Monterey Peninsula Water Supply
The Monterey Peninsula Water Management District (MPWMD) serves approximately 112,000 people within a 170 square mile area consisting primarily of the Monterey Peninsula and Carmel Valley, California. The Carmel River watershed drains 256 square miles. The Seaside Groundwater Basin encompasses 24 square miles
In coastal regions, freshwater meets saltwater in a transition zone that includes the mouth of the river (surface water) and the aquifer (groundwater). Excessive pumping from a coastal aquifer can reduce the seaward flow of freshwater to the extent that saltwater intrudes into the aquifer, rendering it useless for agriculture or consumption. As wells migrate inland to escape saltwater intrusion, they recreate the conditions to extend intrusion farther inland, perpetuating the problem.
For more than a century, agriculture has played a significant role in Monterey County, California. As farms have grown, so has the need for irrigation, eventually depleting the freshwater aquifers to the point that saltwater intrusion was detected in 1946. In following years, tourism and population growth has further exacerbated the demand for freshwater resources.
In 2009, the California State Water Resources Control Board issued a cease-and-desist order to California American Water (CalAm) to stop over-pumping in the Carmel River basin. As conditions worsened, there was talk of imposing water rationing to the tune of 35 gallons per person per day, a third of the national average consumption of 100 gallons per person per day. And that cap didn’t include commercial demand.
The stop-pumping order became fully effective at the end of 2021, requiring CalAm to depend on the Seaside Basin, a natural underground reservoir. The order had serious implications for the local economy.
The obvious, simple solution was to import water, but that alternative was neither cost-effective nor sustainable. It just shifted the problem to other groundwater aquifers, sources that would eventually become overtaxed in turn. Instead, CalAm turned their attention to an infinite supply–the ocean.
CalAm’s goal was to develop a reliable, sustainable water supply for their residential, agricultural, and commercial customers, one that could maximize yield with no foreseeable limits. This included replacing existing Carmel River diversions in excess of CalAm’s legal entitlement of 3,376 acre feet per year (afy), reducing pumping from the Seaside Groundwater Basin from approximately 4,000 afy to 1,474 afy, and paying back the Seaside Groundwater Basinby approximately 700 afy over 25 years.
Early ocean desalination efforts relied on the simple and cost-effective solution of deploying an open pipeline intake in the ocean. This solution created adverse impacts on marine life, prompting the search for less intrusive alternatives, which gave rise to slant wells. A slant well exploits the transition zone between saltwater and groundwater aquifers, with the goal of maximizing the intake of saltwater and minimizing the impact on freshwater aquifers.
CalAm turned to Geoscience, who had already piloted a successful project for the Municipal Water District of Orange County that deployed the first ever successfully constructed, artificially-filter-packed slant well below the ocean floor.
Tests showed that the solution drew 95 percent ocean water and only 5 percent groundwater. A second efficiency emerged as tests showed that, unlike pipeline solutions, slant wells don’t introduce silt density and turbidity issues, eliminating the need for expensive pretreatment.
Another significant goal focused on sustainability. The initial test ran for a week and hit all targets. Continued testing extended for three years with similar results, demonstrating that the solution was sustainable over the long term and produced more-than-adequate capacity.
The final advantage contrasted the throughput of a slant well over a vertical well. For any given thickness of an aquifer, a slant well greatly increases the exposed screen for intake, maximizing throughput.
Contrasted to the legacy method of pumping from freshwater aquifers and aggravating saltwater intrusion, slant wells reverse the process, restoring the seaward flow of freshwater toward the sea, reducing saltwater intrusion while achieving volume and sustainability targets.
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