Researchers Try to Mitigate Desalination Effects

Coastal Engineer Scott Jenkins and Marine Biologist Jeffrey Graham, both from the Scripps Institution of Oceanography at the University of California-San Diego, since 2000 have served as consultants to two desalination projects in Southern California that have been proposed by the Connecticut technology firm Poseidon Resources.

An estimated 13,000 desalination plants operate throughout the world but few studies have tracked the ecological impact of these plants.

At the beginning of the process, intake pipes suck in and destroy plankton, including phytoplankton and the larvae of invertebrates and fish. The removal of these members of the local ecosystem exacts a certain toll, one for which Jenkins has attempted to formulate mitigation measures for Poseidon.

At the end of the process, desalination plants that create a gallon of freshwater from every two gallons of seawater must find a way to dispose of the remainder. The byproduct water has roughly double the salinity of seawater, a concentration well beyond the tolerance of most marine organisms. Many operators of existing desalination plants disregard the danger and simply discharge the hypersaline water back into the ocean. Scientific observations show that some marine organisms have vanished from the areas around underwater outfall pipes where undiluted wastewater is returned to the oceans.

A handful of relatively small ocean desalination plants exist in California, but the state's Coastal Commission is reviewing another 20 proposed projects on a case-by-case basis in an attempt to find a standard for acceptable impact. A Poseidon project in Carlsbad, has come the farthest in the permitting process and could set the bar for the other proposed projects, said Jenkins. In Carlsbad and Huntington Beach., Poseidon has proposed building desalination facilities on the grounds of existing power plants, using a fraction of the seawater used by the plants for cooling as source water for the reverse osmosis process, while the remainder is used to dilute the concentrated saltwater before it returns to the ocean. Each project would produce 50 million gallons of drinking water per day, enough to serve 300,000 people.

It takes about five parts of unaltered seawater to bring one part of hypersaline water to a concentration of 40 parts per thousand. In Carlsbad, Poseidon would build its plant on the grounds of the Encina Power Station, which uses as much as 800 million gallons of seawater a day. Permits issued to the power station regulate consumption of that seawater. Poseidon must plan, however, for a future beyond Encina's possible decommissioning when the environmental responsibility for seawater consumption would be entirely on the desalination plant.

Graham has advised Poseidon on what salinity fishes, marine invertebrates, and plants can tolerate. Jenkins has been working on determining how long the brine discharged from the desalination plant remains concentrated after it returns to the ocean, a process that requires him to model winds, currents, and other physical factors to determine how the hypersaline plume will disperse. His models have suggested that salinity levels in the brine plume would be within 10 percent of natural background salinity in less than 1,000 feet of the discharge channel when flowing at 250 million gallons per day.

The issue at the other end of the process is the loss of potential value of larval plankton. Jenkins studied possibilities for water intake, or entrainment, in a manner that spares a greater proportion of marine life. Two alternatives, one that employs natural seabed filtration and the use of large intake pipe networks known as seafloor galleries, were both found to be inappropriate for the Carlsbad site due to insufficient sediment cover over the bedrock along that section of the coast. The project's environmental impact report determined that continuing to use the power station's existing intake and discharge pipes would cause the least ecological damage.

Jenkins also is involved in assessing the suitability of compensatory measures that could be made at locations other than the Encina Power Station. The Coastal Commission's price for greenlighting Posiedon's Carlsbad project will likely include a condition that Poseidon restore at least 37 acres of wetlands elsewhere in San Diego County. Jenkins is advising the company on where and how wetland restoration projects might be most effective.

Currently, desalinated water costs about $800 per acre-foot to produce, according to Poseidon. That's half of what it cost 10 years ago but is still several hundred dollars more per acre-foot than the cost of bringing water from traditional sources such as the Colorado River or the Sacramento Delta.

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