South Orange County 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 can occur in inland regions where surface water subsumes salt from mineral deposits as it flows into aquifers.

As populations grow in coastal California, 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 can be prohibitive, especially when the proximate source is saltwater.

SCWD’s limited groundwater resources consisted of one well that yielded brackish water, due to their location at the confluence of fresh and salt water aquifers, which required treatment before it could be used for any purpose. Consequently, they relied on imported water for 70% of their resources. Any disruption in the supply chain resulted in service outages for their customers and imported water prices continued to increase with weather variability and local drought conditions.

Their goal: to develop a sustainable and reliable water supply for customers while reducing or eliminating dependence on imported water.

With a limited groundwater basin, but proximity to a potentially infinite water supply due to their position on the coast, it became clear desalination was their best option to create a reliable, sustainable, and more affordable water supply for their growing population.

The challenge facing SCWD was how to create a reliable, safe freshwater supply from ocean saltwater 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.

State regulators and environmental groups prefer drilling coastal wells for desalination plants to protect wildlife and coastal access, but these traditional wells can cause new challenges for other groundwater supplies including the risk of salt water intrusion into fresh water aquifers.

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 data to identify potential subsurface intake locations, followed by the construction of a 350-foot-long slant well angled 23 degrees below horizontal, to access water in the submarine aquifer well below the seabed.

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) over the two year pilot testing.

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 to assess the feasibility of this solution. The groundwater modeling determined the slant well intake system’s potential yield, predicted water quality variations with time, and simulated effects on groundwater levels in the onshore groundwater basin.

The 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.