Laboratory Evaluation to Increase Effectiveness of Field-Scale Soil Flushing in the Hanford 100 Areas

Abstract

This laboratory study was initiated to develop an improved technical understanding of the key controlling geochemical and physical processes of mobilizing residual Cr(VI) in Hanford sediments to provide increased effectiveness in soil flushing activities planned for Hanford 100 Areas. This was accomplished by (1) quantifying Cr mass, release rate, and surface phase changes as Cr is leached from different Hanford sediments; (2) maximizing Cr leaching during soil flushing by evaluating different leach solutions and leach strategies; and (3) minimizing leaching of remaining residual Cr after soil flushing ends. To quantify geochemical controls on Cr leaching from sediments, sequential liquid extractions were used to identify aqueous, adsorbed, and solid Cr surface phases (precipitates or Cr incorporated into other phases) before and after water-saturated and unsaturated leach experiments with pH 8 artificial groundwater and with other amendment solutions. Changes in the Cr release rate from sediments were also correlated to changes in Cr surface phases. During unsaturated infiltration, water advection occurs primarily through larger pores, with less movement through smaller grain layers leading to a slower release of Cr trapped in smaller pores next to smaller grains. To quantify these physical controls on Cr leaching from sediments, 10 ft high 1-D infiltration experiments were conducted at differing leach solution application rates, and Cr leaching during application and subsequent residual water flow was quantified. Qualitative 2-D experiments were also conducted to evaluate surfactant addition to increase flow in low-permeability zones. Cr-contaminated Hanford sediments from the 100-H, 100-K, 100-D, and 200 East Areas exhibited a wide variety of leach behavior from rapid release of a small amount of Cr to fast (i.e., minutes) and slow (i.e., hours or more) release of high Cr. Sediments with low labile Cr (< 0.2 µg/g) exhibited fast Cr release but less overall release, and most (>95%) leached from the sediments within a few pore volumes in water-saturated columns. With nearly no sorption of chromate, aqueous and sorbed Cr (as chromate) were released quickly from sediment within the first few pore volumes, resulting in high leach concentrations, fast initial Cr release rates, and changes in pre- and post-leach extractions. In contrast, sediments with high labile Cr (0.2 to 20 µg/g) exhibited a combination of fast and slow Cr release due to multiple Cr surface phases contributing to Cr release from the sediment at different rates with greater overall release. This resulted in elevated Cr effluent concentrations even after dozens of pore volumes were leached. In addition, the Cr release rate decreased with increasing time during leaching. A decrease in labile Cr (aqueous Cr, adsorbed Cr, pH 5 acetate dissolved precipitates) in post-leach extractions compared to pre-leach extractions showed initial aqueous and adsorbed Cr release, followed by dissolution of high-solubility precipitates such as CaCrO4 (if present), then slow dissolution of calcite that may contain some chromate and possibly BaCrO4. The labile Cr in all sediments correlated well with the Cr release rate (r2 = 0.81). Labile Cr also correlated well with the leached mass in water-saturated columns (r2 = 0.98), and therefore can be used for prediction of potential removal with flushing applications.