PNNL

Abstract

A groundwater plume containing uranium, originating from a combination of purposeful discharges of wastewater to cribs, trenches and ponds, along with some accidental leaks and spills during nuclear fuel fabrication activities, has persisted beneath the Hanford Site 300 Area for many years. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, the goal of less than 30 µg/L has not been achieved within the anticipated 10-year time period. Polyphosphate technology has been demonstrated to delay the precipitation of phosphate phases for controlled in situ precipitation of stable phosphate phases to control the long-term fate of uranium. Precipitation occurs when polyphosphate compounds hydrolyze to yield the orthophosphate molecule. Based on the hydrolysis kinetics of the polyphosphate polymer, the amendment can be tailored to act as a time-released source of phosphate for lateral plume treatment, immediate and sustained remediation of dissolved uranium, and to preclude rapid precipitation which could result in a drastic change in hydraulic conductivity of the target aquifer. Critical to successful implementation of polyphosphate remediation technology is a site specific evaluation and optimization of multi-length polyphosphate amendment formulations. A multi-faceted approach has been taken to provide key fundamental science knowledge regarding optimization of the polyphosphate remedy through: 1) phosphorus-31 nuclear magnetic resonance to quantify the effects of Hanford groundwater and sediment on the degradation of inorganic phosphates, 2) static tests to quantify the kinetics, loading, and stability of apatite as a long-term sorbent for uranium, and 3) single-pass flow through testing to quantify the stability of autunite and apatite under relevant site conditions. Dynamic column tests were utilized to 1) optimize the composition of the polyphosphate formulation for the formation and emplacement of apatite and autunite, 2) understand the rate and extent of reaction between polyphosphate and uranium-bearing phases, 3) evaluate the effect of chemical microenvironments on the degradation of polyphosphate and the formation of autunite, and 4) quantify the mobility of polyphosphate as a function of water content. These activities are being conducted in parallel with a limited field investigation, to more accurately define the vertical extent of uranium in the vadose zone, and in the capillary fringe laterally throughout the plume.