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Desalination, Continued from page 25 Figure 2: Management of Feedwater Salinity Figure 3: Dana Point Test Slant Well and Full Scale 30 mgd Feedwater Supply Dana Point Test Slant Well slant well was constructed off of Doheny of 1,000 ft or more. Variation of feedwater The Municipal Water District of Orange beach in 2006 in order to obtain measure- salinity can be accomplished by varying County (MWDOC) identified a favorable ments of subsurface intake discharge rates, the location and/or angle of slant wells to site for evaluating the feasibility of using aquifer properties, and water quality varia- adapt to a changing fresh water-salt water a subsurface intake system for a 30 mgd tion through performance. Data from an interface. ocean desalination supply at Doheny State 18-month pilot pumping test was recently Beach near the mouth of San Juan Creek, completed and is being used to validate References in Dana Point, California 6. Subsurface ground water model predictions regarding 1. ASTM D 4189-95, 2002. Standard Test deposits of sand and gravel associated with water level changes and feedwater salinity. Method for Silt Density Index (SDI) of the San Juan Creek channel extend off shore These data will also be incorporated into Water. beneath the ocean and provide the needed design of the full-scale RO feedwater 2. Williams, D.E., 2011. Design and filtration to eliminate pre-treatment to the supply system, which will include seven Construction of Slant and Vertical Wells for RO plant. A 350 ft long 12 in. diameter test 1,000 foot-long slant wells (plus two Desalination Intake, Proceedings of the IDA standby wells), with each well producing World Congress, Perth, Western Australia 3,000 gpm (see Figure 3). September 4-9, 2011 Summary and Conclusions 3. Williams, D.E., 1985. Modern Techniques Proven design practices and procedures in Well Design, J.AWWA, 77 developed in the water well industry 4. Roscoe Moss, Co., 1990. Handbook of for vertical wells over the past seven Ground Water Development, J. Wiley & decades are equally applicable to slant Sons wells completed in subsea aquifers for desalination feedwater supply. Near shore 5. Terzaghi, K., and Peck, R.B., 1948. Soil and offshore permeable aquifer systems Mechanics in Engineering Practice. J. Wiley provide the sustainable supplies with low and Sons. silt density indexes eliminating the need for pre-treatment. Subsurface intake systems 6. Williams, D.E., 2008. Horizontal Well also eliminate impingement and entrain- Technology Application in Alluvial Marine ment issues and other environmental Aquifers for Ocean Feedwater Supply and impacts associated with conventional Pretreatment, prepared for State of California intake systems. Silt density indices are Department of Water Resources/Municipal well below the RO design standard for Water District of Orange County. S wells, which have applied Terzaghi’s migra- tion and permeability design standards. Dennis E. Williams, Ph.D., is President of The dual rotary method of drilling allows GEOSCIENCE Support Services, Inc., La Verne, construction of artificially filter packed slant CA. He can be reached at 909-451-6650 or wells beneath the ocean floor to lengths dwilliams@geoscience-water.com. 26 SOURCE summer 2012


SOURCE Summer 2012
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