South Orange Coastal Ocean Desalination Project

Client Profile

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.

Challenge

Brackish water occurs in natural environments where fresh and salt water mix and has salinity levels registering between freshwater and seawater. Primarily, brackish water is found in coastal regions where surface or groundwater mixes with seawater, but is also found 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 to do so is prohibitive, especially when the proximate source is saltwater.

SWCD had limited groundwater resources consisting of one well that yielded brackish water and required treatment before it could be used for any purpose. Consequently, they relied on imported water for 70% of their resources. And any disruption in the supply chain resulted in service outages for their customers. Their goal was to develop a sustainable and reliable water supply for customers with no dependence on imported water.

With a limited groundwater basin next to an infinite supply of water due to their position on the coast, it became clear desalination was their best option to create this reliable, sustainable, and more affordable water supply.

The challenge facing SCWD was how to create a reliable, safe freshwater supply from the ocean saltwater off shore without adversely affecting marine life, one of the downsides of deploying pipelines in the ocean, or impinging on local groundwater reservoirs, which can happen when new wells are drilled on the coast. Drilling coastal wells is often a preferred technology among state regulators and environmental groups for desalination plants to protect wildlife, but they can cause new challenges for other groundwater supplies.

Geoscience Solution

SCWD partnered with Geoscience to innovate a new process for ocean water desalination that would answer their need for a reliable supply while protecting marine life and the nearby freshwater aquifer.

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 a sonic drilling method, two of which were completed as nested monitoring wells.

Initial modeling provided further screening of potential subsurface intake locations, followed by the construction of a 350-foot-long slant well angled 23 degrees below horizontal.

The well began pumping in 2006, and 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 with incorporated comments from a peer review panel of experts in the groundwater modeling field, 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.

Customer results

Using data from the long term test, Geoscience 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 with 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.

The seawater desalination process actually adds volume to the groundwater basin by tapping an infinite alternate source and achieving a two-to-three thousand gallons-per-minute yield.