Opened on December 14, 2015, the entire desalination project cost about $1 billion for the plant, pipelines, and upgrades to existing SDCWA facilities to use the water.

The project to build a desalination plant in San Diego County, California, commenced in 1993 after five years of drought.

[4] Environmentalists opposed the construction due to various concerns, most notably energy consumption, brine discharge[1] and that the ocean water intake could kill fish.

[12] The fresh water output from the plant is sent by a 10 mi (16 km) long, 4.5 ft (1.4 m) diameter pipeline, utilizing six pumps, to connect to the SDCWA distribution system in San Marcos.

[29] Though touted by Poseidon as a "climate-proof" [citation needed] water source, at times the plant has been temporarily closed due to red tides.

[31] The water intake is filtered through gravel, sand, and other media to greatly reduce particulates before going through reverse osmosis filtration.

[16] Half of the saltwater taken into the plant is converted into pure potable water, with the rest discharged as concentrated brine.

[32] The outflow of the plant is put into the discharge from Encina Power Station for dilution, for a final salt concentration about 20% higher than seawater.

Most desalination plants discharge water with about 50% extra salt, which can lead to dead spots in the ocean, because the super-salinated brine does not mix well with seawater.

On July 29, 2015, it argued in a hearing before Superior Court Judge Gregory Pollack that the Authority's long-term water plan (and specifically the Carlsbad desalination plant) violates the California Environmental Quality Act, specifically with respect to energy needs and the greenhouse gases associated with those.