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SOURCE Summer 2012

Advantages of Slant Well vertical leakage through the sea floor and Management of Feedwater Salinity Feedwater Supplies horizontal flow from offshore aquifers. As with Different Angled Slant Wells Slant well feedwater supply systems present such, a constant and reliable source of feed- A system of slant wells can also mini- significant advantages over traditional water to desalination plants can be utilized. mize variation in feedwater salinity (due open ocean intakes or infiltration galleries In addition, the offshore sand and gravel to hydrologic variations of the fresh-salt including: deposits provide a natural filtration, which water interface), by varying the physical eliminates particulate matter eliminating layout and angles (below horizontal) • Avoidance of entrainment and the need for pretreatment to the RO system. of individual slant wells or groups of impingement impacts to marine life; slant wells (e.g. arrays). Shallow angle • Elimination of costly reverse osmosis Slant Well Design Parameters slant well arrays can produce water with pretreatment; For feedwater supply wells producing from higher salinity to stabilize the feedwater subsea or near shore aquifers, the single quality during wet hydrologic cycles (i.e. • No ocean construction impacts; and most important well design parameter is by mixing with the more saline portions • No permanent visual impacts. prevention of fine-grained materials (sand of the subsurface aquifer). Steeper angled and silt) from entering the well. Proper well slant wells arrays can produce water with Geohydrologic Considerations design and construction maximizes aquifer lower salinity to stabilize the feedwater In most coastal areas, numerous rivers and production and stabilizes fine-grained quality during dry hydrologic cycles (i.e. by streams discharge directly to the ocean. materials preventing migration of sand mixing with the less saline portions of the Over geologic time, offshore deposits have to the well and pumping system 5. All subsurface aquifer). Figure 2 illustrates the accumulated beneath the ocean forming wells that follow Terzaghi’s migration and concept of feedwater salinity management permeable subsea aquifer systems. The permeability design standards for artificial using different angled slant wells. recharge to a slant well system completed in filter pack design are suitable for desalina- subsea aquifers contains a high percentage tion feedwater supply systems and will not Continued on page 26 of ocean water with recharge from both require pretreatment. www.ca-nv-awwa.org 25


SOURCE Summer 2012
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