Allometric-kinetic model predictions of concentration ratios for anthropogenic radionuclides across animal classes and food selection.

Protection of the environment from radiation fundamentally relies on dose assessments for non-human biota. Many of these dose assessments use measured or predicted concentrations of radionuclides in soil or water combined with Concentration Ratios (CRs) to estimate whole body concentrations in animals and plants, yet there is a paucity of CR data relative to the vast number of potential taxa and radioactive contaminants in the environment and their taxon-specific ecosystems. Because there are many taxa each having very different behaviors and biology, and there are many possible bioavailable radionuclides, CRs have the potential to vary by orders-of-magnitude, as often seen in published data. Given the diversity of taxa, the International Commission on Radiological Protection (ICRP) has selected 12 non-human biota as reference animals and plants (RAPs), while the U.S. Department of Energy (DOE) uses the non-taxon specific categories of terrestrial, riparian, and aquatic animals. The question we examine here, in part, is: are these RAPs and categorizations sufficient to adequately protect all species given the broad diversity of animals in a region? To explore this question, we utilize an Allometric-Kinetic (A-K) model to calculate radionuclide-specific CRs for common animal classes, which are then further subcategorized into herbivores, omnivores, carnivores, and invertebrate detritivores. Comparisons in CRs among animal classes exhibited only small differences, but there was order of magnitude differences between herbivores, carnivores, and especially detritivores, for many radionuclides of interest. These findings suggest that the ICRP RAPs and the DOE categories are reasonable, but their accuracy could be improved by including sub-categories related to animal dietary ecology and biology. Finally, comparisons of A-K model predicted CR values to published CRs show order-of-magnitude variations, providing justification for additional studies of animal assimilation across radionuclides, environmental conditions, and animal classes.

Allometric-kinetic model predictions of radionuclide dynamics across turtle taxa.

Chelonians (turtles, tortoises, and sea turtles; hereafter, turtles) inhabit a wide variety of ecosystems that are currently, or have the potential in the future to become, radioactively contaminated. Because they are long-lived, turtles may uniquely accumulate significant amounts of the radionuclides, especially those with long half-lives and are less environmentally mobile. Further, turtle shells are covered by scutes made of keratin. For many turtle taxa, each year, keratin grows sequentially creating annual growth rings or layers. Theoretically, analysis of these scute layers for radionuclides could provide a history of the radioactivity levels in the environment, yet there are few previously published studies focused on the dynamics of radionuclide intake in turtles. Using established biochemical and ecological principles, we developed an allometric-kinetic model to establish relationships between the radionuclide concentrations in turtles and the environment they inhabit. Specifically, we calculated Concentration Ratios (CRs - ratio of radionuclide concentration in the turtle divided by the concentration in the soil, sediment, or water) for long-lived radionuclides of uranium and plutonium for freshwater turtles, tortoises, and sea turtles. These CRs allowed prediction of environmental concentrations based on measured concentrations within turtles or vice-versa. We validated model-calculated CR values through comparison with published CR values for representative organisms, and the uncertainty in each of the model parameters was propagated through the CR calculation using Monte Carlo techniques. Results show an accuracy within a factor of three for most CR comparisons though the difference for plutonium was larger with a CR ratio of about 200 times for sea turtles, driven largely by the uncertainty of the solubility of plutonium in sea water.

Radionuclide resuspension across ecosystems and environmental disturbances.

Exposure assessment from radionuclides and other soil-bound contaminants often requires quantifying the amount of contaminant resuspended in the air. Rates and controlling factors of radionuclide resuspension and wind erosion of soil are clearly related but have largely been studied separately. Here, we review both and then integrate wind erosion measurements with the radiological resuspension paradigm to provide better estimates of resuspension factors across a broad range of ecosystems and environmental conditions. Radionuclide resuspension by wind was initially investigated during the era of aboveground nuclear weapons testing. Predictive dose models were developed from empirically-derived ratios of air and soil concentrations, otherwise called the resuspension factor. Resuspension factors were shown to generally predict radionuclide concentrations in air, but they were site-specific and largely derived from the arid and semi-arid environments surrounding nuclear weapons testing locations. In contrast, wind erosion studies from the agricultural and environmental sciences have produced more mechanistic models and a relatively robust data set of wind erosion rates and model parameters across a range of ecosystems. We sequentially show the mathematics linking measured sediment flux from wind erosion rate measurements to resuspension factors using the concept of transport capacity and its relationship to the deposition velocity. We also describe the conceptual framework describing how resuspension factors change through time and the mathematical models describing this decrease. We then show how vertical mass flux measurements across ecosystems were categorized and used to calculate ecosystem-based resuspension factors. These calculations allow generalized estimation of radionuclide resuspension factors across ecosystem types as a function of disturbance and as input for dose calculations.

Validation Tests of Resuspension Models for a Finite and Infinite Site.

Dose assessment for deposited radionuclides often requires estimates of air concentrations that are derived from measured soil concentrations. For this, dose assessors typically use literature resuspension values that, while empirically based, can vary by orders of magnitude making it difficult to provide accurate dose estimates. Despite the complexities of the physical processes involved in resuspension, the models generally used for dose assessment are relatively simplistic and rarely are the models validated for a specific site, thus making prediction of air concentrations or airborne emissions highly uncertain. Additionally, the size of the contaminated area can have an impact on downwind concentrations, yet literature values do not account for smaller-sized contaminated sites adding additional uncertainty. To test resuspension models for soil-bound radionuclides at finite and infinite spatial scales, measurements of soil and air concentrations are made at (1) a location downwind of a former outfall where Pu was released into the environment (a finite site), and (2) uncontaminated locations where regional air sampling provides background measurements of naturally occurring U in sampled dust (an infinite site). Measured air concentrations were compared to those predicted using the resuspension factor model and the mass loading model. An area factor was applied to the smaller contaminated site to account for dilution of dust from the contaminated site with dust originating from offsite locations. Results show that when properly parameterized to site conditions, resuspension models can predict air concentrations to within a factor of 10.

Gamma-Ray Dose From an Overhead Plume.

Standard plume models can underestimate the gamma-ray dose when most of the radioactive material is above the heads of the receptors. Typically, a model is used to calculate the air concentration at the height of the receptor, and the dose is calculated by multiplying the air concentration by a concentration-to-dose conversion factor. Models indicate that if the plume is emitted from a stack during stable atmospheric conditions, the lower edges of the plume may not reach the ground, in which case both the ground-level concentration and the dose are usually reported as zero. However, in such cases, the dose from overhead gamma-emitting radionuclides may be substantial. Such underestimates could impact decision making in emergency situations. The Monte Carlo N-Particle code, MCNP, was used to calculate the overhead shine dose and to compare with standard plume models. At long distances and during unstable atmospheric conditions, the MCNP results agree with the standard models. At short distances, where many models calculate zero, the true dose (as modeled by MCNP) can be estimated with simple equations.

Accuracy of Cloudshine Gamma Dose Calculations in the CAP-88 Dispersion Model.

The U.S. Environmental Protection Agency dispersion model, CAP-88, calculates ground-level dose using the ground-level concentration and the semi-infinite cloud approximation. Doses can be underestimated for elevated plumes during stable atmospheric conditions at receptor locations within a kilometer downwind of a stack. The purpose of this paper is to identify when CAP-88 calculations of gamma dose from cloudshine are inaccurate and provide estimates of the inaccuracy. The method used compares CAP-88 estimates with Monte Carlo N-Particle (MCNP) estimates. Comparisons were made at distances of 800 m and 3,000 m downwind of the stack and for plume heights from 0 to 50 m. For these conditions, the annual dose calculated by CAP-88 is greater than or equal to that calculated by MCNP.

Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose.

Dose assessments typically consider environmental systems as static through time, but environmental disturbances such as drought and fire are normal, albeit infrequent, events that can impact dose-influential attributes of many environmental systems. These phenomena occur over time frames of decades or longer, and are likely to be exacerbated under projected warmer, drier climate. As with other types of dose assessment, the impacts of environmental disturbances are often overlooked when evaluating dose from aeolian transport of radionuclides and other contaminants. Especially lacking are predictions that account for potential changing vegetation cover effects on radionuclide transport over the long time frames required by regulations. A recently developed dynamic wind-transport model that included vegetation succession and environmental disturbance provides more realistic long-term predictability. This study utilized the model to estimate emission rates for aeolian transport, and compare atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances. Specifically, the objective of this study was to utilize the model results as input for a widely used dose assessment model (CAP-88). Our case study focused on low levels of residual plutonium found in soils from past operations at Los Alamos National Laboratory (LANL), in Los Alamos, NM, located in the semiarid southwestern USA. Calculations were conducted for different disturbance scenarios based on conditions associated with current climate, and a potential future drier and warmer climate. Known soil and sediment concentrations of plutonium were used to model dispersal and deposition of windblown residual plutonium, as a function of distance and direction. Environmental disturbances that affected vegetation cover included ground fire, crown fire, and drought, with reoccurrence rates for current climate based on site historical patterns. Using site-specific meteorology, accumulation rates of plutonium in soil were modeled in a variety of directions and distances from LANL sources. Model results suggest that without disturbances, areas downwind to the contaminated watershed would accumulate LANL-derived plutonium at a relatively slow rate (<0.01 Bq m(-2) yr(-1)). However, model results under more realistic assumptions that include environmental disturbances show accumulation rates more than an order-of-magnitude faster. More generally, this assessment highlights the broader need in radioecology and environmental health physics to consider infrequent but normal environmental disturbances in longer-term dose assessments.

Operational experience of continuous air monitoring of smoke for ²³9Pu during a wildfire.

Smoke from a wildfire in northern New Mexico that moved along the border of the Los Alamos National Laboratory (LANL) was monitored for ²³9Pu in the event that the fire might cross into LANL property containing locations with low, but greater than background, levels of ²³9Pu and other alpha-emitting radionuclides. Three Environmental Continuous Air Monitors (ECAMs) in operation at LANL at the time of the fire provided near real-time measurements of the ²³9Pu in the smoke. Sampling data from routine measurements of PM-10 and PM-2.5 concentrations in the city of Los Alamos showed that smoke in the air rose during the fire to several hundred µg m⁻³, which produced limited visibility (several hundred meters) and resulted in poor air quality alerts for about a week-long period. Previous studies have shown that airborne dust can significantly impair continuous air monitors, so the purpose of this study was to assess the performance of the ECAMs under smoky conditions, which is important for many emergency response scenarios. Additionally, ECAMs are not required to be tested in smoke by ANSI standards, so there is little to no published data on performance of any ECAM while sampling smoke. Results show that the deployed ECAMs had reduced flow as the filter clogged with fine particles, but the goodness-of-fit parameter of the peak shape fitting algorithms and the minimum detectable concentration and dose were not impacted until the flow was reduced by more than about 20%, and even then they were within tolerable limits. Overall, ECAM performance was not impacted during the fire even under heavy smoke conditions and fluctuating radon levels, though changing the filters to limit any reductions in flow to less than 20% would maintain optimal ECAM performance.

Interactive effects of grazing and burning on wind- and water-driven sediment fluxes: rangeland management implications.

Rangelands are globally extensive, provide fundamental ecosystem services, and are tightly coupled human-ecological systems. Rangeland sustainability depends largely on the implementation and utilization of various grazing and burning practices optimized to protect against soil erosion and transport. In many cases, however, land management practices lead to increased soil erosion and sediment fluxes for reasons that are poorly understood. Because few studies have directly measured both wind and water erosion and transport, an assessment of how they may differentially respond to grazing and burning practices is lacking. Here, we report simultaneous, co-located estimates of wind- and water-driven sediment transport in a semiarid grassland in Arizona, USA, over three years for four land management treatments: control, grazed, burned, and burned + grazed. For all treatments and most years, annual rates of wind-driven sediment transport exceeded that of water due to a combination of ongoing small but nontrivial wind events and larger, less frequent, wind events that generally preceded the monsoon season. Sediment fluxes by both wind and water differed consistently by treatment: burned + grazed > burned >> grazed > or = control, with effects immediately apparent after burning but delayed after grazing until the following growing season. Notably, the wind:water sediment transport ratio decreased following burning but increased following grazing. Our results show how rangeland practices disproportionally alter sediment fluxes driven by wind and water, differences that could potentially help explain divergence between rangeland sustainability and degradation.

Corrections for measurements of tritium in subterranean vapor using silica gel.

Hazardous contaminants buried within vadose zones can accumulate in soil gas. The concentrations and spatial extent of these contaminants are measured to evaluate potential transport to groundwater for public risk evaluation. Tritium is an important contaminant found and monitored for in vadose zones across numerous sites within the US nuclear weapons complex, including Los Alamos National Laboratory. The extraction, collection, and laboratory analysis of tritium from subterranean soil gas presents numerous technical challenges that have not been fully studied. Particularly, the lack of moisture in the soil gas in the vadose zone makes it difficult to obtain enough sample (e.g., > 5 g) to provide for the required measurement sensitivity, and often, only small amounts of moisture can be collected. Further, although silica gel has high affinity for water vapor and is prebaked prior to sampling, there is still sufficient residual moisture in the prebaked gel to dilute the relatively small amount of sampled moisture; thereby, significantly lowering the "true" tritium concentration in the soil gas. This paper provides an evaluation of the magnitude of the bias from dilution, provides methods to correct past measurements by applying a correction factor (CF), and evaluates the uncertainty of the CF values. For this, 10,000 Monte Carlo calculations were performed, and distribution parameters of CF values were determined and evaluated. The mean and standard deviation of the distribution of CF values were 1.53 ± 0.36, and the minimum, median, and maximum values were 1.14, 1.43, and 5.27, respectively.

Probabilistic model evaluation of continuous air monitor response for meeting radiation protection goals.

Effective continuous air monitor (CAM) programs can eliminate or significantly reduce the amount of inhaled radioactive material following an accidental release. Numerous factors impact the levels of protection CAM programs provide to the workers during these releases. These factors range from those related to the capability of the CAM instrument (e.g., CAM alarm set point and length of counting intervals) to those related to CAM placement in the room relative to dispersion rates and patterns of the released material in a room. While the impact of many of these factors on alarm sensitivity has been investigated in isolation, there are no methods for holistic evaluations of CAM programs relative to radiation protection goals (RPGs) or the contribution of the factors, either individually or combined, toward limiting worker dose. In this study, worker exposure was predicted using CAM response models developed to evaluate protection levels for continuous and acute releases. Monte Carlo simulations of 10,000 releases were performed using various combinations of model parameter values, with associated uncertainty distributions, to assess the expected ability of a CAM program to meet RPGs, and, further, to assess the relative influence of each factor toward lowering worker exposure. Results showed that improvements to CAM instrument capability combined with better ventilation and CAM placement improve worker protection nonlinearly and that these improvements are critical to meet RPGs. The sensitivity analysis showed that ventilation-driven dilution had the greatest impact on exposure reduction with the selected counting interval for alarm decisions and the alarm set point as secondarily important.

Work to save dose: contrasting effective dose rates from radon exposure in workplaces and residences against the backdrop of public and occupational regulatory limits.

Office workers are exposed to radon while at work and at home. Though there are a multitude of studies reporting radon concentrations and potential lung and effective doses associated with radon progeny exposure in homes, similar studies in non-mine workplaces are lacking. Additionally, there are few, if any, comparative analyses of radon exposures at more "typical" workplace with residential exposures within the same county. The purposes of this study were to measure radon concentrations in office and residential spaces in the same county and explore the radiation dose implications. Sixty-five track-etch detectors were deployed for 3-mo sampling periods in office spaces and 47 were deployed in residences, all within Los Alamos County, New Mexico. The measured concentrations were used to calculate and compare effective dose rates resulting from exposure while at work and at home. Results showed that full-time office workers receive on average about 8 times greater exposure at home than while in the office (2.3 mSv y-1 vs. 0.3 mSv y-1). The estimated effective dose rate for a more homebound person was about 3 mSv y-1. Estimating effective doses from background radon exposure in the same county as Los Alamos National Laboratory, with thousands of "radiological workers," highlights interesting contrasts in radiation protection standards that span public and occupational settings. For example, the effective dose rate from background radon exposure in unregulated office spaces ranged up to 1.1 mSv y-1, which is similar to the 1 mSv y-1 threshold for regulation of a "radiological worker," as defined in the Department of Energy regulations for occupational exposure. Additionally, the estimated average effective dose total of >3 mSv y-1 from radon background exposure in homes stands in contrast to the 0.1 mSv y-1 air pathway effective public dose limit regulated by the Environmental Protection Agency for radioactive air emissions, and both these are substantially lower than effective doses associated with priority radon levels in homes of "tens of pCi L-1 and greater" (>370 Bq m-3), as suggested by the Health Physics Society.

Calculating Capstone depleted uranium aerosol concentrations from beta activity measurements.

Beta activity measurements were used as surrogate measurements of uranium mass in aerosol samples collected during the field testing phase of the Capstone Depleted Uranium (DU) Aerosol Study. These aerosol samples generated by the perforation of armored combat vehicles were used to characterize the DU source term for the subsequent Human Health Risk Assessment (HHRA) of Capstone aerosols. Establishing a calibration curve between beta activity measurements and uranium mass measurements is straightforward if the uranium isotopes are in equilibrium with their immediate short-lived, beta-emitting progeny. For DU samples collected during the Capstone study, it was determined that the equilibrium between the uranium isotopes and their immediate short-lived, beta-emitting progeny had been disrupted when penetrators had perforated target vehicles. Adjustments were made to account for the disrupted equilibrium and for wall losses in the aerosol samplers. Values for the equilibrium fraction ranged from 0.16 to 1, and the wall loss correction factors ranged from 1 to 1.92. This paper describes the process used and adjustments necessary to calculate uranium mass from proportional counting measurements.

Adaptive management: a paradigm for remediation of public facilities following a terrorist attack.

Terrorist actions are aimed at maximizing harm (health, psychological, economical, and political) through the combined physical impacts of the act and fear. Immediate and effective response to a terrorist act is critical to limit human and environmental harm, effectively restore facility function, and maintain public confidence. Though there have been terrorist attacks in public facilities that we have learned from, overall our experiences in restoration of public facilities following a terrorist attack are limited. Restoration of public facilities following a release of a hazardous material is inherently far more complex than in industrial settings and has many unique technical, economic, social, and political challenges. For example, there may be a great need to quickly restore the facility to full operation and allow public access even though it was not designed for easy or rapid restoration, and critical information is needed for quantitative risk assessment and effective restoration must be anticipated to be incomplete and uncertain. Whereas present planning documents have substantial linearity in their organization, the "adaptive management" paradigm provides a constructive parallel paradigm for restoration of public facilities that anticipates and plans for uncertainty, inefficiencies, and stakeholder participation. Adaptive management grew out of the need to manage and restore natural resources in highly complex and changing environments with limited knowledge about causal relationships and responses to restoration actions. Similarities between natural resource management and restoration of a public facility after a terrorist attack suggest that integration of adaptive management principles explicitly into restoration processes will result in substantially enhanced and flexible responses necessary to meet the uncertainties of potential terrorist attacks.

Considerations for data processing by continuous air monitors based on accumulation sampling techniques.

Continuous air monitors (CAMs) sample air and alarm when concentration levels of radioactivity in air exceed preset alarm levels. The air concentrations through time are calculated based on accumulation sampling techniques. Accumulation air sampling is the process in which radioactive aerosol is continually deposited onto a collection medium and a radiation detector provides continuous measurements of the radioactivity on the filter. To assess the air concentration, time intervals are established for the counting and sampling times, and the measurement of concentration represents an average over the measurement interval. There are multiple methods that can be used to determine the concentration for the most recent measurement interval, and based on the method used, each can result in significantly different values for concentrations, associated uncertainties, and response times. We evaluate and compare several methods for determining air concentrations based on accumulation counting techniques. Further, we provide a real-life example of accumulation counting and the effects of compensating for background radiation in the context of monitoring for plutonium concentrations against a fluctuating radon progeny background. Results show the importance of selecting a method that provides for a balance of response time, measurement interval, background compensation technique, and uncertainty for optimal protection of workers.

Uranium partition coefficients (Kd) in forest surface soil reveal long equilibrium times and vary by site and soil size fraction.

The environmental mobility of newly deposited radionuclides in surface soil is driven by complex biogeochemical relationships, which have significant impacts on transport pathways. The partition coefficient (Kd) is useful for characterizing the soil-solution exchange kinetics and is an important factor for predicting relative amounts of a radionuclide transported to groundwater compared to that remaining on soil surfaces and thus available for transport through erosion processes. Measurements of Kd for 238U are particularly useful because of the extensive use of 238U in military applications and associated testing, such as done at Los Alamos National Laboratory (LANL). Site-specific measurements of Kd for 238U are needed because Kd is highly dependent on local soil conditions and also on the fine soil fraction because 238U concentrates onto smaller soil particles, such as clays and soil organic material, which are most susceptible to wind erosion and contribute to inhalation exposure in off-site populations. We measured Kd for uranium in soils from two neighboring semiarid forest sites at LANL using a U.S. Environmental Protection Agency (EPA)-based protocol for both whole soil and the fine soil fraction (diameters<45 microm). The 7-d Kd values, which are those specified in the EPA protocol, ranged from 276-508 mL g-1 for whole soil and from 615-2249 mL g-1 for the fine soil fraction. Unexpectedly, the 30-d Kd values, measured to test for soil-solution exchange equilibrium, were more than two times the 7-d values. Rates of adsorption of 238U to soil from solution were derived using a 2-component (FAST and SLOW) exponential model. We found significant differences in Kd values among LANL sampling sites, between whole and fine soils, and between 7-d and 30-d Kd measurements. The significant variation in soil-solution exchange kinetics among the soils and soil sizes promotes the use of site-specific data for estimates of environmental transport rates and suggests possible differences in desorption rates from soil to solution (e.g., into groundwater or lung fluid). We also explore potential relationships between wind erosion, soil characteristics, and Kd values. Combined, our results highlight the need for a better mechanistic understanding of soil-solution partitioning kinetics for accurate risk assessment.

From dust to dose: Effects of forest disturbance on increased inhalation exposure.

Ecosystem disturbances that remove vegetation and disturb surface soils are major causes of excessive soil erosion and can result in accelerated transport of soils contaminated with hazardous materials. Accelerated wind erosion in disturbed lands that are contaminated is of particular concern because of potential increased inhalation exposure, yet measurements regarding these relationships are lacking. The importance of this was highlighted when, in May of 2000, the Cerro Grande fire burned over roughly 30% of Los Alamos National Laboratory (LANL), mostly in ponderosa pine (Pinus ponderosa) forest, and through areas with soils containing contaminants, particularly excess depleted and natural uranium. Additionally, post-fire thinning was performed in burned and unburned forests on about 25% of LANL land. The first goal of this study was to assess the potential for increased inhalation dose from uranium contaminated soils via wind-driven resuspension of soil following the Cerro Grande Fire and subsequent forest thinning. This was done through analysis of post-disturbance measurements of uranium air concentrations and their relationships with wind velocity and seasonal vegetation cover. We found a 14% average increase in uranium air concentrations at LANL perimeter locations after the fire, and the greatest air concentrations occurred during the months of April-June when wind velocities are highest, no snow cover, and low vegetation cover. The second goal was to develop a methodology to assess the relative contribution of each disturbance type towards increasing public and worker exposure to these resuspended soils. Measurements of wind-driven dust flux in severely burned, moderately burned, thinned, and unburned/unthinned forest areas were used to assess horizontal dust flux (HDF) in these areas. Using empirically derived relationships between measurements of HDF and respirible dust, coupled with onsite uranium soil concentrations, we estimate relative increases in inhalation doses for workers ranging from 15% to 38%. Despite the potential for increased doses resulting from these forest disturbances, the estimated annual dose rate for the public was <1 microSv yr(-1), which is far below the dose limits for public exposures, and the upper-bound dose rate for a LANL worker was estimated to be 140 microSv yr(-1), far below the 5 x 10(4) microSv yr(-1) occupational dose limit. These results show the importance of ecosystem disturbance in increasing mobility of soil-bound contaminants, which can ultimately increase exposure. However, it is important to investigate the magnitude of the increases when deciding appropriate strategies for management and long-term stewardship of contaminated lands.