On Modeling pH Buffers and Rebound at An Acid-Contaminated Site.

From 1955 to 1989, low-level radioactive acid waste solutions were disposed into seepage basins at the F-Area of the Savannah River site, South Carolina. As a result, an acidic plume has developed in groundwater beneath the basins. The plume contains many contaminants, including various U isotopes, Sr-90, I-129, Tc-99, tritium, and nitrate, with pH values as low as ~3. After a decade of active remediation, natural attenuation is now being considered as a long-term remediation alternative at this site. 

Although the seepage basins were solidified and capped around 1990, groundwater pH is currently ~3.5 for several hundred meters along the acidic plume axis.  This slow rebound (for nearly 20 years) has taken place despite fast groundwater velocities (~100–150 m/y). As part of a larger study aimed at predicting contaminant behavior at the plume scale, exploratory reactive transport simulations were run to assess reactions potentially controlling pH at this site. Investigated mechanisms include the sorption and/or exchange of H⁺ with the sediments (primarily quartz, kaolinite and goethite), as well as the precipitation of Al silicates, hydroxides, and sulfates. Simulations indicate that H⁺ sorption/exchange could buffer pH at the site for very long periods of time. Another strong pH buffer could result from the potential presence of Al sulfates (jurbanite) and silica below the basins, formed by the dissolution of kaolinite during the period of active disposal.  After closure, these three phases are predicted to buffer pH around 3.5 until Al sulfates have fully re-dissolved.  Simulations results are sensitive to the relative rates of mineral precipitation and dissolution, to the relative rates of reaction versus recharge, and to the type of H⁺ sorption model and parameters.  Efforts are currently underway towards sediment characterization and laboratory experiments to further understand pH controlling mechanisms and to better constrain model predictions.