AWWA CO50840

One of the most serious concerns for recovering drinking quality water from Aquifer Storage and Recovery (ASR) wells in the aquifers of the Atlantic Coastal Plain is the dissolution of metal-bearing minerals upon contact with recharge water containing dissolved oxygen (DO). Dissolution of metal-bearing minerals can release elevated concentrations of metals, particularly iron and manganese into the stored water. Often native groundwater is devoid of dissolved metals, providing little warning of a possible problem. Metal-bearing sulfides and carbonates such as pyrite (FeS2) and siderite (FeCO3) are relatively pervasive in the sand aquifers of the Atlantic Coastal Plain. As an example, analysis of core samples from the Upper Potomac-Raritan-Magothy (PRM) Aquifer at the Evesham Municipal Utilities Authority's Kings Grant site, an ASR facility in New Jersey, indicated that pyrite comprised 1 percent of the bulk rock matrix. Test cycles were performed with, and without pretreating recharge water with a chemical agent to stabilize pyrite in situ. In addition to elevating pH of the recharge water above the solubility limit of pyrite, the OH radical in NaOH reacts with pyrite to form ferric oxyhydroxide coating. This coating isolates the mineral grain from the aquifer environment, preventing dissolution. This coating also adsorbs metal ions including dissolved iron (as a surface precipitation reaction migrating in the groundwater. Includes 4 references, figures.