A deep dive into groundwater, desalination

Marina Beach north of Monterey, near the site of a planned desalination plant. (Photo: Marina Coast Water District)

At the height of the recent drought, the legislature passed and Gov. Brown signed legislation, the Sustainable Groundwater Management Act (SGMA), that for the first time required California water agencies to account for groundwater pumping and held them accountable for the development of sustainable plans for the future. Groundwater accounts for approximately 30% of the state’s water supply.

For many, perhaps most water agencies, this is a very difficult task because the only data provided about the basins and how they flow is provided by wells that are, at best, sporadic providing an incomplete picture.

Along California’s pristine Monterey Bay, groundwater accounts for 99% of the supply and decades of over drafting has left local leaders fighting saltwater intrusion as a result.

Researchers at Stanford University, working with Aqua Geo Frameworks, have deployed an exciting technology, geophysical imaging, that allows a complete three-dimensional mapping of a groundwater basin that is akin to the electromagnetic imaging of the human body. They are able to use magnetic and electrical fields to probe up to 1,000 feet into the ground and measure the electrical resistivity.

Very simply, the most resistive saturated material will be a sand or gravel saturated with freshwater and the least resistive material will be a sediment saturated with saltwater. In Marina, the data were gathered using a large hexagonal frame suspended below a helicopter that methodically flew a 350-mile grid over the groundwater basin; the study also included integrating information from over 300 wells in the area to further confirm the accuracy of the information that was produced.

“The technology that we have employed, to map the subsurface in the Monterey Bay area, is a well-established way to measure the electrical resistivity. The data that were acquired, and our approach to interpretation, have revealed incredible detail about the variation in water quality in the area; information that is essential for sustainable groundwater management,” said Professor Rosemary Knight, Ph.D, of Stanford’s School of Earth, Energy & Environmental Sciences.

Along California’s pristine Monterey Bay, groundwater accounts for 99% of the supply and decades of over drafting has left local leaders fighting saltwater intrusion as a result. One of the key findings uncovered by the Stanford research was that in the working-class seaside community of Marina and the Ord Community (site of the former military base), the Monterey Subbasin was not as intruded by saltwater as had long been thought and contained usable water.

The new data could also potentially jeopardize a planned seawater desalination plant dubbed the Monterey Peninsula Water Supply Project (MPWSP).

More importantly, the study confirmed there is natural recharge in the area that is acting as a barrier in preventing further seawater intrusion.

Turning long believed conventional wisdom on its head is not an easy endeavor, but the results of the Stanford study show the facts are the facts and that natural recharge is occurring in the area which acts as a seawater intrusion barrier and a source of drinking water for refilling our aquifers.

This is a significant finding for our local community and the Marina Coast Water District (MCWD). We are a recognized Groundwater Sustainability Agency (GSA) for the Monterey Subbasin per SGMA and are responsible for developing the sustainability plan which must meet the state mandated goal of meeting groundwater sustainability within 20 years. The Stanford research, which contains important information in developing our sustainability plans, clearly showed the shallow aquifer (Dune Sand Aquifer) flowing seaward and mounding at the coast to create a barrier and then percolating into the lower 180 and 400-foot aquifers that flow inland.

The new data could also potentially jeopardize a planned seawater desalination plant dubbed the Monterey Peninsula Water Supply Project (MPWSP). The MPWSP, proposed by the California American Water Company, would be located on Marina’s coast and would supply up to 6.4 million gallons of water per day for communities of Carmel and Monterey about 5 miles south of the proposed project location.

At minimum, the proposed MPWSP will pump over 15,000 AFY from groundwater wells located in the Dune Sand and 180-foot aquifers, right at the coast where the groundwater basis is most susceptible to seawater intrusion, and right at the location of the existing barrier that is protecting the basin.  Considering MCWD pumps just over 3,300 AFY, from wells much further inland, to serve its current customers in the Marina and Ord Communities, the amount pumped from the MPWSP is massive and at a location that would destroy the beneficial barrier that currently exists.

As part of the long, complex and bureaucratic permitting process, the MPWSP received permission from the California Coastal Commission to operate a test slant well intended to draw subsurface seawater as a potential source for the planned MPWSP.

That test well intake is located in the Monterey Subbasin that the Stanford research clearly showed contains groundwater that is suitable as a source of drinking water and a dedicated barrier, both of which are extremely beneficial sources of water that MCWD depends upon to serve its current and future customers. In other words, data consistently show that the test slant well is drawing usable water from the MCWD Monterey Subbasin.

We strongly believe the single most important job of MCWD is to look out for the interests of our ratepayers and protect our single greatest asset, our water supply. To do that, we need to work collaboratively to identify the best possible sources of future supply and the MPWSP as currently envisioned is not the answer.

The Monterey Region is faced with a limited water supply which is hastened by a cease and desist order issued by the State Water Resources Control Board against California American Water for its overpumping from the Carmel River. However, MCWD and others argue recycled water, storm water capture and existing supplies are a far more feasible option.

Properly situated, desal can be considered a viable option as costs to produce an acre foot of desalinated water are coming down. However, it remains among the most expensive water to produce and could leave residents served by the MPWSP with a significant increase in their water bills that could be as high as $100 a month or more if completed.

Ed’s Note: Keith Van Der Maaten is the general manager of Marina Coast Water District.

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