Historically, more water was pumped out of the Basin than was naturally replenished with rainfall. Extractions remained high into the early 2000s, with over 8,000 acre-feet extracted in 2003. For comparison, the estimated safe level of groundwater extraction in the Basin is 4,870 acre-feet per year; this is referred to as the “sustainable yield” of the Basin. The over-extraction of groundwater led to a condition called “overdraft.” As a result of the overdraft, Basin groundwater levels were 40 to 120 feet below sea level in the mid-1980s to early 1990s allowing active seawater intrusion in some areas. This intrusion is evidenced by the high dissolved chloride concentrations in monitoring wells as shown in the figure from the Basin GSP here. The high chloride concentrations occur near Soquel Point on the west and the Seascape area on the east.
To begin addressing seawater intrusion, water managers in the Basin have focused on reducing overall groundwater use through water efficiency efforts and by shifting groundwater extraction inland away from the coast. Groundwater pumping has declined over the past twenty years as seen in the figure below, with the 2020-2024 5-year average extraction approaching the sustainable yield of the Basin.
As important as reducing overall groundwater demand has been the shifting of extraction away from coast. The groundwater extraction graph here shows that pumping within 1 mile of the coast has been reduced from more than 5,000 acre-feet in the mid-1980s down to less than 2,000 acre-feet by the mid-2020s.
The strategy to reduce groundwater extraction near the coast has proven successful in one example near Soquel Point. This can be seen in the declining chloride concentration in a monitoring well near Moran Lake in the vicinity of Soquel Point. These declines were observed as extraction from a nearby extraction wells was significantly reduced. Note that it took more than 15 years for these concentrations to reduce from about 700 milligrams per liter (mg/L) down to about 40 mg/L.
Unfortunately, simply reducing pumping has not addressed all of the threats associated with seawater intrusion in the Basin. Evidence of this can be seen in monitoring well SC-A2B. Despite reduced groundwater extraction and improved groundwater levels in the Seascape area, chloride concentrations have steadily increased for more than three decades from about 20 mg/L to about 600 mg/L.
