The Sustainable Groundwater Management Act (SGMA) requires the Groundwater Sustainability Agency (GSA) to develop and implement a Groundwater Sustainability Plan (GSP) for managing and using groundwater. The GSP must consider the sustainability indicators described below. The GSP identifies the projects and management actions to bring the Basin into sustainability by 2040.
Seawater Intrusion in Coastal Areas
Seawater intrusion occurs when groundwater is pumped to levels below sea level. When this happens, seawater moves inland to fill the void, making wells salty and no longer useful for water supply. Basin groundwater levels were 40 to 120 feet below sea level in the mid-1980s to early 1990s, allowing seawater intrusion in some areas. Though levels have improved significantly since the 1990s, further seawater intrusion remains a threat. Because of this threat, the California Department of Water Resources (DWR) designates the Basin as “critically overdrafted.”
Groundwater-Level Declines
Over the past fifty years, more water has been pumped out of the ground than has been naturally replenished with rainfall which has lowered groundwater levels. Based upon hydrologists' analyses, over pumping of groundwater in the Basin started in the 1980s. Extractions remained high into the early 2000s, with over 8,000 acre-feet extracted in 2003. Groundwater pumping has declined over the past twenty years (an estimated 4,750 acre-feet was extracted in 2024) but the impacts of historical overdraft persist in some areas. Huge strides have been made to increase groundwater levels through management actions and more efficient use of water by residents, but it has not been enough to fully recover Basin groundwater levels. Over the years, the local agencies have been taking actions to bring the Basin back into balance by promoting water conservation and reducing groundwater withdrawals, moving pumping away from the coast, advancing projects, and working collaboratively with partner agencies. Projects, such as Soquel Creek Water District's Pure Water Soquel and the City of Santa Cruz's Aquifer Storage and Recovery, and other current and future projects are identified in the GSP.
Water Quality
The Plan requires the MGA to monitor groundwater quality to prevent impacts from management activities that could degrade water quality and adversely affect Basin water users.
Interconnected Surface-Water Depletions
In parts of the Basin, streams receive some of their flow from groundwater. Groundwater management must consider impacts to surface water flow. This is particularly important to sensitive species in summer and fall when rainfall is low. Without the addition of groundwater from the Basin, these waterways may not be able to support aquatic plants, fish, and other animals. The MGA monitors groundwater levels and streamflow in the Basin.
Reduction of Groundwater in Storage
Sustainable groundwater management requires groundwater storage is maintained at levels needed to support Basin water use, to preserve or enhance ecological resources, and to provide for a drought reserve when local rainfall is below normal levels.
This video offers information about what seawater intrusion is: