Slant Wells and Submarine Aquifers: The Future of Desalination

California’s next-generation ocean water desalination plants will get their water from submarine aquifers

California’s next-generation ocean water desalination plants will get their water from submarine aquifers – a safer, reliable, and environmentally friendly water source.

The California Coastal Commission’s unanimous approval of South Coast Water District’s Doheny Ocean Desalination Project validated this sustainable ocean water desalination methodology. The project deployed Geoscience’s innovative slant well technology to extract ocean water from submarine aquifers deep under the seabed to protect local marine life and preserve coastal ecosystems while producing reliable water quality and quantities.

Old approaches to ocean water desalination included surprising systemic fragility with numerous risks to both production volume and water quality. The next generation subsurface slant well technology creates a robust water production process for a safe and reliable water supply that meets sustainability and ecological protection requirements.

The Doheny project is the first California municipal ocean water desalination plant approved in 15 years and is the first of the seawater desalination plants of the future.

Subsurface Intakes and Marine Life

Using slant wells to produce water from submarine aquifers meets the California Ocean Plan’s provisions requiring new ocean water desalination facilities to use subsurface intakes and supply water from beneath the ocean floor.

Prior to the Ocean Plan’s subsurface intake requirement, ocean water desalination plants killed marine life sucked into their open intake pipes and were vulnerable to contamination. The California Coastal Commission – a key permitting agency tasked with protecting local marine ecologies – won’t issue permits for new plants with old open ocean intakes.

Geoscience drills deep subsurface slant wells at an angle from the shore to access saltwater aquifers hundreds of feet below the ocean floor. The slant well intake pipes draw in ocean water underground eliminating any possible impacts to marine life – nothing can be sucked into the subsurface pipe – and protecting sensitive desalination equipment from damage.

In addition to preserving offshore ecosystems, subsurface ocean water desalination intakes deliver other valuable benefits by mitigating or eliminating issues threatening previous sea water production methods, as well as delivering several advantages including cost savings and a pre-filtered water supply.

Cost Savings

Next generation ocean water desalination plants with slant well intakes reduce operating costs and use less energy than traditional plants. Water produced from slant wells underneath the ocean floor requires substantially less filtration – called pretreatment – before it passes through reverse osmosis membranes to remove the salt from the water, effectively reducing processing time and filter expenditures.

Open ocean seawater intakes’ extensive pretreatment requirements, including ultrafiltration equipment – membranes that remove particles from water – and older processes such as coagulation and filtration, significantly increase the cost to build and operate ocean water desalination facilities.

Extra filtration steps consume more energy, while additional cleaning chemical requirements, filter replacements, and extra staff to perform pretreatment processes and equipment maintenance dramatically increase costs to operate. Based on a 2004 study conducted by the U.S. Bureau of Reclamation, additional costs for pretreatment in a 12.5 mgd facility would be $41.4 million, with an annual $5.4 million required operation and maintenance for the pretreatment system alone.

Reliable Production

Slant well water intake from aquifers deep underneath the ocean floor insulate California’s next generation desalination plants from risks that take older open-ocean intake plants offline.

For example, by the time returning anglers reported a sheen on the water and noticed black globs on their boats on October 1st, 2021, more than 25,000 gallons of oil had already spilled through a ruptured oil pipeline into Southern California’s coastal waters. The large-volume spill would have shut down any nearby desalination plants as oil infiltrated open-ocean intake pipes, contaminated the water production process, and severely damaged the plant’s pretreatment equipment and reverse osmosis membranes.

Shutdowns from these kinds of events would disrupt water supplies for nearby homes and businesses and cost millions in membrane replacements and filtration equipment cleaning and repair.

Open ocean water intakes are at risk from sewage spills as well, a surprisingly common occurrence in California coastal waters with more than 40 million gallons of sewage spilled last winter. Sewage discharges increase costs for desalination plants producing water with open ocean intake pipes by accelerating filter membrane replacement schedules and increasing the pretreatment filtration and chemical needs. A particularly bad sewage spill, such as the 17-million-gallon untreated sewage discharge into the Santa Monica Bay in 2021, may even shut down the desalination plant entirely.

In addition to sewage and oil, natural contamination from red tides, algae or phytoplankton overgrowths, risk shutting down desalination plants with open ocean intake pipes. Excess biomass can block intake screens on open pipes, or damage treatment equipment and destroy reverse osmosis membranes in. Even more critical, cyanotoxins or domoic acid, potent neurotoxins produced by red tide organisms, build up in the affected costal water and require extra filtration and treatment chemicals to neutralize or remove the toxins and produce safe drinking water. Extensive or persistent red tide events may shut down open ocean intake desalination plants for weeks or months.

Even a regular winter storm can disrupt open ocean water intake desalination plants as runoff debris, sediment, and churned up sand infiltrate or block exposed pipes, and clog filtration equipment. And, in addition to the risk to coastline ecologies, marine life sucked on or into open ocean pipes may also block the water intake or damage filtration equipment.

Such abundant risks make desalination plants with open ocean intake pipes an unreliable and potentially costly water production method.

On the other hand, slant wells producing ocean water from deep, submarine aquifers effectively mitigate all these risks. As sea water percolates down through the ocean floor and into submarine aquifers, contaminants like oil, sewage, storm debris, and algae are naturally filtered out allowing subsurface ocean water intakes to produce water with reliable, consistent water quality with few disruption risks.

Subsurface marine aquifers are the answer to building a robust, dependable water supply ensuring consistent potable water availability through both nature- and human-caused events.

The Slant Well Advantage

Slant wells for subsurface ocean water desalination require appropriate planning, siting, and permitting, as well as expert construction – similar to traditional ocean water production methods. However, once installed, subsurface ocean water desalination delivers significant savings, risk mitigation, and reliability compared to older open ocean water intakes.

Recent test wells and long-term studies prove the advantages of using slant wells for subsurface ocean aquifer water intakes for protecting marine life, minimizing ecosystem impacts, producing energy savings, reducing costs, generating reliable water production, and protecting desalination plants from potential disruption.

With the California Coastal Commission’s approval of our slant well designs, Geoscience will continue working with municipal and private clients to expand application of this innovative technology for subsurface ocean water desalination to secure a reliable, safe, and ecologically friendly water supply.

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