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Cs Activities and 135Cs/137Cs Isotopic Ratios from Soils at Idaho National Laboratory: A Case Study for Contaminant Source Attribution in the Vicinity of Nuclear Facilities
Mathew S. Snow,*,†,‡ Darin C. Snyder,‡ Sue B. Clark,† Morgan Kelley,† and James E. Delmore‡ †
Department of Chemistry, Washington State University PO Box 644630, Pullman, Washington 99164-4630, United States Idaho National Laboratory, PO Box 1625, Idaho Falls, Idaho 83415-2805, United States
‡
S Supporting Information *
ABSTRACT: Radiometric and mass spectrometric analyses of Cs contamination in the environment can reveal the location of Cs emission sources, release mechanisms, modes of transport, prediction of future contamination migration, and attribution of contamination to specific generator(s) and/or process(es). The Subsurface Disposal Area (SDA) at Idaho National Laboratory (INL) represents a complicated case study for demonstrating the current capabilities and limitations to environmental Cs analyses. 137 Cs distribution patterns, 135Cs/137Cs isotope ratios, known Cs chemistry at this site, and historical records enable narrowing the list of possible emission sources and release events to a single source and event, with the SDA identified as the emission source and flood transport of material from within Pit 9 and Trench 48 as the primary release event. These data combined allow refining the possible number of waste generators from dozens to a single generator, with INL on-site research and reactor programs identified as the most likely waste generator. A discussion on the ultimate limitations to the information that 135Cs/137Cs ratios alone can provide is presented and includes (1) uncertainties in the exact date of the fission event and (2) possibility of mixing between different Cs source terms (including nuclear weapons fallout and a source of interest).
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reactor neutron energy spectrum, flux, and duty cycle, the ratio of 135Cs to 137Cs can be a useful forensic tool for discriminating between fission conditions and reactor types. Cs is readily soluble as a monovalent cation that can sorb to environmental media under certain geochemical conditions. After release, its distribution in local environments is influenced by fluvial, pluvial, tidal, groundwater, and/or aeolian transport. These geochemical and atmospheric processes can lead to enrichment in the local environment, and can also provide an opportunity to attribute contamination to a specific industrial source, similar to that demonstrated for toxic metal contamination released during smelting,14 lead recycling,15 and other activities associated with industrial and agricultural development.16−18 While several groups have proposed using 135Cs/137Cs isotope ratios for tracking large releases of anthropogenic Cs from known emission sources (including fallout from nuclear weapons testing,12,19,20 Chernobyl,10 and Fukushima21,22), a currently unexplored area is the application of 135Cs/137Cs
INTRODUCTION Nuclear technologies have been used for the past 80 years to produce energy, medical isotopes, and atomic weapons. These activities have altered the global flux of anthropogenic fission products as a result of emissions during industrial operations,1−3 their associated waste management practices,4−7 accidents such as Chernobyl8 and Fukushima,9 and nuclear weapons tests.3,8 Although the elements released from these industrial activities are not necessarily unique, the isotopic signatures of fission products are almost always distinct from natural sources and from each other, creating an opportunity to attribute them to specific sources or processes. The isotopic signature of natural Cs is 100% 133Cs, whereas 135 Cs and 137Cs are two long-lived isotopes created in high yield by nuclear fission of U and Pu. Under fast fission conditions the total quantities of 135Cs and 137Cs produced approach the cumulative fission yields for each isotope (6.54% and 6.19% for isobars 135 and 137 respectively for thermal fission of 235 U),10,11 whereas under thermal neutron conditions the longer cumulative half-life of the 135 chain and the extremely high thermal neutron capture cross section of 135Cs′s precursor, 135 Xe (>106 barns6), result in a depleted 135Cs/137Cs atom ratio (typically in the 0.2−0.5 range).10,12,13 As the exact rate at which 135Xe undergoes neutron capture is dependent on the © XXXX American Chemical Society
Received: December 4, 2014 Revised: January 28, 2015 Accepted: January 29, 2015
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Figure 1. Map showing the location of Idaho National Laboratory within the state of Idaho.
attribution using Cs data alone. Samples taken in the early 1970′s near the Subsurface Disposal Area (SDA) at Idaho National Laboratory (INL) represent a complicated case study, as three distinct emission sources (representing dozens of different waste generators and over 230 unique processes) have been suggested as the source of the 137Cs around this site.7,24−27 Specifically, this study demonstrates how 137Cs surface soil distribution patterns, 135Cs/137Cs ratios, known Cs chemistry at this site, and historical documents can be utilized together to identify the original source of the 137Cs contamination, the primary release event, and the mechanism(s) of environmental transport. Identification of these attributes in this work enables refining the total number of possible waste generators down from dozens to a single generator. Additionally, a discussion of the ultimate limits to which Cs data alone can be interpreted is presented, along with the ramifications of these limitations on application of these techniques to Cs source term attribution at other nuclear facilities.
ratios for source term attribution of unknown industrial emission sources, including very low level 137Cs releases in the environment. However, accurate attribution of unknown Cs emission sources using 135Cs/137Cs ratios is challenged by several factors: 1. Uncertainties in the Time Stamp of the Fission Event. The difference in half-lives between 135Cs and 137Cs (2.3 × 106 yrs and 30.01 yrs, respectively) results in a steadily increasing 135Cs/137Cs with increasing time. This introduces significant uncertainty in the predicted 135Cs/137Cs ratio for the environmental sample when the time of contaminant release is entirely unknown (for example, a thermal fission product with an initial 135Cs/137Cs ratio of 0.50 will, after 30 years, have a 135 Cs/137Cs ratio of 1.0 and thus could be confused with Cs resulting from a recent fast fission event rather than an aged, thermal fission event). 2. Inability to Identify a Single Emission Source from Multiple Sources Containing Similar 135Cs/137Cs Ratios. An example of this can be seen in ORIGEN calculations by Nishihara et al. for several of the Fukushima Daichi reactor cores; 135Cs/137Cs ratios for several of the cores have similar enough 135Cs/137Cs ratios to make discrimination based on Cs isotope ratios alone challenging (e.g., predicted 135Cs/137Cs ratios of 0.341 and 0.350 as of 3/11/2011 for core 2 and 3 respectively), particularly when considering the possible uncertainties in the actual 135Cs/137Cs ratios for each core (which were not reported in their model).21,23 3. Inability to Separate out Contributions from Multiple Sources with Distinct 135Cs/137Cs Ratios. This is particularly important for extremely low level Cs releases, where the contribution of global nuclear weapons fallout and other background sources in a sample may significantly alter the experimentally observed 135Cs/137Cs ratio from a given emission source. This study demonstrates the current capabilities and limitations of environmental contamination source term
2. BACKGROUND The National Reactor Test Facility, which later became the Idaho National Laboratory (INL), was established in southeastern Idaho in 1949 for the testing of various reactor and fuel types (Figure 1). From 1951 to 2000, a total of 52 different reactors were built and tested.28 To accommodate the large quantities of radioactive waste generated from these operations, a disposal area, called the Subsurface Disposal Area (SDA), was opened in 1952. From 1952 to 1970, radioactive waste from INL operations was buried in unlined excavations (called “Pits”, “Trenches”, and “Vaults”, based upon the specific dimensions of the excavation).29,30 During this time period the SDA received waste from dozens of offsite generators, ranging from medical facilities and research reactor experiments to defense waste from the Rocky Flats, CO site.7,24,25,31 INL also operated a nuclear reprocessing facility, the Idaho Chemical Processing B
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Figure 2. Various possible 137Cs emitters, their predicted 137Cs contamination patterns (based on known 137Cs chemistry and release mechanisms), and their predicted 135Cs/137Cs ratios (estimated from available waste records24,25,28,31,38).
documented but not quantified;3 large releases of 137Cs from ICPP would likely have resulted from the waste evaporation and calcining operations that were conducted during the 1950′s and 1960′s. Airborne deposition of 137Cs particulates from ICPP would be expected to decrease as a function of distance from the ICPP and follow the prevailing wind patterns of the area (e.g., the northeast-southwest direction, consistent with previous observations of actinide contamination patterns in the southeastern Idaho area3,29). A second proposed source of the 137Cs is localized deposition of global + regional nuclear weapons fallout (Figure 2).27 As two pluvial flooding events occurred at the SDA during the 1960′s, it has been suggested that particles containing 137Cs from global + regional fallout could have been selectively deposited in the drainage ditch via surface water sedimentation processes in a manner similar to that observed by Snyder et al. for 137Cs at Lake Mead (albeit on a much smaller scale).12 A third possible source of 137Cs contamination is from 137Cs waste within the SDA; although the inventory of the SDA is not completely known due to inadequate record keeping, > 105 Ci of 137Cs resulting from dozens of different waste generators representing over 230 different processes has been docu-
Plant (ICPP). ICPP is located 7 miles (13 km) to the northeast of the SDA and reprocessed reactor fuel from a variety of onsite and off-site reactors from 1953 to 1992.32 Surface soil samples were collected around the SDA in the early 1970′s by Markham et al; these samples revealed that considerable actinide contamination existed around this site.29 The majority of the actinide contamination has been attributed to waste from the Rocky Flats site which was disposed of in an acid pit called Pit 9;3,26,27,29 material from this excavation is believed to have been spread to the surrounding environment by pluvial flooding of this excavation followed by floodwater evaporation and subsequent particulate resuspension and redistribution via wind transport processes under arid conditions.27,29 More recently, low level 137Cs contamination has been identified in two of these soil samples taken within a drainage ditch on the immediate northeastern SDA perimeter; 26 however, the source and extent of the 137 Cs contamination has yet to be identified. Three possible emission sources for the elevated 137Cs concentrations have been suggested (Figure 2). The first possibility is airborne deposition of 137Cs from the ICPP.26 Stack releases from the ICPP prior to 1970 have been C
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Environmental Science & Technology mented.7,24,25,30 Airborne releases from the SDA could have occurred during normal waste disposal operations or during two fires that occurred in one of the trenches in 1966. Additionally, floods that occurred in 1962 and 1969 could also have released 137Cs. As Cs has been shown to bind rapidly and irreversibly to clays within INL soils,33,34 floodwater transport of discrete 137Cs bearing particles should follow the general surface water flow directions (Figure SI-1, Supporting Information (SI)), whereas continuous low-level and fire releases should follow airborne release patterns (northeastsouthwest dispersion from the emitting locations within the SDA). The wide variety of emission sources and possible release mechanisms make this site a challenging case study in environmental Cs source term analysis.
3. MATERIALS AND METHODS Surface soil samples analyzed in this study were collected in 1972−1974 by Markham et al.29 Samples underwent an initial gamma spectrometry analysis for 137Cs, following which samples were dissolved and prepared for thermal ionization mass spectrometry analysis (TIMS) using the method of Snow et al.35 A detailed discussion of the sample purification approach and method validations can be found in the Supporting Information section (Discussion SI-1, SI).
Figure 3. 137Cs concentrations in soils surrounding the SDA. White dots represent locations analyzed by gamma spectroscopy. Color gradients showing the interpolated concentration gradients were generated using the program MATLAB41 (see Table SI-1, SI for exact 137 Cs activity data).
4.2. 135Cs/137Cs Measurements. Although discrimination between localized global + regional fallout and SDA derived Cs is not definitive based solely on 137Cs concentration data,27 such discrimination is possible through analysis of the 135 Cs/137Cs isotopic ratios.10,12,19 Results from TIMS analyses of the two locations containing the highest concentrations of 137 Cs (locations 1−2 and 1−4) are given in Table 1. 135 Cs/137Cs ratios for all four samples are observed to be statistically similar (within the measurement errors). While the majority of the analyses resulted in relatively small 135Cs/137Cs measurement errors, the larger errors for sample 1−2 #2 are not necessarily surprising as rigorous soil homogeneity studies were beyond the scope of this work (see Discussion SI-1). To determine whether the 135Cs/137Cs isotopic ratios at locations 1−2 and 1−4 are consistent with pure fast fission (including global + regional nuclear weapons fallout), let us consider the sample with the lowest error (sample 1−4 #1). As 137 Cs has a 30.01 year half-life (compared with 135Cs whose half-life is 2.3 × 106 yrs), the original 135Cs/137Cs ratio will continually increase with increasing time since fission. Previous analyses of 135Cs/137Cs ratios from global + regional nuclear weapons fallout, obtained from nearby Sandhole Lake sediment core samples, show that the regional fallout value for 135 Cs/137Cs (decay corrected to the peak deposition of global fallout in 1963) is 0.93 ± 0.17, which corresponds very well with the theoretical, age corrected ratio of ∼1.0.12,19 Thus, if the Cs in sample 1−4 #1 represents a pure fast fission signature (135Cs/137Cs = 1.0 at time =0), it would have to be no older than 22.1 ± 0.9 yrs old, corresponding to a fission event in 1992 ± 0.9 yrs. However, these samples were collected in 1972 and have remained sealed since that date; therefore, the time = 0 135Cs/137Cs isotopic ratios in the SDA samples must lower than 1.0 and thus can only be explained if they contain significant contribution of Cs from a thermal fission source. Therefore, 135Cs/137Cs ratios combined with 137Cs distribution patterns strongly suggest that the 137Cs contamination detected at the SDA perimeter contains thermal fission product Cs derived from waste disposed of within the SDA.
4. RESULTS AND DISCUSSION 4.1. 137Cs Distribution Patterns. Figure 2 shows the three proposed emission sources described in the Background section (e.g., ICPP, global + regional fallout, and the SDA) along with their expected 137Cs contamination patterns and 135Cs/137Cs ratios. Cs has been shown to sorb rapidly and irreversibly to clays (specifically frayed Illite and mica) that are common in INL soils;33 thus, transport of Cs near the SDA is believed to be due primarily to mechanical transport of these discrete Cs bearing clay particles.27,34,36 If the primary transport mode was by transfer and deposition of these particles via surface floodwater flow, contamination should be confined primarily to the area immediately adjacent to the emitting source and concentrated in areas of lower topography (see Figure SI-1, SI). Conversely, if these clay particles are transported by the wind under arid conditions, Cs contamination should be more widely dispersed in the northeast-southwest direction (e.g., the direction of the prevailing winds of the area).3,29 Experimentally observed 137Cs concentrations in soils surrounding the SDA are shown in Figure 3 (see also Figure 4 and Table SI-1, SI). 137Cs concentrations above global + regional fallout levels are only observed within the drainage ditch on the immediate eastern and northeastern SDA perimeter and at locations up to 250 m to the east and northeast of the SDA. 137 Cs distribution patterns are not consistent with those expected from the ICPP; distribution patterns instead suggest that the 137Cs releases originated from the immediate SDA vicinity (consistent with either releases from the SDA or from sedimentation and preconcentration of global + regional fallout). Interestingly, the highest 137Cs concentration observed (location 1−2, see Figure 4) is 6 times greater than the maximum concentration expected for global + regional fallout (Table SI-1, SI). From a practical standpoint, an enrichment of global + regional nuclear weapons fallout of this magnitude (occurring as a result of the two floods and dike expansions) is unlikely. D
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Figure 4. SDA surface features of interest.24,25 Blue, Red, and Green excavations represent excavations open during the 1962 flood, 1966 fires, and 1969 floods (respectively). Dashed lines show the locations of the drainage dikes constructed in late 1962. Open dots represent surface soil samples analyzed in 1969,38 while solid black dots represent samples analyzed by TIMS in this study.
(possibly EBR-1 or EBR-2 waste), 37 while measured Cs/137Cs ratios suggest thermal fission. The 1962 flood is also not the most likely release event, as the drainage dikes were constructed after the 1962 flood and the majority of the 137Cs contamination is observed between the eastern SDA perimeter and drainage dike. Furthermore, neither this study nor surface soil surveys conducted in 1969 observed elevated 137Cs concentrations in the immediate vicinity of excavations open in 1962.38 Unlike continuous releases, the 1962 flood, and the 1966 fires, 137Cs distribution patterns are entirely consistent with those predicted for surface water deposition of discrete 137Cs bearing particles during the 1969 flood (Figures 2-4, see also Figure SI-1, SI). Although several excavations were open during this flood (Pits 9−10 and Trenches 48−49), previous studies have attributed actinide contamination exclusively to material from within Pit 9.3,26,29 However, as both Cs and Pu are transported mechanically under the conditions at the SDA,27,39 the observation of relatively high 137Cs concentrations throughout the eastern-northeastern drainage dike (in contrast to that of Pu, which shows highest concentrations in a small area of the northeastern corner only) suggests that other excavation(s) besides Pit 9 also contributed significantly to the observed 137Cs. Extensive surface soil and water surveys were performed using gamma spectroscopy immediately after the 1969 flood (see Figure 4). Although numerous soil cores were taken near Trench 49 and Pit 10, only a single sample on the eastern SDA boundary within 4 feet of Trench 48 showed detectable 137Cs.38 These results suggest that Pit 10 and Trench 49 did not contribute appreciably to the observed 137Cs contamination on the eastern SDA boundary. Trench 48, however, is located immediately adjacent to the highest 137Cs contamination observed in this study and is known to contain large amounts of mixed fission product waste.24,25,31 Thus, these data suggest that flooding of Pit 9 and Trench 48 are the primary sources of the 137Cs contamination around the SDA, with 137Cs contamination on the southeastern border primarily originating from within Trench 48. Disposal records indicate that Trench 48 and Pit 9 were open for 5 months and 1.2 years (respectively) prior to the 1969 flood.31 During this period only three waste generators
Table 1. Measured 135Cs/137Cs Isotopic Ratios for Samples on the Northeastern Corner of the SDA (as of 2/28/2014) location
sample #
1−4 1−4 1−2 1−2
1 2 1 2
135
135
Cs/137Cs (atom) 1.67 1.83 1.87 2.39
± ± ± ±
0.04 0.10 0.16 0.56
4.3. Determination of the Release Event and Identity of the Original Fission Event/Process. Although the SDA is identified as the 137Cs emission source, dozens of waste generators (spanning more than 230 unique processes) have disposed of waste within the SDA.7,24,25 If a specific 137Cs release event can be determined, the total number of possible waste generators which may have contributed to the observed 137 Cs contamination would be greatly reduced (as the number and identity of waste generators varies considerably with disposal date and location).24 This in turn would result in much more precise attribution of the contamination to a specific nuclear waste generator and/or process. Furthermore, knowledge of a given release event would improve the uncertainty in the original 135Cs/137Cs ratio by narrowing the possible time period during which the fission event could have occurred. This is desirable because while the analytical precision in the 135 Cs/137Cs ratios in Table 1 is sufficient to discriminate between many of the different reactor types that may have generated the material, uncertainties in the 135Cs/137Cs ratio due to the unknown age of the material currently prohibits doing so with these samples (Figure 5a). A detailed analysis of historical records reveals four possible release events that could have occurred at the SDA including (1) continuous airborne releases over the operational history of the SDA, (2) a flood in February 1962, (3) two fires in September 1966, and (4) a flood in January 1969 (see also Discussion SI-2, SI).3,7,24,25,27,29 The locations of excavations involved in each event are shown in Figure 4. Experimentally observed 137Cs surface distribution patterns are not consistent with those anticipated for airborne releases from the SDA, including continuous releases and releases during the 1966 fires. Additionally, the 1966 fires resulted from contaminated alkali metal waste, which suggests that the waste involved with the fires resulted from a fast fission process E
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Figure 5. Variations in the 135Cs/137Cs ratio as a function of the age of the fission event. a) The fission event occurred sometime between the first man made reactor up to the time of soil collection (1943−1972), and b) the material was allowed to cool for a period of 0, 1, 3, or 5 years prior to January 1969.
be expected to vary as a function of the time elapsed since fission (assuming a similar initial fission date and cooling time for all contributing sources). Without information allowing further constraints on the date of the fission event, uncertainties in the age represent a significant limitation in determining the original 135Cs/137Cs ratio(s). A final limitation for further attribution is the likelihood of mixing between fallout and SDA derived Cs. As soils within the drainage dike were disturbed during the 1969 flood and subsequent dike expansions,24,25,29 fallout Cs concentrations cannot be inferred with confidence by comparison to average regional 137Cs concentrations.27,36 Nevertheless, such a correction is essential for accurate determination of the exact 135 Cs/137Cs ratio resulting from the SDA; for example, if for the sake of argument it is assumed that the SDA soils were undisturbed (and thus the concentration of 137Cs resulting from global fallout is equal to the regional background concentration of 0.018 ± 0.008 Bq/g, see Table SI-1, SI), lack of a fallout correction could result in up to a 20% difference in the predicted 135Cs/137Cs ratio for the SDA end member (and thus represent a significant bias in the data). Although a fallout correction cannot be applied based on regional 137Cs background concentrations at this location (due to the soils being highly disturbed), an alternate method which might be employed is to use a secondary nuclide monitor that has a similar environmental mobility to Cs (such as Sr or Pu) and is also present in fallout to predict the concentration of fallout 137Cs. Such an approach has been used in reverse (e.g., using 137Cs to predict Pu concentrations and, consequently, the nonfallout Pu isotopic ratios) by several different groups in the literature.27,36 Although this technique would be challenged at this site (due to the extremely large quantities and variety of actinide, fission product, and activation waste disposed within the SDA), such an approach may be applicable to other studies at other, less complex locations. 4.5. Application of 135Cs/137Cs Ratio for Identifying Low Level 137Cs Contamination at Other Locations. 135 Cs/137Cs ratios provide a sensitive, powerful tool for identifying and attributing anthropogenic 137Cs environmental contamination to specific nuclear industrial processes. In the case of the SDA, a combination of 135Cs/137Cs ratios, 137Cs contamination patterns, knowledge of the Cs chemistry at this
disposed of waste at the SDA: (1) Rocky Flats site, CO, (2) INL routine research and reactor waste, and (3) a minor off-site waste generator. Extensive investigation of the processes involved at the Rocky Flats Site,40 combined with reported waste disposal records from this operation24,25,31,40 show that the quantity of 137Cs disposed by Rocky Flats site at the SDA is negligible. While the exact composition of the waste from the minor offsite generator was not fully documented and thus this generator cannot be completely ruled out, SDA disposal records indicate that INL processes contributed to over 2000 times as much nonactinide waste (by activity) as offsite generators during 1968, including very large quantities of 137 Cs.31,38 Furthermore, while the waste form of the minor offsite generator is not known, INL routine waste was disposed of primarily in cardboard boxes and loose waste which would be particularly susceptible to radionuclide release under flooding conditions.24,25 Therefore, INL related activities represent the most likely contributor to the observed 137Cs contamination. 4.4. Limitations in 135Cs/137Cs Ratios. Although the experimental uncertainties in the measured 135Cs/137Cs ratios (Table 1) are sufficient to discriminate between several reactor designs and operating conditions which may have generated the fission product Cs, three major limitations hamper the ability to do so with these samples. First, multiple fission product sources from INL related operations likely contributed to the waste disposed within Trench 48 and Pit 9. For example, during 1968 several INL facilities contributed fission product waste to the SDA (including operations at the ICPP, NRF, ANL, TAN, and TRA facilities); waste from ICPP alone likely included at least 16 different reactors, of which at least 1 was a fast fission reactor.28,31 As segregation of different sources of fission product waste was almost certainly not performed, and as multiple sources would further have been mixed during the flood, the observed 137Cs likely represents a mixture from a variety of different reactors. The second major limitation to further nuclear process attribution is uncertainty in the exact age of the fission product 137 Cs. The upper limit to the fission date for this material is January 1969 (the date of the 137Cs release); however, the majority of INL waste came from reactor fuels that were most likely allowed to cool for a certain period of time prior to disposal.31 Figure 5b illustrates how the 135Cs/137Cs ratio would F
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ASSOCIATED CONTENT
S Supporting Information *
Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
ACKNOWLEDGMENTS
We thank Jackie Loop from INL for her considerable assistance in acquiring many of the technical documents and reports on the early SDA history critical to this work. We also thank the three anonymous reviewers and the Editor Daniel Giammar for their thoughtful and constructive reviews of this manuscript. This material is based upon work supported in part by the U.S. Department of Homeland Security under Grant Award Number, 2012-DN-130-NF0001-02, and in part, by Battelle Energy Alliance, LLC under Contract No. DE-AC0705ID14517 with the U.S. Department of Energy. Neither the U.S. Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. References herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the U.S. Government or any agency thereof. Views and opinions of the authors expressed herein do not necessarily G
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DOI: 10.1021/es5058852 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
