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Description
Ocotillo Wells is a small desert town 85 miles northeast of San Diego. A ground-water contamination problem in the uppermost drinking water aquifer was first suspected by a homeowner in September 1986. Ground-water sampling has identified the presence of dissolved aromatic hydrocarbon constituents characteristic of gasoline in monitor wells, public supply and domestic supply wells in the town. In this study a ground-water extraction system has been designed to remediate the drinking water aquifer by hydraulic containment of the ground-water plume. The impacted aquifer is under unconfined conditions and consists of fine to very coarse-grained gravelly sand. Depth to ground water ranges from 97 to 125 feet below ground surface. The flow of ground water underlying Ocotillo Wells is toward the south-southeast with a variable gradient ranging from 0.020 to 0.006 feet/foot in the area of contamination. The average of pump test data yields a transmissivity value of 1500 ft2/d. Repeated ground-water sampling and analysis of the monitor wells in Ocotillo Wells indicates an increase in the lateral extent of the plume with each sampling event. Toluene is the aromatic compound that has been measured to be the most widespread contaminant in the aquifer. Therefore the shape of the toluene plume has governed the area of the aquifer that needs to be intercepted by ground-water extraction. The chemical mobility of the different organic compounds detected in the aquifer was assessed. It was estimated that sorption, the partitioning of a solute between the liquid and solid phase, would have the greatest affect on contaminant behavior and transport in the ground water. The degree of sorption was quantified through the calculation of a retardation factor. Benzene is the least retarded and ethylbenzene is the most retarded of the organic compounds. A retardation factor for toluene of 1.52 was utilized in the extraction system design. Two computer programs, THEIS and GWPATH, were used in succession to predict drawdown at the extraction wells and elsewhere in the aquifer, and to estimate horizontal fluid pathlines in the aquifer. After multiple simulations, an extraction design of 3 pumping wells, located downgradient of the plume and pumping at 20 gpm for 2 years was chosen.
