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Injection well. An injection well is the opposite of an extraction well in that water is pumped into the well and returned to the aquifer. Water moves out from the well into the unsaturated soil or groundwater, depending on the location of the well screen. An injection well may be used instead of an infiltration gallery where the water must be injected to a specific desired subsurface location. Injection wells may be constructed similar to extraction wells, but a discharge pipe extends into the water instead of a pump. Wells are subject to clogging as the injected water mixes with formation water and velocities drop. Typically, the specific capacity for injection is less than 50 percent of the specific capacity for extraction from a well. To compensate for the reduced efficiency, more wells or greater head must be i supplied. f The high TDS content and hardness of groundwater at the Disco site indicates that injection wells may clog rapidly and irreversibly. For this reason, injection wells are not recommended. Infiltration Galleries. After above-ground groundwater treatment, groundwater may be reinjected back into the subsurface via an infiltration gallery. The infiltration gallery is similar to a leach field, consisting of a gravel-filled trench through which water may infiltrate into the soil and eventually the groundwater. The infiltration rate is determined by percolation tests or estimated from saturated hydraulic conductivity values (from pumping tests on wells). The gallery is designed to allow all water to percolate at a unit gradient (i.e., no applied head); the soil remains unsaturated. Because infiltration galleries can be easily constructed with excess capacity, they are j generally less susceptible to clogging than injection wells. Infiltration galleries are also cheaper to construct and operate and can be easily enlarged if necessary. For these reasons, an infiltration gallery is recommended for the Disco site. i t Remedial Alternatives �. The above described options can be combined into remedial alternatives for the Disco f site. All recommended alternatives include: • Groundwater monitoring • One sparging well in the contamination source area to stimulate biodegradation and volatilization of hydrocarbons at and above the water table Groundwater extraction wells in and downgradient of the source area to icontrol migration and remove contaminated water SF0316041RP1007.51 4-6