Jon Fancher
CH2M Hill Hanford
3190 George Washington Way, Suite A
Mail Stop X3-40
Richland, WA 99352
509-373-5598
Fax 509-373-9779

Analytic systems currently used by remedial action projects in the Hanford 100 Areas provide data for project management and waste site closeout. The combination of systems most commonly used includes in situ radiological surveys, rapid-turnaround ex situ sample analysis, and full protocol laboratory analysis. Increasing the use of in situ radiological surveys can greatly reduce the number of ex situ samples needed during the decision-making and site closeout processes. This results in a significant decrease in analytic costs.

In the 100 Areas at the U. S. Department of Energy’s Hanford Site in southeast Washington State, there are nine water-cooled, graphite-moderated plutonium production reactors. Leaks in the reactor effluent transfer systems and intentional disposal to cribs and trenches contaminated soils in the vadose zone. Remediation activities, consisting of excavating and transporting contaminated soil and wastes, began in July of 1996. In the first eighteen months of excavation, over 500,000 tons of low-level and mixed waste have been safely excavated and disposed. Excavation and site closeout activities have been guided by the combination of analytic systems described in this paper. Historically, in situ screening has been used at some waste sites as a tool for guiding day-to-day activities. Other waste sites depended largely on ex situ samples. As a result, remediation activities were more costly and took longer to complete at those sites where ex situ sampling was the primary method for determining the remediation approach.

Supplementing ex situ sampling and full protocol laboratory analysis with the field screening technology has greatly improved the time and cost efficiencies of remedial actions. At one waste site, the 116-C-1 Waste Disposal Trench initially estimated the collection of over 310 ex situ samples during site excavation. However, field screening was used extensively in lieu of ex situ sample collection. From this large site, over 100,000 tons of contaminated soils were shipped to Environmental Restoration Disposal Facility (ERDF). Due to utilization of radiological field screening, only 104 ex situ samples were necessary prior to verification sampling at this site. The net result was that over 200 unnecessary projected samples were avoided, resulting in reduced analytic costs of over $200,000 on one waste site.

Field screening methods are used to find "indicator species" of easily measurable contaminants. The full contaminant list for many waste sites often exceeds 13 contaminants. The most commonly used field portable direct reading radiological instruments are most sensitive for gamma-emitting radionuclides such as Cs¹³⁷. Since most contaminants are collocated with other contaminants, initially screening for an indicator is useful in guiding subsequent excavation. Other contaminants, such as beta emitters Sr⁹⁰, Ni⁶³, and U²³⁸ , are not easily detectable with direct reading instruments at low levels, but since they are usually located with Cs¹³⁷ the contaminated soil can be identified and removed.

Prior to waste site excavation, a pre-excavation survey is performed to indicate whether the surface soil is suitable for testing as clean overburden. If contamination exceeds the cleanup criteria, the material is excavated and sent to the ERDF. If no contamination is detected, or it is below cleanup criteria, the overburden is removed and stockpiled for later use as backfill material.

During excavation, in situ radiological surveys are performed to scan for gamma-emitting radionuclides (1) or other contaminants, typically using Sodium Iodide (NaI) detectors. Such surveys guide day-to-day excavation activities and provide information for determining the location for collection of ex situ samples for rapid turn-around analysis. Use of in situ radiological surveys prevents collection of excessive ex situ samples during the ongoing excavation process. In addition, in situ surveys with direct-reading instruments enable technicians to provide immediate results to decision makers. They also facilitate rapid delineation of contaminant plumes, allowing excavations to expand as necessary to include all contaminated areas.

During excavation, using information from in situ radiological surveys, samples are collected for analysis at onsite and commercial rapid-turnaround laboratories. For example, onsite gamma energy analysis results, used to confirm the NaI screening results, are typically available within 2 to 24 hours. This gamma energy analysis speciates most of the easily detectable gamma-emitting contaminants. A mobile analytic laboratory provides Field screening for selected metals via X-ray fluorescence. Results are typically available within 2 to 4 hours. Onsite immunoassay analysis for polychlorinated biphenyls and other contaminants provides data to decision makers in 1 to 2 hours.

Commercial rapid-turnaround laboratory analysis results for radionuclides and metals typically are received within 24 to 72 hours. Rapid analysis for semi-volatiles is available with a 7 day turnaround time. Rapid commercial sample analysis is used to confirm the in situ radiological surveys and identify individual radionuclides. Rapid commercial analysis also is used to confirm field screening and identify non-radionuclide contaminants.

After in situ radiological and rapid ex situ sample analyses indicate site cleanup goals have been attained, verification samples are collected for analysis by a full protocol commercial laboratory. The number of samples collected is statistically based, taking into account sample variability, cleanup goals, and the square footage of the waste site. Each waste site is divided into sampling nodes. Samples are then collected at locations that have been determined using a random number table.

Closeout samples are shipped to a commercial laboratory. Typical turnaround time for analysis is 15 working days. As necessary, full data validation packages accompany closeout sample analyses. A commercial laboratory normally provides data validation packages in 45 days. Upon receipt of sample results, a summary statistics table is prepared that provides the 95^th upper confidence interval of the mean of the samples (2). The summary statistics table is used to provide radionuclide input values for verifying attainment of site cleanup.

The specific 100 Areas Record of Decision contains three remedial action objectives (3,4). The first is to protect from direct exposure to contaminants in the soils. This has been further defined for radionuclides to mean any soil in the shallow zone (less than 15 feet deep) that will result in a dose of 15 mrem/yr or more above background must be removed. A lookup table was produced using the Residual Radioactive Material Guidelines (RESRAD) computer code that is used to guide excavation activities. The lookup table lists each of the contaminants and the individual value in soil that would exceed the 15 mrem/yr cleanup criteria. At Hanford, non-radioactive contaminants must be below the State of Washington Model Toxics Control Act (MTCA) values.

The second remedial action objective is to minimize impacts to the groundwater and the Columbia River. Specifically, residual contaminants should not result in future conditions in which groundwater or surface water would exceed Maximum Contaminant Levels (MCLs). Deep zone sample data provide input values to the RESRAD computer code for assessing the impact of the remaining soil contaminants on groundwater. The third remedial action objective is, to the extent practicable, return soil concentrations to levels that allow for unlimited future use and exposure.

After closeout samples have been collected, the laboratory data is evaluated, validated, and interpreted by project scientists. For determining if the shallow zone (direct exposure) meets the remedial action objective, the 95^th upper confidence interval of the mean of the samples collected in the shallow zone is compared to the single radionuclide lookup values via a sum of the fractions calculation. For determining if the deep zone meets the remedial action objective, the 95^th upper confidence interval of the mean of the deep zone samples is input into RESRAD. The final RESRAD output provides individual groundwater concentrations below the waste site during the selected time period, as well as dose exposure levels and soil radionuclide concentrations.

After completion of the sum of the fractions comparison for the shallow zone and the RESRAD evaluation of the deep zone, if the results indicate that remedial action goals have been met, a final verification package is prepared. After completion of the required elements of the verification package for the waste site, a National Priorities List agreement change form is signed by the lead regulatory agency. Finally, the waste site is backfilled, completing the waste site closeout process.

Basing excavation guidance on in situ screening for gamma-emitting radionuclides is a low-cost, easy to use method, saving hundreds of thousands of dollars. In situ screening is more indicative of contaminant distribution than could be shown through collection of numerous "grab" samples during excavation. Coupling in situ screening with rapid ex situ sample analysis for confirmation of individual contaminants of concern and full protocol sampling for waste site closeout creates an efficient and cost-effective system for large site remedial actions. The RESRAD computer code has proven successful in assessing the remaining dose and implementing site closeout.

1. U.S. Department of Energy, Richland Operations Office, "100-BC-1, 100-DR-1, 100-HR-1 Sampling and Analysis Plan", DOE/RL-96-22, Richland, Washington (1996).
2. U.S. Department of Energy, Richland Operations Office, "Remedial Design Report/Remedial Action Work Plan for the 100 Area", DOE/RL-96-17, Richland, Washington (1996).
3. U.S. Environmental Protection Agency, "Interim Action Record of Decision for the 100-BC-1, 100-DR-1, 100-HR-1 Operable Unite, Hanford Site, Benton County, Washington", U.S. EPA Region 10, Seattle, Washington (1995).
4. U.S. Environmental Protection Agency, "Amendment to the Interim Action Record of Decision for the 100-BC-1, 100-DR-1, 100-HR-1 Operable Unit, Hanford Site, Benton County, Washington ", U.S. EPA Region 10, Seattle, Washington (1997).