Contaminant Release Models Research Articles

Single-pass flow-through test elucidation of weathering behavior and evaluation of contaminant release models for Hanford tank residual radioactive waste

Contaminant release models are required to evaluate and predict long-term environmental impacts of even residual amounts of high-level radioactive waste after cleanup and closure of radioactively contaminated sites such as the DOE’s Hanford Site. More realistic and representative models have been developed for release of uranium, technetium, and chromium from Hanford Site tanks C-202, C-203, and C-103 residual wastes using data collected with a single-pass flow-through test (SPFT) method. These revised models indicate that contaminant release concentrations from these residual wastes will be considerably lower than previous estimates based on batch experiments. For uranium, a thermodynamic solubility model provides an effective description of uranium release, which can account for differences in pore fluid chemistry contacting the waste that could occur through time and as a result of different closure scenarios. Under certain circumstances in the SPFT experiments various calcium rich precipitates (calcium phosphates and calcite) form on the surfaces of the waste particles, inhibiting dissolution of the underlying uranium phases in the waste. This behavior was not observed in previous batch experiments. For both technetium and chromium, empirical release models were developed. In the case of technetium, release from all three wastes was modeled using an equilibrium Kd model. Formore » chromium release, a constant concentration model was applied for all three wastes.« less

Linking HAZUS-MH Risk-Analysis Methodology to Contaminant-Release Models

In seismically active areas, waste-containment systems (WCS) can be damaged by earthquakes that generate significant ground acceleration. In such circumstances, contaminants can be released into the environment. Current state-of-the-art earthquake damage risk-assessment methodologies, Hazards U.S. Multi-Hazard (HAZUS-MH), need to incorporate contaminant-release models to enable more complete environmental impact assessments for existing and planned waste-containment systems. Contaminant-release and transport probabilities that determine the prospect of functional failure of waste-containment systems depend on a wider range of structural, geologic, hydrologic, and human factors than are covered by the structural failure probabilities of conventional structures such as buildings, roads, and bridges. The release of contaminants from WCS, and the cost of remediation depend on conditional probabilities that span the intensity and frequency of the initiating event, the structural damage of the WCS, and the fate and transport of the released contaminants in the subsurface. In this paper, the necessity and approach to linking HAZUS-MH to contaminant-release models are discussed along with relevant factors. The quantitative link that is advocated in this paper would expand the utility of HAZUS-MH in facilities design and environmental impact assessments.

Residual Waste from Hanford Tanks 241-C-203 and 241-C-204. 2. Contaminant Release Model

Release of U and 99Tc from residual sludge in Hanford waste tanks 241-C-203 and 241-C-204 atthe U.S. Department of Energy's (DOE) Hanford Site in southeastern Washington state was quantified by water-leaching, selective extractions, empirical solubility measurements, and thermodynamic modeling. A contaminant release model was developed based on these experimental results and solid-phase characterization results presented elsewhere. Uranium release was determined to be controlled by two phases and occurred in three stages. In the first stage, U release is controlled by the solubility of tejkaite, which is suppressed by high concentrations of sodium released from the dissolution of NaNO3 in the residual sludges. Equilibrium solubility calculations indicate the U released during this stage will have a maximum concentration of 0.021 M. When all the NaNO3 has dissolved from the sludge, the solubility of the remaining cejkaite will increase to 0.28 M. After cejkaite has completely dissolved, the majority of the remaining U is in the form of poorly crystalline Na2U2O7 [or clarkeite Na[(UO2)O(OH)](H20)0-1]. In contact with Hanford groundwater this phase is not stable, and becquerelite becomes the U solubility controlling phase, with a calculated equilibrium concentration of 1.2 x 10(-4) M. For Tc, a significant fraction of its concentration in the residual sludge was determined to be relatively insoluble (20 wt % for C-203 and 80 wt % for C-204). Because of the low concentrations of Tc in these sludge materials, the characterization studies did not identify any discrete Tc solids phases. Release of the soluble fraction of Tc was found to occur concomitantly with NO3-. It was postulated that a NaNO3-NaTcO4 solid solution could be responsible for this behavior. The Tc release concentrations for the soluble fraction were estimated to be 2.4 x 10-6 M for C-203 and 2.7 x 10(-5) M for C-204. Selective extraction results indicated that the recalcitrant fraction of Tc was associated with Fe oxides. Release of the recalcitrant fraction of Tc was assumed to be controlled by dissolution of Fe oxide in the form of ferrihydrite. Based on this assumption and measured values for the ratio of recalcitrant Tc to total Fe in each bulk sludge, the release concentration of the recalcitrant fraction of Tc was calculated to be 3.9 x 10(-12) M for C-203 and 10.0 x 10(-12) M for C-204.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′-DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.

Contaminant dispersal on the Palos Verdes continental margin: III. Processes controlling transport, accumulation and re-emergence of DDT-contaminated sediment particles

A large mass of DDT-contaminated sediment lies buried beneath a thin cover on the Palos Verdes shelf and slope off Los Angeles, California. Analyses, including several types of numerical simulation, have been applied to an extensive data set to evaluate the biological and physical processes controlling the fate of the strongly particle-reactive pollutants. Sequential measurements of the p,p′-DDE (an isomer of DDT) content in cores from monitoring locations indicate that contaminated sediment particles are being re-introduced into the marine environment from the buried historic deposits at a significant rate. There is considerable scatter in the data but the trends for ongoing release over the 8 years between 1983 and 1991 are statistically significant at the 90% level. A comprehensive mathematical model of shelf sediment dynamics (Resuspension Model) has been used to explore sediment erosion and deposition during major storms of the early 1980s and of 1988. This modeling shows that storms alone cannot explain the observed losses of contaminated sediment particles from the historical deposits of the Palos Verdes shelf. On the other hand, it also demonstrates that both severe and common storm events do re-entrain some of the bottom sediment at all water depths across the Palos Verdes shelf. A numerical model (Contaminant Release Model), which couples upward biodiffusion of DDT with storm removal, satisfactorily explains the observed losses. The infaunal activity that creates biodiffusion has been explored in two separate ways. In calibrating the Contaminant Release Model, the depth-dependent biodiffusion coefficient profile is estimated by comparing the predicted and measured depth concentration distributions. A separate analysis has been conducted using the measured vertical distribution of infaunal species (Stull et al. 1995; Swift et al., 1995). The agreement between these two methods is good. Both show that bioturbation extends to the level of the high contamination. Bioturbation is now playing a significant role in redistributing the buried historic contaminants. The Contaminant Release Model, in combination with evaluations of the total sedimentation rate and the natural background sedimentation rate, clearly shows that the flux of solids from the outfall enhances the rate of accumulation of natural sediment on the Palos Verdes shelf. A sharp reduction of the flux of solids, such as would accompany conversion from partial to full secondary treatment, will result in considerably prolonging the high rates with which DDT and other particle-reactive contaminants are re-entering the marine environment.