Geochemical Characterization of Chromate Contamination in the 100 Area Vadose Zone at the Hanford Site

Abstract

The major objectives of this study were to 1) determine the leaching characteristics of hexavalent chromium [Cr(VI)] from contaminated sediments collected from 100 Area spill sites; 2) elucidate possible Cr(VI) mineral and/or chemical associations that may be responsible for Cr(VI) retention in the Hanford Site 100 Areas through the use of macroscopic leaching studies, and microscale characterization of contaminated sediments; and 3) provide information to construct a conceptual model of Cr(VI) geochemistry in the Hanford 100 Area vadose zone that can be used for developing options for environmental remediation.In addressing these objectives, additional benefits accrued were as follows: 1) a more complete understanding of Cr(VI) entrained in the vadose zone that can be utilized in modeling potential Cr(VI) source terms; and 2) accelerating the 100 Area Columbia River Corridor cleanup by providing valuable information to develop remedial action based on a fundamental understanding of Cr(VI) vadose zone geochemistry.A series of column experiments were conducted with contaminated and uncontaminated sediments to study Cr(VI) desorption patterns in aged and freshly contaminated sediments; evaluate the transport characteristics of dichromate liquid retrieved from old pipelines in the 100 Area; and estimate the effect of strongly reducing liquid on the reduction and transport of Cr(VI). Column experiments used the <2-mm fraction of the sediment samples and simulated Hanford Site groundwater solution. Periodic stop-flow events were applied to evaluate the change in elemental concentration during time periods of no flow and greater fluid residence time. The results were fit using a two-site, one-dimensional reactive transport model. Sediments were characterized for the spatial and mineralogical associations of the contamination using an array of microscale techniques including X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray microprobe, and X-ray absorption near-edge structure.The following are conclusions and implications:1. Results from column experiments indicated that most of the contaminant chromium traveled quickly through the sediments and appeared as Cr(VI) in the effluents. However, the fine-grained surface coatings on sediment clasts acted as a porous but restricted medium that was accessible to chromate by diffusion from migrating chromate-laden water.2. The Cr(VI) concentration remained above the drinking water standard of 100 μg/L for many pore volumes. However, the significance of this for groundwater concentrations would depend on the mass flux of recharge to the water table.iii iv 3. Adsorption of Cr(VI) to sediments from spiked Cr(VI) solution was low; calculated retardation coefficients were close to one. During desorption experiments, sediment-dependent tailing was observed.4. Results from column experiments conducted with a strong reductant, such as calcium polysulfide solutions, to characterize and measure solution and sediment r...

Geochemical Characterization of Chromate Contamination in the 100 Area Vadose Zone at the Hanford Site - Part 2

“…Microscopic characterization indicated that Cr was found on grain coatings but some Cr was associated with individual "hot-spots" and in altered minerals. Dresel et al (2008) recently published a report on the EM-funded work.…”

“…The pH of these solutions, buffering capacity, and counter-ion concentration is critical to Na 2 Cr 2 O 7 solution vadose zone geochemistry. While the exact acidity of Hanford Site Cr stock solutions is not known, a 10% Na 2 Cr 2 O 7 (0.82 mol L -1 Cr) has a pH of 3.5, and a 70% Na 2 Cr 2 O 7 (8.96 mol L -1 Cr) may be lower (~1.5 to 2) (Dresel et al 2008). …”

“…The transport of chromate [Cr(VI)O 4 2-] through uncontaminated sediments is not expected to exhibit retardation due to adsorption (sediments may exhibit minimum adsorption capacity under the given conditions of neutral or slightly basic pH because all variable charge sorbents may be slightly negatively charged) (Qafoku et al 2003(Qafoku et al , 2007(Qafoku et al , 2009(Qafoku et al , 2010Ginder-Vogel et al 2005;Dresel et al 2008). However, as clearly shown in some of these references (e.g., Qafoku et al 2003Qafoku et al , 2009Qafoku et al , 2010Dresel et al 2008) A series of column experiments were conducted to investigate Cr(VI) mobility during advective transport under saturated and unsaturated conditions to test the following hypotheses:…”

“…However, as clearly shown in some of these references (e.g., Qafoku et al 2003Qafoku et al , 2009Qafoku et al , 2010Dresel et al 2008) A series of column experiments were conducted to investigate Cr(VI) mobility during advective transport under saturated and unsaturated conditions to test the following hypotheses:…”

Foam Transport in Porous Media - A Review

“…However, drawbacks to delivering amendments to heterogeneous porous media, with or without surfactants, include difficulty in containing amendments to the treatment zone and the potential for desorbed or surfactant-dissolved contaminants migrating to groundwater (Hanson et al 1993;Dresel et al 2008). Because preferential flow paths of the solution in the vadose zone may result, the amendment solution often bypasses the less permeable zones.…”