Background/Objectives: Examining geochemical principles in action in groundwater upwelling through an amended sediment layer can improve anticipated amendment performance.
At the Site being studied, historical Solvay process by-products have created elevated pH conditions in groundwater proximal to a nearby water bodies. Siderite (FeCO3) is an effective treatment method for buffering elevated pH. For example, this approach is being applied at the Onondaga Lake remediation project where a Siderite cap has been installed to reduce pH in water upwelling into the lake. Siderite buffers pH by dissolving into groundwater where dissolved Fe2+ ions from siderite react with hydroxide ions in solution forming iron oxy-hydroxides and thus lowering pH. At the site being studied, the groundwater's geochemistry was limiting the effectiveness of the siderite pH buffering in treatability tests.
Approach: Analyzing the geochemistry of the groundwater at the Site we identified carbonate (CO32-) alkalinity as the factor limiting siderite efficacy. The presence of CO32- limits siderite solubility via the common ion effect, and hence the efficacy of siderite pH buffering. We developed an enhanced approach based on geochemical principles that effectively addressed and restored siderite buffering capacity in elevated pH waters with increased carbonate alkalinity and conducted column testing to demonstrate the geochemical principles in action. Adding gypsum to siderite increased the calcium ion concentration in solution which in turn precipitated with the elevated carbonate alkalinity as CaCO3 reducing the CO32- concentration allowing more Siderite to dissolve and improving the buffering capacity of the system.
Results: This presentation will detail the experimental approach and present comparative data showing the improved performance of Siderite combined with Gypsum in reducing pH in water from a Site with elevated pH.