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Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose
Journal of Environmental Radioactivity 120 (2013) 73e80
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Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose Erika A. Michelotti a, Jeffrey J. Whicker a, *, William F. Eisele a, David D. Breshears b, c, Thomas B. Kirchner d a
Los Alamos National Laboratory, Environmental Stewardship Group, Mail Stop J978, Los Alamos, NM 87544, USA School of Natural Resources and the Environment University of Arizona, Tucson, AZ 85721, USA c Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA d New Mexico State University, Las Cruces, NM 88003, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2012 Received in revised form 8 January 2013 Accepted 8 January 2013 Available online 1 March 2013
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 doseinfluential 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 yr1). 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. Published by Elsevier Ltd.
Keywords: Environmental disturbance Climate change Soil Sediment Contaminant transport
1. Introduction Many of the most fundamental issues in radioecology and environmental health physics span time frames of decades,
Abbreviations: Vegetation Modified Transport, VMTran; Clean Air Act Assessment Package-1988, CAP88; Los Alamos National Laboratory, LANL. * Corresponding author. Tel.: þ1 505 667 2610; fax: þ1 505 665 6071. E-mail addresses: [email protected], [email protected] (J.J. Whicker). 0265-931X/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jenvrad.2013.01.011
centuries, and millennia, which presents unique scientific challenges (Hinton et al., 2013). Notably, regulations for decommissioned sites, spent nuclear fuel repositories, and many other evaluations require assessment of dose to public receptors out to a minimum of 1000 years (EPA, 1982) for longer-lived radionuclides, which requires modeling to predict effective dose rates into the future. Dose assessments for environmental contaminants are implicitly dependent on the environmental context, but assumptions about associated ecosystem dynamics are often not explicitly considered (Whicker and Breshears, 2011; Breshears et al., 2012).
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Ecosystems are complex, dynamic systems which are significantly altered by disturbances (Pickett and White, 1985), and they often are heterogeneous and often in a state of nonequilibrium (Reynolds, 2001). However, models used to predict transport of contaminants through the environment commonly presume stationary or equilibrium conditions, either explicitly or often implicitly, and consequently over-simplify the complex interactions among the components of an ecosystem. Such models are often based on equilibrium ecology, which generally assumes a defined and ideal ecological structure to which, despite disturbances and minor fluctuations, systems will attempt to return (Landis and Wiegers, 2008). Assumptions of stationarity in an ecosystem are compromised when environmental and anthropogenic disturbances are taken into account, including the large potential impacts associated with global climate change. Examples of inadequate management of water resources resulting from stationary assumption have been studied for cases where conditions exceed natural variation (Milly et al., 2008). The deviation from stationary planning assumptions predicted by climate change projections exceeds the natural-variation deviation that has plagued water-resource planners. Probabilistic models that include climate change projections are needed to replace or supplement water distribution and use models. Likewise, models in other environmental disciplines, such as those for soil and natural vegetation patterns, need to be altered to reflect the dynamic complexity of ecosystems, real properties of ecological structures and disturbances, and potential impacts of climate change. Aeolian transport of sediment erodes soil, redistributes nutrients, alters biogeochemical cycles, and, in a radioecological context, can mobilize contaminants; thus, wind transport of soil and sediment needs to be accurately modeled to ensure ecosystem health,
soil productivity and accurate assessments of contaminant transport (Pye, 1987; Chadwick et al., 1999; Toy et al., 2002; Jickells et al., 2005; Whicker et al., 2006; Li et al., 2007; Field et al., 2010; Ravi et al., 2011; Whicker and Breshears, 2011). Regulatory assessments commonly use stationary- or equilibrium-based models that assume, for example, constant emission rates of airborne soil (Yu et al., 2001). However, assuming a constant transport rate without considering the impact of environmental disturbances may underestimate long-term aeolian transport of persistent soil-bound contaminants to off-site locations and preclude accurate evaluations of human health risks (Field et al., 2011; Breshears et al., 2012). For example, most soil contaminant transport models for semiarid ecosystems do not account for changes in vegetation cover as impacted by expected environmental disturbances such as fire, drought, and tree disease, nor of subsequent succession (Fig. 1), all of which have recently been shown to dramatically impact transport rates of contaminated dust over long time periods (Okin, 2008; Breshears et al., 2012). Spatial and temporal patterns and characteristics of vegetation change following periodic disturbances such as fire and drought (Turner et al., 1998; Whicker et al., 2002; Allen, 2007; Romme et al., 2009; Williams et al., in press). Changes invoked by these events depend upon the type and frequency of disturbance, the successional state at the time of the disturbance, and the amount and type of ground cover before and after the disturbance (Breshears et al., 2005a,b). Based on a recently developed conceptual model (Breshears et al., 2009), disturbances typically increase aeolian transport rates by reducing the amount of woody vegetation and altering surface wind flow across the vegetation. Importantly, climate change and anthropogenic activities (land use change including agricultural and logging practices, and emission of green
Fig. 1. Illustration showing ecological succession following closure of a landfill and the impact of environmental disturbances on the vegetation cover. Each of these ecological states has a different wind erosion rate associated with it. From Breshears et al. (2012); figure used with permission.
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house gases) are anticipated to continue to cause an increase in the severity and frequency of environmental disturbances. Including environmental disturbances at different frequencies of occurrence, to simulate the impacts of climate change, could improve the prediction of contaminant transport and accuracy of dose assessments. Many radionuclide-contaminated sites are located in semiarid environments. In contrast to more humid locations, semiarid systems are of particular concern with regard to radionuclide transport because they (1) usually have limited amounts of ground cover, which enables higher potential rates of contaminant mobility, (2) succession after a disturbance can be slow and lead to an alternate state, (3) fire is often a recurrent disturbance processes, (4) extreme climate events related to drought can trigger landscape scale changes in vegetation cover not only through fire but also through droughtinduced tree mortality, and (5) many semiarid regions, including the southwestern USA, are projected to become warmer and drier, thereby potentially further exacerbating the previously highlighted characteristics affecting contaminant transport (Breshears et al. 2012). An important case study where these factors collectively have been studied in detail and where radionuclide transport is of concern is at Los Alamos National Laboratory (LANL), which is part of the Department of Energy’s nuclear weapons complex and where residual radioactivity remains in local soils from past operations. This site in northern New Mexico lies within Southwestern uplands, which are among the most extensive and detailed regional-scale network of fire and drought history available in the world; thus, enabling improved estimates of future disturbance rates (Swetnam and Baisan, 1996; Swetnam et al., 1999; Allen et al., 2002). Additionally, the Southwestern United States is projected to experience a warmer and drier climate in the future (Seager et al., 2007), leading to an increased frequency of disturbances including fire and drought (Breshears et al., 2005b; Westerling et al., 2006; Adams et al., 2010; International Panel on Climate Change, 2007; O’Neal et al., 2005). Such changes would likely increase rates of transport of contaminated soils and need to be accounted for in long-term risk assessments (Breshears et al., 2012). Cases of landscape-scale shifts in vegetation, and associated acceleration of soil loss, have already been documented. In the early 2000’s drought killed 40e97% of pinion trees in a 12,000 km2 site over 3 years (Breshears et al., 2005b). This type of rapid, large-scale change is consistent with other recent tree mortality events around the globe (Allen et al., 2010). Recent forest fires in the Southwestern US can impact increasingly large areas; together with areas impacted by drought-induced tree die-off, at least 18% of Southwestern USA woodlands and forests have been impacted in recent decades (Williams et al., in press). These observations, and current predictions of future conditions, emphasize the need for models which more accurately account for rapid vegetation changes in response to disturbances such as drought, beetle infestations, and fire (Lu et al., 2007; Whicker et al., 2006). 1.1. Objective of study A recently developed dynamic wind-transport model included vegetation succession and environmental disturbance to provide more realistic long-term predictability: the Vegetation Moderated Transport Model (VMTran; Breshears et al., 2012). This study utilized the VMTran model to estimate emission rates for aeolian transport, and compared atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances using a widely used dose assessment model (CAP-88; EPA, 2011). Our case study focused on low levels of residual plutonium found in soils from past operations at LANL in the semiarid southwestern USA. In the recent
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related study using VMTran (Breshears et al., 2012), aeolian transport of contaminated soil was predicted considering environmental disturbance under current and potential changes in climate, focusing on saltation transport (scale of a few meters). Here we build on and extend that work to include suspension of soil and atmospheric transport (scale of km). Using historical data and available modeling tools, our research sought to evaluate the importance of dynamic models and how such models might be utilized to better predict transport rates of plutonium in Los Alamos soil and sediment to distant, downwind locations. For this study, the VMTran model was used to determine the vertical transport (suspension) rate of contaminated soil over a thousand-year time period under static vegetation conditions and cover that varies dynamically due to disturbance and subsequent recovery. The suspension rate estimates for the contaminated soil through time were then linked to an atmospheric dispersion model to predict downwind dispersion and surface deposition rates over a 1000-yr time period. These predictions were calculated for the same site under different environmental conditions. We discuss more broadly how radioecology and environmental health physics need to consider environmental disturbances in longer-term dose assessments. 2. Methods 2.1. Site overview Research focused on Los Alamos Canyon Watershed, located in Los Alamos, New Mexico, U.S.A. Los Alamos has a temperate, semiarid mountain climate. Prior to the 1960’s, LANL released airborne and liquid effluents containing 239/240Pu into the surrounding environment and into the Los Alamos Canyon Watershed. Although remediated to acceptable levels, this watershed has the highest residual plutonium concentrations in soil and sediment relative to other impacted areas (Gallaher et al., 1997). Water, soil, and sediment samples in the canyon have been collected for more than three decades to measure concentrations and determine the spatial distribution of 239/240Pu. Regarding offsite transport of these soils, Los Alamos Canyon Watershed has an ephemeral stream that over the years has transported radionuclides down the canyon during rain storms (Katzman and Reneau, 2004). Additionally, aeolian transport of contaminated soils, formed from stream sediment or from past airborne stack emissions, could be significant contributors to offsite transport of plutonium contaminated soil (Webb et al., 1997; Whicker et al., 2006). Combined, these contaminated soils and sediments are potential sources for off-site transport of plutonium bearing soils by wind. The consideration of climate change and disturbances could significantly impact the predictions of transport from this site. 2.2. Plutonium concentration in soil and sediment The publicly accessible Risk Analysis, Communication, Evaluation, and Reduction Project (RACER) database1 was utilized to obtain concentrations of 239/240Pu for soil and sediment in the Los Alamos Canyon Watershed. In all, over 4000 measurements of plutonium concentrations in soil and sediment were used to calculate the summary statistics. The concentrations appeared lognormally distributed so the median was utilized to provide a single representative concentration for the entire watershed and to avoid an upward bias because of a few high measurements. To
1 http://www.racteam.com/racer.html. Recent version of database found at http://www.intellusnmdata.com/.
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calculate the total number of becquerels of 239/240Pu in the watershed the median was multiplied by the density of soil (1.68 106 g m3), the area of the watershed (1.38 108 m2), and an assumed depth of plutonium in the soil column of 0.3 m. A depth of 0.3 m was selected because the surface deposited plutonium is largely contained in this depth (Whicker and Shultz, 1982), and it is this surface soil that is most susceptible to wind erosion over long time frames. 2.3. Modeling aeolian transport rates To incorporate vegetative succession and disturbances into models of soil movement, the VMTran model was developed based on a suite of empirical dry-land measurements of aeolian dust flux (Breshears et al., 2012). VMTran was designed to estimate local patterns and rates of contaminant redistribution caused by winds, taking into account the impacts of vegetative succession and environmental disturbances, and simulate this transport over long periods of time. A Markov-like transition matrix was used to simulate vegetative succession with vegetative succession (i.e., bare soil to grass to shrubs to woodland to forest) simulated following closure of a waste site and after each disturbance. Successional states modeled were bare soil, litter, grassland, shrubland, woodland, and forest. Plant growth is assumed to be logistic; however, plants can shade other plants and delay succession. Ground cover in intercanopy areas differs between disturbed and undisturbed sites and is reflected in distinguishable transport rates (Breshears et al., 2009). VMTran was utilized to obtain annual horizontal fluxes of soil over a 1000 year time period under three scenarios: 1) static conditions with a constant emission rate, 2) inclusion of environmental disturbances with return frequencies based on current climate conditions, and 3) inclusion of disturbances but with return frequencies that bound (upper and lower) those anticipated under climate change predictions coupled with potential active management of the site. Static conditions assume a constant annual soil transport rate based on continual woodland conditions (i.e., the climax vegetation). Disturbances under assumptions of current climate conditions that were modeled included crown fire (a severe fire where all parts of the tree are burned including the crown), a surface fire (grass and shrubs are burned, but trees survive), and drought (severe enough to kill some portion of trees), at intervals of 30, 50, and 250 years, respectively. Disturbances, including crown fire, surface fire and drought, are stochastically scheduled events in the model based on the mean return time assigned to each type of event. Impacts of the disturbances on vegetation and soil mobility rates vary depending on the type of disturbance, successional state at the time of disturbance, and the amount of ground cover. Because of the large uncertainty regarding the potential impacts of climate change (likely to increase in disturbance frequency) and possible future mitigative land management practices by DOE or successor (e.g., fire suppression, vegetation cover management) on ecological recovery and environmental disturbance rates, upper- and lowerbound rates were modeled. The upper-bound frequency was 15, 25, and 125 years for surface fire, crown fire and drought, respectively, while the lower-bound frequency doubled the historicallybased frequencies in length to 60, 100, and 500 years, respectively. Output from the model includes annual transport rates and the associated percent woody cover as it changes over a 1000 yr period (Breshears et al., 2012). More precisely, transport rates are based on saltation transport and defined as the horizontal mass flux (g m2 yr1) or the amount of soil that is moved horizontally across the land surface, usually a distance of a few meters or less. These saltating particles, upon impact to the soil surface, cause smaller particles to be lifted higher, or suspended from the surface, where
they can be transported long distances (many meters to kilometers) by wind. The suspension rate or vertical flux (g m2 yr1) was calculated from the VMTrans horizontal flux, and because plutonium contamination in surface soil is of primary concern at LANL, the suspension flux was converted to emission rate (i.e., Bq yr1) and used in the atmospheric dispersion model. To account for random error in model simulations, because some of the parameters were stochastically determined, the model was run ten times for each scenario, and the average and standard deviations of horizontal mass flux of the ten runs were determined. Long-distance transport of soil and contaminants are associated with vertical dust flux measurements, which is a measure of emission rate. A relationship of 1% horizontal transport being vertical transport was utilized in this study based on site-specific data (Whicker et al., 2006). Using the vertical mass flux, the radioactivity emission rate (Q), or the amount of plutonium leaving the watershed, was calculated using the 1000 year vertical mass flux rate for each scenario, as shown in Eqn. (1).
Q ¼ VMF*A*C
(1)
where Q is the emission rate (Bq yr1), VMF is vertical mass flux (g m2 yr1) based on the average horizontal mass flux, A is area of the source (m2), and C is the median soil concentration (Bq g1). The radioactivity emission rate was used in CAP88 to estimate dispersion and deposition rates in a variety of directions. 2.4. Air dispersion and deposition modeling The U.S. Environmental Protection Agency (EPA) requires use of the Clean Air Act Assessment Package-1988 (CAP88) to calculate dose equivalents to members of the public. This model calculates atmospheric dispersion and subsequent deposition rates at a variety distances and directions from the identified source using a modified Gaussian plume model (Environmental Protection Agency [EPA], 2011). Despite concerns about the ability of CAP88 to account for turbulence in landscapes with changing terrain, CAP88 has been found to provide reasonable, yet conservative, estimates of the atmospheric dispersion of radionuclides from stack emissions at LANL. For example, CAP88 was used to model the dispersion of tritium releases from LANL stacks to downwind receptors and the results were compared to measurements from ambient air monitoring stations (Michelotti et al., 2013). CAP88 generally predicted higher values than those measured at monitoring stations, but the predicted values averaged over a year were within a factor of 1e3.5, suggesting that it provides reasonable annual estimates of dispersion. These results are consistent with the Tritium Working Group of the International Atomic Energy Agency’s BIOMASS Coordinated Research Programme, which suggest accuracy within a factor of 3 or less in most scenarios is acceptable (IAEA, 2003). Emission rates calculated from VMTrans and local meteorological data were used to calculate dispersion and deposition rates of plutonium from Los Alamos Canyon, Meteorological data inputs into CAP88 were based on climate averages for Los Alamos, including annual precipitation of 45 cm y1, annual ambient temperature of 9 C, annual average atmospheric mixing lid of 1600 m, and an absolute humidity of 5.5 g m3 (Bowen, 1990; Baars, 1997). Releases were modeled as resuspension (i.e., ground release) and the total area of the release set at 1.38 108 m2, the area of the Los Alamos Canyon Watershed.
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Although site-specific model parameters were used in this study, some simplifying assumptions had to be applied in using the CAP88 model, such as assuming uniform concentrations of contamination across the source area and default values for deposition rates. Deposition rates included both wet and dry deposition. Dry deposition was assumed to be proportional to the ground-level concentration and is calculated as the product of the modeled air concentration at ground-level and the deposition velocity (default ¼ 1.8 103 m s1) for particles. The fractional removal rate for wet deposition was calculated as the product of the site precipitation rate (45 cm yr1) and a removal constant of 1 107 yr cm1 s1. 3. Results The median concentration of plutonium in Los Alamos Canyon Watershed was estimated to be 7.1 103 Bq g1 in soil and sediment. For context, the background level of Pu-239 in the Los Alamos area from global fallout is about 7.4 104 Bq g1, an order-ofmagnitude lower than the area median concentration in the area. Though the plutonium soil concentrations are elevated above background, radiation dose projections for living in the study area are still much lower than applicable safety limits for the public (LANL, 2012). Average horizontal transport rates for static conditions and environmental disturbances for a 1000 year time period were used to calculate the vertical flux and radioactivity source term (Fig. 2). Static conditions assume a constant horizontal transport rate of 511 g m2 yr1 for woodland conditions. In contrast, the predicted horizontal fluxes with periodic environmental disturbances ranged from 70 to 44,000 g m2 yr1 (Fig. 3) with an average of w10,000 g m2 yr1 depending on the frequency of disturbances. Increases in horizontal flux are directly associated with environmental disturbance events (Fig. 3) and the fluxes declined as the vegetation, particularly the woody vegetation, recovered. However, if disturbances were very frequent, the woody vegetation could not recover fast enough and the erosion rates remained relatively high. The winds in Los Alamos primarily blow from the southwest to the northeast, so the northeast sector had the greatest deposition
1e+5
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9674
1e+4 6105
-2
-1
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Horizontal Flux Vertical Flux
1e+3 511
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9.7
6.1 5.1
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ti St a
s ion dit on C c
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Fig. 2. Mean soil transport rates and the radioactivity source terms for 1000 years. Vertical fluxes are taken as 1% of the horizontal flux (Whicker et al., 2006). Error bars represent the standard deviation across 10 simulations.
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rates for the 1000 year time period (plotted as a function of distance from the watershed in Fig. 4). Predicted deposition pattern shows no decline in downwind soil concentration out to about 7 km, after which the concentration steadily declines. The flat profile in the first kilometers was expected due to the large source area of the Los Alamos Canyon Watershed that contributed contaminated soil to the closer areas. The source area would become smaller with distance from the watershed, resulting in decreased deposition farther away. The 1000-yr average deposition rates predicted for scenarios that include environmental disturbance were a factor of 19 higher than when static conditions are assumed. When upper- and lowerbound disturbance rates, specifically accounting for potential future impacts of climate change and possible human-influenced environmental mitigation on disturbance or recovery rates were accounted for, the 1000-yr average deposition rates were still 11e 23 times higher than the model would predict under static conditions. 4. Discussion Overall, our results show how transport rates and downwind deposition of contaminated soil increase with disturbance, consistent with recent research (Breshears et al., 2012). Our results suggest that soil transport models that fail to account for vegetative succession and environmental disturbances will likely lead to under predictions and possibly misinformed management decisions. Nonetheless, our modeling results contain several relevant uncertainties. Several important simplifying assumptions are made in this analysis regarding spatial distribution, independence of disturbances, and impact of potential human activities at the site. Both CAP88 and VMTran assume that a finite source of contamination is uniformly scattered across the entire site. In reality concentrations do and will vary spatially across the site though the LANL data are not sufficient to fully characterize this distribution within the watershed with any degree of spatial accuracy. Heterogeneity in soil contamination over the large area will lead to more localized transport because of saltation. In addition, saltation produces resuspension of the contaminated soil that this study shows will be dispersed over much greater areas by wind. In contrast, water tends to concentrate the contaminants into stream beds. The spatial distribution pattern through time will be a combination of these two erosion processes (Field et al., 2009). A median concentration of plutonium distributed evenly across the landscape was a reasonable assumption given a significant fraction of the contamination in the soil was from deposited air emissions and the contaminants in the generally dry stream beds would have had multiple decades to be dispersed by wind. Regardless of the limitation in the data set in spatial resolution, we believe the results suggest that models which include environmental disturbance result in higher predicted aeolian transport regardless of the initial spatial pattern of the contamination. Our results also provide insights into contamination spread as a function of distance in addition to the key importance of including disturbance. As anticipated, larger deposition rates were predicted closer to the source and rates decreased with distance, but in a nonlinear fashion. Deposition rates were relatively constant with distance out to about 7 km before decreasing rapidly. This result was likely due to the large size of the contaminated area modeled and assumptions of CAP88. Order of magnitude differences between static conditions and those with environmental disturbances underscore the complexity of the environment and the importance of accounting for disturbances when modeling aeolian transport. Increasing the frequency of disturbances did not change the range of erosion rates but did impact total erosion through time. More
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1e+4
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1e+1 0
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Fig. 3. Horizontal flux (g m yr ) over a 1000 year time period using historically-based disturbance rates. Dashed line represents the horizontal flux under conditions of climax woodland vegetation without disturbances. The horizontal flux fluctuated with environmental disturbances and ranged from 70 to 44,000 g m2 yr1with an average of w10,000 g m2 yr1 depending on the frequency of disturbances.
disturbances were shown to increase erosion of soil and thus increase the spread of contamination over time. Our assessment does not account for human impacts on the environment, which could affect contaminant transport; these include livestock grazing and fire suppression, or mitigation of elevated erosion by taking such steps as watering during drought, maintaining native species, fire prevention, and limiting construction. Additionally the analysis presented herein does not account for the impact of climate change on all model parameters for atmospheric dispersion; notably, meteorological data was not
changed to reflect potential changes. Such changes may influence the type of vegetation present and alter erosion and deposition rates. This analysis assumes a precipitation rate of 45 cm/year, but this rate will change in a drier (or wetter) climate. Because precipitation rates impact on wet deposition rates, we ran CAP88 using the 5th and 95th percentiles of historical precipitation rates for Los Alamos County. These results showed an increase in precipitation will lead to a small increase in ground deposition and vice versa, but the overall change in ground deposition rates of plutonium changed by less than 10%. Despite this small change in deposition rate
Fig. 4. Deposition rates, averaged over 1000-yr time frame, are shown for static conditions and know disturbance frequencies and an upper and lower bound to provide a range of disturbance frequencies. Deposition rates predicted with known environmental disturbance rates were a factor of 19 times higher than when static conditions are assumed. When upper and lower-bound disturbance frequencies were accounted for, the 1000-yr average deposition rates ranged from 11 to 23 times higher than static conditions.
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modeled by CAP88, the enhancement in deposition of 11e23 times over static conditions will not be altered because the magnitude of the differences between scenarios will remain the same. 5. Conclusions Our results illustrate why model predictions of aeolian transport over long time frames need to consider dynamic vegetation cover as altered by vegetative succession and environmental disturbances, as should other long-term assessments of contaminant transport (Breshears et al., 2005b). Although this model assessment is conservative with respect to disturbance in many ways (Breshears et al., 2012), the results clearly suggest that using static model parameters for simulating aeolian transport predicts significantly lower transport and distant-deposition rates than when environmental disturbances are considered. In addition, our results suggest risk assessments should account for risk from wind-driven as well as water-driven transport, and that such risks over an extended period of time need to account for disturbances; models that account for both would aide site managers and the regulatory community. In summary, our model results suggest that without disturbances, areas downwind to the contaminated watershed are predicted to accumulate LANL-derived plutonium at a relatively slow rate (<0.01 Bq m2 yr1); however, model results under more realistic assumptions that include environmental disturbances predict accumulation rates more than an order-of-magnitude faster. More generally, this assessment highlights the need in radioecology and environmental health physics to consider normal if infrequent environmental disturbances in longer-term dose assessments spanning decades through millenia. Acknowledgments This work was funded primarily through the Department of Energy under contract W7405 ENG-36. Additional support for David D. Breshears was provided through the National Science Foundation (EAR-0724958) and Arizona Agricultural Experiment Station. References Adams, H.D., Macalady, A.K., Breshears, D.D., Allen, C.D., Stephenson, N.L., Saleska, S.R., Huxman, T.E., McDowell, N.G., 2010. Climate-induced tree mortality: earth system consequences. Eos, Trans. Am. Geophys. Union 91 (17), 153e 154. Allen, C.D., 2007. Interactions across spatial scales among forest dieback, fire, and erosion in northern New Mexico landscapes. Ecosystems 10, 797e808. Allen, C.D., Savage, M., Falk, D.A., Suckling, K.F., Swetnam, T.W., Schulke, T., Stacey, P.B., Morgan, P., Hoffman, M., Klingel, J.T., 2002. Ecological restoration of southwestern ponderosa pine ecosystems: a broad perspective. Ecol. Appl. 12 (5), 1418e1433. Allen, C.D., Macalady, A., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, N., Kitzberger, T., Rigling, A., Breshears, D.D., Hogg, E.H., Gonzalez, P., Fensham, R., Zhang, Z., Castro, J., Demidova, N., Lim, J., Allard, G., Running, S.W., Semerci, A., Cobb, N., 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. For. Ecol. Manage. 259 (4), 660e684. Baars, J.A., 1997. Mixing Depth Estimation at Los Alamos: A Preliminary Investigation. Los Alamos National Laboratory. Report LA-UR-97-366. Bowen, B.M., 1990. Los Alamos Climatology. Los Alamos National Laboratory report LA-11735-MS; UC-90Z. National Technical Information Service, Springfield, VA. Breshears, D.D., Nyhan, J.W., Davenport, D.W., 2005a. Ecohydrology monitoring and excavation of semiarid landfill covers a decade after installation. Vadose Zone J. 4, 798e810. Breshears, D.D., Cobb, N., Rich, P.M., Price, K.P., Allen, C.D., Balice, R.G., Romme, W.H., Kastens, J.H., Floyd, M.F., Belnap, J., Anderson, J.J., Myers, O.B., Meyer, C.W., 2005b. Regional vegetation die-off in response to global-change-type drought. Proc. Natl. Acad. Sci. U S A 102 (42), 15144e15148. Breshears, D.D., Whicker, J.J., Zou, C.B., Field, J.P., Allen, C.D., 2009. A conceptual framework for dryland aeolian sediment transport along the grassland-forest continuum: effects of woody plant canopy cover and disturbance. Geomorphology 105, 28e38.
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Los Alamos National Laboratory Environmental Report 2011. Los Alamos National Laboratory report LA-14461ENV. Los Alamos, New Mexico. Lu, J., Vecchi, G.A., Reicher, T., 2007. Expansion of the Hadley cell under global warming. Geophys. Res. Lett. 34 (6), L06805. Michelotti, E., Green, A., Whicker, J.J., Eisele, W.F., Fuehne, D.P., McNaughton, M., 2013. Validation Test for CAP88 Predictions of Tritium Dispersion at Los Alamos National Laboratory. Los Alamos National Laboratory Report LA-UR-12-248556. Los Alamos, New Mexico. Milly, P.C.D., Betancourt, J., Falkenmark, M., Hirsch, R.M., Kundzewicz, Z.W., Lettenmaier, D.P., Stouffer, R.J., 2008. Stationarity is dead: whither water management. Science 319, 573e574. Okin, G.S., 2008. A new model of wind erosion in the presence of vegetation. J. Geophys. Res. 113, F02S10. O’Neal, M.R., Nearing, M.A., Vining, R.C., Southworth, J., Pfeifer, R.A., 2005. Climate change impacts on soil erosion in Midwest United States with changes in crop management. Catena 61, 165e184. Pickett, S.T., White, P.S., 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press Inc., Orlando, FL. Pye, K., 1987. Aeolian Dust and Dust Deposits. Academic Press, Boca Raton, Florida. Ravi, S., D’Ororico, P., Breshears, D.D., Field, J., Goudie, A., Huxman, T., Li, J., Okin, G., Swap, R., Thomas, A., Van, S.P., Whicker, J.J., Zobeck, T., 2011. Aeolian processes and the biosphere: interactions and feedback loops. Rev. Geophys. 49, 1e45. Reynolds, K.M., 2001. Using a logic framework to assess forest ecosystem sustainability. J. For. 99 (6), 26e30. Romme, W.H., Allen, C.D., Bailey, J.D., Baker, W.L., Bestelmeyer, B.T., Brown, P.M., Eisenhart, K.S., Floyd, M.L., Huffman, D.W., Jacobs, B.F., Miller, R.F., Muldavin, E.H., Swetnam, T.W., Tausch, R.J., Weisberg, P.J., 2009. 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Seager, R., Ting, M., Held, I., Kushnir, Y., Lu, J., Vecchi, G., Huang, H., Harnik, N., Leetmaa, A., Lau, N., Li, C., Velez, J., Naik, N., 2007. Model projections of an imminent transition to a more arid climate in Southwestern North America. Science 316 (5828), 1181e1184. Swetnam, T.W., Baisan, C.H., 1996. Historical fire regime patterns in the Southwestern United States since AD 1700. In: Allen, C.D. (Ed.), Fire Effects in Southwestern Forests: Proceedings of the Second La Mesa Fire Symposium, 11e 32. USDA Forest Service, Rocky Mountain Research Station. General Technical Report. RM-GTR-286. Swetnam, T.W., Allen, C.D., Betancourt, J.L., 1999. Applied historical ecology: using the past to manage for the future. Ecol. Appl. 9, 1189e1206. Toy, T.J., Foster, G.R., Renard, K.G., 2002. Soil Erosion: Processes, Prediction, Measurement and Control. John Wiley & Sons, New York. Turner, M.G., Baker, W.L., Peterson, C.J., Peet, R.K., 1998. Factors influencing succession: lessons from large, infrequent natural disturbances. Ecosystems 1 (6), 511e523. Webb, S.B., Ibrahim, S.A., Whicker, F.W., 1997. A three-dimensional spatial model of plutonium in soil near Rocky Flats, Colorado. Health Phys. 73, 340e349. Westerling, A.L., Hidalgo, H.G., Cayan, D.R., Swetnam, T.W., 2006. Warming and earlier spring increase western US forest wildfire activity. Science 313, 940e943.
Whicker, J.J., Breshears, D.D., 2011. Increased dust exposure following forest disturbance: overlooked implications for environmental health. In: Nriagu, J. (Ed.), Encyclopedia of Environmental Health, first ed. Elsevier Science. Whicker, F.W., Shultz, V., 1982. Radioecology: Nuclear Energy and the Environment. CRC Press, Boca Raton, FL. Whicker, J.J., Breshears, D.D., Wasiolek, P.T., Kirchner, T.B., Tavani, R.A., Schoep, D.A., Rodgers, J.C., 2002. Temporal and spatial variation of episodic wind erosion in unburned and burned semiarid shrubland. J. Environ. Qual. 31, 599e612. Whicker, J.J., Pinder, J.E., Breshears, D.D., 2006. Increased wind erosion from forest wildfire: implications for contaminant-related risks. J. Environ. Qual. 35, 468e478. Williams, A.P., Allen, C.D., Macalady, A.K., Griffin, D., Woodhouse, C.A., Meko, D.M., Swetnam, T.W., Rauscher, S.A., Seager, R., Grissino-Mayer, H.D., Dean, J.S., Cook, E.R., Gangodagamage, C., Cai, M., McDowell, N.G. Temperature as a potent driver of regional forest drought stress and tree mortality. Nature Climate Change. in press. Yu, C., Sielen, A.J., Cheng, J.J., Lepoire, D.J., Gnanapragasam, E., Kamboj, S., Arnish, J., Wallo, A., Williams, W.A., Peterson, H., 2001. User’s Manual for RESRAD Version 6. Argonne National Laboratory report ANL/EAD-4. Available electronically from: http://web.ead.anl.gov/resrad/documents/.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Modeling aeolian transport of soil-bound plutonium: considering infrequent but normal environmental disturbances is critical in estimating future dose Erika A. Michelotti a, Jeffrey J. Whicker a, *, William F. Eisele a, David D. Breshears b, c, Thomas B. Kirchner d a
Los Alamos National Laboratory, Environmental Stewardship Group, Mail Stop J978, Los Alamos, NM 87544, USA School of Natural Resources and the Environment University of Arizona, Tucson, AZ 85721, USA c Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ 85721, USA d New Mexico State University, Las Cruces, NM 88003, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 November 2012 Received in revised form 8 January 2013 Accepted 8 January 2013 Available online 1 March 2013
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 doseinfluential 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 yr1). 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. Published by Elsevier Ltd.
Keywords: Environmental disturbance Climate change Soil Sediment Contaminant transport
1. Introduction Many of the most fundamental issues in radioecology and environmental health physics span time frames of decades,
Abbreviations: Vegetation Modified Transport, VMTran; Clean Air Act Assessment Package-1988, CAP88; Los Alamos National Laboratory, LANL. * Corresponding author. Tel.: þ1 505 667 2610; fax: þ1 505 665 6071. E-mail addresses: [email protected], [email protected] (J.J. Whicker). 0265-931X/$ e see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.jenvrad.2013.01.011
centuries, and millennia, which presents unique scientific challenges (Hinton et al., 2013). Notably, regulations for decommissioned sites, spent nuclear fuel repositories, and many other evaluations require assessment of dose to public receptors out to a minimum of 1000 years (EPA, 1982) for longer-lived radionuclides, which requires modeling to predict effective dose rates into the future. Dose assessments for environmental contaminants are implicitly dependent on the environmental context, but assumptions about associated ecosystem dynamics are often not explicitly considered (Whicker and Breshears, 2011; Breshears et al., 2012).
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Ecosystems are complex, dynamic systems which are significantly altered by disturbances (Pickett and White, 1985), and they often are heterogeneous and often in a state of nonequilibrium (Reynolds, 2001). However, models used to predict transport of contaminants through the environment commonly presume stationary or equilibrium conditions, either explicitly or often implicitly, and consequently over-simplify the complex interactions among the components of an ecosystem. Such models are often based on equilibrium ecology, which generally assumes a defined and ideal ecological structure to which, despite disturbances and minor fluctuations, systems will attempt to return (Landis and Wiegers, 2008). Assumptions of stationarity in an ecosystem are compromised when environmental and anthropogenic disturbances are taken into account, including the large potential impacts associated with global climate change. Examples of inadequate management of water resources resulting from stationary assumption have been studied for cases where conditions exceed natural variation (Milly et al., 2008). The deviation from stationary planning assumptions predicted by climate change projections exceeds the natural-variation deviation that has plagued water-resource planners. Probabilistic models that include climate change projections are needed to replace or supplement water distribution and use models. Likewise, models in other environmental disciplines, such as those for soil and natural vegetation patterns, need to be altered to reflect the dynamic complexity of ecosystems, real properties of ecological structures and disturbances, and potential impacts of climate change. Aeolian transport of sediment erodes soil, redistributes nutrients, alters biogeochemical cycles, and, in a radioecological context, can mobilize contaminants; thus, wind transport of soil and sediment needs to be accurately modeled to ensure ecosystem health,
soil productivity and accurate assessments of contaminant transport (Pye, 1987; Chadwick et al., 1999; Toy et al., 2002; Jickells et al., 2005; Whicker et al., 2006; Li et al., 2007; Field et al., 2010; Ravi et al., 2011; Whicker and Breshears, 2011). Regulatory assessments commonly use stationary- or equilibrium-based models that assume, for example, constant emission rates of airborne soil (Yu et al., 2001). However, assuming a constant transport rate without considering the impact of environmental disturbances may underestimate long-term aeolian transport of persistent soil-bound contaminants to off-site locations and preclude accurate evaluations of human health risks (Field et al., 2011; Breshears et al., 2012). For example, most soil contaminant transport models for semiarid ecosystems do not account for changes in vegetation cover as impacted by expected environmental disturbances such as fire, drought, and tree disease, nor of subsequent succession (Fig. 1), all of which have recently been shown to dramatically impact transport rates of contaminated dust over long time periods (Okin, 2008; Breshears et al., 2012). Spatial and temporal patterns and characteristics of vegetation change following periodic disturbances such as fire and drought (Turner et al., 1998; Whicker et al., 2002; Allen, 2007; Romme et al., 2009; Williams et al., in press). Changes invoked by these events depend upon the type and frequency of disturbance, the successional state at the time of the disturbance, and the amount and type of ground cover before and after the disturbance (Breshears et al., 2005a,b). Based on a recently developed conceptual model (Breshears et al., 2009), disturbances typically increase aeolian transport rates by reducing the amount of woody vegetation and altering surface wind flow across the vegetation. Importantly, climate change and anthropogenic activities (land use change including agricultural and logging practices, and emission of green
Fig. 1. Illustration showing ecological succession following closure of a landfill and the impact of environmental disturbances on the vegetation cover. Each of these ecological states has a different wind erosion rate associated with it. From Breshears et al. (2012); figure used with permission.
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house gases) are anticipated to continue to cause an increase in the severity and frequency of environmental disturbances. Including environmental disturbances at different frequencies of occurrence, to simulate the impacts of climate change, could improve the prediction of contaminant transport and accuracy of dose assessments. Many radionuclide-contaminated sites are located in semiarid environments. In contrast to more humid locations, semiarid systems are of particular concern with regard to radionuclide transport because they (1) usually have limited amounts of ground cover, which enables higher potential rates of contaminant mobility, (2) succession after a disturbance can be slow and lead to an alternate state, (3) fire is often a recurrent disturbance processes, (4) extreme climate events related to drought can trigger landscape scale changes in vegetation cover not only through fire but also through droughtinduced tree mortality, and (5) many semiarid regions, including the southwestern USA, are projected to become warmer and drier, thereby potentially further exacerbating the previously highlighted characteristics affecting contaminant transport (Breshears et al. 2012). An important case study where these factors collectively have been studied in detail and where radionuclide transport is of concern is at Los Alamos National Laboratory (LANL), which is part of the Department of Energy’s nuclear weapons complex and where residual radioactivity remains in local soils from past operations. This site in northern New Mexico lies within Southwestern uplands, which are among the most extensive and detailed regional-scale network of fire and drought history available in the world; thus, enabling improved estimates of future disturbance rates (Swetnam and Baisan, 1996; Swetnam et al., 1999; Allen et al., 2002). Additionally, the Southwestern United States is projected to experience a warmer and drier climate in the future (Seager et al., 2007), leading to an increased frequency of disturbances including fire and drought (Breshears et al., 2005b; Westerling et al., 2006; Adams et al., 2010; International Panel on Climate Change, 2007; O’Neal et al., 2005). Such changes would likely increase rates of transport of contaminated soils and need to be accounted for in long-term risk assessments (Breshears et al., 2012). Cases of landscape-scale shifts in vegetation, and associated acceleration of soil loss, have already been documented. In the early 2000’s drought killed 40e97% of pinion trees in a 12,000 km2 site over 3 years (Breshears et al., 2005b). This type of rapid, large-scale change is consistent with other recent tree mortality events around the globe (Allen et al., 2010). Recent forest fires in the Southwestern US can impact increasingly large areas; together with areas impacted by drought-induced tree die-off, at least 18% of Southwestern USA woodlands and forests have been impacted in recent decades (Williams et al., in press). These observations, and current predictions of future conditions, emphasize the need for models which more accurately account for rapid vegetation changes in response to disturbances such as drought, beetle infestations, and fire (Lu et al., 2007; Whicker et al., 2006). 1.1. Objective of study A recently developed dynamic wind-transport model included vegetation succession and environmental disturbance to provide more realistic long-term predictability: the Vegetation Moderated Transport Model (VMTran; Breshears et al., 2012). This study utilized the VMTran model to estimate emission rates for aeolian transport, and compared atmospheric dispersion and deposition rates of airborne plutonium-contaminated soil into neighboring areas with and without environmental disturbances using a widely used dose assessment model (CAP-88; EPA, 2011). Our case study focused on low levels of residual plutonium found in soils from past operations at LANL in the semiarid southwestern USA. In the recent
75
related study using VMTran (Breshears et al., 2012), aeolian transport of contaminated soil was predicted considering environmental disturbance under current and potential changes in climate, focusing on saltation transport (scale of a few meters). Here we build on and extend that work to include suspension of soil and atmospheric transport (scale of km). Using historical data and available modeling tools, our research sought to evaluate the importance of dynamic models and how such models might be utilized to better predict transport rates of plutonium in Los Alamos soil and sediment to distant, downwind locations. For this study, the VMTran model was used to determine the vertical transport (suspension) rate of contaminated soil over a thousand-year time period under static vegetation conditions and cover that varies dynamically due to disturbance and subsequent recovery. The suspension rate estimates for the contaminated soil through time were then linked to an atmospheric dispersion model to predict downwind dispersion and surface deposition rates over a 1000-yr time period. These predictions were calculated for the same site under different environmental conditions. We discuss more broadly how radioecology and environmental health physics need to consider environmental disturbances in longer-term dose assessments. 2. Methods 2.1. Site overview Research focused on Los Alamos Canyon Watershed, located in Los Alamos, New Mexico, U.S.A. Los Alamos has a temperate, semiarid mountain climate. Prior to the 1960’s, LANL released airborne and liquid effluents containing 239/240Pu into the surrounding environment and into the Los Alamos Canyon Watershed. Although remediated to acceptable levels, this watershed has the highest residual plutonium concentrations in soil and sediment relative to other impacted areas (Gallaher et al., 1997). Water, soil, and sediment samples in the canyon have been collected for more than three decades to measure concentrations and determine the spatial distribution of 239/240Pu. Regarding offsite transport of these soils, Los Alamos Canyon Watershed has an ephemeral stream that over the years has transported radionuclides down the canyon during rain storms (Katzman and Reneau, 2004). Additionally, aeolian transport of contaminated soils, formed from stream sediment or from past airborne stack emissions, could be significant contributors to offsite transport of plutonium contaminated soil (Webb et al., 1997; Whicker et al., 2006). Combined, these contaminated soils and sediments are potential sources for off-site transport of plutonium bearing soils by wind. The consideration of climate change and disturbances could significantly impact the predictions of transport from this site. 2.2. Plutonium concentration in soil and sediment The publicly accessible Risk Analysis, Communication, Evaluation, and Reduction Project (RACER) database1 was utilized to obtain concentrations of 239/240Pu for soil and sediment in the Los Alamos Canyon Watershed. In all, over 4000 measurements of plutonium concentrations in soil and sediment were used to calculate the summary statistics. The concentrations appeared lognormally distributed so the median was utilized to provide a single representative concentration for the entire watershed and to avoid an upward bias because of a few high measurements. To
1 http://www.racteam.com/racer.html. Recent version of database found at http://www.intellusnmdata.com/.
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calculate the total number of becquerels of 239/240Pu in the watershed the median was multiplied by the density of soil (1.68 106 g m3), the area of the watershed (1.38 108 m2), and an assumed depth of plutonium in the soil column of 0.3 m. A depth of 0.3 m was selected because the surface deposited plutonium is largely contained in this depth (Whicker and Shultz, 1982), and it is this surface soil that is most susceptible to wind erosion over long time frames. 2.3. Modeling aeolian transport rates To incorporate vegetative succession and disturbances into models of soil movement, the VMTran model was developed based on a suite of empirical dry-land measurements of aeolian dust flux (Breshears et al., 2012). VMTran was designed to estimate local patterns and rates of contaminant redistribution caused by winds, taking into account the impacts of vegetative succession and environmental disturbances, and simulate this transport over long periods of time. A Markov-like transition matrix was used to simulate vegetative succession with vegetative succession (i.e., bare soil to grass to shrubs to woodland to forest) simulated following closure of a waste site and after each disturbance. Successional states modeled were bare soil, litter, grassland, shrubland, woodland, and forest. Plant growth is assumed to be logistic; however, plants can shade other plants and delay succession. Ground cover in intercanopy areas differs between disturbed and undisturbed sites and is reflected in distinguishable transport rates (Breshears et al., 2009). VMTran was utilized to obtain annual horizontal fluxes of soil over a 1000 year time period under three scenarios: 1) static conditions with a constant emission rate, 2) inclusion of environmental disturbances with return frequencies based on current climate conditions, and 3) inclusion of disturbances but with return frequencies that bound (upper and lower) those anticipated under climate change predictions coupled with potential active management of the site. Static conditions assume a constant annual soil transport rate based on continual woodland conditions (i.e., the climax vegetation). Disturbances under assumptions of current climate conditions that were modeled included crown fire (a severe fire where all parts of the tree are burned including the crown), a surface fire (grass and shrubs are burned, but trees survive), and drought (severe enough to kill some portion of trees), at intervals of 30, 50, and 250 years, respectively. Disturbances, including crown fire, surface fire and drought, are stochastically scheduled events in the model based on the mean return time assigned to each type of event. Impacts of the disturbances on vegetation and soil mobility rates vary depending on the type of disturbance, successional state at the time of disturbance, and the amount of ground cover. Because of the large uncertainty regarding the potential impacts of climate change (likely to increase in disturbance frequency) and possible future mitigative land management practices by DOE or successor (e.g., fire suppression, vegetation cover management) on ecological recovery and environmental disturbance rates, upper- and lowerbound rates were modeled. The upper-bound frequency was 15, 25, and 125 years for surface fire, crown fire and drought, respectively, while the lower-bound frequency doubled the historicallybased frequencies in length to 60, 100, and 500 years, respectively. Output from the model includes annual transport rates and the associated percent woody cover as it changes over a 1000 yr period (Breshears et al., 2012). More precisely, transport rates are based on saltation transport and defined as the horizontal mass flux (g m2 yr1) or the amount of soil that is moved horizontally across the land surface, usually a distance of a few meters or less. These saltating particles, upon impact to the soil surface, cause smaller particles to be lifted higher, or suspended from the surface, where
they can be transported long distances (many meters to kilometers) by wind. The suspension rate or vertical flux (g m2 yr1) was calculated from the VMTrans horizontal flux, and because plutonium contamination in surface soil is of primary concern at LANL, the suspension flux was converted to emission rate (i.e., Bq yr1) and used in the atmospheric dispersion model. To account for random error in model simulations, because some of the parameters were stochastically determined, the model was run ten times for each scenario, and the average and standard deviations of horizontal mass flux of the ten runs were determined. Long-distance transport of soil and contaminants are associated with vertical dust flux measurements, which is a measure of emission rate. A relationship of 1% horizontal transport being vertical transport was utilized in this study based on site-specific data (Whicker et al., 2006). Using the vertical mass flux, the radioactivity emission rate (Q), or the amount of plutonium leaving the watershed, was calculated using the 1000 year vertical mass flux rate for each scenario, as shown in Eqn. (1).
Q ¼ VMF*A*C
(1)
where Q is the emission rate (Bq yr1), VMF is vertical mass flux (g m2 yr1) based on the average horizontal mass flux, A is area of the source (m2), and C is the median soil concentration (Bq g1). The radioactivity emission rate was used in CAP88 to estimate dispersion and deposition rates in a variety of directions. 2.4. Air dispersion and deposition modeling The U.S. Environmental Protection Agency (EPA) requires use of the Clean Air Act Assessment Package-1988 (CAP88) to calculate dose equivalents to members of the public. This model calculates atmospheric dispersion and subsequent deposition rates at a variety distances and directions from the identified source using a modified Gaussian plume model (Environmental Protection Agency [EPA], 2011). Despite concerns about the ability of CAP88 to account for turbulence in landscapes with changing terrain, CAP88 has been found to provide reasonable, yet conservative, estimates of the atmospheric dispersion of radionuclides from stack emissions at LANL. For example, CAP88 was used to model the dispersion of tritium releases from LANL stacks to downwind receptors and the results were compared to measurements from ambient air monitoring stations (Michelotti et al., 2013). CAP88 generally predicted higher values than those measured at monitoring stations, but the predicted values averaged over a year were within a factor of 1e3.5, suggesting that it provides reasonable annual estimates of dispersion. These results are consistent with the Tritium Working Group of the International Atomic Energy Agency’s BIOMASS Coordinated Research Programme, which suggest accuracy within a factor of 3 or less in most scenarios is acceptable (IAEA, 2003). Emission rates calculated from VMTrans and local meteorological data were used to calculate dispersion and deposition rates of plutonium from Los Alamos Canyon, Meteorological data inputs into CAP88 were based on climate averages for Los Alamos, including annual precipitation of 45 cm y1, annual ambient temperature of 9 C, annual average atmospheric mixing lid of 1600 m, and an absolute humidity of 5.5 g m3 (Bowen, 1990; Baars, 1997). Releases were modeled as resuspension (i.e., ground release) and the total area of the release set at 1.38 108 m2, the area of the Los Alamos Canyon Watershed.
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Although site-specific model parameters were used in this study, some simplifying assumptions had to be applied in using the CAP88 model, such as assuming uniform concentrations of contamination across the source area and default values for deposition rates. Deposition rates included both wet and dry deposition. Dry deposition was assumed to be proportional to the ground-level concentration and is calculated as the product of the modeled air concentration at ground-level and the deposition velocity (default ¼ 1.8 103 m s1) for particles. The fractional removal rate for wet deposition was calculated as the product of the site precipitation rate (45 cm yr1) and a removal constant of 1 107 yr cm1 s1. 3. Results The median concentration of plutonium in Los Alamos Canyon Watershed was estimated to be 7.1 103 Bq g1 in soil and sediment. For context, the background level of Pu-239 in the Los Alamos area from global fallout is about 7.4 104 Bq g1, an order-ofmagnitude lower than the area median concentration in the area. Though the plutonium soil concentrations are elevated above background, radiation dose projections for living in the study area are still much lower than applicable safety limits for the public (LANL, 2012). Average horizontal transport rates for static conditions and environmental disturbances for a 1000 year time period were used to calculate the vertical flux and radioactivity source term (Fig. 2). Static conditions assume a constant horizontal transport rate of 511 g m2 yr1 for woodland conditions. In contrast, the predicted horizontal fluxes with periodic environmental disturbances ranged from 70 to 44,000 g m2 yr1 (Fig. 3) with an average of w10,000 g m2 yr1 depending on the frequency of disturbances. Increases in horizontal flux are directly associated with environmental disturbance events (Fig. 3) and the fluxes declined as the vegetation, particularly the woody vegetation, recovered. However, if disturbances were very frequent, the woody vegetation could not recover fast enough and the erosion rates remained relatively high. The winds in Los Alamos primarily blow from the southwest to the northeast, so the northeast sector had the greatest deposition
1e+5
11,939
9674
1e+4 6105
-2
-1
Sediment Flux (g m yr )
Horizontal Flux Vertical Flux
1e+3 511
1e+2
11.9
9.7
6.1 5.1
1e+1
ti St a
s ion dit on C c
ce an urb
te Ra
ist is t dD dD un se o a B r-B lly we ica Lo tor s i H
ce an urb
te Ra
is t dD un o r-B pe Up
ce an urb
te Ra
Fig. 2. Mean soil transport rates and the radioactivity source terms for 1000 years. Vertical fluxes are taken as 1% of the horizontal flux (Whicker et al., 2006). Error bars represent the standard deviation across 10 simulations.
77
rates for the 1000 year time period (plotted as a function of distance from the watershed in Fig. 4). Predicted deposition pattern shows no decline in downwind soil concentration out to about 7 km, after which the concentration steadily declines. The flat profile in the first kilometers was expected due to the large source area of the Los Alamos Canyon Watershed that contributed contaminated soil to the closer areas. The source area would become smaller with distance from the watershed, resulting in decreased deposition farther away. The 1000-yr average deposition rates predicted for scenarios that include environmental disturbance were a factor of 19 higher than when static conditions are assumed. When upper- and lowerbound disturbance rates, specifically accounting for potential future impacts of climate change and possible human-influenced environmental mitigation on disturbance or recovery rates were accounted for, the 1000-yr average deposition rates were still 11e 23 times higher than the model would predict under static conditions. 4. Discussion Overall, our results show how transport rates and downwind deposition of contaminated soil increase with disturbance, consistent with recent research (Breshears et al., 2012). Our results suggest that soil transport models that fail to account for vegetative succession and environmental disturbances will likely lead to under predictions and possibly misinformed management decisions. Nonetheless, our modeling results contain several relevant uncertainties. Several important simplifying assumptions are made in this analysis regarding spatial distribution, independence of disturbances, and impact of potential human activities at the site. Both CAP88 and VMTran assume that a finite source of contamination is uniformly scattered across the entire site. In reality concentrations do and will vary spatially across the site though the LANL data are not sufficient to fully characterize this distribution within the watershed with any degree of spatial accuracy. Heterogeneity in soil contamination over the large area will lead to more localized transport because of saltation. In addition, saltation produces resuspension of the contaminated soil that this study shows will be dispersed over much greater areas by wind. In contrast, water tends to concentrate the contaminants into stream beds. The spatial distribution pattern through time will be a combination of these two erosion processes (Field et al., 2009). A median concentration of plutonium distributed evenly across the landscape was a reasonable assumption given a significant fraction of the contamination in the soil was from deposited air emissions and the contaminants in the generally dry stream beds would have had multiple decades to be dispersed by wind. Regardless of the limitation in the data set in spatial resolution, we believe the results suggest that models which include environmental disturbance result in higher predicted aeolian transport regardless of the initial spatial pattern of the contamination. Our results also provide insights into contamination spread as a function of distance in addition to the key importance of including disturbance. As anticipated, larger deposition rates were predicted closer to the source and rates decreased with distance, but in a nonlinear fashion. Deposition rates were relatively constant with distance out to about 7 km before decreasing rapidly. This result was likely due to the large size of the contaminated area modeled and assumptions of CAP88. Order of magnitude differences between static conditions and those with environmental disturbances underscore the complexity of the environment and the importance of accounting for disturbances when modeling aeolian transport. Increasing the frequency of disturbances did not change the range of erosion rates but did impact total erosion through time. More
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1e+4
-2
-1
Horizontal Flux (g m yr )
1e+5
1e+3
1e+2
1e+1 0
200
400
600
800
1000
1200
Years post closure 2
1
Fig. 3. Horizontal flux (g m yr ) over a 1000 year time period using historically-based disturbance rates. Dashed line represents the horizontal flux under conditions of climax woodland vegetation without disturbances. The horizontal flux fluctuated with environmental disturbances and ranged from 70 to 44,000 g m2 yr1with an average of w10,000 g m2 yr1 depending on the frequency of disturbances.
disturbances were shown to increase erosion of soil and thus increase the spread of contamination over time. Our assessment does not account for human impacts on the environment, which could affect contaminant transport; these include livestock grazing and fire suppression, or mitigation of elevated erosion by taking such steps as watering during drought, maintaining native species, fire prevention, and limiting construction. Additionally the analysis presented herein does not account for the impact of climate change on all model parameters for atmospheric dispersion; notably, meteorological data was not
changed to reflect potential changes. Such changes may influence the type of vegetation present and alter erosion and deposition rates. This analysis assumes a precipitation rate of 45 cm/year, but this rate will change in a drier (or wetter) climate. Because precipitation rates impact on wet deposition rates, we ran CAP88 using the 5th and 95th percentiles of historical precipitation rates for Los Alamos County. These results showed an increase in precipitation will lead to a small increase in ground deposition and vice versa, but the overall change in ground deposition rates of plutonium changed by less than 10%. Despite this small change in deposition rate
Fig. 4. Deposition rates, averaged over 1000-yr time frame, are shown for static conditions and know disturbance frequencies and an upper and lower bound to provide a range of disturbance frequencies. Deposition rates predicted with known environmental disturbance rates were a factor of 19 times higher than when static conditions are assumed. When upper and lower-bound disturbance frequencies were accounted for, the 1000-yr average deposition rates ranged from 11 to 23 times higher than static conditions.
E.A. Michelotti et al. / Journal of Environmental Radioactivity 120 (2013) 73e80
modeled by CAP88, the enhancement in deposition of 11e23 times over static conditions will not be altered because the magnitude of the differences between scenarios will remain the same. 5. Conclusions Our results illustrate why model predictions of aeolian transport over long time frames need to consider dynamic vegetation cover as altered by vegetative succession and environmental disturbances, as should other long-term assessments of contaminant transport (Breshears et al., 2005b). Although this model assessment is conservative with respect to disturbance in many ways (Breshears et al., 2012), the results clearly suggest that using static model parameters for simulating aeolian transport predicts significantly lower transport and distant-deposition rates than when environmental disturbances are considered. In addition, our results suggest risk assessments should account for risk from wind-driven as well as water-driven transport, and that such risks over an extended period of time need to account for disturbances; models that account for both would aide site managers and the regulatory community. In summary, our model results suggest that without disturbances, areas downwind to the contaminated watershed are predicted to accumulate LANL-derived plutonium at a relatively slow rate (<0.01 Bq m2 yr1); however, model results under more realistic assumptions that include environmental disturbances predict accumulation rates more than an order-of-magnitude faster. More generally, this assessment highlights the need in radioecology and environmental health physics to consider normal if infrequent environmental disturbances in longer-term dose assessments spanning decades through millenia. Acknowledgments This work was funded primarily through the Department of Energy under contract W7405 ENG-36. Additional support for David D. Breshears was provided through the National Science Foundation (EAR-0724958) and Arizona Agricultural Experiment Station. References Adams, H.D., Macalady, A.K., Breshears, D.D., Allen, C.D., Stephenson, N.L., Saleska, S.R., Huxman, T.E., McDowell, N.G., 2010. Climate-induced tree mortality: earth system consequences. Eos, Trans. Am. Geophys. Union 91 (17), 153e 154. Allen, C.D., 2007. Interactions across spatial scales among forest dieback, fire, and erosion in northern New Mexico landscapes. Ecosystems 10, 797e808. Allen, C.D., Savage, M., Falk, D.A., Suckling, K.F., Swetnam, T.W., Schulke, T., Stacey, P.B., Morgan, P., Hoffman, M., Klingel, J.T., 2002. Ecological restoration of southwestern ponderosa pine ecosystems: a broad perspective. Ecol. Appl. 12 (5), 1418e1433. Allen, C.D., Macalady, A., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, N., Kitzberger, T., Rigling, A., Breshears, D.D., Hogg, E.H., Gonzalez, P., Fensham, R., Zhang, Z., Castro, J., Demidova, N., Lim, J., Allard, G., Running, S.W., Semerci, A., Cobb, N., 2010. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. For. Ecol. Manage. 259 (4), 660e684. Baars, J.A., 1997. Mixing Depth Estimation at Los Alamos: A Preliminary Investigation. Los Alamos National Laboratory. Report LA-UR-97-366. Bowen, B.M., 1990. Los Alamos Climatology. Los Alamos National Laboratory report LA-11735-MS; UC-90Z. National Technical Information Service, Springfield, VA. Breshears, D.D., Nyhan, J.W., Davenport, D.W., 2005a. Ecohydrology monitoring and excavation of semiarid landfill covers a decade after installation. Vadose Zone J. 4, 798e810. Breshears, D.D., Cobb, N., Rich, P.M., Price, K.P., Allen, C.D., Balice, R.G., Romme, W.H., Kastens, J.H., Floyd, M.F., Belnap, J., Anderson, J.J., Myers, O.B., Meyer, C.W., 2005b. Regional vegetation die-off in response to global-change-type drought. Proc. Natl. Acad. Sci. U S A 102 (42), 15144e15148. Breshears, D.D., Whicker, J.J., Zou, C.B., Field, J.P., Allen, C.D., 2009. A conceptual framework for dryland aeolian sediment transport along the grassland-forest continuum: effects of woody plant canopy cover and disturbance. Geomorphology 105, 28e38.
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