California has been in a record-breaking drought over the past five years, leading policymakers to ask pertinent questions: How can we improve California's water supply management and pursue cost-effective and environmentally friendly water supply alternatives? In our NRDC Issue Brief: Proceed with Caution II, we conclude that conventional desalination should be the last resort, and only pursued after preferred water solutions – such as water conservation, water efficiency, stormwater capture, and recycling efforts – have been fully implemented.
What is desalination?
Desalination is an engineered process that turns seawater, brackish water (water with more salt than freshwater, but less than ocean water) and other impure waters into cleaner water for drinking or other uses. There are three main types of desalination: seawater desalination, brackish surface water desalination and brackish groundwater desalination (described in the figure below).
Are we currently desalinating water in California?
Yes, and most of it is coming from brackish groundwater desalination plants. There has been a recent rise in seawater desalination production (given Carlsbad – the largest desalination plant in the hemisphere—came online in December 2015) and attempts to increase the number of seawater desalination plants. But brackish groundwater desalination plants still produce more potable water.
There are 23 brackish groundwater desalination plants that produce 139,627 acre-feet per year of clean water, while only 12 active seawater desalination plants that produce 62,840 acre-feet per year, less than half of the capacity of brackish groundwater desalination facilities.
Desalination has adverse impacts on the environment
Seawater desalination plants often have "open ocean intakes" or large intake pipes that kill marine life on their sceens as they take in water. Seawater desalination plants also discharge "brine" or highly concentrated saline water that can harm marine life. These plants can damange California's network of marine protected areas (MPAs), threatening the diversity of marine life and destroying habitats.
Brackish surface desalination plants have many of the same environmental impacts for estaurine environments as seawater desalination plants do for marine environments. Both open ocean intakes and the discharge of brine can harm important ecosystems and aquatic biodiversity.
Brackish groundwater desalination plants are less harmful than these other types of desalination plants; however, there is the possibility of groundwater overdraft, which can lead to land subsidence and poor water quality.
Preferred water resources should be implemented first
We recommend first implementing conservation, water efficiency, stormwater capture and recycling. If these resources cannot meet local water demand, we recommend brackish groundwater desalination plants that mitigate their GHG emissions, use safe brine disposal strategies and are sited in groundwater basins that are sustainably managed.
Seawater and brackish surface water desalination plants have significant negative impacts on marine and estuarine environments, have high costs and are energy intensive (as my colleague Sierra explains). These plants should be the last option. And even then, they should be designed to minimize their impacts, following this evaluative matrix:
Seawater and brackish surface water desalination plants plants should have "subsurface intakes" that draw water from under the seafloor, sustainable brine disposal strategies and be sited outside MPA boundaries. For seawater desalination, these policies follow the State Water Resources Control Board policy detailed in the Ocean's Plan Amendment; however, we recommend that these guidelines are followed for brackish surface water plants as well. Least impactful desalination plants mitigate their GHG emissions and are designed to meet local water needs, rather than to produce the most water.
In light of all these drawbacks, California must proceed with caution when considering desalination.