Reconstruction of atmospheric concentrations and deposition of uranium and decay products released from the former uranium mill at Uravan, Colorado

Journal of Environmental Radioactivity 99 (2008) 1258–1278

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Reconstruction of atmospheric concentrations and deposition of uranium and decay products released from the former uranium mill at Uravan, Colorado Arthur S. Rood a, *, Paul G. Voilleque´ b, Susan K. Rope c, Helen A. Grogan d, John E. Till e a

K-Spar, Inc., 4835 West Foxtrail Lane, Idaho Falls, ID 83402, United States MJP Risk Assessment, 7085 East Bayaud Avenue, Denver, CO 80220, United States c Environmental Perspectives, Inc., 5800 S. Marbrisa Lane, Idaho Falls, ID 83406, United States d Cascade Scientific, 1678 NW Albany Avenue, Bend, OR 97701, United States e Risk Assessment Corporation, 417 Till Road, Neeses, SC 29107, United States b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 August 2007 Received in revised form 27 February 2008 Accepted 7 March 2008 Available online 29 April 2008

Radionuclide concentrations in air from uranium milling emissions were estimated for the town of Uravan, Colorado, USA and the surrounding area for a 49-yr period of mill operations beginning in 1936 and ending in 1984. Milling processes with the potential to emit radionuclides to the air included crushing and grinding of ores; conveyance of ore; ore roasting, drying, and packaging of the product (U3O8); and fugitive dust releases from ore piles, tailings’ piles, and roads. The town of Uravan is located in a narrow canyon formed by the San Miguel River in western Colorado. Atmospheric transport modeling required a complex terrain model. Because historical meteorological data necessary for a complex terrain model were lacking, meteorological instruments were installed, and relevant data were collected for 1 yr. Monthly average dispersion and deposition factors were calculated using the complex terrain model, CALPUFF. Radionuclide concentrations in air and deposition on ground were calculated by multiplying the estimated source-specific release rate by the dispersion or deposition factor. Timedependent resuspension was also included in the model. Predicted concentrations in air and soil were compared to measurements from continuous air samplers from 1979 to 1986 and to soil profile sampling performed in 2006. The geometric mean predicted-to-observed ratio for annual average air concentrations was 1.25 with a geometric standard deviation of 1.8. Predicted-to-observed ratios for uranium concentrations in undisturbed soil ranged from 0.67 to 1.22. Average air concentrations from 1936 to 1984 in housing blocks ranged from about 2.5 to 6 mBq m3 for 238U and 1.5 to 3.5 mBq m3 for 230Th, 226 Ra, and 210Pb. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Uranium Uranium processing Air dispersion modeling Air deposition Model validation

1. Introduction The former uranium mill at Uravan was located in western Colorado near the Utah border in a narrow canyon cut by the San Miguel River (Fig. 1). Uravan lies within the Colorado Plateau in an area known as the Uravan Mineral Belt. Uranium- and vanadiumbearing ore deposits, primarily in the form of the mineral carnotite [K2(UO2)2(VO4)2$3H2O], reside in the fluvial sands of the Salt Wash Member of the Morrison Formation, which forms the upper benches above Uravan. Uranium ores mined in the Uravan region during the assessment period (1936–1984) were moderate to low grade, containing about 0.1–0.3% U3O8. Around 1911, Standard Chemical located claims on the San Miguel River in an area known as the Ford Camp. A radium

* Corresponding author. Tel.: þ1 208 528 0670. E-mail address: [email protected] (A.S. Rood). 0265-931X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2008.03.009

concentrator known as the Joe Jr. Mill was the first mill in the region and was constructed on the south side of the San Miguel River in about 1912. Carnotite ores mined in the region, often containing over 1% U3O8, were processed at the mill from 1912 to 1923 when Joe Jr. Mill ceased operations. United States Vanadium Corporation (USV) bought the mill and mining claims from Standard Chemical in 1928 and overhauled the mill which was called the Hillside Mill for vanadium processing; however, operations of the Hillside Mill did not begin until 1936 after construction of facilities for ore crushing and grinding and salt roasting were completed (Alexandroff, 1995). At this time, construction of the town site began and the name Uravan (for the uranium and vanadium present in the ore) was adopted (Hamrick et al., 2002). The town was built near the mill and extended about 1.5 km down the San Miguel River valley. The housing units were identified as blocks that were labeled alphabetically. New housing units were added in 1950s and in the early 1960s. Unlike other western towns such as Grand Junction,

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Fig. 1. Atmospheric modeling domain for Uravan.

Colorado, tailings were not used intentionally in the construction of housing at Uravan. The population of the town fluctuated throughout its years of operation, ranging from a high of about 800 in 1970 (Dames and Moore, 1978) to a low of about 200 in 1984 before the mill shut down permanently (NRC, 1984). The town supported a school, medical clinic, general store, and gas station. During operation of the Hillside Mill, ore was hauled to the top of Club Mesa where it was gravity-fed to a crushing and grinding facility on the hillside. The crushed ore was then mixed with salt and roasted at 700–900  C to increase the solubility of vanadium. After the ore was roasted, it was quenched and leached in a water bath to obtain water-soluble sodium metavanadate. Subsequent treatment with sulfuric acid led to precipitation of red cake (sodium hexavanadate), which was filtered and washed prior to drying and fusion in a multiple-hearth furnace to produce purified V2O5. At this time, uranium was a byproduct of the process. In 1938, a pilot process to recover uranium from the vanadium tailings was added. The pilot plant facility became known as the WAA Plant and served as a prototype for the WSP Plant that extracted uranium from the accumulated vanadium tailings as part of the Manhattan Project. The WSP Plant operated from 1943 to 1945. The process involved an acid leach of the tailings and precipitation of a sludge containing both uranium and vanadium. The sludge was partially dried and shipped to Grand Junction, Colorado, for further processing.

The Hillside Mill and WAA and WSP Plants were closed in 1945 after the end of World War II. After renovations, mill operations were restarted in 1950 to produce uranium under contract with the Atomic Energy Commission. Initial operations employed salt roasting of the ore, followed by recovery of both uranium and vanadium. The A- and B-Plants (Fig. 2) were constructed during the period of 1950–1955 to replace the salt roasting operations used in the Hillside Mill. The B-Plant was entirely new, while the A-Plant incorporated some features of the Hillside Mill. B-Plant operations took place on top of Club Mesa (Dames and Moore, 1978) where incoming uranium ore was crushed, sampled for uranium content, then subjected to grinding, leaching, and counter-current decantation. A-Plant operations took place on the valley floor where the concentrated uranium liquor was transferred via gravity-feed pipeline from B-Plant to either ion exchange or solvent extraction processes. Yellowcake drying and packaging also were performed in A-Plant. The A- and B-Plants operated until 1984, when the mill was permanently shut down due to low prices for yellowcake. The current owner, Umetco Minerals Corporation, demolished the town and mill in the early 1990s and is currently completing remediation of the entire property. The only remaining structures at Uravan are the field office and shop and historical structures that include the bunk house and community center. In this study, atmospheric source terms from 1936 to 1984 were reconstructed, and dispersion and deposition modeling

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Fig. 2. Detailed map showing locations of release points, tailings’ piles, and housing blocks. Buildings shown are circa 1975.

was performed using the CALPUFF1 (Scire et al., 2000a,b) atmospheric dispersion model. Releases prior to 1936 from the Joe Jr. Mill were not addressed in this study. Meteorological parameters were measured at the site because historical meteorological data for atmospheric transport modeling using a complex terrain model were lacking. Predicted air and soil concentrations were compared to measured values to evaluate model uncertainty. Model uncertainty and source term uncertainty were propagated through the model using Monte Carlo sampling to estimate the uncertainty in predicted concentrations in air and soil for the assessment period. This paper focuses on the atmospheric dispersion and deposition modeling of particulate releases from stacks and fugitive dust sources. Radiation doses resulting from inhalation of airborne particulates, as well as doses from exposure to radon, external radiation, and ingestion of locally grown foodstuffs, were calculated and reported in RAC (2007).

1 CALPUFF includes three modules: a meteorological model (CALMET), a dispersion model (CALPUFF), and a post-processor module (CALPOST).

2. Overview of methodology Atmospheric source terms were developed for uranium and its decay products from milling facilities and operations that released radionuclide effluent to the air. The radionuclides explicitly considered were 238U, 234U, 230Th, 226Ra, and 210Pb, although 234U was assumed to be in secular equilibrium with 238U in the ore and yellowcake product. The 210Pb decay products, 210Bi and 210Po, were assumed to be in secular equilibrium with their parent when released, but were then treated separately for computations involving atmospheric deposition, inhalation dose, and radionuclide build-up in soil. Radon-222 was not addressed through modeling because indoor radon measurements were available and were used to estimate radon exposure (RAC, 2007). The 238U decay products, 234 Th, 234Pa, and 234mPa, were assumed to be in secular equilibrium with their parent. For each identified source, dispersion and deposition factors were calculated across a grid covering the modeling domain using a complex terrain atmospheric transport model and 1 yr of sitespecific meteorological data. The grid had a 75-m node spacing. The dispersion factor (X/Q) represented the average concentration (X) divided by a unit release rate (Q). The deposition factor (j/Q) represented the atmospheric deposition rate (j) divided by a unit

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

release rate (Q). The product of the actual source emission rate and the dispersion factor provided the estimated air concentration from the source. Likewise, the product of the deposition factor and the actual source emission rate provided an estimate of the deposition from the source. A soil model was used to compute decay, ingrowth, and leaching from the soil layers of various depths. Inhalation doses for residents could then be calculated using age-specific dose coefficients and breathing rates. A resident could be placed at multiple locations in the model domain for any length of time within the assessment period beginning in 1936 and ending in 1984. Uncertainty in model estimates was based on the distribution of predicted-to-observed ratios (P/O) for the period 1979–1986 when continuous air monitoring data were available to establish the overall uncertainty in the model. The uncertainty factor established by the distribution of P/O ratios coupled with an uncertainty estimate for the source term was used to estimate the uncertainty in modeled concentrations before 1979. After 1979, uncertainty was based only on the distribution of P/O ratios. 3. Source terms Airborne effluent sources at Uravan included stacks, tailings’ piles, and suspension from contaminated roads (Table 1). Sources operated at different times during the assessment period. In the case of tailings’ piles, source geometry and the corresponding spatially integrated release rate changed with time. Emission rates for stacks were computed from process knowledge, ore processing rates, yellowcake and other production rates, and data from periodic measurements of stack effluents. Crushing and grinding operations also considered data published in NRC (1987) and EPA (1986) to estimate release rates. Tailings pile and road emissions rates were computed using the methodology in NRC (1987) and EPA (2006). Details of the derivation of emission rates are provided in RAC (2007). 3.1. Stack releases Location and release parameters (stack height, release velocity, and release temperature) for the A- and B-Plant stacks were obtained from Bartram (1980), digital aerial photographs, and plan

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maps of the A- and B-plants. Table 1 provides an overview of the different sources and release points, and Fig. 2 shows their locations. In January 1981, a new yellowcake facility was completed and went on line in A-Plant. The new yellowcake stack was located about 30 m northwest of the original stack. For convenience, the modeled stack was located between the old and new yellowcake stack coordinates. Building wake effects for the yellowcake facility were originally modeled using BPIP software (EPA, 1995). However, because the yellowcake building changed location and orientation in the early 1980s and other buildings were constructed and demolished in the vicinity of the yellowcake building, it was decided to model building wake for the entire operational history of the yellowcake stack in terms of initial sigmas (sy and sz). Initial sigmas define the initial horizontal (sy) and vertical (sz) dimensions of a puff introduced to the modeling domain. For modeling purposes, the four Aerofall stacks at the B-Plant were combined into a single effective stack. This was not expected to impact concentrations in the valley significantly because of the distance of B-Plant from the valley. Emissions from the leach acid kill stack also included releases from the primary leach stack. Building wake effects were not modeled for any of the B-Plant stacks because the B-Plant was approximately 500 m from the nearest Uravan resident. The salt roasting process involved multiple roasters and stacks enclosed in building located at the base of the hillside below the crushing and grinding operations. Emissions from the salt roasting facility were modeled as a single stack located on the hillside-edge of the salt roasting building. The stacks were located on the slope of the hillside, and, therefore, their base elevation was above the base elevation of the roaster building. Stack height above the base of the roaster building was estimated from historical photographs. The location of the roaster building and roaster stacks was estimated from a 1948 USV Uravan site map contained in a report by Burwell (1946). Building wake effects were estimated using the BPIP software, assuming the building and stack were on a flat surface. The release temperature was estimated from process knowledge and stack gas measurements documented in USV reports. The hillside crushing and grinding operations took place in a building with no exhaust stack. It was presumed that releases from this building were due to building leakage from open

Table 1 Uravan mill sources, release parameters, and total radionuclide emissions Source

Yellowcake stack Aerofall mill Leach acid killa Fine ore bin Hillside Mill Hillside Mill – Grizzly conveyerb Salt roaster process WAA dryerc WSP dryer WSP/WAA tailing haulingd Sample plante North river pondsf Tailings’ piles 1 and 2 Tailings pile 3 Ore hauling roads Ore storage pad a b c d e f g

Location

A-Plant B-Plant B-Plant B-Plant Canyon rim Club Mesa A-Plant A-Plant Valley Valley B-Plant Valley Club Mesa Club Mesa Valley and Club Mesa B-Plant

Years of operation

Stack height (m)

Release velocity (m s1)

Release temperature (K)

Particle size (mm AED)

Total release quantity (MBq)g

1950–1984 1956–1984 1956–1984 1956–1984 1936–1955 1936–1955 1936–1957 1944–1945 1943–1945 1944–1945 1956–1984 1936–1971 1956–1967 1968–1986 1936–1984

21.3 11.6 10.4 13.4 11 n/a 20.5 21.3 15 n/a n/a n/a n/a n/a n/a

3.25 11 6.3 15.9 n/a n/a 17 3.25 11 n/a n/a n/a n/a n/a n/a

323 284 284 284 284 n/a 628 323 433 n/a n/a n/a n/a n/a n/a

3 1.5 1.5 1.5 1.5 7.75 15 3 3 7.75 7.75 7.75 7.75 7.75 7.75

4.16  105 2.87  104 4.32  104 6.79  104 3080 7630 3.00  106 1.54  104 1.23  104 854 2270 19 229 64 616

3830 2.90  104 4.36  104 6.85  104 3110 8320 3.05  106 7780 6200 861 2290 75 13,400 3760 804

66 2.90  104 4.36  104 6.85  104 3110 8320 3.04  106 7780 6200 861 2290 88 15,830 4450 562

2960 2.90  104 4.36  104 6.85  104 3110 8320 3.04  106 7780 6200 861 2290 102 55,210 15,500 689

1936–1984

n/a

n/a

n/a

7.75

29

28

31

32

238

U

Includes releases from the primary leach stack. Releases from this source were included in releases from the ore storage pad. Releases from this source were included in releases from the yellowcake stack. Releases from this source were included in releases from the north river ponds. Releases from this source were included in releases from the ore storage pad. The north river ponds were originally a tailings pile that was converted to ponds in 1971. In general, 234U releases were the same as 238U, and releases of 210Po and 210Bi were assumed to be in secular equilibrium with

230

210

Th

Pb.

226

Ra

210

Pb

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windows. A point source with initial horizontal and vertical dispersion coefficients was used to model this facility. Based on historical photographs, the building was estimated to be about three stories (36 ft or 11 m) high. Horizontal dimensions of the building were estimated from a digital scan of the 1948 Uravan site map (USV, 1948). Release factors for all crushing operations ranged from 9.9  104 to 2.2  103 kg h1 per MT d1 of processed ore and were derived from NRC (1987), EPA (1986), and site-specific measurements (RAC, 2007). These release factors combined with ore processing rates were used to estimate radionuclide releases from hillside crushing and grinding operations. For modeling purposes, releases from the WAA Mill were assigned to the yellowcake stack because the WAA Mill was located close to the yellowcake building (it was likely the same structure) and the dryer used in the process was the same dryer that was later used in the yellowcake facility. Stack height and release parameters for the WSP Mill were obtained from Burwell (1946).

Disposals to tailings’ piles 1 and 2 were assumed to cease once tailings pile 3 was operational. The tailings pile on the north side of the San Miguel River was assumed to begin receiving tailings in 1936 and to stop receiving tailings in 1956 when B-Plant was constructed. Releases from the tailings pile on the north side of the San Miguel River ceased after 1971 when the tailings were excavated for construction of the North River ponds. This source also included releases from the loading and unloading of tailings that were processed for uranium during the Manhattan Project. Radionuclide concentrations in ore and tailings were obtained from Bartram (1980). The size of the ore storage pile was assumed to remain constant; therefore, no adjustment was necessary. The ore storage pad source area also included releases from the sampling plant stack (B-Plant), the coarse ore crusher (grizzly) operations for both the Hillside Mill and B-Plant, and releases from ore and tailings conveyance. In general, ore and tailings conveyance released more radioactivity to the air than suspension from ore and tailings’ piles because the generally light wind speeds observed in the vicinity of Uravan results in little suspension of particulate matter.

3.2. Tailings’ piles and ore storage areas 3.3. Roads Tailings’ piles 1, 2, and 3; the ore storage pad located on Club Mesa; and the old vanadium/uranium tailings on the north side of the San Miguel River were modeled for fine particle releases (Fig. 2). The sources were modeled as area sources in CALPUFF using the methodology in NRC Regulatory Guide 3.59 (NRC, 1987), which defines fine particles as having an aerodynamic equivalent diameter (AED) of less than 7.75 mm. The AED for spherical particles is given by NRC (1987) as rffiffiffiffiffi rp AED ¼ Dp (1) ru where Dp ¼ physical particle diameter (mm); rp ¼ particle density (g cm3); ru ¼ unit particle density (1 g cm3). Coarse particle releases were not included because the particle diameter (54 mm AED) is not respirable and the mean wind speed was lower than the threshold necessary to suspend particles of that size. The release rates are in terms of particulate mass flux per square meter. The total particulate flux is found by multiplying the release rate by the area of the source. Because the area of the pile changed with time (as the pile accumulated tailings) and only a single area was modeled in CALPUFF, the release rate was adjusted for the area of the pile. The area of the pile was assumed to be proportional to the ratio of the integrated ore processed at a given year to the total ore processed. The adjusted flux is then: Qi;j ¼ AFj RFw ðuÞCi j P Pk

AFj ¼

(2)

k¼1

PT

where Qi,j ¼ release rate of radionuclide i at year j (Bq m2 s1); RFw(u) ¼ wind speed-weighted suspension flux for tailings (g m2 s1); Ci ¼ concentration of contaminant i in tailings (Bq g1); AFj ¼ area factor for year j; Pk ¼ total ore processed from start of tailings pile to year j (MT); PT ¼ total ore processed during all years of operation of the tailings pile (MT). The wind speed-weighted suspension flux for tailings was obtained from NRC (1987) and was characterized in terms of six mean wind speed classes: 0.67, 2.5, 4.5, 6.9, 9.6, and 12.5 m s1. For the first two wind speed classes, the value of RFw was zero. The remaining four wind speed classes had RFw values of 3.9  107, 9.7  106, 5.7  105, 2.1 104 g (m2 s)1 and showed the marked dependence of suspension on the mean wind speed. Mean wind speeds were calculated using the meteorological data collected from a station on Club Mesa. Tailings’ piles 1 and 2 were begun in 1956, and tailings pile 3 was begun in 1968 (Hamrick et al., 2002).

The main haulage routes from the surrounding mines to the Uravan Mill included Colorado Highway 141 (paved), County Road Y11 (unpaved), and County Road E-22 (partially paved). Because unpaved roads release significantly more dust than paved roads, only releases from unpaved road segments were included. Road segments were modeled as 16 point sources distributed along County Road Y-11, County Road E-22, and along the haulage road to B-Plant. Each point source was modeled with initial sigma (s) values. The initial sy (horizontal dispersion) was set at 100 m, and the sz (vertical dispersion) was set at 2 m. The initial sigma sy of 100 m results in essentially an area source emission. The number of points representing the road source was limited by computational run times and maximum allowable output file size. Adding more points to the road source made little difference to the predicted concentrations in the housing blocks. Percent silt content and radionuclide concentrations in silt were obtained from Bartram (1980). The source term for the roads was calibrated to measured concentrations of airborne particulates and is discussed in Section 4.5. 3.4. Particle size Particle sizes (Table 1) for most stacks and tailings pile effluent were obtained from US Nuclear Regulatory Commission Regulatory Guide 3.51 (NRC, 1982). Based on the EPA Air Pollution Emission Factors AP-42 (EPA, 2006), a particle size of 1.5 mm AED was used for the Hillside Mill and road sources. A median particle size of 15 mm AED was used for the salt roaster based on emissions from zinc roasting in the 1972 edition of AP-42 (EPA, 1972) and data from Pazin and Meintin (1974). The geometric standard deviation for all particle sizes was set at a nominal value of 2. 4. Atmospheric modeling The air dispersion modeling environment at Uravan can be characterized as complex terrain. The town site and A-Plant were located in the San Miguel River valley at an elevation of about 1530 m (5020 ft) (Fig. 2). The valley is northwest–southeast trending and is about 350 m (1150 ft) wide at the town site. The BPlant was located about 100 m (330 ft) above the valley floor on the Club Mesa canyon rim. The Hillside Mill, as the name suggests, was located on the canyon wall between A- and B-Plants. Air flow in the San Miguel River valley was reported to be different from air flow on Club Mesa where B-Plant existed (Momeni, 1981). The Lagrangian puff dispersion model CALPUFF (Scire et al., 2000a,b) was

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selected to perform air dispersion calculations on the basis of its ability to simulate complex terrain, three-dimensional wind fields, deposition, and plume depletion. The CALPUFF model is composed of three modules: a meteorological module to compute threedimensional wind fields (CALMET), a Lagrangian puff transport module (CALPUFF), and a post-processor module (CALPOST). 4.1. Model domain and computational grid The model domain for the CALPUFF simulations (Fig. 1) encompasses an area of 27.5 km2 (6795 acres). The domain measures 5.5 km (3.4 miles) east–west and 5.1 km (3.1 miles) north–south. The southwest corner of the domain has coordinates (in UTM NAD27) 694000E, 4247000N. The extent of the domain was chosen to include the Uravan town site, the milling facilities including waste piles and ponds, major topographic features, and some of the surrounding area. The town of Uravan is situated slightly to the east of the center of the domain. The steep terrain at Uravan required relatively small grid spacing to resolve the canyon walls of the San Miguel River valley. A grid node spacing of 75 m was shown to adequately represent the topographic features while limiting the total number of nodes to 4891. Terrain elevations were obtained from US Geological Survey (USGS) 7.5-min Digital Elevation Model (DEM) data with a grid resolution of 30 m. The gridded terrain elevations are contoured and plotted in Figs. 1 and 2 and show major topographic features including the San Miguel River valley, Hieroglyphic Canyon, and Atkinson Creek Canyon. The vertical air column was discretized into seven layers with heights (above land surface) of 2, 60, 100, 300, 600, 1200, and 1500 m. In addition to terrain elevations, CALPUFF also requires land use data. Land use is important for defining the energy balance at each grid node and the surface roughness height. Land use data were obtained in the USGS Composite Theme Grid (CTG) format. The CTG resolution was less than the CALPUFF computational grid and in such cases, CALPUFF assigns a default land use value. It was necessary to modify by hand the default land use assigned by CALPUFF to values that reflected the historical land use. 4.2. Meteorological data Meteorological data required by CALMET include both surface observations and upper air observations. Surface observations include those taken routinely at airports and archived by the National Climatic Data Center and site-specific data. Upper air data are routinely taken at major airports around the country and characterize synoptic weather patterns in the upper atmosphere. Grand Junction Airport (Walker Field) was the station nearest to Uravan from where upper air data were obtained. Site-specific meteorological data were collected at A- and BPlant from 1974 to 1987, however, hourly records were only available from 1982 to 1987. These data included wind speed, wind direction, and temperature, but did not provide all the data required by CALPUFF. Therefore, site-specific meteorological data were collected over a 1-yr period to characterize the meteorology within the San Miguel River valley and Club Mesa. 4.2.1. Meteorological monitoring Two meteorological stations were installed in the Uravan vicinity: one in the valley near the current Umetco office and one on Club Mesa (Table 2). Data were electronically stored in a Campbell2 CR1000 data logger and recorded every 15 min. Meteorological data were downloaded to a portable computer every two to three weeks for a year beginning on 26 April 2005. During each

2

Campbell Scientific, Logan, UT.

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download, the data were reviewed for inconsistency and irregular patterns and instruments were visually inspected for damage. Data downloads and other observations were recorded in an instrument log book. The site-specific data were supplemented with cloud cover and ceiling height data obtained from the National Climatic Data Center for Grand Junction Walker Field Airport. The valley location required an AC power source for the heating element in the rain gage, and the Umetco office was the closest source of readily accessible AC power. The tower was mounted on a 2.7-m cargo storage shed that was located about 60 m northwest of the Umetco office. The storage shed had a readily accessible AC power source and was a sufficient distance from the office building (>10  the building height) to avoid building wake effects. The height of the anemometer (9.22 m) was also above the building wake from the storage shed (2.5  building height or 6.75 m, EPA, 2000). The Club Mesa station (URAVAN2) was located in an open, gently sloping area on the southwest corner of the region identified as the Club Mesa area in Fig. 1. Placement of the station near the former B-Plant was not possible because of the obstruction presented by tailings’ piles and ongoing remediation operations. The area selected was free of trees and physical obstructions and was sparsely vegetated with brush. Power to the station was provided by a 10-W solar panel. Except for the period beginning on 9 March 2006 at 20:45 and ending on 10 March at 03:45, there were no missing records from URAVAN1. During this period, a wet heavy snow was reported that appears to have temporarily frozen the anemometer (based on the measured temperature), resulting in a recorded wind speed and direction of zero. After the temperature warmed to above freezing, the anemometer returned to normal operation. A similar condition existed on the Club Mesa station (URAVAN2) during the period beginning on 11 March 2006 at 02:15 and ending on 11 March 2006 at 09:15. 4.2.2. Meteorological data processing All data were processed by the CALMET surface meteorological preprocessor, SMERGE. Meteorological variables that were recorded represented the average value during a 15-min accumulation period. Because the time resolution of CALMET data is 1 h, the 15min site-specific observations were converted to hourly average observations. Time-averaging was performed using the EPA timeaveraging protocol (EPA, 2000). Solar radiation measurements cannot be used directly in CALMET. Instead, cloud cover data are used to estimate the incoming solar radiation flux. However, direct solar radiation measurements can be used to estimate equivalent cloud cover. The incoming short-wave radiation is given in CALMET by (Scire et al., 2000a)   Qsw ¼ ða1 sin 4 þ a2 Þ 1 þ b1 N b2 (3) where Qsw ¼ incoming short-wave radiation (W m2); a1 ¼ net radiation constant from Holtslag and van Ulden (1983) (990 W m2); a2 ¼ net radiation constant from Holtslag and van Ulden (1983) (30 W m2); b1 ¼ net radiation constant from Holtslag and van Ulden (1983) (0.75); b2 ¼ net radiation constant from Holtslag and van Ulden (1983) (3.4); 4 ¼ solar elevation angle (degrees); N ¼ fraction of sky covered by clouds. Eq. (3) can be solved for N during daylight hours when Qsw is greater than zero: N ¼



Qsw 1 a1 sin 4 þ a2

 1=b2 b1

(4)

The sine of the solar elevation angle (sin 4) was calculated using the routine in CALMET, which is based on the work of Scire et al. (1984). Eq.

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Table 2 Description of stations and data used in the CALPUFF/CALMET simulation Station name

Description

UTM east (m)

UTM north (m)

Base elevation (m)

Data collected

URAVAN1

Valley location near Umetco office Club Mesa Grand Junction Airport

697,210

4,249,828

1527

696,251 713,594

4,248,322 4,332,793

1731 1481

Wind speed, wind direction, barometric pressure, temperature, relative humidity, precipitation. Wind speed, wind direction, temperature, relative humidity, solar radiation. Wind speed, wind direction, cloud cover, temperature, relative humidity, ceiling height. Upper air data (wind speed, direction, and temperature).

URAVAN2 GJTa a

Operated by the National Weather Service. Data obtained from the National Climatic Data Center, Ashville, North Carolina.

(4) was used to calculate N during daylight hours. For nighttime conditions, N was interpolated for each hour of nighttime using a starting value of N an hour before sunset and an ending value of N one hour after sunrise of the next day. Cloud cover estimates using this technique were also supplemented with observations in Grand Junction. 4.2.3. Wind fields, stability class, temperature, and precipitation Wind roses for URAVAN1 and URAVAN2 are illustrated in Fig. 3. The number of hours represented in the wind rose was 8877 and 8876 for URAVAN1 and URAVAN2, respectively. The wind rose at the valley location (URAVAN1) shows channeling of wind up and down the San Miguel River valley and is similar to the wind rose for the valley location reported in Coffey and Maxwell (1980). The wind rose for the Club Mesa site (URAVAN2) exhibits no channeling from the San Miguel River valley. The mean wind speed recorded at URAVAN1 was 1.9 m s1 (4.3 mph) at the 9.27-m level, which is slightly less than the mean wind speed reported in Coffey and Maxwell (1980) of 2 m s1 (4.5 mph) at the 10-m level. The mean wind speed at the Club Mesa site was 2.2 m s1 (4.9 mph) at the 6.4-m level. The number of hours with hourly averaged wind speeds less than the anemometer stall speed of 0.4 m s1 was 186 (2.1%) at URAVAN1 and 383 (4.1%) at URAVAN2. For comparison purposes, a joint frequency distribution (JFD) of wind speed, wind direction, and atmospheric stability was constructed. The JFD is not used in CALPUFF, but is a convenient way to summarize meteorological data taken. Stability class was calculated using the sA lateral turbulence method (EPA, 2000). Wind speeds were corrected to the 10-m level and a roughness height of 15 cm per EPA (2000) guidance. At URAVAN1, stability class A (unstable) and F (stable) were observed 37% and 44% of time respectively, while other stability classes (B, C, D, and E) were observed 19% of

the time. At URAVAN2, stability classes A and F were observed 34% and 39% of the time, respectively, while the remaining stability classes (B, C, D, and E) were observed 27% of the time, with stability class D (neutral) conditions occurring 12% of the time. Analysis of modeled wind vectors and calculated stability classes indicated that the meteorology at Uravan can be characterized into three diurnal regimes: (1) nocturnal drainage flow down the San Miguel River Valley coupled with stable conditions and inversions, particularly during winter months; (2) unstable conditions coupled with light and variable winds within the valley during daylight hours; and (3) evening and morning transitional periods. Morning transitional periods are characterized by air masses ascending the southwest-facing canyon walls in response to heating during morning hours and being replaced by sinking air from the northeast facing canyon wall and lower portions of Club Mesa. This phenomenon was observed by Momeni (1981) using visible tracers. Of particular importance for inhalation exposure were inversion conditions during evening hours resulting in pollutant trapping in the valley, and morning transitional periods when pollutant entrained in the air mass from Club Mesa sinks into the valley. The mean temperature at URAVAN1 during the measurement period was 12.0  C and ranged from 15.9 to 40.2  C. The mean temperature at URAVAN2 during the measurement period was 11.6  C and ranged from 17.3 to 39.3  C. The total precipitation recorded at URAVAN1 was 24.0 cm. Temperature and precipitation were also measured at a National Oceanic and Atmospheric Administration (NOAA) station located about 20 m north of URAVAN1. Comparison of daily temperature extremes and total precipitation indicated small differences between the URAVAN1 and the NOAA stations. Temperature and precipitation data at URAVAN1 were corrected to NOAA data because the temperature probe for URAVAN1

Fig. 3. Wind roses from April 2005 to April 2006 at the URAVAN1 and URAVAN2 meteorological towers.

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

was not at the standard 2-m height and the precipitation gage may have been located too close to a building. The correction involved adjusting the minimum, maximum, and 8:00 a.m.-observed temperature recorded at the URAVAN1 station to the corresponding values recorded by NOAA. Temperature extremes for a given day were defined by the maximum and minimum NOAA values. A comparison of the temperature and precipitation data taken at URAVAN1 with 23 yr (1960–1983) of historical records provided in Jones (1984) shows that the annual averages and extremes measured at URAVAN1 are within statistical norms. 4.2.4. Dispersion coefficients and terrain algorithm The default dispersion coefficient option in CALPUFF (PGT or Pasquill–Gifford–Turner) was not used, but instead, dispersion coefficients were computed from micrometeorological variables and similarity theory. The PGT dispersion scheme is best suited for regulatory compliance calculations. Likewise, the default complex terrain algorithm (Partial Plume Path Adjustment) was not used, but instead, the CALPUFF-type terrain adjustment was selected. This terrain algorithm adjusts the properties of the puff based on local strain to the flow imparted by the underlying terrain. It was found that the default dispersion option and terrain algorithm did not appropriately allow for B-Plant sources to disperse within the valley air mass, which made a significant difference in modeled air concentrations. Using the default complex terrain algorithm, concentrations of radionuclides in air from B-Plant sources were approximately three times less than those calculated using the CALPUFF-type terrain adjustment.

1265

point sources, Bq m2 s1 for area sources); Dtm ¼ time step for month m (year); UQj,k ¼ source uncertainty factor for source j and year k (unitless); M1 ¼ starting month; M2 ¼ ending month. The source uncertainty factor was assumed to be lognormally distributed having a geometric mean (GM) of 1.0 and a geometric standard deviation (GSD) of 2 and was only applied to years where no suitable measurement data existed in which to compare modeled air concentrations. The source uncertainty factor was assumed because there was little information in which to quantitatively derive an uncertainty factor. For example, stack measurements exhibited a large amount of variability, but were unreliable indicators of uncertainty over long periods of time (years) because the measurement value only reflected the conditions in the plant during the measurement period, which was typically on the order of several hours. Therefore, an uncertainty factor for the source was assumed for calculations prior to 1979. The uncertainty factor was intended to include both lack of knowledge and errors and imprecision in estimating ore grades, production rates, and release fractions. The source uncertainty factor was sampled once for each model realization. The model was designed so that if the source uncertainty changed over the exposure period, then it was sampled every time the uncertainty distribution changed for each model realization. However in this application, source uncertainty was assumed to be the same for all years of the simulation. The time-integrated concentration for multiple years of exposure is given by 0 1 Y2 X zi;j ¼ @ xi;j;k AUDi;j (6) k ¼ Y1

4.2.5. Wet and dry depositions Dry and wet depositions were modeled for particulate releases. Dry deposition velocity was calculated using algorithms in the CALPUFF model that are based on an approach that expresses the deposition velocity as the inverse of the sum of resistances plus gravitational settling. Gravitational settling is governed by the aerodynamic equivalent diameter (AED) of the particle and is important for particle sizes greater than about 10 mm AED. Wet deposition was modeled in terms of a scavenging coefficient. The default scavenging coefficients for particles of 1 104 for liquid precipitation and 3  105 for frozen precipitation were used in the simulation.

where zi,j ¼ time-integrated concentration for receptor i and source j (Bq yr m3); UDi,j ¼ dispersion uncertainty factor for receptor i and source j (unitless); Y1 ¼ starting year; Y2 ¼ ending year. For each model realization, the dispersion uncertainty factor was sampled once for each receptor and each source. The dispersion uncertainty factor was based on the distribution of predicted-to-observed ratios and is discussed in Section 4.5. The time-integrated exposure concentration for an individual at multiple exposure locations from all sources is given by TIC ¼

4.3. Calculation of air and soil concentration The CALPUFF model was used to calculate monthly average X/Q and j/Q values at each of the 4891 nodes in the model domain. Monthly average X/Q and j/Q values were calculated because monthly differences in X/Q values varied on average by a factor of w2.7, and as much as a factor of 12 for some locations in the Uravan area. Winter X/Q values were generally higher than X/Q values during the summer months because inversion and stable conditions persist longer during the winter months. The source term was also discretized by month when sufficient information was available, and, in those cases, concentrations were computed on a monthly basis. 4.3.1. Air concentrations from direct emissions The time-integrated concentration at a receptor for a year and a single source was computed by 0 1 M2  X @ xi;j;k ¼ (5) X=Qi;j;m Qj;k;m Dtm AUQ j;k m ¼ M1

where xi,j,k ¼ time-integrated concentration at receptor i and source j for year k (Bq yr m3); X/Qi,j,m ¼ X/Q value for receptor i and source j and month m (s m3 for point sources, s m1 for area sources); Qj,k,m ¼ source term for source j, year k, and month m (Bq s1 for

NR X NS X i¼1 j¼1

zi; j

(7)

where TIC ¼ time-integrated concentration for the individual exposure period (Bq yr m3); NR ¼ number of receptor locations; NS ¼ number of sources. 4.3.2. Deposition and soil concentrations Deposition is addressed in a similar fashion to Eq. (5) where the j/Q value replaces the X/Q value: 0 1 M2  X 4i;j;k ¼ @ (8) j=Qi;j;m Qj;k;m Dtm CFAUQ j;k m ¼ M1

where 4i,j,k ¼ deposition at receptor i from source j and year k (Bq m2); j/Qi,j,m ¼ j/Q value for receptor i and source j and month m (m2 for point sources, unitless for area sources); CF ¼ conversion factor from years to seconds (3.1536  107). Total deposition from all sources at a receptor is given by 00 1 1 Y2 NS X X @ @ A (9) ui;j ¼ 4i;j;k UDi;j A j¼1

k ¼ Y1

The deposition uncertainty factor was applied in a similar manner to the dispersion uncertainty factor and was assumed to be

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lognormally distributed having a GM of 1 and a GSD of 2. Accumulation of activity in the surface soil accounts for leaching and radioactive decay and ingrowth. The system is described by the differential equation:

resuspension factor (RF, m1). The resuspension factor was calculated using an equation adapted from NRC (1982):

X dQSi;n ¼ ji;j;n ðtÞ  ðlLn þ ln ÞQSi;n þ ln1 QSi;n1 dt j

where lR ¼ the resuspension decay factor equivalent to a 50-day half-life (5.06 yr1); RFS ¼ short-term resuspension factor (m1); RFL ¼ long-term resuspension factor (m1). The short and long-term resuspension factors are given in NRC (1982) as 105 and 109, respectively. The time-integrated concentration from resuspension (TICRS) over multiple years of deposition is found by convolution of the resuspension function, decay and ingrowth functions, and source terms:

(10)

where ji,j,n,(t) ¼ time-variable deposition flux of radionuclide n at receptor i from source j equivalent to j/Q  Q (atoms m2 yr1); QSi,n ¼ inventory of radionuclide n at receptor i (atoms m2); ln ¼ decay rate constant for radionuclide n (yr1); lLn ¼ leach rate constant for radionuclide n (yr1). The last term in Eq. (10) (ln1QSi,n1) is omitted for the first member of the decay chain (238U). Eq. (10) was solved using convolution of the source and loss terms. 20 1 3 Z tX Y2 NS NS X X 4 @ A QSi;n ¼ ji;j;n ðsÞaðt  sÞ dsy ji;j;k;n aY2 k 5 dt 0 j¼1

k ¼ Y1

j1

(11) where aY  k ¼ decay-ingrowth and leach factor at time, Y2  k. 2 The term Y2  k is the age of the deposit. If radionuclide ingrowth is ignored, a is given by aY2 k ¼ expð  ðl þ lL ÞðY2  kÞÞ

(12)

For decay, ingrowth, and leaching calculations, a was calculated using a matrix solution to ordinary differential equations described in Birchall (1986). Decay and ingrowth calculations are performed in units of atoms and subsequently converted to activity units. The leach rate constant is given by lL ¼

I qTð1 þ

 Kd r

(13)

q

where I ¼ infiltration rate (m yr1); q ¼ moisture content (m3 m3); T ¼ thickness of soil layer (0.03 m for resuspension); r ¼ bulk density (g cm3); and Kd ¼ linear sorption coefficient (mL g1). The leach rate constant for uranium (0.0194 yr1) was estimated based on uranium depth profiles taken at five of the six undisturbed soil sampling sites and is discussed in Section 4.5 of this paper. Using the leach rate constant for uranium estimated from the soil data and nominal values for net infiltration (0.01 m yr1), bulk density (0.78 g cm3 based on soil measurements), and moisture content (0.06), a uranium Kd value of 22 mL g1 was back-calculated by solving Eq. (13) for Kd. The calculated uranium Kd compared well with uranium Kd values in the literature for sandy loam soils like those found in Uravan. Sheppard and Thibault (1990) report a geometric mean Kd for uranium of 35 mL/g for sand and 15 mL/g for loam. Using data from EPA (1999) for sand-loam soils, a geometric mean Kd value of 20 mL/g (n ¼ 15) was calculated. The uranium Kd value reported in NCRP (1996) was 15 mL g1 and the Kd values for the other radionuclides were taken from this reference. The Kd values were 3200 mL g1 for thorium, 500 mL g1 for radium, 270 mL g1 for lead, 120 mL g1 for bismuth, and 150 mL g1 for polonium. The empirically determined uranium leach rate constant corresponds to residence half-time of w36 yr in the top 3 cm of soil. For comparison, the NRC Regulatory Guide 3.51 (NRC, 1982) uses a leach rate constant of 0.0139 yr1 for all nuclides in surface soils, which corresponds to a half-time of 50 yr.

4.3.3. Resuspension The air concentration from resuspension is given by the product of the surface soil concentration (4, Bq m2) and the time-dependent

RF ¼ RFS elRt þ RFL

TICRSi;j ðtÞ ¼

Z

t 0

4i;j ðsÞRFðt  sÞaðt  sÞ dsy

(14)

Y2 h i X 4i;j;k RFY2 k aY2 k dt k ¼ Y1

(15) where Y2  k is the age of the deposit and RFY2 k ¼ RFS elRðY2 kÞ þ RFL . The decay, ingrowth, and leaching factor (a) was calculated using a matrix solution to ordinary differential equations described in Birchall (1986). These calculations were performed on an annual basis. Because concentrations are reported on an annual basis, the short-term resuspension factor was integrated and averaged over a 1-yr period resulting in an average integrated short-term resuspension factor of 2  106 m1. For leaching calculations, the soil depth was assumed to be 3 cm because resuspension is a function of the surface soil concentration. The resuspension factor is highly variable and values in the literature vary by orders of magnitude. However, resuspension was shown to be a minor contributor to the total time-integrated concentration while the mill was operating. After cessation of operations, resuspension is the primary source of radionuclides in ambient air, although these air concentrations are generally much lower than the air concentrations during operations. Because the focus of this assessment was air concentrations during the time the mill operated, uncertainty in the resuspension factor was not considered because it would have made little difference in the predicted concentrations during mill operations. 4.4. X/Q and j/Q values Concentration and deposition fluxes were calculated at each of the 4891 nodes and six discrete receptors for each of the modeled sources. Each source was modeled using a steady-state unit emission rate (1 g s1 for point sources, 1 g m2 s1 for area sources). The ratio of the concentration or deposition flux to the emission rate is the X/Q or j/Q value, respectively. The X/Q and j/Q values were extracted for each month of the year at all 4891 nodes and stored in ASCII files. The X/Q or j/Q value used to calculate an air or soil concentration for a receptor was obtained by determining the node closest to the receptor location and then extracting the X/Q or j/Q values for that location. Given a node spacing of 75 m, the maximum difference between the reported receptor location and the actual location used in the calculations was ((37.5 m)2 þ (37.5 m)2)1/2 ¼ 53 m. X/Q and j/Q values for important sources are tabulated in Tables 4 and 5, respectively, for locations in six Uravan housing areas that were closest to the sources. Salt roaster X/Q values were about a factor of 7 less than the X/Q values for yellowcake stack, and j/Q values were about a factor of 2 higher. The lower salt roaster X/Q values relative to the yellowcake stack are a result of the high release temperature (and subsequently greater plume rise) of the salt roaster effluent. The higher salt roaster j/Q values relative to the yellowcake stack are a result of the larger particle size of salt roaster effluent. Spatially, C-Block had the highest X/Q values for all five sources (Table 3), and it received the greatest deposition from sources on

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

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Table 3 X/Q values for selected sources and selected locations in the Uravan town site (s m3  106) Source

Location

January

February

March

April

May

June

July

August

October

November

December

Yellowcake

A-Block B-Block C-Block D-Block E-Block F-Block

5.26 5.72 7.80 9.36 5.33 4.51

5.84 6.63 9.18 8.19 4.05 3.21

5.08 3.88 4.95 5.64 2.56 2.07

5.73 4.87 6.22 3.04 1.48 1.31

4.54 4.55 6.13 5.96 2.90 1.93

5.01 4.44 6.07 4.54 2.50 1.71

3.91 4.89 6.82 5.28 2.03 1.29

3.66 5.04 7.37 4.23 1.66 1.16

5.18 5.76 8.19 4.44 1.85 1.34

6.89 7.43 10.3 9.65 4.60 3.49

8.03 8.86 12.0 7.35 3.87 2.60

6.90 7.90 10.4 8.46 4.28 3.27

Leach AK

A-Block B-Block C-Block D-Block E-Block F-Block

4.36 4.38 4.61 4.20 4.03 4.28

4.40 4.76 5.21 3.58 2.97 3.04

3.49 3.25 3.66 2.61 2.49 2.35

3.51 4.17 4.80 2.26 2.15 1.88

4.26 5.10 6.16 1.99 1.54 1.68

4.95 5.57 6.75 2.79 1.84 1.38

4.47 5.83 7.26 1.45 0.70 0.51

6.37 8.01 9.83 1.94 0.97 0.76

6.83 7.68 9.37 2.43 1.30 0.85

5.90 7.01 8.15 4.35 3.65 3.36

6.51 7.39 8.23 4.45 3.84 2.52

4.74 5.11 5.58 3.64 3.18 3.23

Aerofall

A-Block B-Block C-Block D-Block E-Block F-Block

3.32 3.22 3.29 3.37 3.40 3.22

3.23 3.81 4.14 2.76 2.42 2.45

2.50 2.63 2.96 1.82 1.77 1.66

3.90 4.67 5.49 1.60 1.33 1.39

4.58 5.77 6.90 2.18 1.99 1.77

6.31 7.48 8.93 2.79 1.83 1.55

5.37 7.04 8.83 2.02 1.14 0.78

7.63 10.1 12.4 2.35 1.34 1.05

8.31 10.0 12.1 2.97 1.56 1.04

6.21 7.36 8.62 4.38 3.72 3.34

6.24 6.92 7.72 3.75 3.24 2.17

3.59 4.03 4.20 2.75 2.35 2.39

Fine OB

A-Block B-Block C-Block D-Block E-Block F-Block

2.94 2.79 2.76 3.25 3.02 2.53

1.61 1.86 1.95 1.96 1.80 1.80

1.54 1.65 1.76 1.48 1.43 1.49

2.07 2.53 2.80 1.35 1.05 1.19

2.74 3.40 3.74 1.86 1.74 1.66

2.64 3.08 3.24 2.17 2.01 1.75

2.69 3.34 3.87 1.60 1.42 0.93

3.25 4.46 5.05 1.43 1.12 1.07

3.80 4.44 4.97 1.57 1.05 0.85

3.83 4.63 5.16 3.09 2.77 3.13

4.10 4.33 4.50 3.22 2.95 1.90

2.87 3.00 2.98 2.71 2.45 2.32

Salt roaster

A-Block B-Block C-Block D-Block E-Block F-Block

0.713 0.706 0.781 0.842 0.611 0.674

0.613 0.829 0.939 0.606 0.451 0.468

0.654 0.477 0.513 0.483 0.288 0.318

0.804 0.634 0.677 0.402 0.230 0.230

0.629 0.653 0.736 0.732 0.348 0.314

0.850 0.605 0.666 0.449 0.234 0.251

0.603 0.766 0.911 0.725 0.311 0.318

0.386 0.425 0.525 0.482 0.208 0.185

0.770 0.793 0.884 0.360 0.166 0.173

0.643 0.956 1.112 0.748 0.525 0.612

0.894 1.100 1.296 0.553 0.371 0.397

0.789 0.913 1.061 0.499 0.387 0.483

the mesa (Table 4). The yellowcake stack is predicted to deposit most heavily on D- and A-Blocks and the salt roaster on D-Block, followed by A- and C-Blocks. Seasonal variations are evident by examination of monthly values in Tables 4 and 5. Another factor that was found to be important was the difference between the day and the night X/Q values. Fig. 4 shows hourly X/Q contour plots for the yellowcake stack for 1 and 2 October 2005. During the evening hours, nocturnal drainage flow coupled with stable atmospheric conditions and temperature inversion resulted in the plume being confined to the San Miguel River valley. By 10:00 in the morning, daytime heating resulted in an unstable atmosphere and upslope flow causing favorable mixing conditions. The X/Q values are substantially lower compared to the nighttime X/Q values. These conditions persisted until 18:00 h when the onset of nighttime cooling and stable atmospheric conditions resulted in a tightly confined plume. Finally, at hour 00:00 on October 2, nocturnal drainage flow and stable conditions were reestablished in the valley. This diurnal pattern of stable atmospheric conditions and temperature inversions during the evening, and upslope flow and unstable atmospheric conditions during the daytime hours was observed by Momeni (1981) using smoke tracers. 4.5. Model calibration and comparison with measurements This section contains a general description of the model calibration procedure employed in this assessment. The CALPUFFgenerated X/Q values and j/Q values, combined with the source terms and the preceding methodology, were used to calculate annual average concentrations and deposition at selected locations where continuous air monitoring stations were operating. The predicted concentration includes direct air emissions from stacks, fugitive dust emissions from tailings’ piles and roads, and resuspension. These predicted concentrations were then compared to annual average measured concentrations and the

September

predicted-to-observed ratio was computed (P/O). The geometric mean (GM) and geometric standard deviation (GSD) of the distribution of P/O ratios were then calculated and evaluated against a target GM of 1.0. Source terms were then reevaluated and, if sufficient justification was found, were adjusted to bring the modeled concentrations in line with the measured values. That is, source term adjustments were made to bring the GM P/O ratio as close to the target value of unity. The data set used for model calibration was the annual average concentrations of radioactive particulates in air measured at continuous sampling stations from 1979 to 1986 (see Figs. 1 and 2 for locations of samplers). Air monitoring data were compiled from historical monitoring records held by Umetco Minerals Corporation in its Grand Junction, Colorado office and summarized in RAC, 2007. High-volume air samplers manufactured by General Metals Works were mounted on wooden towers 2 m above ground. Particulates were collected on 8  10 in. glass microfiber filters, which were changed out weekly and composited every two to four weeks. Radionuclide analyses were conducted by a laboratory in Grand Junction. The environmental monitoring program at Uravan, with associated quality assurance procedures, was approved and periodically reviewed by the Colorado Department of Health. In 1984, the mean reported lower limits of detection (LLD) for samples collected at the clarifier (S1), based on actual volumes of air sampled and analytical uncertainties, were U-nat – 0.00018 mBq m3, 230 Th – 0.000063 mBq m3, 226Ra – 0.0042 mBq m3, and 210Pb – 0.000086 mBq m3. Air sampling data were compared to predicted concentrations for small particle sizes, which are efficiently collected by high-volume air samplers. No bias correction was applied to the reported concentrations. The air monitoring data from 1978 were excluded because it was not a complete year of monitoring. The 1985 and 1986 air concentration data were taken while the plant was shut down and were used to calibrate long-term resuspension releases and

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Table 4 j/Q values for selected sources and selected locations in the Uravan town site (m2  109) Source

Location

January

February

March

April

May

June

July

August

September

October

November

December

Yellowcake

A-Block B-Block C-Block D-Block E-Block F-Block

9.76 6.59 8.19 32.9 9.58 8.66

17.70 9.11 10.27 29.8 8.65 6.95

46.33 18.70 17.18 34.6 8.31 7.15

54.32 20.07 19.25 36.7 9.42 7.37

40.00 15.98 16.89 42.7 17.20 9.87

54.50 20.14 20.36 44.5 10.09 8.69

14.02 9.82 11.76 47.3 12.76 8.75

10.22 7.57 9.71 32.0 7.20 6.52

36.72 15.01 15.99 32.2 8.09 6.22

30.20 15.36 16.72 30.2 8.87 8.47

12.07 9.07 11.40 40.7 11.47 10.56

13.12 8.81 10.18 24.2 6.43 6.90

Leach AK

A-Block B-Block C-Block D-Block E-Block F-Block

1.04 1.04 1.13 1.00 0.954 1.13

1.27 1.52 1.73 1.09 0.874 0.973

1.21 1.44 1.78 1.22 1.03 1.01

1.40 2.08 2.66 1.10 0.974 0.817

1.21 1.64 2.07 0.894 0.635 0.769

2.19 2.58 3.28 0.970 0.657 0.690

1.15 1.50 1.86 0.562 0.327 0.426

1.62 1.94 2.39 0.799 0.460 0.478

2.03 2.46 3.10 0.902 0.576 0.752

1.94 2.50 2.99 1.39 1.18 1.33

1.51 1.72 1.95 1.58 1.34 1.32

1.57 1.51 1.73 1.15 0.983 1.20

Aerofall

A-Block B-Block C-Block D-Block E-Block F-Block

0.945 0.837 0.912 1.00 0.919 1.07

1.18 1.56 1.98 1.05 0.824 0.947

1.10 1.76 2.37 1.31 0.969 1.02

2.04 3.44 4.64 1.29 0.913 0.968

1.65 2.43 3.18 1.33 0.897 0.955

2.99 3.50 4.53 1.30 0.840 0.903

1.43 1.85 2.33 0.991 0.584 0.696

1.95 2.53 3.15 1.07 0.633 0.698

2.84 3.52 4.45 1.25 0.786 1.14

2.20 2.98 3.66 1.67 1.34 1.49

1.46 1.63 1.87 1.79 1.39 1.49

1.56 1.25 1.33 0.884 0.724 1.33

Fine OB

A-Block B-Block C-Block D-Block E-Block F-Block

0.730 0.725 0.757 0.852 0.782 0.796

0.696 0.904 1.04 0.650 0.587 0.713

0.756 1.13 1.43 1.06 0.818 0.854

1.009 1.77 2.28 0.836 0.584 0.643

0.893 1.33 1.63 0.955 0.703 0.790

1.848 2.08 2.59 0.951 0.773 0.797

0.786 0.974 1.16 0.610 0.480 0.530

0.951 1.17 1.35 0.718 0.521 0.574

1.30 1.78 2.16 0.606 0.469 0.734

1.41 2.00 2.38 1.146 0.998 1.286

0.980 1.05 1.14 1.328 1.157 1.228

1.26 0.909 0.953 0.768 0.656 0.953

Salt roaster

A-Block B-Block C-Block D-Block E-Block F-Block

18.9 16.2 17.4 28.5 15.8 19.5

26.4 22.9 24.0 28.5 14.7 16.7

45.3 27.3 26.8 27.8 11.8 13.7

64.8 40.4 37.8 31.8 12.6 14.3

41.8 31.5 33.1 56.2 22.0 20.2

releases from tailings’ piles. The uranium concentration in air at sampler S1 was 80 times lower in 1985–1986 than in 1980, reflecting the importance of uranium emissions from the yellowcake drying operations. Little radium and thorium were emitted from the yellowcake drying operations and therefore, radium and thorium concentrations in air were only 2.6 and 5.5 times lower, respectively, at sampled S1 in 1985–1986 compared to 1980. Measured radionuclide concentrations taken prior to 1978 consisted of 1-h ‘‘grab’’ samples taken on a monthly or semi-annual basis. It was inappropriate to compare an annual average concentration to a 1-h grab sample, and therefore, these measurements were not directly used in the calibration procedure. However, a valid comparison of these measurements can be made by comparing the distribution of hourly predicted concentrations to the distribution of measured hourly concentrations. These comparisons were used as an additional check on the performance of the model. Measured concentrations were corrected for background by subtracting a site-specific background concentration from each annual average measured concentration. Nuclide-specific background values recommended by the Colorado Department of Health for the Uravan valley floor (Hazle and Weaver, 1984) were 0.0005 pCi m3 (0.018 mBq m3) for U-nat (238U þ 234U), 0.0003 pCi m3 (0.011 mBq m3) for 230Th, 0.0001 pCi m3 (0.0037 mBq m3) for 226 Ra, and 0.015 pCi m3 (0.56 mBq m3) for 210Pb. 4.5.1. Model calibration using 1979–1986 continuous air monitoring data Calibration of tailings pile releases, resuspension, and road dust was performed first. Measurement data from 1985 to 1986 were compared to model predictions for the same period. The tailings’ piles continued to be sources for suspension of radionuclides by winds after milling operations ceased in late 1984. The initial results indicated model under-prediction, particularly for radium,

74.8 39.4 41.7 36.5 14.9 15.8

30.0 25.4 30.8 53.7 18.7 21.0

20.1 16.5 20.3 36.5 13.4 13.2

53.8 34.2 35.8 25.3 10.1 11.5

32.1 31.0 32.2 30.8 16.3 19.8

27.3 25.9 29.2 29.4 14.0 17.5

23.6 22.2 24.4 19.0 11.1 13.8

thorium, and lead isotopes. Adjustments were made in the 230Th, Ra, and 210Pb source term for tailings. The source term adjustment factors in tailings were 10 for 230Th and 226Ra, and 30 for 210 Pb. The higher radionuclide releases from tailings may be the result of several factors. Assuming the dispersion aspects of the model are reasonably accurate, the apparent underestimation of radionuclide concentrations in air from tailings releases could be the result of (1) higher radionuclide concentrations in the fine particle fraction (<10 mm) of the tailings that are more susceptible to suspension, (2) higher proportion of the fines near the surface of the tailing pile and subject to suspension, and (3) underestimation of the wind speed-dependent suspension factors used in the calculation. The radionuclide concentration measurements reported in Bartram (1980) were suspected to be the bulk concentration in tailings and not the concentration in the fine material that was subject to suspension. Schwendiman et al. (1980) measured the activity concentration in slurry that comes from the mill to the tailings pond and in the surface of a tailings pile and reported that a relatively large fraction of the radioactivity was associated with particles smaller than 7 mm and that activity density (activity per unit mass) of the smaller particles is about an order of magnitude greater than that of the larger particles. Furthermore, about 40% of the activity in the tailings pile surface was associated with particles less than 7 mm while in the slurry sample, only about 12% of the activity was associated with particles less that 7 mm. These data suggest that there is an enrichment in the effective tailings concentration susceptible to suspension by more than an order of magnitude relative to the bulk radionuclide concentration in tailings. Additionally, the wind speed-dependent suspension factors developed by NRC are subject to considerable uncertainty and possibly bias. Some of the model under-prediction could have also been due to the presence of tailings within Uravan proper and in close proximity to the samplers. After this adjustment was made, 226

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

1269

Table 5 Results of soil sampling profiles at Uravan Sample numbera

Layer thickness (cm)

Layer bottom depth (cm)

Effective soil density (g cm3)

137 Cs (Bq g1)

238

U (Bq g1)

137

1-A 1-B 1-C 1-D Total

3 3 4 5

3 6 10 15

0.84 1.01 0.91 0.97

0.016 0.072 0.000 0.000

0.611 0.391 0.159 0.147

0.40 2.18 0.00 0.00 2.58

2-A 2-B 2-C 2-D Total

3 3 4 5

3 6 10 15

0.78 1.34 1.07 1.29

0.018 0.006 0.001 0.000

0.794 0.317 0.183 0.101

3-A 3-B 3-C 3-D 3-E Total

3 3 4 5 5

3 6 10 15 20

0.55 0.83 0.91 1.79 1.21

0.032 0.020 0.007 0.000 0.001

4-A 4-B 4-C 4-D 4-E Total

3 3 4 5 6

3 6 10 15 21

0.91 1.09 0.77 0.57 1.20

5-A 5-B 5-C 5-D 5-E Total

3 3 4 5 6

3 6 10 15 21

6-A 6-B 6-C 6-D 6-E Total

3 3 4 5 5

3 6 10 15 20

a b

Csb (kBq m2)

238

Ub (kBq m2)

137 Cs percentage in each layer

238

15.45 11.87 5.75 7.14 40.2

15.53 84.47 0.00 0.00

38.43 29.51 14.30 17.75

0.43 0.23 0.03 0.01 0.70

18.5 12.7 7.85 6.53 45.6

60.64 32.70 4.96 1.69

40.53 27.92 17.22 14.32

2.320 1.954 0.635 0.105 0.083

0.52 0.50 0.27 0.00 0.03 1.32

37.9 48.4 23.1 9.40 5.03 123.9

39.44 37.55 20.64 0.00 2.38

30.61 39.08 18.66 7.59 4.06

0.030 0.014 0.000 0.000 0.000

1.465 2.442 0.232 0.061 0.027

0.83 0.47 0.00 0.00 0.00 1.30

39.9 80.2 7.19 1.72 1.93 130.9

63.53 36.47 0.00 0.00 0.00

30.46 61.26 5.49 1.32 1.47

0.81 1.07 0.80 0.76 0.55

0.025 0.008 0.001 0.000 0.000

0.586 0.232 0.089 0.065 0.053

0.60 0.26 0.03 0.00 0.00 0.89

14.2 7.47 2.84 2.45 1.74 28.7

67.70 29.25 3.06 0.00 0.00

49.50 26.01 9.90 8.54 6.05

0.52 0.87 0.77 0.74 1.04

0.036 0.037 0.010 0.002 0.000

0.038 0.046 0.035 0.026 0.020

0.57 0.96 0.30 0.08 0.01 1.92

0.59 1.21 1.09 0.95 1.02 4.86

29.59 49.97 15.53 4.21 0.70

12.23 24.82 22.47 19.56 20.92

U percentage in each layer

The A, B, C, D, and E designations in sample number refer to the depth level at each sampling site. Inventory ¼ mass concentration (Bq g1)  layer thickness (cm)  effective density (g cm3).

the GM P/O ratio for the years 1985 and 1986 was 0.91 with a GSD of 3.0. Road emissions were reduced based on an analysis of the particulate matter estimated to have been suspended from the roads and the measured particulate matter in air as reported in Bartram (1980). The road emission model coupled with the road X/Q values estimated particulate air concentrations that were about twice the measured values when background and other sources were considered. It was also noted that state highway CO 141 was rerouted to the north side of the San Miguel River and paved in 1961, which would have resulted in a significant reduction in road emissions. To compensate for overestimation of particulate matter in air and the paving and rerouting of CO 141, road emission sources were reduced by a factor of 10 after 1961 and by a factor of 2 prior to 1961. The distribution of annual average P/O ratios for 1979–1984 was calculated for 130 data pairs. The data set included concentrations of 238U, 230Th, 226Ra, and 210Pb. The GM P/O ratio was 1.9 with a GSD of 2.1. Many of the over-predicted modeled concentrations were for 238 U after 1981. In 1981, a new effluent treatment system was installed on the yellowcake stack, which reduced emissions by about a factor of 4 as indicated by stack measurements. However, ambient air measurements indicated otherwise and showed a more pronounced decrease in uranium concentrations at most samplers. On average, 238U concentrations in air at samplers 1 and 2 were lower by about a factor of 10 for 1981 and up to a factor of 100 for 1984 compared to 1980 values. Since the yellowcake stack was the primary contributor of uranium in air at these samplers, the releases

for the yellowcake stack from 1981 to 1984 were reduced by the same factor of reduction observed in the ambient air measurements. After this correction was made, the GM P/O ratio was 1.25 with a GSD of 1.8 and no further adjustments were made to the release estimates. Discrepancies between predicted and observed concentrations could have been a result of many factors including source term, X/Q values, and timing of the release (daytime vs. nighttime). Corrections to the yellowcake stack emissions brought the 238U predicted concentrations closer in line with the measured values. In contrast, predicted concentrations of 230Th, 226Ra, and 210Pb showed many inconsistencies with measurements even after adjustment. One complicating factor with uranium progeny was that tailings were used as backfill for water and sewer lines in Uravan and tailings from early operations were randomly dispersed throughout the town. These tailing may have been a source of airbone thorium and radium particulates that affected measurements made at the samplers and that were not directly accounted for in the model. Figs. 5–11 show a comparison of the predicted and observed concentrations at each of the seven samplers where continuous air monitoring was performed. Uncertainty was calculated using a lognormal dispersion uncertainty factor having a GM of 1.0 and a GSD of 1.8. Source term uncertainty was not considered for this period because the overall uncertainty of the model was already reflected in the P/O ratios. For the years prior to 1978, however, a lognormal uncertainty factor having a GM of 1.0 and a GSD of 2 was applied. Plots were generated from 500 model realizations.

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A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

Fig. 4. Hourly X/Q isopleths maps, 1 and 2 October 2005, for the yellowcake stack. During the evening hours, nocturnal drainage flow and stable atmospheric conditions confine the plume within the canyon walls downstream of the stack. During daytime hours, unstable conditions result in significant dispersal of material emitted to the atmosphere.

4.5.2. Comparison with 1-h grab samples One-hour outdoor grab samples for 238U were taken at numerous locations and times in Uravan between 1960 and 1977. Sample locations included the housing blocks, bunk house, hospital, post office, school, and some mill facilities. These hourly data were pooled across year and location, and the percentiles and the mean of the distribution of net hourly concentrations (gross concentration

minus background) were calculated. Modeled concentrations were calculated using the hourly average X/Q values at six of the sampling locations where continuous samplers were stationed from 1978 to 1986. The predicted concentrations were then pooled across location and year, and percentiles and the mean of the distribution of predicted concentrations were calculated and compared to the distribution of measured concentrations. No source corrections

1x102

U-238 Clarifier

1x101

Predicted (50th Percentile) Measured

1x100 1x10-1 -2

1x10

1x10-3 1979

1980

1981

1982

1983

1984

1985

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

1x102

U-238 Sewage Plant

1

Predicted (50th Percentile)

1x10

Measured

1x100 1x10-1 1x10-2 1x10-3 1x10-4 1979

1986

1980

1981

Predicted (50th Percentile) Measured 1x100

1x10-1

1x102 1980

1981

1982

1983

1984

1985

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

Th-230 Clarifier

1979

1x101

1x10-1

1981

1982

Measured

1x10-1

1x10-2 1980

1981

1983

1984

1985

1x101

Measured 1x100

1x10-1

1x10-2 1984

1986

Year

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1984

1985

1986

Measured

1x100

1x10-1

1x10-2 1980

1981

1982

1983

1984

1985

1986

Year

Predicted (50th Percentile)

1982

1983

Predicted (50th Percentile)

1979

1986

Pb-210 Clarifier

1980

1982

Ra-226 Sewage Plant

Year 1x101

1986

1x100

1979

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1x10

1980

1985

Year

Predicted (50th Percentile) Measured

1x10-2 1979

1984

Predicted (50th Percentile)

1986

Ra-226 Clarifier

0

1983

Th-230 Sewage Plant

Year 1x101

1982

Year

Year 1x101

1271

1x101

Pb-210 Sewage Plant Predicted (50th Percentile) Measured

1x100

1x10-1 1979

1980

1981

1982

1983

1984

1985

1986

Year

Fig. 5. Predicted and measured radionuclide concentrations in air at the sampler designated S1, located at the clarifier.

Fig. 6. Predicted and measured radionuclide concentrations in air at the sampler designated S2, located at the sewage plant.

were applied except for the roads, and concentrations were computed for the hours beginning at 06:00 and ending at 18:00 because samples were taken only during the dayshift. The mean and 50th percentile of the measured concentration were 2.2 mBq m3 (6  1014 Ci m3) and 1 mBq m3 (2.7  1014 Ci m3), respectively. The mean and 50th percentile of the modeled concentration were 1.9 mBq m3 (5.2  1014 Ci m3) and 0.44 mBq m3 (1.2  1014 Ci m3), respectively. P/O ratios ranged from 0.311 for the 5th percentile to 0.815 for the 95th percentile. From this comparison, it was determined

that the model was predicting the higher hourly concentrations reasonably well, but perhaps under-predicting lower hourly concentrations. Overall, these discrepancies were captured in the uncertainty, and, therefore, no additional adjustments to the model were made. 4.5.3. Comparison with soil concentrations Numerous soil measurements have been made in Uravan and its vicinity. Previous measurements, however, were made for site

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

1x101

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1272

U-238 West Pile #2

1x100 1x10-1 1x10-2

Predicted (50th Percentile) Measured

1x10-3 1981

1982

1983

1984

1985

1986

1x102

1x101

1x100

1x10-1

1x10

1980

1x10-1 Predicted (50th Percentile) Measured 1x10-2 1983

1984

1985

1986

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1x100

1982

1x101

1983

1984

1983

1984

1x10-1 Predicted (50th Percentile) Measured 1x10-2 1981

1982

Ra-226 West Pile #2

1x100

1x10-1

-2

1x10

1982

1983

1984

1985

1986

Year

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1984

Year

1981

1x101

Ra-226 C-Block

1x100

1x10-1 Predicted (50th Percentile) Measured -2

1x10

1980

1981

1x100

Predicted (50th Percentile) Measured 1x10-1 1981

1982

1982

Year

Pb-210 West Pile #2

1983

1984

1985

1986

Year Fig. 7. Predicted and measured radionuclide concentrations in air at the sampler designated S3, located west of tailings’ piles 1 and 2.

characterization and post-remediation surveys, and not for determination of atmospheric deposition. For these reasons, additional sampling was performed in July 2006 at six locations around Uravan in undisturbed soil. Samples 1 and 2 were taken on the mesa southeast of Uravan in a place known locally as Eagle Basin. Samples 3 and 4 were taken west of tailings’ piles 1 and 2. Sample 5 was taken at a location downstream of Uravan on a bench on the north side of CO 141 and below the confluence of the San Miguel River with Atkinson Creek. This sampling site lay 0.5 km west of the model

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1983

Th-230 C-Block

1980

Predicted (50th Percentile) Measured

1x101

1982

1x100

Year 1x101

1981

Year

Th-230 West Pile #2

1981

Predicted (50th Percentile) Measured

-2

Year 1x101

U-238 C-Block

1x101

Pb-210 C-Block Predicted (50th Percentile) Measured

1x100

1x10-1 1980

1981

1982

1983

1984

Year Fig. 8. Predicted and measured radionuclide concentrations in air at the sampler designated S4, located in C-Block.

domain, and the nearest model node was used for comparison with sample 5. Sample 6 was taken on a bench several kilometers up a canyon formed by Tabeguache Creek from its junction with CO 141. Tabeguache Creek lies 1.6 km east of the eastern margin of the model domain and is upstream of Uravan. The sample was taken to establish background.

1x101

U-238 Fusion Plant Predicted (50th Percentile) Measured

1x100 1x10-1 1x10-2 1x10-3 1981

1982

1983

1984

1985

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

1x100

1273

U-238 Swimming Pool

1x10-1 1x10-2 1x10-3

Predicted (50th Percentile) Measured

1x10-4 1981

1986

1982

1983

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

Year 1x101

Th-230 Fusion Plant

1x100

1x10-1 Predicted (50th Percentile) Measured 1x10-2 1981

1x100

1983

1984

1985

1x10-1

1x10-2 1982

1983

1986

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

1x100

1x10-1 Predicted (50th Percentile) Measured -2

1x10

1x101

1982

1983

1984

1985

Year Predicted (50th Percentile) Measured

1x100

1x10-1 1981

1982

1983

1984

1985

1x100

1986

Year Fig. 9. Predicted and measured radionuclide concentrations in air at the sampler designated S5, located at the fusion plant.

The sampling procedure followed was based on Environmental Measurements Laboratory procedures (EML, 1990) and used by other researchers (Little et al., 1980; Webb et al., 1997). The procedure involved excavation of a 25  25-cm pit in an undisturbed location and removal of specific layers. Target layer depths were 0– 3, 3–6, 6–10, 10–15, and 15–20 cm, although the presence of bedrock limited samples at depth for some locations. Each sample was double bagged in a plastic lock-sealed bag and labeled. Sampling equipment consisted of a trowel, trenching shovel, and chisel.

1985

1986

Ra-226 Swiming Pool Predicted (50th Percentile) Measured

1x10-1

1x10-2 1981

1982

1983

1984

1985

1986

Year

1986

Pb-210 Fusion Plant

1984

Year

Air Concentration (mBq m-3)

Air Concentration (mBq m-3)

Ra-226 Fusion Plant

1981

1986

Predicted (50th Percentile) Measured

Year 1x101

1985

Th-230 Swimming Pool

1981 1982

1984

Year

1x100

Pb-210 Swimming Pool

1x10-1

Predicted (50th Percentile) Measured 1x10-2 1981

1982

1983

1984

1985

1986

Year Fig. 10. Predicted and measured radionuclide concentrations in air at the sampler designated S6, located at the new swimming pool.

Sampling tools were cleaned between samples. Sample sites were selected in the vicinity of pinion and juniper trees where surface soil showed no signs of significant erosion by either wind or water or disturbance by livestock or wild animals. Samples were analyzed for 238U and 137Cs by Paragon Analytics, Fort Collins, Colorado (Table 5). Cesium-137 was used to confirm that the site was undisturbed. If the site was undisturbed, there should have been a clear decrease in 137Cs concentration with depth. Disturbance would have been evident by a 137Cs profile that

Air Concentration (mBq m-3)

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A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

1x100

U-238 F-Block

1x10-1 1x10-2 1x10-3

Predicted (50th Percentile) Measured

1x10-4 1981

1982

1983

1984

1985

1986

Air Concentration (mBq m-3)

Year 1x100

Th-230 F-Block Predicted (50th Percentile) Measured

1x10-1

1x10-2 1981

1982

1983

1984

1985

1986

Air Concentration (mBq m-3)

Year 1x100

Ra-226 F-Block Predicted (50th Percentile) Measured

1x10-1

1x10-2 1981

1982

1983

1984

1985

1986

Year Air Concentration (mBq m-3)

Fig. 12. Predicted and measured 238U soil profile at sampling sites 3 and 4. The measured concentration represents the average of samples 3 and 4 at each depth interval. Predicted concentrations were estimated using the calibrated Kd value and the Mixing Cell Model. The background depth profile taken at sample site 6 is shown for comparison.

1x100

Pb-210 F-Block Predicted (50th Percentile) Measured

1x10-1

1x10-2 1981

1982

1983

1984

1985

expected to find greater concentrations at depth in response to leaching. The salt roasting facility was suspected as the primary contributor to offsite uranium because of the relatively large particles sizes and estimated release amounts. Model results showed that the salt roaster accounted for about 97% of the total anthropogenic uranium deposited from the atmosphere at the sample sites. The uranium profiles were also used to estimate a leach rate constant. Salt roaster operation ended in 1957; therefore, about 50 yr have elapsed from time of deposition. On average, 38% of the total uranium mass in the profile resided in the 0–3-cm layer. Assuming a first-order model, the leach rate constant can be derived as ln(0.38)/50 yr ¼ 0.0194 yr1. Using this rate constant for the first 0–3-cm layer, an equivalent Kd can be calculated by solving Eq. (13) for Kd and using the nominal values for moisture content and infiltration. The bulk density was the average of the five samples in the 0–3-cm depth (0.78 g cm3). Using these data and solving Eq. (13) for Kd yields a uranium Kd of 22 mL g1. To evaluate the calibrated Kd value on the entire soil profile, the Mixing Cell Model (Rood, 2004, 2005) was used to compute the

1986

Year Fig. 11. Predicted and measured radionuclide concentrations in air at the sampler designated S7, located in F-Block.

deviated from the expected undisturbed pattern. All samples showed a clear decrease in 137Cs concentrations with depth and in most cases, over 90% of the total 137Cs inventory in the profile was found in the top 6 cm. Uranium profiles were used to establish if the uranium was indeed from atmospheric deposition and to estimate the leach rate constant for uranium. If uranium were deposited from the atmosphere, then it would also have shown a clear profile with depth. Because uranium is more mobile in soil compared to cesium, we

Table 6 Predicted-to-observed ratios for total Sample number

Description

238

U in soil

Observed (kBq m2)

1 and 2a Eagle Basin SE of mouth 42.9 of Hieroglyphic Canyon 127.4 3 and 4b West of tailings’ piles 1 and 2 5 Downstream of confluence 28.7 of San Miguel River and Atkinson Creek a

Predicted (kBq m2)

P/O Predicted þ backgroundc 2 (kBq m )

47.7

52.5

1.22

106.8

111.6

0.88

14.3

19.1

0.67

The observed concentration represents the average of sample numbers 1 and 2. The observed concentration represents the average of sample numbers 3 and 4. c The background concentration was from sample number 6 taken upstream of Uravan (4.86 kBq m2). b

A.S. Rood et al. / Journal of Environmental Radioactivity 99 (2008) 1258–1278

Fig. 13. Isopleth map of

238

1275

U deposition in the Uravan model domain (kBq m2).

Table 6 shows the predicted and observed ratios for the total uranium in the soil profile at five of the sampling sites. Predictedto-observed ratios ranged from 0.67 at sampling site 5 to 1.22 at sampling sites 1 and 2. Overall, the model predictions compared very favorably with measured values, and the general pattern and magnitude of 238U deposition patterns were consistent with model estimates. Umetco Minerals also took samples in the vicinity of samples 3 and 4 (Junge, 2006). These confirmatory samples represented a 0– 15-cm (0–6-in.) plug in soil. Samples were analyzed for 238U, 230Th, and 226Ra. Average soil concentrations were 0.26 Bq g1 for 226Ra,

depth profiles at sampling sites 3 and 4. An upper boundary condition of the time-dependent deposition flux and zero concentration initial condition was imposed. Fig. 12 shows a very good agreement with the predicted and measured 238U depth profile at sampling sites 3 and 4. It should be noted that the calibrated Kd value is dependent on the nominal values selected for infiltration and moisture content. These values were chosen to represent a semi-arid site and a sandy soil. Moisture content was computed using the van Genuchten (1980) fitting parameters for sand as reported in Carsel and Parrish (1988). No attempt was made to measure the site-specific net infiltration and moisture content values.

Table 7 Median (50th percentile) predicted time-averaged concentrations of radionuclides in air with 95% confidence interval (in parentheses) at selected locations in Uravana Location

238

A-Block B-Block C-Block D-Block E-Block F-Block H-Block J-Block

4.8 4.9 5.9 4.6 2.4 2.2 3.7 4.8

a

U (mBq m3) (1.4–19) (1.2–19) (1.5–22) (1.3–17) (0.67–9.3) (0.58–9.0) (0.97–15) (1.3–20)

234

U (mBq m3)

4.9 4.9 5.9 4.6 2.4 2.3 3.8 4.9

(1.4–19) (1.2–19) (1.5–22) (1.3–17) (0.67–9.4) (0.59–9.1) (0.98–15) (1.3–20)

Averaging period is from 1936 to 1984.

230

Th (mBq m3)

3.0 2.9 3.3 2.4 1.4 1.4 2.6 3.4

(0.84–13) (0.79–13) (0.89–14) (0.65–12) (0.37–6.2) (0.38–6.8) (0.68–12) (0.88–16)

226

Ra (mBq m3)

3.0 2.9 3.2 2.4 1.4 1.4 2.6 3.4

(0.83–13) (0.77–13) (0.87–14) (0.64–11) (0.36–6.1) (0.38–6.6) (0.66–12) (0.85–16)

210

Pb (mBq m3)

3.1 3.0 3.4 2.4 1.4 1.4 2.6 3.5

(0.85–13) (0.82–13) (0.91–14) (0.67–11) (0.38–6.1) (0.38–6.7) (0.70–12) (0.90–16)

210

Bi (mBq m3)

3.1 3.0 3.4 2.4 1.4 1.4 2.6 3.5

(0.85–13) (0.82–13) (0.91–14) (0.67–11) (0.38–6.1) (0.38–6.7) (0.70–12) (0.90–16)

210

Po (mBq m3)

3.1 3.0 3.4 2.4 1.4 1.4 2.6 3.5

(0.85–13) (0.82–13) (0.91–14) (0.67–11) (0.38–6.1) (0.38–6.7) (0.70–12) (0.90–16)

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Fig. 14. Predicted 238U concentration in air as a function of time at A-Block for Uravan sources. Concentrations for each source represent the direct emissions to air from the source and resuspension from material deposited by the source.

0.51 Bq g1 for 230Th, and 0.32 Bq g1 for 238U. These results corroborate that the yellowcake stack could not have been the source of the enhanced uranium concentrations observed in the soil profile samples because if it were, then 230Th and 226Ra soil concentrations would have been substantially less in the Umetco samples. The spatial distribution of predicted 238U deposition in the model domain (Fig. 13) shows enhanced deposition along the southern canyon wall and the elevated terrain west of tailing piles 1 and 2. The deposition plume also shows the effects of wind channeling induced by the canyon.

4.6. Predicted concentrations The distribution of predicted time-averaged concentrations from 1936 to 1984 is shown in Table 7 for the Uravan housing blocks. Housing block concentrations were computed at a single node that was located at the center of the block. Concentration distributions were developed from the simple random sampling of model parameters for 500 model realizations. Differences among housing blocks ranged from about a factor of 2 for uranium isotopes to about a factor of 2.3 for the remaining

Fig. 15. Predicted 230Th concentration in air as a function of time at A-Block for Uravan sources. Concentrations for each source represent the direct emissions to air from the source and resuspension from material deposited by the source.

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Fig. 16. Isopleth map of time-averaged resuspension.

238

1277

U concentration in air (mBq m3) from 1936 to 1984 for Uravan sources. Air concentration does not include contributions from

radionuclides. The 95% confidence interval (2.5th percentile– 97.5th percentile) typically spanned about a factor of 15 or less. The highest time-averaged concentrations were in C-Block and the lowest time-averaged concentrations were in F-Block. C-Block was located downwind of most of the Uravan facilities and close to the northeast facing hillside where effluent released from BPlant would tend to migrate during morning hours when heating of the southwest-facing cliffs opposite Uravan occurred. F-Block was located the farthest upwind of any of the housing blocks and, therefore, had air concentrations that were substantially lower relative to the other housing blocks. Predicted air concentrations as a function of year and by source at A-Block are shown in Figs. 14 and 15 for 238U and 230Th, respectively. The concentrations represent direct emissions to air plus the resuspension component associated with each source. Dominant sources for uranium include the salt roasters for releases up to 1957 and the yellowcake stack for releases after 1957. Dominant sources for 230Th include the leach acid kill, fine ore bin, and aerofall stacks for post-1957 releases, and the salt roaster for pre1957 releases. The remaining sources (tailings’ piles and road emissions) contributed typically less than 10% to the total.

The spatial distribution of the 1936–1984 time-averaged 238U concentration (Fig. 16) shows the plume responding to wind channeling in the canyon. The concentrations in Fig. 16 represent contributions from all sources and all particle sizes but do not include the resuspension component. Highest average concentrations are found near sampler S5, which is on the hillside between A- and B-Plants. However, the high predicted concentrations at this sampler are attributed to the salt roaster, which ceased operation before the sampler was installed. Therefore, the higher concentrations at this sampler suggested by the predicted time-averaged concentration in Fig. 16 are not reflected in the available measurement data for 1981–1986. 5. Summary and conclusions Ambient air concentrations of uranium decay series radionuclides were calculated from Uravan mill emissions from 1936 to 1984. Predicted air concentrations included contributions from stacks, suspension emissions from tailings’ piles, emissions from roads, and resuspension of deposited material on soil. The complex terrain model CALPUFF was used to compute air concentrations using 1 yr of

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site-specific meteorological data collected at two locations in 2005 and 2006. A soil module was used to compute the mass balance of radionuclides at various depths in the soil profile, and radionuclide concentrations in air and soil were compared to corresponding measurements. In general, the average predicted radionuclide concentrations in air were higher than the corresponding measurements (the GM P/O ratio was 1.25). Therefore, on average, the predicted air concentrations can generally be regarded as over-estimates of the actual air concentrations that existed in Uravan. Complex terrain influenced the dispersion patterns and conditions within the model domain by confining the plume during stable atmospheric conditions to the San Miguel River valley. Winds within the valley were channeled up and down the valley, but showed a clear preference for blowing down the valley. Therefore, predicted concentrations were higher in housing blocks that were located downwind (northwest) of Uravan mill facilities compared to housing blocks that were located upwind (southeast). The salt roaster process and yellowcake dryer were the primary effluent sources for uranium isotopes. Emissions of the other radionuclides (230Th, 226Ra, 210Pb, 210Bi, and 210Po) were dominated by the salt roaster process and emissions from B-Plant facilities (leach tanks and Aerofall mills). Releases from tailings’ piles and roads were minor contributors to the emissions. Analysis of soil data indicated the presence of anthropogenic uranium deposited from airborne plumes. The salt roasting process was believed to be the major source of this uranium. Soil profile sampling and modeling indicated that approximately 60% of the uranium deposited had leached from the surface soil. Acknowledgments The authors would like to thank Gene Greenwood and Ray Junge for their assistance in the field and logistical support and Union Carbide Corp. for funding the study. Dr. Till has provided an expert report regarding the reconstruction of historical doses from radionuclides released to the environment by the Uravan milling site. References Alexandroff, M., 1 February 1995. Historic Context of Uravan, Colorado 1881 to 1984. Winter and Company, Boulder, CO (available from the authors upon written request). Bartram, B.W., 1980. 40 CFR 190 Related Radiological Doses Due to the Uravan Uranium Mill. NUS Corporation, Denver, CO (available from the authors upon written request). Birchall, A., 1986. A microcomputer algorithm for solving compartmental models involving radionuclide transformations. Heath Physics 50, 389–397. Burwell, B., 1946. Construction, Operation, and Maintenance Report of Uranium Sludge Plants Operated by the United States Vanadium Corporation in the Colorado Area. United States Vanadium Corporation Uravan Unit. Available from Umetco Minerals, Grand Junction Colorado (available from the authors upon written request). Carsel, R.F., Parrish, R.S., 1988. Developing joint probability distribution of soil water retention characteristics. Water Resource Research 25 (5), 755–769. Coffey, M.E., Maxwell, D.R., 1980. A Review of Available Meteorological Data Representative of the Uravan Uranium Mill Site Vicinity. NUS Corporation, Denver, CO (available from the authors upon written request). Dames and Moore Corp., 31 August 1978. Environment Report Uravan Uranium Project Montrose County, Colorado for Union Carbide Corporation. Dames and Moore Corporation (available from the authors upon written request). EML (Environmental Measurements Laboratory), 1990. EML Procedure Manual, 27th ed., vol. 1. US Department of Energy, New York, NY. EPA (U.S. Environmental Protection Agency), 1972. Compilation of Air Pollution Emission Factors (Revised). US EPA Office of Air Quality and Planning and Standards, Research Triangle Park, NC. EPA, August 1986. Section 11.24 ‘‘Metallic Minerals Processing’’, Compilation of Air Pollution Emission Factors (reformatted in January 1995).

EPA, 1995. User’s Guide to the Building Profile Input Program. EPA-454/R-93-038. US Environmental Protection Agency, Research Triangle Park, NC. EPA, 1999. Understanding Variation in Partition Coefficient, Kd, Values, Review of Geochemistry and Available Kd Values for Cadmium, Cesium, Chromium, Lead, Plutonium, Radon, Strontium, Thorium, Tritium (3H) and Uranium, vol. II. EPA 402-R-99–004B. US Environmental Protection Agency, Office of Air and Radiation, Washington, DC. EPA, 2000. Meteorological Monitoring Guidance for Regulatory Modeling Applications. EPA-454/R-99–005. US EPA Office of Air Quality and Planning and Standards, Research Triangle Park, NC. EPA, November 2006. Section 13.2.2, ‘‘Unpaved Roads’’ of AP-42, Compilation of Air Pollution Emission Factors. van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal 44, 892–898. Hamrick, J.S., Kocis, D.E., Shepherd, S.E., June 2002. Uravan, Colorado One Hundred Years of History. Umetco Minerals Corporation, Grand Junction, Colorado (available from the authors upon written request). Hazle, A.J., Weaver, K.L., 22 May 1984. Preliminary Executive Licensing Review Summary (PELRS) for Evaluation of the Application for a Radioactive Materials License and the Environmental Report. Colorado Department of Health Radiation Control Division, p. 418 (available from the authors upon written request). Holtslag, A.A.M., van Ulden, A.P., 1983. A simple scheme for daytime estimates of the surface fluxes from routine weather. Journal of Climatology and Applied Meteorology 22, 517–529. Jones, R.K., 1984. Annual Environmental Monitoring Data Review. Umetco Minerals Corporation. Uravan, Colorado Mill. SUA-673. Umetco Minerals, Grand Junction, CO (available from the authors upon written request). Junge, W.R., 25 August 2006. Uravan Site Manager, Umetco Minerals. Letter to Phil Stoffey, Program Manager, Colorado Department of Health. Subject: Area E. Windblown Area (available from the authors upon written request). Little, C.A., Whicker, F.W., Winsor, T.F., 1980. Plutonium in a grassland ecosystem at rocky flats. Journal of Environmental Quality 9 (3), 350–354. Momeni, M.H., June 1981. Environment in the Vicinity of Uravan Mill: Characterization of Radioactive Effluents and Airborne Radionuclide Concentrations. NUREG/CR-2286. ANL/ES-110. Argonne National Laboratory for the US Nuclear Regulatory Commission. NCRP (National Council on Radiation Protection and Measurements), 1996. Screening Models for Releases of Radionuclides to Atmosphere, Surface Water, and Ground. NCRP Report No. 123. National Council on Radiation Protection and Measurements, Bethesda, Maryland. NRC (US Nuclear Regulatory Commission), 1982. Calculational Models for Estimating Radiation Doses to Man from Airborne Radioactive Materials Resulting from Uranium Milling Operations. Regulatory Guide 3.51. Nuclear Regulatory Commission, Washington, DC. NRC, August 1984. Uravan Colorado: Radiation and Regulation. A Report by a Special Study Staff of the US Nuclear Regulatory Commission, Region IV. Uranium Recovery Field Office, Denver, CO (available from the authors upon written request). NRC, 1987. Methods for Estimating Radioactive and Toxic Airborne Source Terms for Uranium Milling Operations. Regulatory Guide 3.59. Nuclear Regulatory Commission, Washington, DC. Pazin, L.M., Meintin, L.M., August 1974. Operation of roaster gas cleaning installations. Metallurigist 18 (8), 628–630. RAC (Risk Assessments Corporation), 2007. Reconstruction of Historical Doses from Radionuclides Released to the Environment by the Uravan Mining and Milling Site. Risk Assessment Corporation, Neeses, SC. Rood, A.S., 2004. A mixing-cell model for assessment of contaminant transport in the unsaturated zone under steady-state and transient flow conditions. Environmental Engineering Science 21 (6). Rood, A.S., 2005. Mixing Cell Model: A One-Dimensional Numerical Model for Assessment of Water Flow and Contaminant Transport in the Unsaturated Zone. ICP/EXT-05-00748. Idaho National Laboratory, Idaho Falls, ID. Schwendiman, L.C., Sehmel, G.A., Horst, T.W., Thomas, C.W., Perkins, R.W., 1980. A Field and Modeling Study of Windblown Particulates from a Uranium Mill Tailings Pile. NUREG/CR-1407, PNL-3345. Battelle Pacific Northwest Laboratory, Richland, WA. Scire, J.S., Lurmann, F.W., Bass, A., Hanna, S.R., 1984. User’s Guide to the MESOPUFF II Model and Related Processor Programs. EPA-600/8-84-013. U.S. EPA, Research Triangle Park, NC. Scire, J.S., Robe, F.R., Fernau, M.E., Yamartino, R.J., 2000a. A User’s Guide for the CALMET Meteorological Model Version 5. Earth Tech Inc., Concord, MA. Scire, J.S., Strimaitis, D.G., Yamartino, R.J., 2000b. A User’s Guide for the CALPUFF Dispersion Model Version 5. Earth Tech Inc., Concord, MA. Sheppard, M.I., Thibault, D.H., 1990. Default soil solid/liquid partition coefficients, Kds for four major soil types: a compendium. Health Physics 59 (4), 471–482. Webb, S.B., Ibrahim, S.A., Whicker, F.W., 1997. A three dimensional spatial model of plutonium in soil near Rocky Flats, Colorado. Health Physics 73 (2), 340–349.