AQUIFER TREATMENT METHODS FOR PREVENTING IRON CONCENTRATIONS IN THE RECOVERED WATER FROM AQUIFER STORAGE AND RECOVERY WELLS INSTALLED IN CLAYEY SAND AQUIFERS OF THE ATLANTIC COASTAL PLAIN

摘要

Reduced iron-bearing minerals in the matrix of clayey sand aquifers in the Atlantic Coastal Plain present unique geochemical problems for recovering drinking quality water below the Federal Drinking Water Standard of 0.3 mg/l for iron from Aquifer Storage and Recovery (ASR) wells. During ASR recharge, these minerals are exposed to oxygenated water, producing dissolved iron, decreasing pH, and increasing or decreasing alkalinity and sulfate depending on the predominance of pyrite (FeS_2) or siderite (FeCO_3) in the aquifer matrix.rnTwo methods for stabilizing iron-bearing minerals insitu have been tested at three ASR sites in New Jersey. The first method involved pretreating the aquifer with 35 million gallons of low pH (less than 2.0), deoxygenated water to dissolve siderite insitu. Unfortunately, recovered iron concentrations were above 0.3 milligrams per liter (mg/l), along with low pH and increasing sulfate. This pretreatment method was successful in removing siderite, but the low pH treatment stripped existing ferric oxyhydroxide coatings on pyrite, making the mineral more reactive during subsequent recharge and recovery cycles.rnThe second method consisted of increasing pH of recharge water from 7.5 to 8.6 with sodium hydroxide. The hydroxyls react with pyrite and siderite to form ferric oxyhydroxide coating, preventing further reaction with the oxygenated recharge water. Furthermore, the coating adsorbs dissolved iron migrating in the storage zone. Over ten ASR cycles at the three sites, with storage volumes from 3 to 175 million gallons, the recovered iron concentrations have been below the Federal Standard. Other chemical parameters (major cations and anions, pH, Eh and manganese) indicate recovered water quality improves and that aquifer mineralogy is progressively oxidized with successive cycles.