Robert G. Riley

Preconcentration and Analysis of Strontium-90 and Technetium-99 from Hanford Groundwater Using Solid Phase Extraction

Solid-phase extraction disks produced by 3M and Eichrom were evaluated for routine use in supporting the Hanford Groundwater Monitoring Project. Both disk formats contain Sr- or Tc-selective extractants, bound in a filter support, that act to preconcentrate and isolate the isotope of interest. The 3M Empore™ Sr Rad Disks and Tc Rad Disks were tested with respect to precision, accuracy, radiochemical yields, interferences, and volume-load variation. The Empore™ and Eichrom solid-phase extraction disks were applied to the 90Sr and/or 99Tc determination in representative Hanford groundwater samples with varying chemical and isotopic compositions. Results were compared to standard analytical methods. Both the Empore™ and Eichrom Tc extraction disks produced consistently higher radiochemical yields, lower detection limits, and greater accuracy than the standard analysis method. The Empore™ Sr extraction disks produced comparable radiochemical yields, detection limit, and accuracy relative to the standard method; however, total uncertainties were lower.

Carbon Tetrachloride and Chloroform Partition Coefficients Derived from Aqueous Desorption of Contaminated Hanford Sediments

Researchers at PNNL determined CCl4 and CHCl3 groundwater/sediment partition coefficients (Kd values) for contaminated aquifer sediments collected from borehole C3246 (299-W15-46) located in the 200 West Area adjacent to the Z-9 trench. Having realistic values for this parameter is critical to predict future movement of CCl4 in groundwater from the 200 West Area.

Desorption behavior of carbon tetrachloride and chloroform in contaminated low organic carbon aquifer sediments

Slow release behavior of carbon tetrachloride (CCl(4)) and chloroform (CHCl(3)) in low organic carbon (<0.1%) deep aquifer sediments was quantified by 1-D column desorption studies with intact cores. The compounds had been in contact with the sediments for 30years. Comparison of the CCl(4) distribution coefficient (K(d)) from this study with those from short contact time experiments suggested that CCl(4)K(d)s calculated from site contaminated sediments of long contact time are likely a factor of 10 or more higher than those calculated from short contact-time lab experiments. A significant portion of the CHCl(3) mass (55% to more than 90%) was resistant to aqueous desorption in sediments with clay contents ranging from 2.0% to 36.7% and organic carbon content ranging from 0.017% to 0.088%. In contrast, CCl(4) showed greatest mass retention (31% or more) only in the highest clay and organic carbon content sediment. Relatively easy solvent extraction of the residual masses of CCl(4) and CHCl(3) from the sediments indicated the compounds were not permanently sequestered. Tracer breakthrough in columns was well behaved, indicating interparticle diffusion was not causing the slow release behavior. Diffusion out of intraparticle pores is suggested to be the main process governing the observed behavior although, diffusion out of natural organic matter cannot be ruled out as a potential contributing factor. The half-life for release of the slow fraction of CHCl(3) mass from sediments was estimated to be in the range of weeks (100h) to months (1100h). Neither CCl(4) or CHCl(3) were detected at measurable levels in the column effluent of one of the sediments even though a significant mass fraction of CHCl(3) was found present on the sediment following desorption suggesting that our estimate of hundreds to thousands of hours for complete release of CHCl(3) masses from such sediment is conservative.

Carbon Tetrachloride Partition Coefficients Measured by Aqueous Sorption to Hanford Sediments from Operable Units 200-UP-1 and 200-ZP-1

Kd values obtained on sediment samples from 200-UP-1 and 10-ZP-1 contribute to a larger Kd database that exists for other Hanford sediments, and contains significant desorption data for CCl4. Adsorption results presented here validate the use of a linear adsorption isotherm (Kd) to predict short contact time CCl4 adsorption to sediments in 200-UP-1 groundwater plume for a distinct ranges in CCl4 concentration. However, this does not imply that values of Kd will be constant if the groundwater chemical composition at 200-UP-1 changes with space or time. Additionally, results presented here suggest the potential significance of slower intraparticle diffusion on the long-term fate of CCl4 within the subsurface Hanford environment. Such behavior could afford prolonged desorption of CCl4 and serve as a long-term source of contaminant CCl4 to the aquifer. Further evaluation of possible bimodal sorption behavior for CCl4 and the mechanism of CCl¬4 sequestration should be the subject of future investigations to provide a thorough, mechanistic understanding of the retention and long-term fate of CCl4. Comparison of previous data with new results (e.g., from this study) will allow inferences to be made on how the 200-UP-1 Kd values for CCl4 may compare with sediments from other Hanford locations. This site-specific sorption data, when complemented by the chemical, geologic, mineralogic, hydrologic, and physical characterization data that are also being collected (see Sampling and Analysis Plan for the 200-UP-1 Groundwater Monitoring Well Network, DOE 2002) can be used to develop a robust, scientifically defensible data base to allow risk predictions to be generated and to aid in future remediation decisions for the 200-UP-1 and 200-ZP-1 operable units.

Quantification of Volatile Organics in Soil Aging Experiments Using Fourier Transform Infrared Spectroscopy

On-line Fourier transform infrared (FT-IR) spectroscopy was applied to monitor the concentration of halogenated volatile organic compounds in a sample-preparation process that simulates long-term, slow accumulation of contaminants in soils (i.e., aging). Artificial aging is conducted by circulating a supercritical fluid solution containing the contaminant(s) of interest through a packed soil column. Mid-infrared spectra of several volatile halocarbons were measured in supercritical Xe and CO2 to evaluate possible interferences from the strong absorption of CO2. Although some of the C–X bands were partially masked in supercritical CO2, all of the compounds studied had distinct spectral features in the region 1400–700 cm−1 and could be monitored in either solvent. Quantitative measurements of halogenated volatile organics in supercritical CO2 were demonstrated with CCl4. Excellent results were obtained over the range 7–280 mM. Representative artificial aging experiments were conducted on two test soils using CCl4 as the contaminant. On-line (FT-IR) estimates of the aged soil concentrations were 1.3–4.4 times higher than off-line concentrations obtained by gas chromatography/mass spectrometry. The discrepancies were primarily ascribed to post-aging losses that occurred during depressurization and subsequent sample handling. FT-IR spectroscopy is shown to be a powerful tool for monitoring soil loading behavior and for developing artificial aging protocols.

DESORPTION BEHAVIOR OF TRICHLOROETHENE AND TETRACHLOROETHENE IN U.S. DEPARTMENT OF ENERGY SAVANNAH RIVER SITE UNCONFINED AQUIFER SEDIMENTS

The DOE Savannah River Site (SRS) is evaluating the potential applicability of the monitored natural attenuation (MNA) process as a contributor to the understanding of the restoration of its unconfined groundwater aquifer known to be contaminated with the chlorinated hydrocarbon compounds trichloroethylene (TCE) and tetrachloroethylene (PCE). This report discusses the results from aqueous desorption experiments on SRS aquifer sediments from two different locations at the SRS (A/M Area; P-Area) with the objective of providing technically defensible TCE/PCE distribution coefficient (Kd) data and data on TCE/PCE reversible and irreversible sorption behavior needed for further MNA evaluation.

Slow Desorption of Phenanthrene from Silica Particles: Influence of Pore Size, Pore Water, and Aging Time

When micro-porous and meso-porous silica particles were exposed to aqueous phenanthrene solutions for various durations it was observed that sorbed-phase phenanthrene concentrations increased with aging time only for meso-porous but not micro-porous silicas. Desorption equilibrium was reached almost instantaneously for the micro-porous particles while both the rate and extent of desorption decreased with increasing aging time for the meso-porous silicas. These findings indicate that phenanthrene can be sequestered within the internal pore-space of meso-porous silicas while the internal surfaces of micro-porous silicas are not accessible to phenanthrene sorption, possibly due to the presence of physi - or chemi-sorbed water that may sterically hinder the diffusion of phenanthrene inside water-filled micro-pores. By contrast, the internal surfaces of these micro-porous silicas are accessible to phenanthrene when incorporation methods are employed which assure that pores are devoid of physi-sorbed water. Cons equently, when phenanthrene was incorporated into these particles using either supercritical CO2 or via solvent soaking, the aqueous desorption kinetics were extremely slow indicating effective sequestration of phenanthrene inside micro-porous particles. Finally, a two - compartment conceptual model is used to interpret the experimental findings and the implications for contaminant fate and transport are discussed.

Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments

Provide a better description of the processes by which non-polar compounds are retained by sediments and subsequently released. The objective will be reached through a combination of theory and experimentation with model porous materials and natural sediments. Focus is on the behavior of carbon tetrachloride in aquifer sediments.

Annual Report submitted on the PNNL portion of EMSP Project No. 86729

A migration-resistant fraction (MRF) is a portion of a polluted sediments contaminant inventory that exhibits slow release. Slow release is a key process that controls organic contaminant transport and fate in a plume long after the major portion of the contaminant inventory of a source term has been depleted or removed. Slow release rates are not well understood nor are they commonly accounted for in subsurface numerical transport models. In this project, we propose to study the accumulation and slow-release behavior of carbon tetrachloride (CCl4) MRF as a function of time, contaminant concentration and different physicochemical properties of sediments. Both model materials that mimic the physical/chemical properties of sediments and natural sediments will be used in project studies. Experiments will be conducted at macro- and microscopic scales under both unsaturated (Washington State University-WSU) and saturated conditions (Pacific Northwest National Laboratory-PNNL). The results will be used to (1) develop a mechanistic description of slow release of CCl4 in the subsurface environment and (2) lay the groundwork for improving the robustness of numerical models that predict organic contaminant transport and fate under natural conditions. The outcomes of this study are expected to improve the conceptual model of CCl4 subsurface transport and fate at different physical scales and have an impact on remediation and site closure decision-making at Department of Energy (DOE) sites, especially in situations involving the potential application of natural attenuation. This report summarizes work performed on the PNNL component of the project after the first 8 months of a three-year project. Progress on the WSU component of the project is addressed under a separate annual report submission.