Rain water seeps into the aquifers and slowly flows underground to the ocean. The outflowing fresh groundwater that meets seawater along the coast forms a buffer that keeps seawater from moving inland. This buffer is the key to protecting our local groundwater supply. Freshwater pressure from a healthy aquifer prevents seawater from moving inland and contaminating groundwater supply wells. As groundwater is pumped to keep up with local water demand, and as groundwater flows out to adjacent basins, the size of this freshwater wedge is reduced, and seawater faces less resistance moving inland.

Both the Pure Water Monterey facility in Marina that recycles wastewater and the Aquifer Storage and Recover facilities that capture Carmel River water store the water produced in the Seaside Groundwater Basin. However, these projects are only designed to meet consumer demand. The stored water only remains in the Basin a short period of time before it is pumped to customers. As groundwater levels continue to decline, it is only a matter of when, not if, seawater will render the groundwater supply and water stored in the Basin by the projects unusable.

There is a threat more imminent than seawater intrusion directly from the ocean. As groundwater levels continue to decline, seawater naturally resident in and traveling along the shallower sand aquifer that overlies the declining groundwater aquifer surface below, in a healthy basin buoyed by adequate underlying groundwater levels, is drawn down towards the base of pumping wells causing seawater contamination.

Changing Levels: Since the 1960s, groundwater levels have fallen on average 0.6 feet per year. Groundwater elevations in the Basin are in some areas 40 feet or more below sea level. Adjacent down-gradient groundwater-depleted basins pull hundreds of millions of gallons of water out of the Seaside Basin each year. Recharge from rain has not been sufficient to keep groundwater at seawater intrusion protective levels.

Recent studies performed for the Watermaster concluded that 1,000 to 3,600 acre-feet per year (up to 11 billion gallons) of water would need to be injected into the Seaside Basin every year to replenish it and raise groundwater levels high enough to prevent seawater intrusion from occurring. The risk and replenishment need will increase each year that groundwater levels continue to fall and remain below sea level. The current or expanded water supply projects, limited by availability of source water, do not accomplish this. A source of replenishment water for the Basin is desperately needed. Desalination has been determined to be the only source of replenishment water that this and adjacent basins can rely on for ultimate health and protection.