During remedial action at a nearby contaminated site (the Ashland 2 site), Rattlesnake Creek was identified as an area of concern containing radiologically-contaminated sediments. The U.S. Army Corps of Engineers (USACE) was responsible for cleaning up this property, and developed site-specific derived concentration guideline level (DCGL) cleanup requirements consistent with the Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM). Because of uncertainty about the distribution of contamination within the creek, the USACE used the Triad Approach to collect high-resolution sediment data and design the remedial action. Systematic planning helped target the areas of concern, develop a CSM, and identify data gaps for the remedial design. The resulting pre-remediation sampling and analysis plans were designed to support excavation planning needs and closure requirements in areas where contamination was not encountered above DCGL standards. Judicious use of real-time technologies such as XRF and gamma walkover surveys minimized expensive off-site alpha spectrometry analyses, and at the same time, provided the ability to respond to unexpected field conditions. A demonstration of method applicability (DMA) study for the XRF instrument showed that total uranium could be used as an indicator chemical (a "surrogate") for the radionuclide Thorium-230. A decision rule to differentiate "clean" vs. "dirty" areas of the creek was derived based on total uranium for application of XRF in the field. Pre-excavation data collection using this decision rule successfully delineated site contamination in the Spring and Summer of 2004. Excavation of sediments followed in Summer 2005, and the site was officially closed in September 2006.
|Site Name||Rattlesnake Creek|
|Site Type||Uranium Mining|
|Project Lead Organization||USACE - Buffalo District|
|Project Lead Type||U.S. Army Corps of Engineers Lead|
|Triad Project Status||Field Program Completed|
|Reuse Objective Identified||Yes|
Rattlesnake Creek is one of multiple sites in the Buffalo area that are associated with the processing of uranium ores in the 1940s by the Manhattan Engineer District (MED), a predecessor of the U.S. Department of Energy (DOE). The neighboring Ashland 1, Ashland 2, and Seaway sites contained surface and subsurface soils contaminated with radionuclides from MED activities. Rattlesnake Creek is a natural intermittent channel that originates on the Ashland 1 site, passes under the Seaway site via a culvert, and then flows through the Ashland 2 site before joining a tributary of the Niagara River. A Record of Decision (ROD) was issued by the USACE for the Ashland 1, Ashland 2, and Seaway sites in 1998 and identified Uranium-238, Radium-226, and Thorium-230 as contaminants of concern (COCs). The ROD further determined, based on the distribution of COCs, that if soil containing more than 40 picocuries per gram (pCi/g) Thorium-230 was removed from the Ashland sites, the residual concentrations of other COCs would be protective of human health and the environment, and also in compliance with other applicable or relevant and appropriate requirements (ARARs) for the sites. Therefore, the principal remedial action objective (RAO) for the Ashland sites was to excavate soil exceeding 40 pCi/g Thorium-230 and transport this soil off site for licensed and permitted disposal.
The remedial investigation (RI) work originally conducted prior to the ROD for the Ashland 1, Ashland 2, and Seaway sites did not identify Rattlesnake Creek as an area of concern. As excavation proceeded at the Ashland 2 site, however, it became clear that contaminated soils had been carried into the streambed and deposited as sediments within the primary stream flood plain. Additional investigative work confirmed that Rattlesnake Creek contained radionuclide contamination that had originated from the Ashland and Seaway properties.
The proposed remedial action for Rattlesnake Creek was the same as for the other Ashland sites: excavation and removal of soils contaminated above a site-specific cleanup level. In addition to the lack of historical data, however, the creek presented other unique challenges in developing an efficient characterization approach and remedial design. For example, it appeared that the contaminated sediment layer was covered in many places by more recent sedimentation. The gamma walkover surveys that had been very effective field characterization tools for the other Ashland sites are useful only for surficial soils and cannot be used to detect buried contamination. In addition, Thorium-230 was the principal contaminant in Rattlesnake Creek, and appeared to be the driver for remedial efforts. However, Thorium-230 was not identifiable in the field with currently available real-time measurement technologies. Quantitative estimates of Thorium-230 at risk-based concentrations of interest require alpha spectroscopy, an expensive and time-consuming procedure usually conducted in a fixed-based laboratory setting. The project team was able to address these issues through careful systematic planning, allowing for successful application of field-based HRSC techniques.
Collection of high-resolution pre-excavation data using the Triad Approach took place in the spring and summer of 2004. More than 350 direct push technology (DPT) cores were obtained down the length of the creek, and more than 2,000 XRF analyses were performed on individual 15-cm intervals in these cores. In conjunction with the XRF analyses, more than 800 samples were sent for more definitive off-site alpha spectroscopy analyses. The end result of the HRSC work was that approximately six acres of the Rattlesnake Creek bed was identified as requiring remediation, with the proposed excavation footprint primarily derived from the spatially dense XRF results.
Remedial action was performed at Rattlesnake Creek in May through September 2005. Approximately 18,700 cubic meters (33,300 tons) of soil was excavated over the six-acre footprint. Completeness of the excavation and residual radioactivity were assessed through gamma walkover surveys along with sampling and off-site laboratory analysis in accordance with site closure guidance in MARSSIM and State of New York quality assurance/quality control (QA/QC) requirements. The confirmation sampling program demonstrated that residual concentrations of COCs were below applicable risk-based criteria. Redevelopment of the site as an industrial park began in late 2005, and the site was officially closed by USACE and the State of New York in the summer of 2006.
The project objective was to completely delineate radionuclide contamination in Rattlesnake Creek sediments using a Triad-based high-resolution approach in a single expedited field investigation, incorporating site closure requirements and excavation planning needs so that cleanup could proceed immediately in a subsequent mobilization.
Using a Triad Approach within a MARSSIM framework provided a means for expediting and increasing the efficiency of the decision-making and data collection process. The Rattlesnake Creek site provides an example of how the two approaches can be implemented in tandem to better integrate pre-remedial design, remediation support, and data collection in compliance with MARSSIM Final Status Survey (FSS) requirements for closure.
Initial estimates identified approximately 41,000 square meters (10 acres) of contaminated sediments in the creek bed, with an estimated excavation volume of 25,000 cubic meters. The pre-excavation HRSC sampling program identified additional contaminated zones, but at the same time was able to refine and reduce the overall area requiring remediation. The USACE had enough confidence in the contaminated soil volume estimate to use a fixed price contract for removal. The final excavation based on the high-resolution Triad data covered approximately 6 acres and removed slightly less than 20,000 cubic meters of sediment. The remediation was completed on schedule and within budget. State acceptance of the Triad data set and the subsequent excavation allowed redevelopment to begin shortly after the excavation program was completed, followed by complete closure of the site within a year (with only two years from site investigation to closeout).
Cost and time savings were not quantified. However, it is believed the Triad Approach helped to reduce the overall project cost by saving time and money in the following ways:
In the assessment of the project team, the use of real-time field tools under the Triad Approach significantly reduced overall analytical costs and delays by minimizing the number of samples requiring expensive alpha spectroscopy analyses for radionuclides at off-site laboratories.
A FSS plan was developed for the creek bed, consistent with MARSSIM guidance. The FSS development process included establishment of a CSM that described the likely contamination scenario, outlined the area of concern, and identified data gaps and needs to support remediation and closure. Consideration of the CSM indicated that the distribution of the COCs in the sediments of the creek was different than the distribution of those same radionuclides in the soils at the other Ashland sites, as a result of the way the material was transported and differences in solubility of the radionuclides. In order to address these differences, the USACE developed unique site-specific DCGLs in accordance with MARSSIM for use during the remediation of the Rattlesnake Creek area. An Explanation of Significant Differences (ESD) document was prepared in compliance with CERCLA for the Rattlesnake Creek portion of the Ashland sites to address the additional creek contamination and to document the DCGLs.
Upon completion of the FSS plan, a pre-excavation Field Sampling Plan (FSP) was developed to fill the data gaps in the CSM (i.e., to provide data that would support better contaminated volume estimates, and provide more definitive excavation footprints). Both the FSS plan and the FSP benefited from using the USACE Technical Project Planning (TPP) process during development. The project team met with data users and decision-makers, including regulators, to identify issues, data needs, data quality objectives (DQOs) and technical approaches, prior to and during the development of the plans.
In devising a technical approach for the FSP, the challenge was to find a surrogate analyte for Thorium-230 that could be measured using real-time techniques. The surrogate selected was total uranium and the real-time technique employed was field-portable (FP-XRF). A review of limited existing sample results revealed that almost all samples with Thorium-230 results greater than the DCGL had total uranium values greater than 90 parts per million (ppm). The majority (>80%) of samples that had a total uranium value greater than 300 ppm also had Thorium-230 greater than DCGL requirements. Total uranium results between 90 and 300 ppm constituted an initial "uncertainty region" where decisions regarding the presence or absence of thorium above its DCGL could not be made with acceptable certainty using uranium measurements. Uranium at these levels in soil is difficult to detect with gamma sensing equipment in the field but is well within range of the XRF.
Project management and technical leadership (engineering oversight) for the project was the responsibility of the USACE Buffalo District. Additional technical support for the pre-excavation sampling program and Triad facilitation was provided by DOEs Argonne National Laboratory. Cabrera Services provided contractor technical support to USACE for the FSS, and Sevenson Environmental Services provided field contractor support for the remediation program. Technical oversight and concurrence on the site remediation and closure was provided by the New York State Department of Environmental Conservation (NYSDEC).
The Triad dynamic work strategy (DWS) included surveying each soil core at 15-cm intervals with a FP-XRF in a field-laboratory for real-time measurements of uranium concentrations. Locations for which all soil core intervals contained less than 90 ppm total uranium were deemed ready for FSS sampling (that is, closure verification sampling) that included alpha spectroscopy analysis of a surface sample and a sample homogenized over the length of the subsurface core. Locations that yielded one or more core intervals with greater than 300 ppm total uranium were assumed to contain greater than the Thorium-230 DCGL and identified as requiring remediation. Locations where the highest core interval total uranium value was between 90 and 300 ppm were deemed suspect, and a sample from those intervals was sent for alpha spectroscopy analysis for Thorium-230 to manage the analytical uncertainty involved with using uranium as a surrogate analyte. Locations with cores that yielded elevated uranium in the bottom interval were re-cored to a greater depth to make sure contamination was vertically bounded.
The availability of real-time XRF data allowed the characterization work to respond to several unexpected conditions. For example, contrary to the initial CSM, there was evidence that the contaminated layer extended deeper than 1 m in some areas. In these cases, the GeoProbe DPT rig was used to retrieve a deeper core to fully characterize contamination depths. As another example, soil mounds were observed along the creek bed for one segment of the stream. Preliminary XRF analysis of these mounds identified contamination that then led to a more thorough investigation. The mounds apparently resulted from historical creek trenching activities. Finally, the XRF detected the presence of other heavy metals (particularly lead) in the Rattlesnake Creek sediments. While not COCs from a FUSRAP perspective, lead levels were high enough to pose potential Resource Conservation and Recovery Act (RCRA) disposal issues. The real-time XRF data allowed for more accurate costing of disposal and waste profiling to ensure that requirements of potential disposal facilities would be met.
The primary decision rule for the project is introduced in the "Systematic Planning" and "Dynamic Work Strategy" sections of this profile. Over the course of the field investigation, total uranium concentrations from the XRF were found to be an excellent and cost-effective way to identify the impacted sediment zones. This zone was approximately 45 cm thick and ranged in depth from 0 to 30 cm below the surface. However, the decision rule was refined based on the data collected. For most of the creek, comparison of the XRF total uranium results with DCGL exceedances from off-site laboratory samples over the course of the study suggested that 40 ppm total uranium would be a more appropriate investigation level for the XRF than the 90 ppm initially derived from historical data.
In the north branch of the creek, Thorium-230 detections in the absence of uranium indicated that any decision rule based on XRF total uranium concentrations could not be applied in this area. Thus, the characterization program in the north branch had to rely on alpha spectroscopy Thorium-230 analyses from an off-site laboratory.
Because the performance of the XRF was critical to the success of the DWS, a DMA study was performed prior to the initiation of fieldwork. This study made use of selected archived samples from the limited previous characterization activities at the Rattlesnake Creek site. The study demonstrated that practical detection capabilities were well below 90 ppm for total uranium. As noted previously, samples with total uranium results between 90 and 300 ppm were sent off site for alpha spectroscopy analysis during the pre-excavation sampling program. The results from these samples were used both to verify the presence or absence of Thorium-230 contamination above DCGL requirements, and to monitor the performance of the 90 and 300-ppm investigation levels as surrogate measurements for Thorium-230.
The XRF total uranium results displayed good correlation with uranium concentrations determined by alpha spectroscopy (r2 = 0.719) see Exhibit 2 of Case Study (PDF, 74 KB), with observed detection limits around 20-ppm total uranium. The scatter in the XRF results relative to the laboratory method was attributed to the differences in sample preparation and analytical parameters measured between the two methods. In the case of XRF, the surface concentration of a homogenized, air-dried sub-sample was measured (i.e., a sample approximately 2 cm x 2 cm x 1-2 mm). In the case of alpha spectroscopy, these air-dried sub-samples were subjected to further sub-sampling and extraction at the laboratory before the spectroscopy was performed. XRF measures total uranium directly while alpha spectroscopy provides an estimate of Uranium-238 activity concentrations, with the total uranium mass concentration inferred by assuming naturally occurring uranium isotopic ratios.
TQRS not prepared
The Triad-based, high-resolution pre-excavation sampling program was subject to routine USACE QA/QC activities. Contractor project QC was maintained through the implementation of project specific Quality Control Plans (QCPs) and Quality Assurance Project Plans (QAPPs.) The USACE QA process included having a USACE construction inspector and/or health physicist on-site to ensure that plans and proper procedures were implemented.
A QC review of field data was performed as it was generated. The review included an examination of global positioning system (GPS) printout data, instrument calibration and QC checks (blanks and reference materials), review of procedures, and discussion of findings. Upon completion of the QC process, USACE performed a QA review of the field and laboratory data sets. Argonne National Laboratory performed an independent review of field characterization data, as well as all closure (FSS) data.
Field duplicates and QA splits were compared to the original samples as a measure of precision. All samples used to close-out the Ashland sites were found to meet the required quality standards. NYSDEC also collected many splits and biased samples. NYSDEC shared the results of their sampling with USACE, and NYSDEC results were used to adjust decision-making as appropriate.
Field analytical data was managed using XRF instrument software and downloaded to spreadsheets for review and evaluation. The RESidual RADioactivity computer code (RESRAD) version 6.10 was used to derive the DCGLs. Bayesian Approaches to Adaptive Sampling (BAASS) software was used for volume estimation and uncertainty calculations. ArcView software was used for mapping and data presentation purposes. A secure password-protected web site was used for posting and distributing maps and datasets during remediation work. This web site was updated daily.
|Site Closeout Report for the Ashland 1 (Including Seaway Area D), Ashland 2, and Rattlesnake Creek FUSRAP Sites (2.3 MB)|
|Triad Case Study: Rattlesnake Creek. (184 KB)|
|USACE. Technical Project Planning (TPP) Process (1.3 KB)|
To update this profile, contact Cheryl T. Johnson at Johnson.Cheryl@epa.gov or (703) 603-9045.