Multiple Advantages of Slant Wells for Ocean Desalination Feedwater Supply

 

Seawater desalination is a viable alternative for coastal communities due to limited or poor inland ground water quality and a decreasing reliability of imported water supplies.  Cost effective and efficient subsurface intake systems use low angled wells (slant wells) which produce ground water from offshore subsea aquifer systems.  These subsurface intakes provide a number of significant advantages over open ocean intakes including:  lower unit cost, avoidance of entrainment and impingement impacts to marine life, reduction of costly reverse osmosis (RO) pretreatment, no ocean construction or permanent visual impacts, salvage of ground water losses to the ocean, and seawater intrusion control.  Slant wells can also minimize variation in feedwater salinity (due to hydrologic variations of the fresh-salt water interface), by varying the physical layout and vertical angles (below horizontal) of individual slant wells or groups of slant wells (e.g. arrays).  Shallow angle slant well arrays can produce water with higher salinity to stabilize the feedwater quality during wet hydrologic cycles (i.e. by mixing with the more saline portions of the subsurface aquifer).  Steeper angled slant well arrays can produce water with lower salinity to stabilize the feedwater quality during dry hydrologic cycles (i.e. by mixing with the less saline portions of the subsurface aquifer).  This flexibility will become increasingly important due to future climatic changes and sea level rise.  Subsea aquifer systems provide natural filtration from suspended organic matter and sediment particularly during storm surges and heavy precipitation.  An 18 month pilot test is currently underway as part of the South Orange Coastal Ocean Desalination Project--a phased program designed to provide a 30 mgd desalination feed water supply.  Located beneath the sands of Doheny Beach near Dana Point in Southern California, a 350 ft long slant well was completed in the subsea aquifer system to depths of approximately 140 ft.