UC Santa Cruz

Terrestrial water resources are scarce in arid and semi-arid regions of the world and increasing demands for water worldwide are adding additional pressures on limited water resources. Seawater desalination provides a reliable source of potable water. The process of desalination creates a high-salinity byproduct that is discharged back into the coastal environment by various methods (pipes, diffusers, channels). The brine effluent is often mixed with chemicals used at the desalination facilities and can, without dilution; reach salinities twice of the ambient feed-water. Despite a growing desalination capacity worldwide, the effects of that brine discharge on local coastal environments are not fully understood or constrained.

This thesis investigates the impacts of desalination brine discharge on coastal biology and chemistry, and is comprised of three chapters: Chapter 1 investigates the in-situ impacts of desalination discharge measured before and after the Carlsbad Desalination Plant (California) became operational on both water chemical parameters and benthic macro fauna; Chapter 2 investigates the impacts of desalination brine on corals from The Gulf of Aqaba in a 4-week incubation experiment with salinities 10% above ambient and addition of phosphonate-based antiscalants; and Chapter 3 reviews current knowledge of the impacts of brine discharge on a wide range of coastal organisms.

The Carlsbad Desalination Plant became operational in December 2015, and water, sediment and biological samples were collected prior to operations (December 2014 and September 2015) and after operations began (May and November 2016). A distinct brine plume of salinity 34-37 (ambient salinity 33.3) extending 600m offshore from the discharge point was observed after the plant started discharging. This is despite the coastal area being “high-energy” and previous computer models predicting mixing of the brine to be better. The salinity plume does not have an immediate effect on the benthic environment: Macro fauna abundance (tube-forming Polychaetes) significantly increases compared to post-operation surveys, however the in-faunal organisms show no change. The benthic area around the outfall has same level of disturbance pre and post operations, possibly because of the continuous discharge from the local power plant. A 5-week bioassay of brittle stars in brine treatments indicated a trend towards decreased growth and impaired agility in increased salinities but no significant change was observed. Finally, using mean wave height along the California coast as a proxy for coastal mixing, locations of proposed desalination facilities are compared to the location of Carlsbad Desalination Plant to evaluate the expected mixing properties of those areas.

In the brine incubation experiment of hard corals (Stylophora pistillata, Acropora tenuis, and Pocillopora verrucosa) there were an overall decrease in coral performance and changes in the corals’ physiology. Symbiodinium abundance, protein content and heterotrophic abundances were significantly reduced at the end of the incubation in all three species. In particular, the addition of phosphonate-based antiscalant to a 10% salinity increase reduced the amount of corals tissue symbionts, calcification rates and oxygen production rates. These results suggest that coral reefs may be susceptible to exposure to SWRO discharge and calls for careful selection of locations of SWRO facilities around coral reefs.

Various marine species have various response and resilience to increased salinity and brine discharge. To increase SWRO facilities sustainability and minimize their footprint in the coastal environment, sufficient mixing of the brine effluent is important. Diffusor systems generally provide the best mixing, but co-location of SWRO facilities with power plants, already discharging cooling water, minimizes additional impacts of the brine. Local monitoring of the marine benthic environment is encouraged in locations of proposed desalination facilities.