Monterey boreholes reveal good slant-well conditions
California American Water (Cal Am) reported on 20 December 2013 that data from its ongoing investigation into subsurface slant well intakes for its proposed Monterey desalination plant were showing promising results.Over the last four months, Cal Am has drilled eight geotechnical boreholes in three areas along the Monterey coast in its study of preferred sites for a subsurface intake for its Monterey Peninsula Water Supply Project. This will be based on a seawater desalination plant with a capacity of 6.4‑9.6 MGD (24,000‑36,000 m³/d).
Two of the three drilling sites - Potrero Road and further down the coast on property owned by Cemex Ltd - show highly favorable conditions for locating the subsurface slant wells. The Cemex boreholes indicated an almost continuous layer of sands and gravels to a depth of 240 ft (73 m). The Potrero Road boreholes revealed a thick layer of clay at a depth of approximately 140 ft (43 m), indicating a separation, also known as an aquitard, between the proposed ocean intake zone above and the lower aquifers.
"The results thus far are very promising," said CalAm director of engineering Rich Svindland. "At the northern site near Potrero Road, we have a nice, deep layer of sand filled with salt water with a well-formed aquitard below, which, if drawn from, would likely avoid impacts to the Salinas Basin. At the Cemex site we have a very thick sand layer below the ocean floor which will work nicely for the subsurface slant well sea water supply."
The third site, at Moss Landing, was not as promising. There, intermittent clay layers mixed with silt and fine sand, without enough continuous sand layers to use any type of subsurface intake system efficiently. Although Moss Landing was not one of Cal Am's preferred intake locations, the company agreed to drill boreholes there as part of a settlement agreement to study alternative sites proposed by private developers.
"Now that we have affirmed sufficient geological conditions, we will install a test slant well under the ocean floor to assure we have suitable water flow and quality for a fully operational desalination plant," Svindland added. Additional onshore monitoring wells will be drilled in and around the test slant well site to monitor the well's effects on surrounding groundwater aquifers.
California American Water has proposed the variable-sized desalination facility as part of a three-pronged project to address the Monterey Peninsula's impending water supply shortage. The proposal also includes aquifer storage recovery and recycled water projects, which are presently advancing in planning and development.