Original Date: 09/14/1998
Revision Date: 01/18/2007
Best Practice : Pressurized Recharge Wells
Technology has improved since Raytheon Missile Systems Company (RMSC) implemented the Installation Restoration program at AFP44. To obtain the best performance out of its existing equipment, the company developed several cutting-edge cleanup practices for recharge wells. Two such improvements are re-engineering the well heads to inject pressure into the wells, and removing some of the control mechanisms at AFP44.
RMSC’s recharge wells were originally constructed with computer-actuated valves which controlled the flow of water into the wells. The computer received information on the well’s water level via a pressure transducer, and then opened or closed the valve to maintain the water level at 15 feet below ground surface. However, various situations can interfere with this process. Since water flows in under the force of gravity, a computer or valve malfunction can cause a well to overflow water onto the ground. A well can also lose its recharge capacity due to the clogging effects of calcium carbonate, so every year, RMSC had to redevelop its wells at a cost of $1,800 a piece. Prior to the injection step, the water at AFP44 was pH-adjusted with sulfuric acid and filtered through sand filters. RMSC theorized that the calcium carbonate was precipitated in the well as the water fell in the injection tubing. The precipitation occurred because the water would be under vacuum as it fell and carbon dioxide would be lost, thus changing the carbonate balance. To counteract this effect, the company converted the wells so they could be injected under pressure. This process involved redeveloping the wells one more time; removing the level sensors and computer-controlled valves; sealing the well heads; and adding air relief valves to the casings. With these changes, the water is now delivered under pressure to the wells from computer-controlled pumps at the treatment plant. Since converting the wells in 1991, RMSC has not needed to redevelop a single well, and has experienced only a small drop in recharge capacity at each well.
The pump and treat groundwater remediation system at RMSC also uses pressurized recharge wells to return treated groundwater to the aquifer along the margins of the contaminant plume. The remediation system is designed to treat groundwater contaminated with trichloroethylene (TCE) and 1,1-dichloroethylene (DCE) which is pumped from a two-square mile plume. The system operates at 3,000 gpm, and must reinject this water continuously into ten recharge wells. Most recharge wells are 180 to 200 feet in depth and have a water table at 90 to 120 feet below the surface. The wells are gravel packed and constructed with well screens that have openings of 0.08 to 0.125 inch. The three- or four-inch diameter injection pipes extend below the water table. Water is injected into the wells under pressure, with a maximum of 15 pounds per square inch as measured at the well head. The wells are equipped with air relief valves at the top of the casings, allowing air to flow in and out of the top of the well, but preventing water from flowing out. The valve performs two functions: (1) lets air out when the well is turned on, allowing the water level to rise in the casing, and (2) lets air in which breaks the vacuum when the well is turned off, allowing the water level to fall in the casing.
These modifications have enabled RMSC to save $1,800 per well every year since 1991. By removing the computer-actuated valves and level sensors, the company eliminated surface spills, and simplified its maintenance and operation. RMSC’s injection well modifications could produce similar benefits in other injection well situations, if the water to be reinjected contains enough calcium carbonate to cause precipitation problems, and if the water has been filtered and pH-adjusted below 8.0. Under these conditions, pressurized recharge will maximize the sustained recharge rate into a well and minimize the need for well redevelopment.
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