ML032190383
| ML032190383 | |
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| Issue date: | 09/19/2003 |
| From: | James Shepherd, Gerry Stirewalt NRC/NMSS/DWM |
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3D Geospatial Property Models for Characterization and Analysis of Uranium in Groundwater: An Innovative Approach to Regulatory Decision-Making By Gerry L. Stirewalt, MANDEX, Inc. and James C. Shepherd, U.S. Nuclear Regulatory Commission Introduction The primary goal of this modeling effort was to develop three-dimensional (3D) geospatial property models for characterization, conceptualization, and analysis of uranium contamination in groundwater at a former uranium processing facility in Oklahoma. The models were a critical part of the regulatory decision process because they made it possible to visualize variations in subsurface uranium contamination in the complex geohydrologic setting at the site and consequently assisted staff of the U.S. Nuclear Regulatory Commission (NRC) with assessment of the site for the purpose of facility decommissioning.
The facility, owned by Sequoyah Fuels Corporation (SFC), is located in Sequoyah County, east-central Oklahoma, about 25 miles southeast of Muskogee near the confluence of the Arkansas and Illinois Rivers. The main industrial processing area comprised 85 acres of the total 600-acre site. Spills of nitric acid solutions containing uranium and radioactive decay products in this area near the main processing (MPB) and solvent extraction (SXB) buildings resulted in contamination of groundwater during operation of the facility between 1970 and 1993. Additional release of radioactive elements to surface water impoundments from the sanitary water system and laundry as well as leaching of radioactive materials from discarded equipment and stored waste containers also contaminated groundwater at the site during this time.
Site Hydrostratigraphy and Data Based on hydrologic properties of stratigraphic units at the site, SFC staff defined three distinct groundwater system aquifers (Terrace, Shallow, and Deep) and intervening aquitards as shown in Table 1. Pennsylvanian-age bedrock comprises the seven indurated stratigraphic units of the aquifer systems and the two aquitards (SFC, 1998). Bedrock units are interbedded shales and sandstones of the Atoka Formation having a regional dip of one to four degrees south-southwest and a total thickness of about 390 feet in the facility area (SFC, 1998). Shales are fractured and form water-bearing units, while thicker indurated sandstones are aquitards (SFC, 1998).
The uppermost Terrace groundwater system (TGWS) aquifer is composed of unconsolidated Quaternary-age Pleistocene terrace and alluvial deposits and a basal Pennsylvanian shale of the Atoka. The Shallow groundwater system (SGWS) aquifer, made up of three shale and two sandstone units of the Atoka Formation, is separated from the overlying TGWS and underlying Deep groundwater system (DGWS) aquifers by Atoka sandstone aquitards (i.e.,
Sandstones 1 and 4, respectively, in Table 1). TGWS units do not extend beyond property lines of the facility, and unconsolidated deposits of the TGWS, interpreted as a perched aquifer, are not everywhere saturated (SFC, 1998).
Unconsolidated TGWS deposits are thickest (16.4 feet) near the southwest corner of the MPB in the east-central part of the site and thin in all directions away from that area (SFC, 1998). TGWS and SGWS units are discontinuous on the western side of the site adjacent to the Illinois River due to erosion. The Sandstone 4 aquitard underlies the entire facility area, as does Shale 5 of the DGWS (SFC, 1998). The only significant fresh water in the facility area, suitable for irrigation and watering stock, occurs in alluvium along the Illinois and Arkansas Rivers where up to 900 gallons per minute flow rates have been measured (SFC, 1998).
Table 1. Groundwater System Aquifers and Associated Stratigraphic Units Groundwater System Aquifer Stratigraphic Units Terrace (TGWS)
Terrace Deposits/Alluvium, Shale 1 Aquitard Sandstone 1 Shallow (SGWS)
Shale 2, Sandstone 2, Shale 3, Sandstone 3, Shale 4 Aquitard Sandstone 4 Deep (DGWS)
Shale 5
Although groundwater data were collected by SFC staff during the entire operational period of the facility, annual data systematically acquired from MW-series monitoring wells in the three groundwater system aquifers since 1991 were used for construction of 3D geospatial property models. This annual sampling program, designed to characterize lateral and vertical distribution of uranium in groundwater over time in the three aquifer systems, has continued to the present. Highest uranium concentrations are consistently located in the vicinity of industrial processing and storage facilities, and data clearly indicate elevated concentrations (i.e., >10,000 micrograms/liter) in both the TGWS and SGWS aquifers in the vicinity of the MPB and SXB.
SFC willingly provided the database employed for developing 3D geospatial property models of the site. Sample location data for MW-series monitoring wells consisted of coordinates (x,y,z) defining sample location points (x,y) at the ground surface and sample depths (z) below the ground surface. The datum from which sample depths (z) were measured was developed from ground surface elevation data by construction of a 2D grid representing surface topography. Results of laboratory analyses of uranium concentrations (micrograms/liter) for annual groundwater samples collected at coordinates (x,y,z) in the TGWS, SGWS, and DGWS aquifers since 1991 made up the property data set. Data on elevation of top of bedrock enabled construction of a 2D grid for topography of the buried bedrock surface and assessment of thickness of unconsolidated units of the TGWS aquifer overlying bedrock.
Model Development Analyzing subsurface contaminant data has commonly involved construction of 2D maps showing contoured concentration levels for each successive sample depth. The innovative approach applied for developing 3D geospatial property models for characterization, conceptualization, and analysis of uranium contamination in groundwater at the SFC site involved use of EarthVision (EV), a 3D geospatial modeling software developed by Dynamic Graphics, Inc. of Alameda, CA. Minimum tension gridding was used in construction of final property models rather than the optional kriging approach since this method honored the data closely.
A viable 3D geohydrologic framework model was initially constructed employing EV software so property data could be spatially located with accuracy in relation to subsurface geohydrologic units. The 3D framework model contained three hydrostratigraphic zones equivalent to the TGWS, SGWS, and DGWS aquifers and two aquitards.
The model was constrained by field data on mapped extent of stratigraphic units which make up the three aquifer systems. Three hydrostratigraphic zones and two intervening aquitards comprise the framework model, even though well log data indicate the zones and aquitards are composed of ten stratigraphic units as shown in Table 1. This simplification was deemed necessary because tops and bottoms of all units could not be accurately determined from well log information. Also, sampling intervals were set up to collect property data across stratigraphic units of each aquifer system, rather than in individual units of a system. Consequently, uranium concentrations could not be specified for each of the ten individual stratigraphic units.
Employing EV software and property data collected in April, May, June 1991 and April 1996, seasonally equivalent months for two non-consecutive years of data from the annual sampling program, 3D geospatial property models were constructed to illustrate variations in distribution of uranium contamination over time at the SFC site. Property data were formatted and incorporated into the geohydrologic framework model as ASCII files, and 3D contours (i.e.,
isoshells) were constructed to define subsurface contaminant plumes in 3D space. Figure 1 presents a portion of the April 1996 model focused on the main processing area to illustrate contaminant plumes in the three aquifer systems during that time period. Uranium concentrations reflect processing spills in the vicinity of the MPB and SXB. The 1991 model showed a similar relationship between plumes and processing facilities and clearly delineated the same source areas for uranium contamination in groundwater as the 1996 model.
Model Analysis and Uses The 3D geospatial property models proved invaluable for characterization, conceptualization, and analysis of uranium contamination in groundwater at the SFC site. The models made it possible to assess distribution of subsurface data and contaminants and variations in plume geometry in space and time; delineate probable contaminant source areas; define apparent vertical and lateral migration pathways; and calculate contaminant plume volumes for a range of concentration values. Visual analysis of subsurface data distributions in the models enabled identification of areas of sparse data and resulted in collection of additional site information from boreholes, resistivity studies, and trenching activity by SFC.
The property models suggest the DGWS aquifer was relatively isolated from the two overlying aquifer systems. In Figure 1, for example, monitoring well MW012B in the DGWS aquifer, with 8.5 micrograms/liter uranium in April 1996, is observed to occur below higher values in the TGWS (i.e., well MW012 with 270 micrograms/liter uranium) and SGWS (well MW012A with 12,300 micrograms/liter) aquifers. Such higher-than-background concentrations in the DGWS aquifer are not common in the data sets, and are believed to have resulted from downward movement of groundwater along boreholes from overlying aquifers during drilling of MW-series wells rather than from vertical connectivity along natural fractures. Contaminant plumes were not defined for the DGWS aquifer by the sparse data collected in that aquifer system. Although some uranium concentrations in the DGWS aquifer were higher than background, all were less than 44 micrograms/liter and consequently below the maximum contaminant level (MCL) for uranium in drinking water defined in 40 CFR 141.66(e) by the U.S. Environmental Protection Agency (U.S.
EPA, 2000). The MCL currently specified by the EPA for uranium in drinking water is 30 micrograms/liter.
Vertical connectivity was suggested between the TGWS and SGWS aquifers for both the 1991 and 1996 data sets, possibly along fractures in the intervening sandstone aquiclude (i.e., Sandstone 1) or along boreholes. Monitoring wells MW014 and MW014A in Figure 1 suggest vertical connectivity between these two aquifers in April 1996, as do wells MW025 and MW025A which exhibited April 1996 uranium concentrations, respectively, of 5000 and 1000 micrograms/liter. Well MW025A is not shown in Figure 1 since it is located in the SGWS aquifer and is therefore below well MW025 in the TGWS aquifer.
Lateral movement of uranium also appeared to have occurred in the two upper aquifer systems over time as evidenced by changes in locations of high concentrations and contaminant plume geometry. Lateral migration in the TGWS is suggested between 1991 and 1996 along an elongate, south-southwest-trending, shallow depression in the bedrock surface buried beneath terrace deposits near the southwest corner of the MPB. This depression, thought to exist by SFC prior to confirmatory modeling of this feature, was clearly depicted in the 2D grid of the buried bedrock surface constructed from elevation data for top of bedrock. High uranium concentrations from 1996 data in holes MW018 and MW010 (i.e., 8400 and 4700 micrograms/liter, respectively, in Figure 1) are located in this elongate depression. The 1991 data set showed 4160 micrograms/liter uranium in MW018. MW010 was not sampled in April, May, June of 1991 so comparison with seasonal 1996 data could not be made. Another pathway illustrated by the 2D grid of the buried bedrock surface, trending west-northwest from the SXB and partially reflected by current surface topography, was suggested as the probable lateral migration route for an April 1996 concentration of 840 micrograms/liter measured in well MW035. The source for this high was thought to be in the vicinity of the SXB, so in this case the 3D model revealed a lateral pathway not previously defined. Well MW035 is not located in the field of view for the portion of the April 1996 model shown in Figure 1.
Definition of contaminant plume movement over time is particularly important for assessing direction and rate of migration and the potential need for preventive measures to control contamination of groundwater outside facility property lines. By defining variations in plume movement and contaminant concentrations in 3D space over time using EV software, optimum locations for monitoring and collection wells can be specified. SFC did subsequently install monitoring and collection wells to limit movement of uranium contamination along the elongate depression in the bedrock surface extending from the southwest corner of the MPB.
Plume volumes were calculated in EV for different concentration ranges to determine amounts of saturated material in the TGWS and SGWS aquifers possibly requiring consideration for excavation or pump and treat operations at the SRC site. These volumes, calculated with due consideration for regulatory release limits, directly influence cost of potential site remediation activities. Based on preliminary calculations using the 1996 model (Figure 1), the volume of rock in which uranium concentrations in groundwater exceeded 50 micrograms/liter in the TGWS aquifer, and consequently greater than the current EPA drinking water standard of 30 micrograms/liter MCL (U.S.
EPA, 2000), was about 5.5 million cubic feet. If pump-and-treat operations are implemented at the site, this volume must be considered for determining treatment cost.
Finally, the 3D geospatial property models were employed to interactively illustrate data on extent, volume, and quantitative amounts of uranium contamination in the subsurface at the SFC site to managers who were required to make informed regulatory decisions related to site decommissioning. The models were also used as an exceptional visualization tool to explain extent of subsurface contamination in public meetings because they made it possible to examine distribution of contaminants in space and time in a manner which it is not possible to achieve with
traditional 2D maps. In addition, the 3D geohydrologic framework model developed for accurately locating property data in relation to subsurface geohydrologic units provides a conceptual model which should be considered when defining hydrostratigraphic units for flow and transport analyses, even if some simplification of reality must be implemented for undertaking those analyses.
Acknowledgments The authors are grateful to SFC for permission to publish results of these independent modeling efforts. All work was performed in collaboration with NRC staff under Contract Number NRC-02-00-008 with MANDEX, Inc.
Information presented in this paper does not necessarily reflect views or regulatory position of the NRC.
References Cited Sequoyah Fuels Corporation (SFC), 1998, Site Characterization Report.
U.S. Environmental Protection Agency (EPA), 7 December 2000, 40 CFR Part 141, Subpart G - National Primary Drinking Water Regulations:
Vol. 65, No. 236, p. 76748.
Biographical Sketches Gerry L. Stirewalt, MANDEX, Inc., 12500 Fair Lakes Circle, Suite 125, Fairfax, VA 22033-3804 Telephone (301) 415-5265; Fax (301) 415-5398; Email gls3@nrc.gov Gerry L. Stirewalt, Senior Engineering/Environmental Geologist, is Program Manager for 3D geospatial modeling and GIS work performed by MANDEX, Inc. for the U.S. Nuclear Regulatory Commission (NRC). He develops 3D models for visualization and analysis of subsurface geology and groundwater contamination at sites under NRC purview. He holds a PhD in geology and is a registered professional geologist in North Carolina and Oregon.
J. C. Shepherd, MS T7 F27, U.S. Nuclear Regulatory Commission, Washington, DC 20555 Phone: 301-415-6712 Fax: 301-415-5397 E-mail: jcs2@nrc.gov James Shepherd serves as a Project Engineer in the Decommissioning Branch of the U.S. Nuclear Regulatory Commission. He performs and directs activities related to the decommissioning and license termination of nuclear materials sites and former nuclear reactors. He received an M.S. from the University of Washington in Nuclear Engineering. He is a member of NGWA and NYAS.