According to statistics from the city of Santa Cruz and the Soquel Creek Water District, it requires approximately 1.4 to 2.1 kilowatt hours per thousand gallons (kWh/kgal) to collect, treat and distribute the traditional water supply—that’s surface water and groundwater—for the city and the water district.
A desalination facility requires about 13 kWh/Kgal. If the city and water district build a desal plant, the percentage of household energy for this new water supply would be about 1 to 1.6 percent of overall household energy use.
Heidi Luckenbach and Melanie Mow Schumacher, the desalination program coordinators for the city and the district, respectively, are quick to say that no decision has been made on desalination. The next step is to study the proposal in an Environmental Impact Report. The first EIR scoping meetings happen Wednesday, Dec. 8.
But Luckenbach and Mow Schumacher also say the current water supply isn’t sustainable. The city, which serves about 90,000 people and primarily relies on surface water captured in local reservoirs and streams, doesn’t have an adequate water supply during drought years. And Soquel Creek Water District, with its 49,000 customers, replies solely on groundwater. Its aquifers are over-pumped, and salty seawater threatens to contaminate the groundwater. The current annual water use for the district exceeds available water supply—even in rainy years.
“No decision has been made on desalination,” says Mow Schumacher. “We can’t say it’s desalination or no desalination; what we’re saying is: based on previous studies and previous options, desalination is an option to pursue.”
If the two agencies built a 2.5 million-gallon-per-day plant, it would provide water to each during different times of the year. The city would have priority to use the desalted seawater during the summer, although water officials expect to only use it during drought years. During the rest of the year, the plant would provide water to Soquel Creek Water District customers, offsetting the water that would otherwise be pumped from the aquifers, which would allow the groundwater to return to target levels.
While the environmental review process will look at different desalination alternatives, basically, it would work like this.
First, a seawater intake system will draw in water either through sub-seafloor intakes or screened, open-ocean intakes. Sub-seafloor intake options include:
• Vertical beach wells, which are similar to typical groundwater wells. They draw brackish water through the sand, which filters organisms out.
• Slant wells, which use a new type of well technology where the wells are installed at an angle and drilled from or near the beach.
• Offshore radial collector wells. These have never been constructed in an offshore marine environment. Each would include a central caisson extending down into the sub-seafloor sand with lateral well screens fanning out horizontally. Water would flow through the horizontal well screens into the caisson and be pumped to the desal plant.
• An offshore infiltration gallery—also new technology—consists of a group of well screens or perforated collection pipes buried horizontally with engineered coarse sand above it. Seawater percolates through the engineered sand and into the pipes.
Screened open-ocean intakes, on the other hand, involve narrow-slot cylindrical wedgewire screens that attach to pipelines that run to an onshore pump station. Seawater flows through the intake screens, which are intended to keep marine critters out, and is pumped to the desal plant. The agencies are looking at two potential locations: near the abandoned outfall pipeline at Mitchell’s Cove Beach and near the Santa Cruz Municipal Wharf.
Both sub-seafloor and open-ocean intakes have environmental advantages and drawbacks, all of which will be wildly debated in the months ahead.
“The infiltration gallery option might work,” says Tom Luster, an environmental scientist with the California Coastal Commission, which will be involved in the permitting phase of a future desal plant. “But it would have to be fairly small and there are limited sites for it. Those work best when there’s not a whole lot of mud or silt in the water column that gets moved along the ocean floor. They’ve also done some good screening studies; I don’t know which intake system we’d like to see. It may be any combination of those.”
Regardless of the intake system, the seawater will then be piped to a desalination plant on the Westside, likely located at the Natural Bridges Industrial Park or an area on Shaffer Road. The plant will remove the salt by reverse osmosis, a pressure-driven process that pushes seawater through semi-permeable membranes. This produces up to 2.5 million gallons per day of product water and also brine, which is twice as salty as ocean water.
Desalinated water will be treated so it’s drinkable before being piped to existing potable water distribution systems. A different pipeline will carry the brine to the city’s wastewater treatment plan, where it will mix with effluent—diluting the salt—and then discharge it into the ocean.
Meanwhile, the two agencies want the public to comment on the proposals and get involved in the process. Says Mow Schumacher: “We want to hear the good, the bad the ugly—everything in between.”
THE DESAL EIR SCOPING MEETINGS are Wednesday, Dec. 8, noon-2:30pm at First Congregational Church, 900 High St., Santa Cruz; and 6:30-9pm at New Brighton Middle School, 250 Washburn Ave., Capitola. For info visit www.soquelcreekwater.org
