The Role of Desalination in Southern California’s Water Future

Joe Grindstaff

Oct 2

8 min read

Blue industrial water treatment tanks and pumps in a clean room, with gray piping and label MA2, lit bright and sterile.

Southern California is surrounded by water, but most of it cannot be used for drinking without extensive treatment.

Seawater contains too much salt for normal drinking water treatment. Some local groundwater basins also contain elevated salt levels that make the water difficult or impossible to use without additional treatment.

Desalination changes that by removing salt and other dissolved materials from water.

For a region facing population growth, climate variability, Colorado River uncertainty, and limits on imported water, desalination can provide a dependable local water supply. However, it also requires significant energy, infrastructure, environmental protections, and long-term investment.

Desalination is not a complete answer to Southern California’s water challenges. It is one tool that can become more valuable when combined with conservation, recycled water, groundwater storage, regional partnerships, and responsible imported water management.

What Is Desalination?

Desalination is the process of removing salt and other dissolved materials from water so it can be used as a drinking water supply.

The most common modern treatment method is reverse osmosis.

During reverse osmosis, pressure pushes water through tightly designed membranes. Water molecules pass through the membranes, while salts and many other dissolved substances are separated from the treated water.

The process creates two main streams:

Treated freshwater that can be prepared for distribution
A concentrated salty waste stream, generally called brine or concentrate

After reverse osmosis, the treated water must be stabilized, disinfected, tested, and adjusted so it meets drinking water standards and can move safely through the distribution system.

Although the basic concept is straightforward, operating a desalination facility requires carefully managed treatment, energy, monitoring, membrane maintenance, and brine disposal.

Seawater and Brackish Groundwater Are Different

Discussions about desalination often focus on the ocean, but desalination can treat several types of water.

The two most relevant sources for Southern California are seawater and brackish groundwater.

Seawater desalination

Seawater desalination removes salt from ocean water. Because seawater contains a high concentration of salt, it requires substantial pressure and energy to push the water through reverse osmosis membranes.

Facilities must also manage how ocean water is collected and how concentrated brine is returned to the environment.

Brackish-groundwater desalination

Brackish groundwater contains more salt than normal freshwater but significantly less salt than the ocean.

The salinity may occur naturally or result from historical land use, irrigation, or other activities that cause salts to accumulate in the groundwater basin.

Because brackish water is less salty than seawater, it generally requires less pressure and energy to treat. It also allows inland communities to develop a local drinking water supply from groundwater that would otherwise have limited use.

This is the type of desalination Eastern Municipal Water District currently uses.

How Does EMWD Use Desalination?

Portions of EMWD’s service area contain brackish groundwater with salt levels that make the water unsuitable for regular drinking water use without advanced treatment.

EMWD operates groundwater desalination facilities in Menifee that use reverse osmosis to remove salts and produce a safe local drinking water supply.

The district’s third facility, the Perris II Desalter, opened in 2022. According to Eastern Municipal Water District, the facility can treat approximately 5.4 million gallons of water per day.

Combined, EMWD’s groundwater desalination facilities can produce more than 14 million gallons of drinking water each day. That is enough to serve more than 30,000 households annually and meet nearly 10 percent of EMWD’s water demand.

The program also removes approximately 65,000 tons of salt from the groundwater basin every year. The concentrated brine is conveyed through the Inland Empire Brine Line for discharge to the Pacific Ocean.

This means the program provides two connected benefits: it creates a local water supply and helps improve the long-term quality of the groundwater basin.

EMWD Is Preparing to Expand Its Program

EMWD has continued seeking state and federal support to expand its brackish-groundwater desalination program.

In January 2025, EMWD announced that the federal Water Resources Development Act increased the total authorization for its desalination program to $100 million.

The additional federal funding authorization can support groundwater wells, pipelines, and treatment infrastructure.

The planned improvements could increase EMWD’s desalination capacity by another 5.4 million gallons per day, enough water to serve approximately 15,000 additional households annually.

Expanding local desalination reduces the amount of water the region must import from hundreds of miles away. It also gives EMWD another option when drought, infrastructure problems, environmental regulations, or negotiations affect imported supplies.

Why Is Desalination Considered Drought-Resilient?

Many water supplies depend directly on precipitation.

Reservoirs need rain and snow. Rivers depend on runoff. Groundwater basins must be replenished. Even recycled water supply depends partly on the amount of water used within a community.

Seawater availability is not directly controlled by rainfall or snowpack. Brackish groundwater can also provide a relatively stable source when the basin is carefully managed.

That makes desalination a drought-resilient water supply.

A desalination facility can continue producing water during a dry year, assuming the source remains available and the infrastructure has adequate energy, treatment capacity, and permits.

This reliability can be especially valuable during emergencies or prolonged droughts.

However, drought resilience does not mean unlimited supply. A plant can only produce the amount of water its wells, intake system, treatment equipment, pipelines, and permits allow.

Desalination facilities also require routine maintenance and can be affected by energy prices, equipment problems, source-water conditions, and environmental requirements.

Seawater Desalination Already Supports San Diego County

The Claude “Bud” Lewis Carlsbad Desalination Plant provides a large-scale Southern California example of seawater desalination.

According to the San Diego County Water Authority, the plant can produce up to 50 million gallons of drinking water per day.

The facility provides approximately 10 percent of San Diego County’s water supply and reduces the region’s dependence on imported water.

This local supply is available regardless of annual snowpack or Colorado River runoff. That makes it an important part of San Diego County’s diversified water portfolio.

At the same time, the project required major investments in treatment, pipelines, environmental mitigation, energy, and long-term water purchase agreements.

Carlsbad demonstrates both sides of the desalination discussion: it can provide a highly reliable local supply, but that reliability comes with significant costs and responsibilities.

What Are the Financial Challenges?

Desalination is generally more expensive than treating high-quality freshwater.

The process may require:

New groundwater wells or ocean intake systems
Reverse osmosis equipment
High-pressure pumps
Significant electricity
Specialized treatment and monitoring
Membrane replacement
Pipelines and storage facilities
Brine management
Environmental studies and mitigation
Trained operators and maintenance personnel

The cost varies depending on the water source, salt concentration, facility size, energy prices, location, environmental requirements, and existing infrastructure.

Brackish-groundwater desalination is typically less energy-intensive than seawater desalination because the water contains less salt. However, it still requires significant capital investment and a reliable method for managing the concentrate.

State and federal grants can reduce the portion paid directly by local customers. EMWD has received public funding for wells, pipelines, treatment facilities, and program expansion.

Those funds help make local water development more affordable, but the system still has ongoing operating, maintenance, energy, and replacement costs.

Water agencies must compare those costs with the value of supply reliability. A source that is more expensive during normal conditions may become extremely valuable during a severe drought or imported water shortage.

What Are the Environmental Concerns?

Seawater desalination presents different environmental concerns than brackish-groundwater desalination.

Ocean facilities must take in large volumes of seawater. Depending on the intake design, small fish, larvae, eggs, plankton, and other marine organisms may be trapped or drawn into the facility.

The treatment process also produces concentrated brine. If brine is not diluted and dispersed appropriately, increased salinity near the discharge location could affect marine life.

California’s Ocean Plan requirements for seawater desalination facilities address facility location, design, technology, intake systems, brine discharge, and mitigation.

The rules require facilities to use the best feasible site, design, and technology to minimize harm to marine life. Projects may also be required to offset environmental effects that cannot be avoided.

Brackish-groundwater desalination avoids some ocean-intake concerns, but it still produces concentrate that must be managed. Inland projects may require brine pipelines, evaporation facilities, injection, or another approved disposal method.

Both types of desalination also use energy. The environmental effect depends partly on how much electricity the facility consumes and how that electricity is generated.

These issues do not automatically make desalination unacceptable. They mean each project must be evaluated based on its location, design, source water, energy use, discharge method, and available alternatives.

Can Desalination Help the Colorado River?

Desalination may also create opportunities beyond the community where the treated water is produced.

In 2026, water agencies and federal partners began exploring an interstate exchange framework involving desalinated seawater produced in San Diego County.

Under a potential exchange, San Diego County would use its local desalinated water. In return, another Colorado River Basin agency could leave an agreed-upon amount of water in the Colorado River system or make water available elsewhere.

The physical desalinated water would not need to be pumped from the California coast to another state. The benefit would be created through accounting, agreements, and coordinated water management.

The San Diego County Water Authority’s interstate partnership is still exploratory. Any future exchange would require legal agreements, funding, infrastructure capacity, and protections for local ratepayers.

However, the concept shows how local desalination could create regional value. A dependable coastal supply might reduce demand on the Colorado River while supporting communities that have already invested in desalination infrastructure.

Why Desalination Is Not a Complete Solution

It is tempting to look at the Pacific Ocean and conclude that Southern California has an unlimited water supply waiting to be treated.

The reality is more complicated.

Desalination requires energy, money, infrastructure, permits, and environmental mitigation. Not every coastal location is appropriate for a plant. Inland agencies must have a practical way to manage brine. Transporting treated water from the coast to inland communities can require additional pipelines and pumping.

Desalination also does not eliminate the need to:

Conserve water
Maintain existing canals and aqueducts
Manage groundwater sustainably
Expand recycled water
Capture stormwater
Store water during wet years
Protect watersheds
Negotiate fair Colorado River agreements
Repair aging local infrastructure

Relying too heavily on any single source creates risk.

Desalination is most valuable when it is part of a balanced water supply portfolio.

Desalination Can Provide More Than Additional Water

A desalination project should not be evaluated only by its cost per acre-foot.

It may also provide:

A local supply that does not depend directly on snowpack
Greater independence from imported water
Emergency supply capacity
Improved groundwater quality
More flexibility during drought
Reduced demand on rivers and aqueducts
Opportunities for regional water exchanges
Predictable production from existing facilities

These benefits have financial value, even when they are difficult to include in a simple comparison.

For EMWD, brackish-groundwater desalination also allows the district to use water that is already located within its service area. Removing salt improves the basin while producing drinking water.

That combination makes groundwater desalination especially relevant for western Riverside County.

Building a Diverse Southern California Water Supply

Southern California’s future water reliability will not come from one major project.

It will come from many strategies working together.

Conservation can reduce unnecessary demand. Recycled water can replace drinking water for irrigation and other appropriate uses. Groundwater storage can preserve supplies for dry periods. Regional transfers can move available water to where it is needed. Reservoirs can capture water during wet conditions. Desalination can turn otherwise unusable water into a dependable local supply.

Each option has benefits, costs, and environmental considerations.

The goal should not be to declare one source the winner. It should be to develop a portfolio that can continue serving communities when conditions change.

EMWD’s brackish-groundwater desalination program demonstrates that desalination is not only a future idea. It already contributes to Riverside County’s water supply, supports more than 30,000 households annually, and improves groundwater quality.

Future expansions and regional seawater partnerships could increase that contribution.

Desalination is not a silver bullet, but it can be an important part of a more reliable, flexible, and locally controlled Southern California water future.