of the previous durations of these climate cycles, we have likely transitioned from a multi-decadal wet cycle that ended in 1998, and are now returning to a period of dry climate similar to what prevailed in California from 1945-1977. Such a transition in climate will put increasing pressures on already limited supplies of fresh water, making the development of alternative sources in California a necessity.

The Los Angeles Department of Water and Power (LADWP) plans to construct and operate a reverse osmosis (R.O.) desalination plant to be located on the site of the Scattergood Generating Station, 12700 Vista Del Mar, Los Angeles CA. Potentially, up to 50 million gallons per day (mgd) of product drinking water produced by this plant will be blended with other supplies to provide supplemental water to water utilities served by LADWP’s service area. The source of water for the desalination plant will be seawater drawn from the Santa Monica Bay, about 1,600 feet, ft, offshore. The source water will be pre-treated and filtered through reverse osmosis membranes to produce high quality drinking water. The plant’s product drinking water will be blended with other sources and distributed to consumers. The concentrated seawater produced by the reverse osmosis process (brine) will be mixed with the cooling water and then conveyed through one or more of three existing outfall structures: 1) the 17.5 ft diameter thermal outfall servicing Scattergood Generating Station, located 1,200 ft offshore, 2) the 12 ft diameter Hyperion emergency outfall located 5,384 ft offshore; and 3) the 12 ft diameter Hyperion deep outfall located 27,539 ft offshore. The net physical effect of desalination on the ocean receiving waters is in principle no different than the effects of evaporation; except that it would take 2,100 desalination plants of the size being proposed by LADWP at Scattergood to match the evaporative losses occurring naturally in the waters of the Southern California Bight. (The Southern California Bight is a water body bounded by the coastline between Point Conception and the United States-Mexican border, and extending offshore to the island arc formed by the Channel Islands, Catalina Island, San Clemente Island and the Coronado Islands).

The following study has utilized a hydrodynamic model to evaluate the brine dilution and dispersion for each of the three possible discharge options over the historical range of ocean receiving water conditions and host generating station operations. Product water production by the desalination plant was varied in the model between 12 and 50 mgd to evaluate the “carrying capacity” of each discharge option in the presence of long-term ocean variability and host plant operations. Carrying capacity was judged according to how the modeled brine dilution fields compared with the scientific consensus of the salinity tolerance limits of the marine biota indigenous to the Southern California Bight. (Generally, the salinity tolerance limit for indefinite exposure is believed to be 38 parts per thousand, ppt, as compared to an average salinity in the receiving water of 33.5ppt). The hydrodynamic model analysis employed for this purpose was the SEDXPORT modeling system that was developed at Scripps Institution of Oceanography for the US Navy’s Coastal Water Clarity System and Littoral Remote Sensing Simulator; that has been peer reviewed multiple times and has been calibrated and validated in the Southern California Bight for 4 previous desalination design projects.

Based on hydrodynamic model results derived from 20 years of ocean monitoring data and Scattergood and Hyperion operating data, four primary conclusions have been formed:

1) If the production rate of product water by the desalination plant is limited to 12- 25 mgd, then the Scattergood outfall located 1,200 ft offshore provides adequate brine dilution in the receiving waters under all circumstances;

2) If the production rate of product water by the desalination plant is increased to 50 mgd, then brine discharges from the Scattergood outfall still remain below marine biology tolerance limits 82 % of the time. During the remaining 18 % of the time when bottom salinity exceeds the marine biology tolerance threshold, an area of benthic habitat covering 51 acres is impacted by hyper-salinity, some of which is in the surf zone;

3) Brine discharges from the Hyperion 1-mile emergency outfall will exceed marine biology tolerances 98% of the time if product water is produced by the desalination plant at a rate of 12 mgd. If product water production is increased to 50 mgd, then marine biology tolerances are exceeded 100% of the time. The Hyperion 1-mile outfall is not a viable discharge option unless the brine is diluted with supplemental seawater prior to being discharged. A dilution ratio of 3.25 to 1 is required with supplemental sea water to eliminate potential benthic hyper-salinity impacts associated with brine discharge through Hyperion 1-mile outfall;

4) Brine discharges from the Hyperion 5-mile deep outfall cause no hyper- salinity impacts on marine biology and will reduce the footprint of the Hyperion waste field by as much as 42% depending on seasonal and decadal variability of ocean and meteorological conditions. The Hyperion 5-mile outfall offers the lowest risk alternative for marine benthic impacts while allowing the largest desalination production capacity at the Scattergood Generating Station;

5) Discharges from the Hyperion 5-mile deep outfall present no significant impacts on the source water quality derived from the intake flow of the Scattergood Generating Station. Dilution factors for the Hyperion waste field at the intake to the Scattergood Generating Station are greater than 108 to 1 for both the pre- and post-project conditions.