Environ. Sci. Technol. 2003, 37, 2060-2066
Assessment of the Disposal of Radioactive Petroleum Industry Waste in Nonhazardous Landfills Using Risk-Based Modeling KAREN P. SMITH,* JOHN J. ARNISH, GUSTAVIOUS P. WILLIAMS,† AND DEBORAH L. BLUNT Argonne National Laboratory, Environmental Assessment Division, 9700 South Cass Avenue, Argonne, Illinois 60439
Certain petroleum production activities cause naturally occurring radioactive materials (NORM) to accumulate in concentrations above natural background levels, making safe and cost-effective management of such technologically enhanced NORM (TENORM) a key issue for the petroleum industry. As a result, both industry and regulators are interested in identifying cost-effective disposal alternatives that provide adequate protection of human health and the environment. One such alternative, currently allowed in Michigan with restrictions, is the disposal of TENORM wastes in nonhazardous waste landfills. The disposal of petroleum industry wastes containing radium-226 (Ra226) in nonhazardous landfills was modeled to evaluate the potential radiological doses and health risks to workers and the public. Multiple scenarios were considered in evaluating the potential risks associated with landfill operations and the future use of the property. The scenarios were defined, in part, to evaluate the Michigan policy; sensitivity analyses were conducted to evaluate the impact of key parameters on potential risks. The results indicate that the disposal of petroleum industry TENORM wastes in nonhazardous landfills in accordance with the Michigan policy and existing landfill regulations presents a negligible risk to most of the potential receptors considered in this study.
Introduction A number of industrial processes, such as oil and gas production and processing activities, can cause naturally occurring radioactive materials (NORM) to become concentrated at levels above natural background in byproduct waste streams. NORM that accumulates in industrial waste streams is referred to as “technologically enhanced NORM”, or TENORM. For the petroleum industry, the sources of TENORM radioactivity are the isotopes uranium-238 (U-238) and thorium-232 (Th-232) and their progeny, which are naturally present in the subsurface formations from which oil and gas are produced. The primary radionuclide of concern is radium-226 (Ra-226). One of the primary concerns for industry, related to the occurrence of TENORM, has been increased waste management costs as a result of increased regulation. In general, * Corresponding author e-mail:
[email protected]; phone: (303)986-1140, ext 267; fax: (303)986-1311. † Present address: Remote Sensing Laboratory, Las Vegas, NV. 2060
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TENORM regulations have limited the available disposal options to methods that provide greater isolation of the waste streams, thereby increasing disposal costs. The identification, evaluation, and acceptance of alternative disposal methods that are adequately protective of public health and safety are likely to have the effect of lowering waste management costs. One such potential alternative is the disposal of TENORM wastes in municipal solid waste (MSW) landfills (i.e., landfills permitted to accept primarily nonhazardous wastes). The primary objective of this paper is to present the results of a study that evaluated the potential health risks associated with disposing of petroleum industry TENORM wastes in a MSW landfill on the basis of modeled fate and transport of Ra-226 (1). Risks were calculated for a variety of receptors, including those that would be potentially impacted during the disposal action (operational-phase scenarios) and those that would be potentially impacted as a result of some future use of the landfill property post-closure (future-use scenarios). A full discussion of the methods and assumptions used in the study, the results, and the related analyses of regulatory constraints and disposal costs is contained in the original report by Smith et al. (1).
Background Description of Petroleum Industry TENORM. The primary radionuclides of concern in petroleum industry TENORM are Ra-226 and radium-228 (Ra-228) and their associated progeny (Figure 1). The waste streams most likely to be contaminated by elevated radium concentrations include (i) produced water, which is formation water that is brought to the surface along with the produced oil and gas; (ii) scale, which is a hard, insoluble deposit that accumulates on the surfaces of equipment and solid debris that come in contact with produced water; and (iii) sludge, which is a slightly granular, usually hydrocarbon-rich deposit that accumulates in the bottom of some storage and process vessels. Radium, which is slightly soluble, can be mobilized in the liquid phases of a subsurface formation and transported to the surface in the produced water stream. As the produced water is brought to the surface, some of the dissolved radium precipitates out in solid form, resulting in concentrated levels of radium in the precipitate. Most commonly, the radium coprecipitates with barium sulfate scales; however, it also can coprecipitate to form other complex sulfates and carbonates. The radium content in produced water varies geographically. Radium solubility and mobility are influenced by a number of factors, including the chemical regime of the solution water, temperature, pH, and pressure (2). A variety of factors appear to affect the degree to which the radium in solution in produced water will precipitate out in solid form. For example, as the produced water is brought to the surface, temperature and pressure both decrease, allowing solids to precipitate. In general, radium concentrations tend to be highest closest to the wellhead where these changes are greatest. Mixing waters from different formations also can promote precipitation. In addition, sulfate content of the produced water is a factor given the strong correlation between barium sulfate scale occurrence and radium precipitation. Wells that do not have significant associated scale formation generally do not have a TENORM problem. Radium that remains in solution in the produced water typically is disposed of along with the produced water stream. In the United States, most produced water is disposed of via subsurface injection, and the radium content in re-injected water is not regulated. Radium content in scales and sludges, however, may be regulated, particularly in states that have 10.1021/es0261729 CCC: $25.00
2003 American Chemical Society Published on Web 04/12/2003
FIGURE 1. Uranium-238 and thorium-232 decay series. enacted TENORM regulatory programs. Periodically, the scales and sludges that accumulate inside pieces of oilfield equipment are removed. Radium-bearing scales and sludges pose a waste management issue if the radium content exceeds specified exemption levels. Similarly, pieces of equipment containing residual quantities of TENORM-bearing scales and sludges and surface soils impacted by these wastes can present a waste management problem. Regulation of TENORM. In the United States, by definition, TENORM that does not contain more than 0.05% uranium or thorium by weight, or any combination thereof, is not subject to regulatory control under the Atomic Energy Act of 1954 because it does not meet the definitions for a source material, special nuclear material, or byproduct material (42 U.S.C. Section 2011-2259). TENORM, also by definition, is not subject to regulatory control under the Low Level Radioactive Waste Policy Act. Under that Act, lowlevel radioactive waste (LLW) is defined as material that (i) is not a high-level radioactive waste, spent nuclear fuel, or byproduct material and (ii) has been classified by the Nuclear Regulatory Commission (NRC) as a LLW (42 U.S.C. Section 2021b-2021j). At this time, the NRC has not classified NORM as a LLW. Other U.S. Federal regulations do not specifically address TENORM, and as a result, regulatory control exists at the state level. As the presence of TENORM and the need to regulate it have become subjects of increasing debate. A number of states, most with significant levels of oil and gas production, have responded by enacting regulatory programs. Several other states are evaluating the need to do so, while the remaining states have determined that existing radiationcontrol regulations adequately address the issues currently presented by TENORM. Landfill Disposal of TENORM. In 1996, the Michigan Department of Environmental Quality (MDEQ) issued a set of guidelines that allow the disposal of bulk wastes contaminated with Ra-226, including petroleum industry TENORM, in MSW landfills (3). MSW landfills are landfills designed and permitted primarily for the disposal of non-
hazardous solid wastes. In the United States, the Resource Conservation and Recovery Act (RCRA) defines nonhazardous solid waste as any discarded, abandoned, recycled, or inherently waste-like material that is not listed as a hazardous waste, does not exhibit any of four hazardous characteristics (i.e., ignitability, corrosivity, reactivity, and toxicity), or is not otherwise exempted. Specifically, the MDEQ policy allows wastes containing an average concentration of 1.85 Bq/g (50 pCi/g) Ra-226 to be disposed of in a Type II landfill (Michigan’s classification for a MSW landfill), provided no single sample contains a concentration greater than 3.7 Bq/g (100 pCi/g) Ra-226. In Michigan, a Type II landfill is permitted to accept only nonhazardous household wastes, municipal solid wastes, incinerator ash, sewage sludge, commercial wastes, and industrial wastes. Type II landfills are regulated under Part 4 of the MDEQ’s Part 115 Administrative Rules, which establish requirements equivalent to or more substantive than the U.S. Environmental Protection Agency’s RCRA Subtitle D regulations (in 40 CFR Part 258). These regulations establish requirements pertaining to location, construction, design, operation, monitoring, closure, and post-closure care. Type II landfills are required to have a cover system with infiltration barriers and a double-barrier system with a leachate capture and monitoring system to prevent and monitor failure. By allowing wastes containing low concentrations of Ra226 to be disposed of in a MSW landfill, the MDEQ is providing a disposal alternative that may be substantially less expensive than other disposal options available to the petroleum industry (Table 1). This disposal alternative, however, has not been met with widespread acceptance among other state regulators or the solid waste management industry.
Methods Landfill Construction and Operation. The parameters defining the landfill modeled in the study were based upon an existing landfill in Michigan and the Michigan Type II VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary of Costs Associated with Various TENORM Disposal Alternativesa disposal method TENORM/LLW landfill company A company B company C downhole encapsulationb underground injectiond on-site off-site commercial, company D off-site commercial, company E on-site landspreadinge commercial landfarmsf municipal solid waste (MSW) landfill company F, Michigan company G, Michigan
disposal costs ($/55 gal) 500-550 60-90 27-515 792-3333c 20-393 196 132 45-56 12-39 10-14 5-6
a All costs based on information gathered for 1999 unless otherwise indicated. Any changes in these cost estimates since 1999 are likely to be relatively consistent between disposal methods. b Downhole encapsulation entails placement of waste into the subsurface casing of a well that is to be plugged and abandoned. c Ref 14; based on costs for 1992. This disposal alternative is no longer used because of high costs. d Underground injection entails the placement of waste into the subsurface formation typically through pressurized injection of a slurried mixture of waste and water. e Landspreading entails the placement of waste over the surface of a tract of land; typically the spread wastes are mixed into the upper portion of the surface soils. f Commercial landfarms are facilities permitted in the State of Louisiana where wastes are mixed with clean materials and diluted to a lower concentration level.
landfill regulations. The landfill was assumed to contain nine disposal cells of varying size with a total disposal capacity of approximately 7.3 × 106 m3. It was assumed that the cell that would receive the TENORM wastes had a capacity of 14 500 m3 and that, when the landfill was full, it would be about 25 m thick. The landfill was assumed to be constructed with a liner system consisting of a 1-m-thick layer of compacted clay at the base, overlain successively by a 1-cmthick high-density polyethylene (HDPE) liner, a 0.5-cm-thick drainage net, a 0.5-cm-thick bentonite layer, a second HDPE liner, and a 1-m-thick gravel drainage layer. The municipal wastes would be placed directly over the gravel drainage layer. Upon closure of the cell, the municipal wastes would be covered by a cap composed of, from the bottom up, a
0.6-m-thick layer of compacted clay, a 0.5-m-thick gravel layer, and a 0.5-m-thick layer of topsoil. In the base case, it was assumed that the TENORM wastes were placed in a single cell within the landfill in a layer about 2.5 m thick (Figure 2) and then overlain by a 0.3-m-thick layer of clean soil as part of normal landfill operations, which require placement of cover material at the end of each day of operations. The soil layer was assumed to be overlain by about 2.5 m of municipal wastes and subsequently by a 1.6m-thick cap, as required for final closure of the cell. Risk Assessment Methodology. Potential doses and health risks associated with the disposal of petroleum industry TENORM wastes in a municipal nonhazardous landfill were calculated for workers and the general public. A number of models were used to calculate concentrations and exposures (4-8). Evaluations were performed for operational-phase receptors (i.e., individuals who could be exposed as a result of waste placement activities) and future-use receptors (i.e., individuals who could be exposed as a result of future use of the property following closure of the landfill or consumption of groundwater impacted by landfill leachate). Doses were calculated for the maximally exposed receptor for each scenario. Collective doses were also estimated for the offsite population that could be exposed during waste placement operations. For this assessment, radiation doses were converted to carcinogenic risks by using risk factors recommended by the International Commission on Radiological Protection (ICRP) (9). The ICRP risk factor is 5 × 10-9 per mSv (5 × 10-7 per mrem) for the public and 4 × 10-7 per mSv (4 × 10-7 per mrem) for workers. Risks are expressed as the increased probability of fatal cancer over a lifetime. Health impacts associated with nonradiological constituents of the TENORM wastes and other wastes in the landfill were not evaluated. Operational-Phase Scenarios. The operational-phase receptors that were evaluated included (i) a driver working at the landfill, (ii) a waste-placement operator working at the landfill, (iii) a landfill worker involved in pumping and disposing of landfill leachate, and (iv) off-site residents living adjacent to the landfill or within a 80-km radius of the landfill. For each of the landfill workers, the primary exposure pathway was assumed to be external irradiation. Inhalation of contaminated particulates also was considered to be a potential pathway for waste-placement operators for cases
FIGURE 2. Schematic diagram of base-case assumptions regarding placement of NORM waste within the landfill. 2062
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TABLE 2. Exposure Parameters Used To Model Worker Scenarios input parametera
driver
wasteplacement operator
exposure time (h) distance to truck (m) shielding thickness (cm) airborne respirable dust concn (mg/m3) drive time to disposal area (min) truck bed/tank size (m) length width height
0.75b 0.6 0 nad
0.75b 1.5 or 3c 5 3.3 × 10-7 e
0.5 1 1.3 na
15
na
na
7.6 f 1.8 f 1.8 f
7.6 f 1.8 f 1.8 f
1.7 0.7 1.7
leachate worker
a Value used for each parameter was based on engineering judgment, unless other reference or rationale is noted. b 0.5 h assumed for inspection; 15 min assumed for disposal. c 1.5 m assumed for inspection; 3 m assumed for disposal operations. d na indicates not applicable. e Ref 10. f Default value from TSD-DOSE code (4).
TABLE 3. Exposure Parameters Used To Model Operational Phase Off-Site Resident Scenario input parametera exposure time (h) exposure frequency (d/yr) distance to source (km) wind speed (m/s) frequency wind blows in direction population density (persons/km2) 0-32 km radius 0-80 km radius
maximally exposed individual
population located within 80 km
24 365 0.3 4 0.5
24 365 0-80 4 na
nab na
29c 83c
a TSD-DOSE default values were used for all input parameters unless other rationale or reference is specified (4). b na indicates not applicable. c Value derived from site-specific data.
in which the wastes were not disposed of in containers. For the off-site residents, the primary pathway of exposure, assuming the wastes were not containerized, was inhalation of contaminated particulates. For these residents, external irradiation, incidental ingestion of contaminated particulates, and ingestion of contaminated foodstuff also were evaluated for completeness. Doses for operational-phase receptors were evaluated by using the TSD-DOSE computer code developed by Argonne National Laboratory (4). This model estimates radiological doses to treatment, storage, and disposal (TSD) facility workers and the surrounding public as a result of processing and disposing of waste that is slightly contaminated with radionuclides. The values used in this study for exposure parameters considered in the TSD-DOSE code are summarized in Table 2 for the workers and in Table 3 for the off-site residents. To calculate doses resulting from the inhalation pathway, Ra-226 releases were calculated by multiplying the Ra-226 concentration in the waste by the fraction of respirable particles (i.e., particles that are less than 10 µm in size) released during dumping operations. The fraction of respirable particles was estimated to be 3 × 10-7 on the basis of a methodology developed by the U.S. Environmental Protection Agency (10). Future-Use Scenarios. The future-use receptors evaluated in this study included (i) an on-site resident, (ii) an on-site industrial worker, (iii) a recreational visitor, and (iv) an offsite resident consuming groundwater impacted by leachate from the landfill. For the future-use scenarios, it was assumed
that the integrity of the landfill cap would be maintained and that there would not be any direct exposure to the TENORM wastes. The on-site resident scenario was the most conservative scenario evaluated in this study (i.e., the scenario expected to result in the greatest risk). For this scenario, it was assumed that an individual lived on the site in a home constructed on a slab (i.e., with no basement or crawl space) without a radon-reduction system. It also was assumed that the individual produced most of his or her food on site, including vegetables, milk, meat, and fish. The primary exposure pathways for the on-site resident were assumed to be external irradiation and inhalation of indoor and outdoor radon. Although unlikely, given that the integrity of the landfill cap would be maintained, other pathways were also evaluated, such as inhalation of contaminated particulates; inadvertent ingestion of contaminated soil; and ingestion of crops, milk, and meat grown on the contaminated property. Given existing regulatory prohibitions on breaching the landfill cap, it was assumed the resident’s water supply was from an unaffected off-site source, such as a municipal drinking water system. For the on-site industrial worker and recreational visitor, the primary pathways of exposure were assumed to be external irradiation and inhalation of radon. Inhalation of contaminated particulates and inadvertent ingestion of soil were also evaluated for completeness, although they are considered unlikely pathways, assuming the integrity of the landfill cap would be maintained. For both of these scenarios, it was assumed that the receptors would use water from only an unaffected off-site supply. The off-site residential scenario evaluated potential doses to an off-site resident from consumption of groundwater impacted by leachate from the landfill. The only exposure pathway to this receptor was assumed to be ingestion of groundwater. The RESRAD computer code (Version 5.782) (5) developed by Argonne National Laboratory was used to calculate potential doses to an on-site resident, industrial worker, and recreational visitor from all applicable pathways. (The current version of this code is RESRAD 6.21.) The RESRAD code estimates the time-integrated annual dose and excess lifetime cancer risk to individuals exposed to sites with radiological contamination. The RESRAD code focuses on radioactive contaminants initially in the soil and subsequently transported in air, water, and biological media to a single receptor. In this assessment, doses were projected over a period of 1000 yr. The source was adjusted over time to account for radioactive decay and ingrowth, leaching, erosion, and mixing. The values used in this study for exposure parameters considered in the RESRAD code are summarized in Table 4. Potential radiological doses to the off-site groundwater receptor were calculated from the estimated radionuclide concentrations projected by the leachate and groundwater transport models (discussed below), EPA-recommended exposure parameters for maximum residential exposures (11), and radionuclide-specific ingestion dose conversion factors (DCFs). Dose Conversion Factors. TSD-DOSE and RESRAD use the most conservative DCFs as the default values in cases where more than one DCF is defined for a specific radionuclide and exposure pathway. Ingestion DCFs are defined on the basis of the fraction of ingested material that will be absorbed from the gastrointestinal tract into bodily fluids. Inhalation DCFs are defined on the basis of the radionuclides’ lung retention times (i.e., the rates at which deposited materials are removed from the respiratory tract). The ingestion and inhalation DCFs depend highly on the radionuclide’s chemical form. This analysis used the most conservative DCFs from the EPA’s Federal Guidance Report No. 11 (12). These values VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Exposure Parameters Used To Model Future-Use Scenarios scenariob input parametera (m2)
area cover depth (m) waste layer thickness (m) density of waste layer (g/cm3) density of cover material (g/cm3) exposure time (h/d) indoor outdoor exposure frequency (d/yr) ingestion rate soil (g/d) meat (kg/yr) plant (kg/yr) groundwater (L/d) inhalation rate (m3/h) Rn-222 emanation coefficient foundation depth below surface (m) erosion rate (mm/yr) plant/soil transfer factor radium lead thorium fraction of food from site a
on-site resident
on-site industrial worker
recreational visitor
off-site resident
13.6 4.3 2.4 2.0 1.6
13.6 4.3 2.4 2.0 1.6
13.6 4.3 2.4 2.0 1.6
13.6 4.3 2.4 na na
12 6 365
6 2 250
0 4 20
na na 365
0.1 63 160 na 0.96 0.04 0.3 1.0
0.1 na na na 0.96 0.04 0.3 1.0
0.1 na na na 0.96 0.04 na na
na na na 2 na 0.04 na na
6.8 × 10-5 3.3 × 10-5 1.7 × 10-6 0.5
na na na na
na na na na
na na na na
may overestimate the doses because the solubility for radium sulfate scales is extremely low. Modeling the Fate and Transport of Radium. Definition of Source Term. In this study, 2000 m3 of TENORM wastes, having an average Ra-226 concentration of 1.85 Bq/g (50 pCi/g), were disposed of in a MSW landfill. It was assumed that Ra-228 was also present at a ratio of 3:1 Ra-226/Ra-228 (13). Ingrowth of Pb-210 was assumed for 10 yr at the start of the analysis. It was assumed that the radium was in the form of relatively insoluble radium/barium sulfate, which is typical of TENORM wastes generated by the petroleum industry. For this study, it was assumed that the radium dissolved instantly to its solubility limit of 2 × 10-6 g/L upon contact with leachate moving through the landfill. No kinetic or common-ion effects were evaluated. These assumptions resulted in overpredictions of radium concentrations in the leachate and added a degree of conservatism to the dose and risk calculations. Leachate and Groundwater Transport Calculations. Three separate models were used individually and in series to evaluate leachate and groundwater transport. The Hydrologic Evaluation of Landfill Performance (HELP) model (6) was used to calculate the amount of fluid that could percolate through the surface cover of the landfill and the amount of fluid that could leak through the containment system, located at the base of the landfill, under various conditions. A series of runs was made to determine how these volumes were affected by the quality of both the clay cap and the liner system at the base of the landfill. Climatic conditions from central Michigan were used for these calculations (1). An analytical model developed by Tomasko (7) was used to estimate the movement of dissolved Ra-226 from the initial position of the waste in the landfill to the bottom of the landfill cell. Because radionuclide transport within the landfill is driven by leachate percolation, this model required quantification of the amount of fluid that would leak through the landfill surface cover; the HELP model results were used to define this value. Ra-226 concentrations in the leachate were estimated for three different waste layer thicknesses at three different locations within the landfill: (i) at the liner 9
site-specific site-specific site-specific mixture of soil/NORM (15) RESRAD default engineering judgment
RESRAD default
RESRAD default values were used for input parameters not listed.
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b
RESRAD default ref 16 engineering judgment RESRAD default ref 17
engineering judgment
na indicates not applicable.
immediately below the TENORM waste layer; (ii) at the base of the cell containing the TENORM; and (iii) within the entire landfill, assuming leachate from all of the landfill cells is mixed. The SWIFT II model (8) was then used to evaluate groundwater transport of Ra-226 and its progeny, radon-222 (Rn-222), from the point of release below the landfill liner to a receptor located about 300 m downgradient of the landfill at a depth of 1.5 m below the base of the landfill. The HELP model (6) was used to estimate the volume of leachate that could leak from the landfill, while the model developed by Tomasko (7) was used to estimate the TENORM concentration in the leachate and the length of time that the leachate would contain TENORM.
Results Leachate and Groundwater Transport Calculations. Predictably, the results of the HELP model (6) evaluations of landfill performance indicated that potential leakage through the bottom of the landfill increased with increasing hydraulic conductivity of the clay cap and decreasing quality or absence of the geomembrane liners. Potential leakage rates, however, were quite low (i.e., on the order of 4.2 m3/yr/ha or less). It was anticipated that these low leakage rates would translate into negligible radium transport rates in the drinking water aquifer; therefore, a leakage rate was calculated for a worstcase scenario in which the landfill was constructed without geomembrane liners. Without the geomembrane liners, the HELP model estimated a leakage rate of about 1900 m3 yr-1 ha-1. The absence of geomembrane liners in the worst-case scenario conservatively increased the leakage rate through the bottom of the landfill by a factor of about 450 relative to the more realistic scenario. The results of the analytical model (7) used to calculate Ra-226 concentrations in the leachate indicated that the Ra226 concentration below the TENORM waste layer increased with increasing thickness of the waste layer. This increase was due to the longer duration of the source term for the thicker waste layers. In addition, the results of the analytical model (7) indicated that the Ra-226 concentrations decreased
TABLE 5. Estimated Peak-Year Doses and Carcinogenic Risks for Disposal of NORM-Impacted Wastes in a Nonhazardous Landfilla receptor
dose, mSv/yr (mrem/yr)
risk
Operational-Phase Scenarios driver 0.003 (0.3) waste-placement operator 0.017 (1.7) leachate worker 2 × 10-6 (2 × 10-4) off-site resident 6.6 × 10-6 (6.6 × 10-4) general populationb 2.7 × 10-7 (2.7 × 10-5) (80-km radius)
1 × 10-7 7 × 10-7 8 × 10-11 3 × 10-10 1 × 10-8
Future-Use Scenarios on-site resident 0.074 (7.4) on-site industrial worker 0.022 (2.2) recreational visitor 1.2 × 10-9 (1.2 × 10-7) off-site resident 3.2 × 10-5 (3.2 × 10-4)
4 × 10-6 1 × 10-6 6 × 10-14 2 × 10-10
a Doses are for bulk disposal of 2000 m3 of radium-bearing wastes having an average Ra-226 concentration of 1.85 Bq/g (50 pCi/g). b Dose for the general population is in person-Sievert (person-rem).
as the leachate was mixed with leachate generated from larger areas within the landfill. For example, assuming the TENORM waste layer was 2.5 m thick, the Ra-226 concentration in leachate immediately below this waste layer was 27.4 Bq/L (740 pCi/L). Dilution of the leachate by mixing with leachate generated from other wastes placed in that cell decreased the Ra-226 concentration to about 8 × 10-3 Bq/L (0.22 pCi/ L). Further dilution by mixing with leachate generated throughout the landfill decreased the Ra-226 concentration to about 1 × 10-3 Bq/L (0.03 pCi/L). A series of computer simulations was made using the SWIFT II model (8) to estimate the Ra-226 concentrations at a receptor located 300 m downgradient of the landfill at a depth of 1.5 m below the base of the landfill under various conditions. Even when very conservative assumptions were made regarding the scenario, estimated concentrations at the receptor location were very low. For example, the Ra-226 concentration was only 1.2 × 10-5 Bq/L (3.3 × 10-4 pCi/L), even when it was assumed that (i) there were no geomembrane liners present in the landfill, (ii) the leachate was diluted only with leachate from within the individual landfill cell, and (iii) the groundwater aquifer was in direct contact with the base of the landfill. When a separate run was made assuming the presence of poor-quality geomembrane liners (i.e., liners with about 10 flaws/acre), the estimated Ra-226 concentration was reduced by about 5 orders of magnitude to a value of 2.1 × 10-10 Bq/L (5.7 × 10-9 pCi/L). Similarly, when the depth of the groundwater was assumed to be 3-10 m below the base of the landfill, the Ra-226 concentrations were reduced by an additional 3 orders of magnitude. Calculated Doses and Health Risks. Table 5 presents the estimated doses and associated health risks for each receptor evaluated in this study. These doses and risks are related to disposing of 2000 m3 of TENORM wastes containing an average Ra-226 concentration of 1.85 Bq/g (50 pCi/g). To place the estimated doses in perspective, the currently accepted public dose limit recommended by the ICRP is 1 mSv/yr (100 mrem/yr) from all sources (9). Unless workers involved in the disposal of TENORM are classified as radiation workers, this dose limit is applicable to them as well. For the operational-phase worker scenarios, the results indicated that the waste-placement operator would receive the highest potential dose; however, this value was very low, less than 0.02 mSv/yr (2 mrem/yr). Potential doses to the other workers and the general public were negligible. For the future-use scenarios, the results indicated that the onsite residential receptor would receive the highest potential dose; however, this value also was low, at only 0.07 mSv/yr
(7.4 mrem/yr). The potential doses to other future users of the site were very low to negligible. Sensitivity Analyses. Sensitivity analyses were conducted on several input parameters for the on-site resident, on-site industrial worker, and recreational visitor scenarios. Only those parameters related to the radon pathway were analyzed because this was the only pathway that contributed significantly to dose. These parameters included depth of the TENORM waste layer below the landfill surface, radon emanation coefficient, area and thickness of the TENORM waste layer, and source concentration. In addition, breach of the landfill cap in home construction was analyzed for the residential scenario. The results of the sensitivity analyses indicated that all of the parameters evaluated except the areal extent of the TENORM waste layer had an impact on estimated doses. The two parameters potentially having the greatest impact on doses to the on-site residential receptor were the depth of the TENORM waste layer and the integrity of the landfill cap. In the case of waste layer depth, the sensitivity analyses indicated that potential doses could be unacceptably high (1.25 mSv/yr [125 mrem/yr]) if the waste layer was shallower than about 3 m. Similarly, potential doses could also be unacceptably high (0.6 mSv/yr [63 mrem/yr]) if the landfill cap were breached during home construction. The effect on potential doses associated with the use of a radon-reduction system or other radon mitigation measures was not assessed.
Discussion On the basis of the results presented above, the disposal of petroleum industry TENORM wastes in MSW landfills presents a negligible risk to all potential receptors considered in this study provided that (i) the average Ra-226 concentration of the TENORM wastes is 1.85 Bq/g (50 pCi/g) or less, (ii) the TENORM waste layer is placed at least 3 m below the landfill cap, and (iii) the integrity of the landfill cap is maintained into the future. The disposal of wastes containing higher concentrations of Ra-226, up to a few Bq/g (or a few hundred pCi/g), may also present negligible risk to most receptors, although this should be considered on a caseby-case basis. The sensitivity analysis completed for this study covered a wide range of climatic conditions and showed that the study results are applicable for most conditions found in the continental United States. In addition, assumptions used in this study about the soil and hydraulic conditions were conservative; as a result, calculated doses and risks are not likely to be exceeded in the continental United States where other soil and hydraulic conditions may exist.
Acknowledgments This study was funded by the U.S. Department of Energy (DOE), Office of Fossil Energy and National Petroleum Technology Office, under Contract W-31-109-ENG-38. Cofunding was received from the American Petroleum Institute (API). The authors would like to acknowledge the help of John Ford, DOE National Petroleum Technology Office; Glenda Smith, formerly with API; Lewis Cook, formerly with Chevron Research and Technology Company; Kevin Grice, Chevron Texaco Energy Research and Technology Company; and David Minnaar and Bob Skowronek, MDEQ, Radiological Protection Section.
Literature Cited (1) Smith, K. P.; Blunt, D. L.; Williams, G. P.; Arnish, J. J.; Pfingston, M.; Herbert, J.; Haffenden, R. A. An Assessment of the Disposal of Petroleum Industry NORM in Nonhazardous Landfills; U.S. Department of Energy, National Petroleum Technology Office: Tulsa, OK, 1999; DOE/BC/W-31-109-Eng-38-8. (2) Smith, A. L. J. Pet. Technol. 1987, 39 (6), 697-706. VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2065
(3) Cleanup and Disposal Guidelines for Sites Contaminated With Radium-226; Michigan Department of Environmental Quality, Drinking Water and Radiological Protection Division: Lansing, MI, 1996. (4) Pfingston, M. R.; et al. TSD-DOSE: A Radiological Dose Assessment Model for Treatment, Storage, and Disposal Facilities; Argonne National Laboratory: Argonne, IL, 1998; ANL/EAD/ LD-4 (Revision 1). (5) Yu, C.; et al. Manual for Implementing Residual Radioactive Material Guidelines Using RESRAD, Version 5.0; Argonne National Laboratory: Argonne, IL, 1993; ANL/EAD/LD-2. (6) Schroeder, P. R.; Dozier, T. S.; Zappi, P. A.; McEnroe, B. M.; Sjostrom, J. W.; Peyton, R. L. The Hydrologic Evaluation of Landfill Performance (HELP) Model: Engineering Documentation for Version 3; U.S. Environmental Protection Agency, Office of Research and Development: Washington, DC, 1994; EPA/ 600/R-94/168b. (7) Tomasko, D. Modeling Vertical and Horizontal Solute Transport for the Weldon Spring Site Remedial Action Project; Argonne National Laboratory: Argonne, IL, 1992; ANL/EAIS/TM-77. (8) Reeves, M.; et al. Theory and Implementation for SWIFT II: The Sandia Waste-Isolation Flow and Transport Model for Fractured Media, Release 4.84; Sandia National Laboratories: Albuquerque, NM, 1986; SAND83-1159. (9) International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection; Pergamon Press: Oxford, UK, 1991; Publication 60. (10) Air/Superfund National Technical Guidance Study Series, Vol. III-Estimation of Air Emissions from Cleanup Activities at a
2066
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 10, 2003
(11)
(12)
(13) (14)
(15)
(16)
(17)
Superfund Site; U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards: Research Triangle Park, NC, 1989; EPA-450/1-89-003. Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation Manual, Interim Final; U.S. Environmental Protection Agency, Office of Emergency and Remedial Response: Washington, DC, 1991; EPA/540/R-92/003. Limiting Values of Radionuclide Intake and Air Concentrations and Dose Conversion Factors for Inhalation, Submersion, and Ingestion; Federal Guidance Report 11; U.S. Environmental Protection Agency: Washington, DC, 1988; EPA 520/1-88-020. Wilson, A. J.; Scott, L. M. Health Phys. 1992, 63 (6), 681-685. A Naturally Occurring Radioactive Material (NORM) Disposal Cost Study; American Petroleum Institute: Washington, DC, 1996; Publication 7100. U.S. Environmental Protection Agency. DRAFT Diffuse NORM Wastes-Waste Characterization and Preliminary Risk Assessment; Office of Radiation and Indoor Air: Washington, DC, 1993. Rood, A. S.; Kendrick, D. T. NORM/NARM: Regulation and Risk Assessment; Proceedings of the 29th Midyear Topical Meeting of the Health Physics Society, Scottsdale, AZ, 1996; pp 139-144. Auxier & Associates, Inc. Leachate Analysis of Martha Oil Field Wastes, Martha, Kentucky; Auxier & Associates: Knoxville, TN, 1996.
Received for review September 19, 2002. Revised manuscript received March 12, 2003. Accepted March 17, 2003. ES0261729
