REPORTS: The potential for brackish desalination in California

In 2017, a nationwide assessment by the U.S. Geological Survey found that brackish groundwater in the nation’s aquifers is more than 800 times the amount currently used yearly.  California’s Coastal Basin and Central Valley aquifers contain close to 7 billion acre-feet of brackish water, which, if desalinated, could provide enough water for the state’s needs for the next 160 years.

Brackish water has salts and other dissolved solids and must be desalinated to make it suitable for human use.  However, since brackish water has a lower salinity than seawater, desalinating it requires less treatment, less energy, and creates less brine discharge.

Governor Newsom’s Water Supply Strategy’s Action1.2 targets expanding brackish desalination by 28,000 acre-feet by 2030 and 84,000 acre-feet by 2040.  The Strategy directed the Department of Water Resources and the State Water Board to identify brackish desalination projects with the potential to be operational by 2030 and no later than 2040, and to determine the volume of water available for brackish groundwater desalination.

Last month, the Department of Water Resources released the report, Projected Brackish Water Desalination Projects in California,  and the State Water Resources Control Board released the report, Water Available for Brackish Groundwater Desalination, in fulfillment of their charge.

Brackish desalination in California

The Department of Water Resources determined that in 2020, about 106,000 acre-feet of brackish water was desalinated for drinking water, representing two-thirds of the desalinated water produced and used in California.  DWR’s analysis determined that the state will easily meet the 2030 Water Supply Strategy goal, but few facilities are planned beyond that at this time.

The State Water Board report identified that appropriate brine management and disposal options are often the limiting factor for inland areas.  They found the highest potential for brackish groundwater desalination exists in coastal groundwater basins with access to existing infrastructure for brine disposal and where groundwater pumping is consistent with the local Groundwater Sustainability Plan.  They note basins or subbasins already impacted by seawater intrusion may be strong candidates for brackish groundwater desalination and warrant further evaluation.

Although State Water Board staff could not estimate the total amount of brackish groundwater available, their report includes an inventory of brackish groundwater basins that could potentially be used for brackish desalination.

In California, most brackish desalination uses brackish groundwater as the water source.  However, brackish surface water can also be a source; in 2021, the city of Fort Bragg began desalinating water from its Noyo River intake because of seawater intrusion during high tide, and in 2025, the city of Antioch will begin desalinating water from its San Joaquin River intake because of increasing saline bay water intrusion.  Other communities are also considering future desalination of brackish surface water to augment or replace existing vulnerable supplies.

Opportunities and challenges

Since brackish groundwater is less salty than seawater, brackish water desalination requires less treatment and energy and produces less brine than seawater desalination; optimizing existing water infrastructure for brine management could further reduce costs.  Brackish desalination could be considered for other uses requiring less treatment, such as cooling water for power generation, irrigation, livestock watering, or aquaculture.

However, project-specific modeling of aquifer impacts is needed to ensure a project does not mobilize potential contaminants or exacerbate seawater intrusion or subsidence in vulnerable basins.   Some chemical constituents may be found in brackish groundwater at high concentrations that may require additional treatment or render the groundwater unsuitable for certain uses.

Recommendations include conducting site-specific evaluations that include the availability of lower-cost alternatives, funding, affordability, local hydrogeologic conditions, treatment requirements, permitting considerations, and brine disposal options; and expanding water quality data collection of brackish groundwater, and filling data gaps by expanding the GAMA program.