South Orange Coastal Ocean Desalination Project
South Coast Water District (SCWD) provides potable water, recycled water, and wastewater services to 35,000 residents, 1,000 businesses, and 2 million visitors per year in South Orange County, California.
Brackish water occurs in a natural environment that produces a salinity level registering between freshwater and seawater. It occurs in coastal regions where surface or groundwater mixes with seawater and in inland regions where surface water subsumes salt from mineral deposits as it flows into aquifers.
As populations grow in coastal California, the demands on groundwater resources often outstrip capacity and sustainability. The most practical alternative is artificially recharging local aquifers, but the cost of bringing in freshwater is prohibitive, especially when the proximate source is saltwater. SWCD had limited groundwater resources consisting of one well that yielded brackish water and therefore had to be treated. Consequently, they relied on imported water for 70 percent of their resources. Any disruption in the supply chain resulted in service outages for their customers.
With a limited groundwater basin and an infinite supply of water on the coast, it became clear that a desalination project was their best option. The challenge facing SCWD was how to transmute saltwater into freshwater without adversely affecting marine life, which is the downside of deploying pipelines from the ocean, or impinging on local groundwater reservoirs, which can result from drilling new wells on the coast, the preferred technology among state regulators and environmental groups for desalination plants.
Their goal was to develop a sustainable and reliable water supply for customers with no dependence on imported water.
SCWD partnered with Geoscience to innovate a new process. In 2005, Geoscience began a multi-phase study, beginning with a comprehensive literature review and initial screening of subsurface intakes. The next step was to identify sites for four exploratory boreholes on Doheny beach using the sonic drilling method, two of which were completed as nested monitoring wells. Initial modeling provided further screening of potential subsurface intakes and construction of a 350 ft long slant well 23 degrees below horizontal.
The slant well, which began pumping in 2006, was the first ever successfully constructed artificially-filter-packed slant well completed below the ocean floor. It produced approximately three million gallons per day (mgd) in two years of pilot testing. To assess the feasibility of this alternative, Geoscience developed and calibrated a variable density groundwater model that incorporated comments from a peer review panel of experts in the field of groundwater modeling, as well as feedback from the San Juan Basin Authority and other agencies. The groundwater modeling work determined the potential yield of a slant well intake system, predicted water quality variations with time, and simulated effects on groundwater levels in the onshore groundwater basin.
Using data from the long term test, GSSI designed a full-scale 30 mgd water supply, consisting of seven 800 foot slant wells and two standby wells. As is common with slant wells, most of the yield was saltwater, but the yield also included some brackish water from the groundwater basin. SCWD had permits to cover the five percent overlap, processed the brackish water, and returned an equivalent amount of freshwater to the aquifer, resulting in zero net impact on the basin.
By contrast, desalinating seawater actually adds volume to the groundwater basin by tapping an infinite alternate source to process, achieving a yield of two to three thousand gallons per minute.
Contact Brian Villalobos to learn more about this case study.
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