Precipitate Formation Potential of Resin Regeneration Effluent in the 100-HR-3 Operable Unit

Abstract

Calculations performed as part of this study indicate that injection of treated groundwater containing treated regenerant solution has a high potential for precipitate formation that could lead to plugging of formation porosity surrounding the injection well. Precipitate formation could lead to substantial loss in porosity on an annual basis at high injection rates, but is minimal at low injection rates. In the worst case scenario, substantial plugging could occur within a year of the initiation of injection. For example, assuming an injection rate of 600 gallons/minute, 24 hours/day and 365 days/year, an annual porosity loss of 7.3% is predicted for an initial solution pH of 7.0. At pH 8.0, the annual porosity loss would increase to 16% per year. For cases in which solution injection is assumed to occur at a rate of 600 gallons/minute, 15 hours/day and 1 day/week, the annual porosity loss drops considerably. For these assumptions, the annual porosity loss is 0.65% for an initial solution pH of 7.0 and 1.4% for an initial solution pH of 8.0. Some uncertainty is associated with respect to these analyses. The uncertainty results from the fact that equilibrium with the most stable mineral assemblage cannot always be assumed and that slow precipitation rates could occur and reliable estimates of precipitation kinetics under Hanford aquifer conditions are not available. It is recommended that the potential of calcium phosphate precipitation be investigated further using a combination of laboratory and field investigations to reduce this uncertainty.