Energy's Hanford Site Research Articles

(99)Tc(VII) Retardation, Reduction, and Redox Rate Scaling in Naturally Reduced Sediments.

An experimental and modeling study was conducted to investigate pertechnetate (Tc(VII)O4(-)) retardation, reduction, and rate scaling in three sediments from Ringold formation at U.S. Department of Energy's Hanford site, where (99)Tc is a major contaminant in groundwater. Tc(VII) was reduced in all the sediments in both batch reactors and diffusion columns, with a faster rate in a sediment containing a higher concentration of HCl-extractable Fe(II). Tc(VII) migration in the diffusion columns was reductively retarded with retardation degrees correlated with Tc(VII) reduction rates. The reduction rates were faster in the diffusion columns than those in the batch reactors, apparently influenced by the spatial distribution of redox-reactive minerals along transport paths that supplied Tc(VII). X-ray computed tomography and autoradiography were performed to identify the spatial locations of Tc(VII) reduction and transport paths in the sediments, and results generally confirmed the newly found behavior of reaction rate changes from batch to column. The results from this study implied that Tc(VII) migration can be reductively retarded at Hanford site with a retardation degree dependent on reactive Fe(II) content and its distribution in sediments. This study also demonstrated that an effective reaction rate may be faster in transport systems than that in well-mixed reactors.

Nitrate bioreduction in redox-variable low permeability sediments

Low permeability zone (LPZ) can play an important role as a sink or secondary source in contaminant transport in groundwater system. This study investigated the rate and end product of nitrate bioreduction in LPZ sediments. The sediments were from the U.S. Department of Energy's Hanford Site, where nitrate is a groundwater contaminant as a by-product of radionuclide waste discharges. The LPZ at the Hanford site consists of two layers with an oxidized layer on top and reduced layer below. The oxidized layer is directly in contact with the overlying contaminated aquifer, while the reduced layer is in contact with an uncontaminated aquifer below. The experimental results showed that nitrate bioreduction rate and end-product differed significantly in the sediments. The bioreduction rate in the oxidized sediment was significantly faster than that in the reduced one. A significant amount of N2O was accumulated in the reduced sediment; while in the oxidized sediment, N2O was further reduced to N2. RT-PCR analysis revealed that nosZ, the gene that codes for N2O reductase, was below detection limit in the reduced sediment. Batch experiments and kinetic modeling were performed to provide insights into the role of organic carbon bioavailability, biomass growth, and competition between nitrate and its reducing products for electrons from electron donors. The results revealed that it is important to consider sediment redox conditions and functional genes in understanding and modeling nitrate bioreduction in subsurface sediments. The results also implied that LPZ sediments can be important sink of nitrate and a potential secondary source of N2O as a nitrate bioreduction product in groundwater.

Field Soil Water Retention of the Prototype Hanford Barrier and Its Variability with Space and Time

Engineered surface barriers (or covers) are used to isolate underlying contaminants from water, plants, animals, and humans. To understand the flow processes within a barrier and the barrier's ability to store and release water, the field hydraulic properties of the barrier need to be known. A multiyear test of the evapotranspiration (ET) barrier at the US Department of Energy's Hanford Site in the southeast of the state of Washington has yielded in situ soil water content and pressure data from multiple locations for a 9‐yr period, offering the opportunity to estimate soil water retention properties at different locations and times. The upper 2‐m layer of the ET barrier is a silt loam, and the top 1 m also contains 15% (w/w) pea gravel. Within this layer, valid monitoring data from 1995 to 2003 for four depths at 12 monitoring stations were used to determine the field water retention of the silt loam, with and without gravel. The data covered a wide range of wetness, from near saturation to the permanent wilting point, and each retention curve contained 51 to 96 data points. The data were described well with the commonly used van Genuchten water retention model. It was found that the spatial variation of the saturated and residual water content and the pore size distribution parameter was relatively small, while that of the van Genuchten α was relatively large. The effects of spatial variability of the retention properties appeared to be larger than the combined effects of added pea gravel and plant roots on the properties. Neither the primary wetting process in the winter season nor the drying process in the summer season nor time had a detectable effect on the water retention of the silt loam barrier.

Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

Volatile loss of radioactive technetium-99 (99Tc) to off-gas is a major challenge when vitrifying low-activity waste (LAW) at the U.S. Department of Energy's Hanford Site in Washington State. We investigated the partitioning and incorporation of rhenium (Re) (a nonradioactive surrogate for 99Tc) into the glass melt during crucible melting of two simulated LAW feeds that have exhibited a large difference in 99mTc/Re retention in glass from small-scale melter tests. Each feed was prepared from a simulated liquid LAW and additives (boric acid, silica sand, etc.). The as-mixed slurry feeds were dried at 105°C and heated to 600–1100°C at 5K/min. The dried feeds and heat-treated samples were leached with deionized water for 10min at room temperature followed by 24-h leaching at 80°C. Chemical compositions of the resulting solutions and insoluble solids were analyzed. Volume expansion measurements and X-ray diffraction (XRD) analyses were performed on dried feeds and heat-treated samples to characterize the progress of feed-to-glass conversion reactions. We found that incorporation of Re into the glass melt was virtually completed during the major feed-to-glass conversion reactions that occurred at ≤700°C. The results of our study suggest that the different compositions of the salt phases formed during early stages of melting at ≤700°C are responsible for the large difference in Re incorporation into the glass melt in these two feeds.

Immobilization and exchange of perrhenate in sodalite and cancrinite

Highly alkaline nuclear tank wastes containing key anionic contaminants have leaked into the subsurface at the U.S. Department of Energy's Hanford Site. Laboratory studies showed that when simulated caustic tank wastes contact subsurface sediments, mineral dissolution and precipitation result in feldspathoid formation. Feldspathoids are environmentally important for waste management and disposal purposes because of their purported potential to sequester contaminants of interest (e.g., 99TcO4−, 137Cs+, 90Sr2+) into their structure. We investigated the incorporation of perrhenate (ReO4−), a surrogate for TcO4−, in the presence of competing anions (X) including OH−, NO2−, NO3− and Cl− on feldspathoid formation and under conditions mimicking tank waste solution compositions. The resulting solids were characterized by their chemical composition, structure and morphology. Regardless of solution pH, sodalite formed in the presence of Cl− and NO2− whereas NO3− promoted either cancrinite formation in 16-mol OH−/kg (16 m) or mixed sodalite/cancrinite phases in 1-mol OH−/kg (1 m) solutions. In the presence of Cl−, NO2−, and NO3− less than 0.02 mol fraction of ReO4− was incorporated into the feldspathoid phase(s). Although, the NO2-sodalite and mixed NO3-cancrinite/sodalite phases incorporated significantly more ReO4− than NO3-cancrinite or Cl-sodalite phases, the total ReO4− fraction was ∼1% or less of the total sites. The ReO4− immobilized in ReO4-sodalite, NO2-sodalite, mixed NO3-cancrinite/sodalite and NO3-cancrinite was resistant to ion exchange with either NO2− or NO3−. The results imply that ReO4−, and thus by analogy 99TcO4−, does not compete well with smaller ions for incorporation into feldspathoids, but, once sequestered, is difficult to exchange.

Solid-phase zirconium and fluoride species in alkaline zircaloy cladding waste at Hanford

The United States Department of Energy Hanford Site, near Richland, Washington, USA, processed plutonium between 1944 and 1987. Fifty-six million gallons of waste of various origins remain, including waste from removing zircaloy fuel cladding using the so-called Zirflex process. The speciation of zirconium and fluoride in this waste is important because of the corrosivity and reactivity of fluoride as well as the (potentially) high density of Zr-phases. This study evaluates the solid-phase speciation of zirconium and fluoride using X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS). Two waste samples were analyzed: one waste sample that is relatively pure zirconium cladding waste from tank 241-AW-105 and another that is a blend of zirconium cladding wastes and other high-level wastes from tank 241-C-104. Villiaumite (NaF) was found to be the dominant fluoride species in the cladding waste and natrophosphate (Na7F[PO4]2·19H2O) was the dominant species in the blended waste. Most zirconium was present as a sub-micron amorphous Na–Zr–O phase in the cladding waste and a Na-Al–Zr–O phase in the blended waste. Some zirconium was present in both tanks as either rounded or elongated crystalline needles of Na-bearing ZrO2 that are up to 200μm in length. These results provide waste process planners the speciation data needed to develop disposal processes for this waste.

Relationship between Anisotropy in Soil Hydraulic Conductivity and Saturation

Anisotropy in unsaturated hydraulic conductivity is saturation‐dependent. Accurate characterization of soil anisotropy is very important in simulating flow and contaminant transport (e.g., radioactive nuclides at the U.S. Department of Energy's Hanford Site). A tensorial connectivity–tortuosity (TCT) concept describes the hydraulic conductivity tensor of the unsaturated anisotropic soils as the product of a scalar variable, the symmetric connectivity tortuosity tensor, and the saturated hydraulic conductivity tensor. In this study, a model based on the TCT concept is used to quantify soil anisotropy in unsaturated hydraulic conductivity. The TCT model can describe different types of soil anisotropy; for example, the anisotropy coefficient can monotonically increase or decrease with saturation and can vary from greater than unity to less than unity and vice versa. Soil anisotropy is independent of soil water retention properties and can be characterized by the ratio of the saturated hydraulic conductivities and the difference of the tortuosity–connectivity coefficients in two directions. The anisotropy coefficient is log‐linearly proportional to the effective saturation. This log‐linear relationship allows the saturation‐dependent anisotropy to be determined using regression with the measurements of the directional hydraulic conductivities at a minimum of two water content levels, one of which may be at full saturation. The model was tested using measured or simulated directional hydraulic conductivities.

Effect of Subgrid Heterogeneity on Scaling Geochemical and Biogeochemical Reactions: A Case of U(VI) Desorption

The effect of subgrid heterogeneity in sediment properties on the rate of uranyl[U(VI)] desorption was investigated using a sediment collected from the U.S. Department of Energy Hanford site. The sediment was sieved into 7 grain size fractions that each exhibited different U(VI) desorption properties. Six columns were assembled using the sediment with its grain size fractions arranged in different spatial configurations to mimic subgrid heterogeneity in reactive transport properties. The apparent rate of U(VI) desorption varied significantly in the columns. Those columns with sediment structures leading to preferential transport had much lower rates of U(VI) desorption than those with relatively homogeneous transport. Modeling analysis indicated that the U(VI) desorption model and parameters characterized from well-mixed reactors significantly overpredicted the measured U(VI) desorption in the columns with preferential transport. A dual domain model, which operationally separates reactive transport properties into two subgrid domains, improved the predictions significantly. A similar effect of subgrid heterogeneity, albeit to a lesser degree, was observed for denitrification, which also occurred in the columns. The results imply that subgrid heterogeneity is an important consideration in extrapolating reaction rates from the laboratory to field.

Determination of uranium metal concentration in irradiated fuel storage basin sludge using selective dissolution

Irradiated uranium metal fuel was stored underwater in the K East and K West storage basins at the US Department of Energy Hanford Site. The uranium metal under damaged cladding reacted with water to generate hydrogen gas, uranium oxides, and spalled uranium metal particles which intermingled with other particulates to form sludge. While the fuel has been removed, uranium metal in the sludge remains hazardous. An expeditious routine method to analyze 0.03 wt% uranium metal in the presence of >30 wt% total uranium was needed to support safe sludge management and processing. A selective dissolution method was designed based on the rapid uranium oxide dissolution but very low uranium metal corrosion rates in hot concentrated phosphoric acid. The uranium metal-bearing heel from the phosphoric acid step then is rinsed before the uranium metal is dissolved in hot concentrated nitric acid for analysis. Technical underpinnings of the selective dissolution method, including the influence of sludge components, were investigated to design the steps and define the reagents, quantities, concentrations, temperatures, and times within the selective dissolution analysis. Tests with simulant sludge proved the technique feasible. Tests with genuine sludge showed a 0.0028 ± 0.0037 wt% (at one standard deviation) uranium metal analytical background, a 0.011 wt% detection limit, and a 0.030 wt% quantitation limit in settled (wet) sludge. In tests using genuine K Basin sludge spiked with uranium metal at concentrations above the 0.030 wt% ± 25 % (relative) quantitation limit, uranium metal recoveries averaged 99.5 % with a relative standard deviation of 3.5 %.

Colloid mobilization in an undisturbed sediment core under semiarid recharge rates

The semiarid US Department of Energy Hanford site has a deep vadose zone with low recharge rates. Contaminants originating from nuclear waste processing are expected to move slowly through the vadose zone. The movement of certain contaminants can be facilitated by colloids. We hypothesized that the low recharge rates and low water contents in semiarid regions, however, tend to inhibit movement of colloidal particles, thereby reducing the risk for colloid‐facilitated contaminant transport. The goal of this study was to investigate whether in situ natural colloids can be mobilized and transported in undisturbed, deep vadose zone sediments at the Hanford site under typical, semiarid recharge rates. We sampled an undisturbed sediment core (i.d. 50 cm, 59.5 cm height) from a depth of 17 m below ground at the Hanford 200 Area. The core was set up as a laboratory lysimeter and exposed to an infiltration rate of 18 mm/yr by applying simulated pore water onto the surface. Particle concentrations were quantified in the column outflow, and selected samples were examined microscopically and for elemental composition (transmission electron microscopy and energy dispersive X‐ray). Measured water contents and potentials were used to calibrate a numerical model (HYDRUS‐1D), which was then applied to simulate colloid mobilization from the sediment core. During 5.3 years of monitoring, natural colloids like silicates, aluminosilicates, and Fe‐oxides were observed in the core outflow, indicating the continuous mobilization of in situ colloids. The total amount of particles mobilized during 5.3 years corresponded to 1.1% of the total dispersible colloids inside the core. Comparison of the amounts of colloids released with weathering rates suggests that mineral weathering can be a major source of the mobilized colloids. The fitted colloid release rate coefficient was 6 to 7 orders of magnitude smaller than coefficients reported from previous studies, where disturbed Hanford sediments and higher flow rates were used. Our findings demonstrate that even under low recharge rates and water contents typical for semiarid, deep vadose zone sediments, particles can continuously be mobilized, although the total mass of particles is low.

Characterization of Persistent Volatile Contaminant Sources in the Vadose Zone

AbstractEffective long‐term operation of soil vapor extraction (SVE) systems for cleanup of vadose‐zone sources requires consideration of the likelihood that remediation activities over time will alter the subsurface distribution and configuration of contaminants. A method is demonstrated for locating and characterizing the distribution and nature of persistent volatile organic contaminant (VOC) sources in the vadose zone. The method consists of three components: analysis of existing site and SVE‐operations data, vapor‐phase cyclic contaminant mass‐discharge testing, and short‐term vapor‐phase contaminant mass‐discharge tests conducted in series at multiple locations. Results obtained from the method were used to characterize overall source zone mass‐transfer limitations, source‐strength reductions, potential changes in source‐zone architecture, and the spatial variability and extent of the persistent source(s) for the Department of Energy's Hanford site. The results confirmed a heterogeneous distribution of contaminant mass discharge throughout the vadose zone. Analyses of the mass‐discharge profiles indicate that the remaining contaminant source is coincident with a lower‐permeability unit at the site. Such measurements of source strength and size as obtained herein are needed to determine the impacts of vadose‐zone sources on groundwater contamination and vapor intrusion, and can support evaluation and optimization of the performance of SVE operations.

Facilitated strontium transport by remobilization of strontium-containing secondary precipitates in Hanfod Site subsurface

Significantly enhanced immobilization of radionuclides (such as 90Sr and 137Cs) due to adsorption and coprecipitation with neo-formed colloid-sized secondary precipitates has been reported at the U.S. Department of Energy's Hanford Site. However, the stability of these secondary precipitates containing radionuclides in the subsurface under changeable field conditions is not clear. Here, the authors tested the remobilization possibility of Sr-containing secondary precipitates (nitrate-cancrinite) in the subsurface using saturated column experiments under different geochemical and flow conditions. The columns were packed with quartz sand that contained secondary precipitates (nitrate-cancrinite containing Sr), and leached using colloid-free solutions under different flow rates, varying pH, and ionic strength conditions. The results indicate remobilization of the neo-formed secondary precipitates could be possible given a change of pH of ionic strength and flow rate conditions. The remobility of the neo-formed precipitates increased with the rise in the leaching solution flow rate and pH (in a range of pH 4–11), as well as with decreasing solution ionic strength. The increased mobility of Sr-containing secondary precipitates with changing background conditions can be a potential source for additional radionuclide transport in Hanford Site subsurface environments.

Geophysical Monitoring of Foam Used to Deliver Remediation Treatments within the Vadose Zone

Foam is a promising vehicle for delivering amendments into the vadose zone for in situ remediation; it is an approach being considered for in situ treatment and stabilization of metals and radionuclides located within the deep vadose zone of the Department of Energy's Hanford Site in the state of Washington. A central aspect of evaluating the effectiveness of this approach is the ability to monitor foam distribution, its transformation, and the reactions that it induces in the subsurface, ideally in a noninvasive manner. In this study, we performed laboratory experiments to evaluate the potential of geophysical methods (complex resistivity and time‐domain reflectometry [TDR]) as tools for monitoring foam‐assisted amendment delivery in the deep vadose zone. Our results indicated great sensitivity of electrical methods to foam transportation and evolution in unsaturated porous media that were related to foam bubble coalescence and drainage processes. Specifically, we observed (i) a decrease in electrical resistivity (increase in electrical conductivity) by more than an order of magnitude in both silica sand and natural sediment matrices during foam transportation; (ii) an increase in resistivity (decrease in conductivity) of more than twofold during foam coalescence and drainage; and (iii) a distinct phase and imaginary conductivity signature related to the evolution of water films on sediment grains during foam injection and evolution processes. To assist with the interpretation of these data, TDR measurements were used to monitor moisture content, which provided complementary information about foam distribution and drainage. Our results clearly demonstrated the sensitivity of electrical and TDR signals to foam transportation and evolution in unsaturated porous media and suggest the potential of these methods for monitoring the response of a system to foam‐based remediation treatments at field scales.

Submerged RadBall® Deployments in Hanford Site Hot Cells Containing 137CsCl Capsules

The overall objective of this study was to demonstrate that a new technology, known as RadBall®, could locate submerged radiological hazards. RadBall® is a novel, passive, radiation detection device that provides a 3-D visualization of radiation from areas where measurements have not been previously possible due to lack of access or extremely high radiation doses. This technology has been under development during recent years, and all of its previous tests have included dry deployments. This study involved, for the first time, underwater RadBall® deployments in hot cells containing 137CsCl capsules at the U.S. Department of Energy's Hanford Site. RadBall® can be used to characterize a contaminated room, hot cell, or glovebox by providing the locations of the radiation sources and hazards, identifying the radionuclides present within the cell, and determining the radiation sources' strength (e.g., intensities or dose rates). These parameters have been previously determined for dry deployments; however, only the location of radiation sources and hazards can be determined for an underwater RadBall® deployment. The results from this study include 3-D images representing the location of the radiation sources within the Hanford Site cells. Due to RadBall®'s unique deployability and non-electrical nature, this technology shows significant promise for future characterization of radiation hazards prior to and during the decommissioning of contaminated nuclear facilities.

Separating and Stabilizing Phosphate from High-Level Radioactive Waste: Process Development and Spectroscopic Monitoring

Removing phosphate from alkaline high-level waste sludges at the Department of Energy's Hanford Site in Washington State is necessary to increase the waste loading in the borosilicate glass waste form that will be used to immobilize the highly radioactive fraction of these wastes. We are developing a process which first leaches phosphate from the high-level waste solids with aqueous sodium hydroxide, and then isolates the phosphate by precipitation with calcium oxide. Tests with actual tank waste confirmed that this process is an effective method of phosphate removal from the sludge and offers an additional option for managing the phosphorus in the Hanford tank waste solids. The presence of vibrationally active species, such as nitrate and phosphate ions, in the tank waste processing streams makes the phosphate removal process an ideal candidate for monitoring by Raman or infrared spectroscopic means. As a proof-of-principle demonstration, Raman and Fourier transform infrared (FTIR) spectra were acquired for all phases during a test of the process with actual tank waste. Quantitative determination of phosphate, nitrate, and sulfate in the liquid phases was achieved by Raman spectroscopy, demonstrating the applicability of Raman spectroscopy for the monitoring of these species in the tank waste process streams.

Two-Layer Model for Horizontal Pipe Flow of Newtonian and Non-Newtonian Settling Dense Slurries

The steady-state flow of Newtonian and non-Newtonian dense aqueous slurries in horizontal pipes has been analyzed using a two-layer model, consisting of a top layer of flowing suspension and a bottom bed of stationary or moving particles. The coarse solids used have a wide range of particle density and particle size distributions. The fluids studied were designed to simulate U.S. Department of Energy Hanford site waste slurries. The most important changes from previous models include an independent settling analysis for different particle size fractions, effects of particle shape on settling velocity, and a new correlation for the turbulent particle dispersivity. The results indicate that the turbulent dispersivity of settling particles is sensitive to particle size and density. The model gives good estimation of the critical deposition velocity as the minimum of the pressure drop versus superficial slurry velocity relation. The existence of a stationary layer can be observed and predicted by the model under laminar and turbulent flow conditions.

Carbon-14 Bioassay for Decommissioning of Hanford Reactors

The production reactors at the U.S. Department of Energy Hanford Site used large graphite piles as the moderator. As part of long-term decommissioning plans, the potential need for ¹⁴C radiobioassay of workers was identified. Technical issues associated with ¹⁴C bioassay and worker monitoring were investigated, including anticipated graphite characterization, potential intake scenarios, and the bioassay capabilities that may be required to support the decommissioning of the graphite piles. A combination of urine and feces sampling would likely be required for the absorption type S ¹⁴C anticipated to be encountered. However, the concentrations in the graphite piles appear to be sufficiently low that dosimetrically significant intakes of ¹⁴C are not credible, thus rendering moot the need for such bioassay.

Improved Model for Hydrogen Generation Rate of Radioactive Waste at the Hanford Site

Hydrogen is the major flammable gas observed in the dome space of each million-gallon radioactive waste storage tank at the U.S. Department of Energy Hanford Site. Semiempirical rate equations are derived to estimate hydrogen generation based on chemical reactions, radiolysis of water and organic compounds, and corrosion. The rate equations account for tank waste composition, temperature, radiation dose rate, and liquid fraction. Numerical parameters are established by the analysis of gas generation kinetic data from actual waste samples, literature data, and waste characterization and field surveillance data. The model improvement includes development of refined water radiolysis equations, accounting of total alpha radiation contribution to both water and organic radiolysis, new parameterization on the rate equations of organic thermolysis and radiolysis with extra tank waste gas generation test data, and revised corrosion rate equations. A comparison of the generation rates observed in the field with the rates calculated for 28 tanks shows agreement within a factor of 3. The model serves as a useful tool to evaluate flammable gas issues to support Hanford operations.

Diffusion and Adsorption of Uranyl Carbonate Species in Nanosized Mineral Fractures

Atomistic simulations were performed to study the diffusion and adsorption of Ca(2)UO(2)(CO3)3 and of some of its constituent species, i.e., UO(2)2+, CO(3)2–, and UO(2)CO3, in feldspar nanosized fractures. Feldspar is important to uranium remediation efforts at the U.S. Department of Energy Hanford site as it has been found in recent studies to host contaminants within its intragrain fractures. In addition, uranyl carbonate species are known to dominate U(VI) speciation in conditions relevant to the Hanford site. Molecular dynamics (MD) simulations showed that the presence of the feldspar surface diminishes the diffusion coefficients of all of the species considered in this work and that the diffusion coefficients do not reach their bulk aqueous solution values in the center of a 2.5 nm fracture. Moreover, the MD simulations showed that the rate of decrease in the diffusion coefficients with decreasing distance from the surface is greater for larger adsorbing species. Free energy profiles of the same species adsorbing on the feldspar surface revealed a large favorable free energy of adsorption for UO(2)2+ and UO(2)CO3, which are able to adsorb to the surface with their uranium atom directly bonded to a surface hydroxyl oxygen, whereas adsorption of CO(3)2– and Ca(2)UO(2)(CO3)3, which attach to the surface via hydrogen bonding from a surface hydroxyl group to a carbonate oxygen, was calculated to be either only slightly favorable or unfavorable.

Microbial community response to addition of polylactate compounds to stimulate hexavalent chromium reduction in groundwater

To evaluate the efficacy of bioimmobilization of Cr(VI) in groundwater at the Department of Energy Hanford site, we conducted a series of microcosm experiments using a range of commercial electron donors with varying degrees of lactate polymerization (polylactate). These experiments were conducted using Hanford Formation sediments (coarse sand and gravel) immersed in Hanford groundwater, which were amended with Cr(VI) and several types of lactate-based electron donors (Hydrogen Release Compound, HRC; primer-HRC, pHRC; extended release HRC) and the polylactate-cysteine form (Metal Remediation Compound, MRC). The results showed that polylactate compounds stimulated an increase in bacterial biomass and activity to a greater extent than sodium lactate when applied at equivalent carbon concentrations. At the same time, concentrations of headspace hydrogen and methane increased and correlated with changes in the microbial community structure. Enrichment of Pseudomonas spp. occurred with all lactate additions, and enrichment of sulfate-reducing Desulfosporosinus spp. occurred with almost complete sulfate reduction. The results of these experiments demonstrate that amendment with the pHRC and MRC forms result in effective removal of Cr(VI) from solution most likely by both direct (enzymatic) and indirect (microbially generated reductant) mechanisms.