Chlorine-36 as a tracer of perchlorate origin

Perchlorate and chlorate assessment in drinking water in northern Chilean cities

Perchlorate and chlorate are endocrine disruptors considered emerging contaminants (ECs). Both oxyanions are commonly associated with anthropogenic contamination from fertilizers, pesticides, explosives, and disinfection byproducts. However, the soils of the Atacama Desert are the most extensive natural reservoirs of perchlorate in the world, compromising drinking water sources in northern Chile. Field campaigns were carried (2014-2018) to assess the presence of these ECs in the water supply networks of twelve Chilean cities. Additionally, the occurrence of perchlorate, chlorate and other anions typically observed in drinking water matrices of the Atacama Desert (i.e., nitrate, chloride, sulfate) was evaluated using a Spearman correlation analysis to determine predictors for perchlorate and chlorate. High concentrations of perchlorate (up to 114.48 μg L-1) and chlorate (up to 9650 μg L-1) were found in three northern cities. Spatial heterogeneities were observed in the physicochemical properties and anion concentrations of the water supply network. Spearman correlation analysis indicated that nitrate, chloride, and sulfate were not useful predictors for the presence of perchlorate and chlorate in drinking water in Chile. Hence, this study highlights the need to establish systematic monitoring, regulation, and treatment for these EC of drinking water sources in northern Chilean cities for public health protection.

Occurrence, latitudinal gradient and potential sources of perchlorate in the atmosphere across the hemispheres (31°N to 80°S)

Perchlorate (ClO₄^-) is harmful to human health, and knowledge on the levels and sources of natural ClO₄^- in different environments remains rather limited. Here, we investigate ClO₄^- in aerosol samples collected along a cross-hemisphere ship cruise between China and Antarctica and on a traverse between coastal East Antarctica and the ice sheet summit (Dome Argus). Perchlorate concentrations range from a few to a few hundred pg m^-3. A clear latitudinal trend is found, with elevated ClO₄^- concentrations near populated areas and in the southern mid-high latitudes. Spatial patterns of atmospheric ClO₄^- over oceans near the landmasses support that terrestrial ClO₄^- is not transported efficiently over long distances. In the southern mid-latitudes, higher ClO₄^- concentrations in March than in November-December may be caused by significant stratospheric inputs in March. Perchlorate concentrations appear to be higher in the warm half than in the cold half of the year in the southern high latitudes, suggesting seasonal difference in main atmospheric sources. ClO₄^- may be formed in the reactions between chlorine free radical (Cl·) and ozone (O₃) in the stratosphere when Antarctic ozone hole occurs during September-October. And the stratosphere-produced ClO₄^- is moved to the boundary layer in several months and may be responsible for the high ClO₄^- concentrations in the warm half of the year. Perchlorate produced by photochemical reactions between O₃ and Cl· in the Antarctic stratosphere is likely responsible for the higher ClO₄^- concentrations in Antarctica than in Arctic.

Isotopic Tracing of Perchlorate Sources in the Environment

Perchlorate (ClO4−) is an emerging persistent pollutant that is ubiquitous in the environment at trace concentrations. Perchlorate ingestion poses a risk to human health because it interferes with thyroidal hormone production. The identification of perchlorate sources in groundwater is a primary concern. Chlorine and multi-oxygen isotopic tracing of perchlorate (δ37Cl, 36Cl/Cl, δ18O, and Δ17O) can provide a unique tool for identifying the origin and transport of perchlorate in groundwater. Along with the kinetic fractionation of chlorine and oxygen isotopes, the Δ17O value, 36Cl/Cl ratio, and ε18O/ε37Cl (the fractionation coefficient of oxygen and chlorine isotopes) are constant, potentially indicating the biodegradation of perchlorate, without disguising its source information. Therefore, comprehensive characterization of stable chlorine and poly-oxygen isotopes is expected to provide direct evidence for identifying the source of perchlorate in groundwater. However, further studies are needed to increase the amount of isotopic data of different perchlorate sources, to make the end-member model available to broader regions. It is critically important to understand the range of values and differences of isotopes among natural perchlorate sources and the perchlorate formation mechanisms.

Isotopic composition of natural and synthetic chlorate (δ18O, Δ17O, δ37Cl, 36Cl/Cl): Methods and initial results

Natural chlorate (ClO₃^-) is widely distributed in terrestrial and extraterrestrial environments. To improve understanding of the origins and distribution of ClO₃^-, we developed and tested methods to determine the multi-dimensional isotopic compositions (δ¹⁸O, Δ¹⁷O, δ³⁷Cl, ³⁶Cl/Cl) of ClO₃^- and then applied the methods to samples of natural nitrate-rich caliche-type salt deposits in the Atacama Desert, Chile, and Death Valley, USA. Tests with reagents and artificial mixed samples indicate stable-isotope ratios were minimally affected by the purification processes. Chlorate extracted from Atacama samples had δ¹⁸O = +7.0 to +11.1‰, Δ¹⁷O = +5.7 to +6.4‰, δ³⁷Cl = -1.4 to +1.3‰, and ³⁶Cl/Cl = 48 × 10^-15 to 104 × 10^-15. Chlorate from Death Valley samples had δ¹⁸O = -6.9 to +1.6‰, Δ¹⁷O = +0.4 to +2.6‰, δ³⁷Cl = +0.8 to +1.0‰, and ³⁶Cl/Cl = 14 × 10^-15 to 44 × 10^-15. Positive Δ¹⁷O values of natural ClO₃^- indicate that its production involved reaction with O₃, while its Cl isotopic composition is consistent with a tropospheric or near-surface source of Cl. The Δ¹⁷O and δ¹⁸O values of natural ClO₃^- are positively correlated, as are those of ClO₄^- and NO₃^- from the same localities, possibly indicating variation in the relative contributions of O₃ as a source of O in the formation of the oxyanions. Additional isotopic analyses of ClO₃^- could provide stronger constraints on its production mechanisms and/or post-formational alterations, with applications for environmental forensics, global biogeochemical cycling of Cl, and the origins of oxyanions detected on Mars.

Perchlorate - properties, toxicity and human health effects: an updated review

Interest in perchlorate as environmental pollutant has increased since 1997, when high concentrations have been found in the waters of the Colorado River, USA. Perchlorate is very persistent in nature and it is slowly degraded. Although harmful effects of large doses of perchlorate on thyroid function have been proven, the environmental effects are still unclear. The primary objective of the present review is to collect prevailing data of perchlorate exposure and to discuss its impact on human health. The results show that more than 50% of reviewed works found significant associations of perchlorate exposure and human health. This review consists of the following sections: general information of perchlorate sources, its properties and determination methods, role and sources in human body including food and water intake, overview of the scientific literature on the research on the effect of perchlorate on human health from 2010 to 2020. Finally, conclusions and recommendations on future perchlorate studies concerning human exposure are presented.

Sources and behavior of perchlorate in a shallow Chalk aquifer under military (World War I) and agricultural influences

Perchlorate (ClO₄^ö) has been detected at concentrations of concern for human health on a large scale in groundwater used for drinking water supplies in NE France. Two sources are suspected: a military source related to World War I (WWI) and an agricultural source related to past use of Chilean nitrate fertilizers. The sources and behavior of ClO₄^ö have been studied in groundwater and rivers near the Reims city, by monitoring monthly the major ions and ClO₄- concentrations for two years (2017-2019), and by measuring the isotopic composition of ClO₄^ö and NO₃^ö in water samples. ClO₄^ö was detected throughout the study area with high concentrations (> 4 μg⋅L^-1) detected mainly downgradient of the Champagne Mounts, where large quantities of ammunition were used, stored and destroyed during and after WWI. A WWI military origin of ClO₄- is inferred from isotopic analysis and groundwater ages. Different tendencies of ClO₄- variation are observed and interpreted by a combination of ClO₄- concentrations, aquifer functioning and historical investigations, revealing major sources of ClO₄- (e.g., unexploded ordnance, ammunition destruction sites) and its transfer mechanisms in the aquifer. Finally, we show that concentrations of ClO₄^ö in groundwater seems unlikely to decrease in the short- to medium-term.

Worldwide occurrence and origin of perchlorate ion in waters: A review

Perchlorate (ClO₄^-) is a persistent water soluble oxyanion of growing environmental interest. Perchlorate contamination can be a health concern due to its ability to disrupt the use of iodine by the thyroid gland and the production of metabolic hormones. Its widespread presence in surface water and groundwater makes the aquatic environment a potential source of perchlorate exposure. However, the amount of published data on perchlorate origins and water contamination worldwide remains spatially limited. Here, we present an overview of research on perchlorate origins and occurrences in water, and the methodology to distinguish the different perchlorate sources based on isotope analysis. All published ranges of isotopic content in perchlorate from different sources are presented, including naturally occurring and man-made perchlorate source types, as well as the effects of isotope fractionation that accompanies biodegradation processes. An example of a case study in France is presented to emphasize the need for further research on this topic.

The perchlorate record during 1956-2004 from Tienshan ice core, East Asia

Perchlorate concentration in a shallow ice core at Tienshan, East Asia ranged between 0.55 and 52.1 ng L^-1, with significant temporal variations during 1956-2004. Since the 1980s, a clear increasing trend of perchlorate was observed in the Miaoergou ice core, possibly the result of elevated stratospheric chlorine levels caused by emissions of anthropogenic volatile chlorine compounds. Although differences in trends and amounts were observed, the 1956-2004 perchlorate data from this study compares well with the perchlorate data from the High Arctic ice cores. The spatial and temporal differences of the perchlorate in Miaoergou ice core may be due to differences in anthropogenic sources. Such as, the nitrate ore field in Turpan-Hami Basin in eastern Xinjiang, China, may be the primary anthropogenic source. From the organic chlorine species emission data, HCFC-141b, HCFC-142b and HCFC-124 were identified as the primary anthropogenic sources responsible for the two perchlorate spikes observed for 1980-1996 and 1997-2001. The Miaoergou ice core covering the 1956-2004 period provides further evidence for the perchlorate deposition variations between mid-latitudes and the High Arctic regions.

Evidence of Influence of Human Activities and Volcanic Eruptions on Environmental Perchlorate from a 300-Year Greenland Ice Core Record

A 300-year (1700-2007) chronological record of environmental perchlorate, reconstructed from high-resolution analysis of a central Greenland ice core, shows that perchlorate levels in the post-1980 atm were two-to-three times those of the pre-1980 environment. While this confirms recent reports of increased perchlorate in Arctic snow since 1980 compared with the levels for the prior decades (1930-1980), the longer Greenland record demonstrates that the Industrial Revolution and other human activities, which emitted large quantities of pollutants and contaminants, did not significantly impact environmental perchlorate, as perchlorate levels remained stable throughout the 18^th, 19^th, and much of the 20^th centuries. The increased levels since 1980 likely result from enhanced atmospheric perchlorate production, rather than from direct release from perchlorate manufacturing and applications. The enhancement is probably influenced by the emission of organic chlorine compounds in the last several decades. Prior to 1980, no significant long-term temporal trends in perchlorate concentration are observed. Brief (a few years) high-concentration episodes appear frequently over an apparently stable and low background (∼1 ng kg^-1). Several such episodes coincide in time with large explosive volcanic eruptions including the 1912 Novarupta/Katmai eruption in Alaska. It appears that atmospheric perchlorate production is impacted by large eruptions in both high- and low-latitudes, but not by small eruptions and nonexplosive degassing.

Perchlorate contamination in Chile: Legacy, challenges, and potential solutions

This paper reviews the unique situation of perchlorate contamination in Chile, including its sources, presence in environmental media and in the human population, and possible steps to mitigate its health impacts. Perchlorate is a ubiquitous water contaminant that inhibits thyroid function. Standards for drinking water range from 2 to 18 µg L^-1 in United States and Europe. A major natural source of perchlorate contamination is Chile saltpeter, found in the Atacama Desert. High concentrations of perchlorate have presumably existed in this region, in soils, sediments, surface waters and groundwaters, for millions of years. As a result of this presence, and the use of Chile saltpeter as a nitrogen fertilizer, perchlorate in Chile has been found at concentrations as high as 1480 µg L^-1 in drinking water, 140 µg/kg^-1 in fruits, and 30 µg L^-1 in wine. Health studies in Chile have shown concentrations of 100 µg L^-1 in breast milk and 20 µg L^-1 in neonatal serum. It is important to acknowledge perchlorate as a potential health concern in Chile, and assess mitigation strategies. A more thorough survey of perchlorate in Chilean soils, sediments, surface waters, groundwaters, and food products can help better assess the risks and potentially develop standards. Also, perchlorate treatment technologies should be more closely assessed for relevance to Chile. The Atacama Desert is a unique biogeochemical environment, with millions of years of perchlorate exposure, which can be mined for novel perchlorate-reducing microorganisms, potentially leading to new biological treatment processes for perchlorate-containing waters, brines, and fertilizers.

Stable isotope analyses of oxygen (18 O:17 O:16 O) and chlorine (37 Cl:35 Cl) in perchlorate: reference materials, calibrations, methods, and interferences

RATIONALE

Perchlorate (ClO₄^- ) is a common trace constituent of water, soils, and plants; it has both natural and synthetic sources and is subject to biodegradation. The stable isotope ratios of Cl and O provide three independent quantities for ClO₄^- source attribution and natural attenuation studies: δ³⁷ Cl, δ¹⁸ O, and δ¹⁷ O (or Δ¹⁷ O or ¹⁷ Δ) values. Documented reference materials, calibration schemes, methods, and interferences will improve the reliability of such studies.

METHODS

Three large batches of KClO₄ with contrasting isotopic compositions were synthesized and analyzed against VSMOW-SLAP, atmospheric O₂ , and international nitrate and chloride reference materials. Three analytical methods were tested for O isotopes: conversion of ClO₄^- to CO for continuous-flow IRMS (CO-CFIRMS), decomposition to O₂ for dual-inlet IRMS (O2-DIIRMS), and decomposition to O₂ with molecular-sieve trap (O2-DIIRMS+T). For Cl isotopes, KCl produced by thermal decomposition of KClO₄ was reprecipitated as AgCl and converted into CH₃ Cl for DIIRMS.

RESULTS

KClO₄ isotopic reference materials (USGS37, USGS38, USGS39) represent a wide range of Cl and O isotopic compositions, including non-mass-dependent O isotopic variation. Isotopic fractionation and exchange can affect O isotope analyses of ClO₄^- depending on the decomposition method. Routine analyses can be adjusted for such effects by normalization, using reference materials prepared and analyzed as samples. Analytical errors caused by SO₄^2- , NO₃^- , ReO₄^2- , and C-bearing contaminants include isotope mixing and fractionation effects on CO and O₂ , plus direct interference from CO₂ in the mass spectrometer. The results highlight the importance of effective purification of ClO₄^- from environmental samples.

CONCLUSIONS

KClO₄ reference materials are available for testing methods and calibrating isotopic data for ClO₄^- and other substances with widely varying Cl or O isotopic compositions. Current ClO₄^- extraction, purification, and analysis techniques provide relative isotope-ratio measurements with uncertainties much smaller than the range of values in environmental ClO₄^- , permitting isotopic evaluation of environmental ClO₄^- sources and natural attenuation. Copyright © 2016 John Wiley & Sons, Ltd.

Application of chlorine-36 technique in determining the age of modern groundwater in the Al-Zulfi province, Saudi Arabia

The present study aims to estimate the residence time of groundwater based on bomb-produced (36)Cl. (36)Cl/Cl ratios in the water samples are determined by inductively coupled plasma mass spectrometry and liquid scintillation counting. (36)Cl/Cl ratios in the groundwater were estimated to be 1.0-2.0 × 10(-12). Estimates of residence time were obtained by comparing the measured bomb-derived (36)Cl concentrations in groundwater with the background reference. Dating based on a (36)Cl bomb pulse may be more reliable and sensitive for groundwater recharged before 1975, back as far as the mid-1950s. The above (36)Cl background concentration was deduced by determining the background-corrected Dye-3 ice core data from the frozen Arctic data, according to the estimated total (36)Cl resources. The residence time of 7.81 × 10(4) y is obtained from extrapolated groundwater flow velocity. (36)Cl concentration in groundwater does not reflect the input of bomb pulse (36)Cl, and it belongs to the era before 1950.

Chlorine isotopic composition of perchlorate in human urine as a means of distinguishing among exposure sources

Perchlorate (ClO4(-)) is a ubiquitous environmental contaminant with high human exposure potential. Natural perchlorate forms in the atmosphere from where it deposits onto the surface of Earth, whereas synthetic perchlorate is manufactured as an oxidant for industrial, aerospace, and military applications. Perchlorate exposure can potentially cause adverse health effects in humans by interfering with the production of thyroid hormones through competitively blocking iodide uptake. To control and reduce perchlorate exposure, the contributions of different sources of perchlorate exposure need to be quantified. Thus, we demonstrate a novel approach for determining the contribution of different perchlorate exposure sources by quantifying stable and radioactive chlorine isotopes of perchlorate extracted from composite urine samples from two distinct populations: one in Atlanta, USA and one in Taltal, Chile (Atacama region). Urinary perchlorate from the Atlanta region resembles indigenous natural perchlorate from the western USA (δ(37)Cl=+4.1±1.0‰; (36)Cl/Cl=1 811 (±136) × 10(-15)), and urinary perchlorate from the Taltal, Chile region is similar to natural perchlorate in nitrate salt deposits from the Atacama Desert of northern Chile (δ(37)Cl=-11.0±1.0‰; (36)Cl/Cl=254 (±40) × 10(-15)). Neither urinary perchlorate resembled the isotopic pattern found in synthetic perchlorate. These results indicate that natural perchlorate of regional provenance is the dominant exposure source for the two sample populations, and that chlorine isotope ratios provide a robust tool for elucidating perchlorate exposure pathways.

Perchlorate in indoor dust and human urine in China: contribution of indoor dust to total daily intake

Perchlorate is used in fireworks and China is the largest fireworks producer and consumer in the world. Information regarding human exposure to perchlorate is scarce in China, and exposure via indoor dust ingestion (EDI indoor dust) has rarely been evaluated. In this study, perchlorate was found in indoor dust (detection rate: 100%, median: 47.4 μg/g), human urine (99%, 26.2 ng/mL), drinking water (100%, 3.99 ng/mL), and dairy milk (100%, 12.3 ng/mL) collected from cities that have fireworks manufacturing areas (Yueyang and Nanchang) and in cities that do not have fireworks manufacturing industries (Tianjin, Shijiazhuang, Yuxi and Guilin) in China. In comparison with perchlorate levels reported for other countries, perchlorate levels in urine samples from fireworks sites and nonfireworks sites in China were higher. Median indoor dust perchlorate concentrations were positively correlated (r = 0.964, p < 0.001) with outdoor dust perchlorate levels reported previously. The total daily intake (EDI total) of perchlorate, estimated based on urinary levels, ranged from 0.090 to 27.72 μg/kg body weight (bw)/day for all studied participants; the percentage of donors who had EDI total exceeding the reference dose (RfD) recommended by the United States Environmental Protection Agency (US EPA) was 79%, 48%, and 25% for toddlers (median: 1.829 μg/kg bw/day), adults (0.669 μg/kg bw/day), and children (median: 0.373 μg/kg bw/day), respectively. Toddlers (0.258 μg/kg bw/day) had the highest median EDI indoor dust, which was 2 to 5 times greater than the EDI indoor dust calculated for other age groups (the range of median values: 0.044 to 0.127 μg/kg bw/day). Contribution of indoor dust to EDItotal was 26%, 28%, and 7% for toddlers, children, and adults, respectively. Indoor dust contributed higher percentage to EDI total than that by dairy milk (0.5-5%).

Perchlorate in the Great Lakes: isotopic composition and origin

Perchlorate is a persistent and mobile contaminant in the environment with both natural and anthropogenic sources. Stable isotope ratios of oxygen (δ(18)O, Δ(17)O) and chlorine (δ(37)Cl) along with the abundance of the radioactive isotope (36)Cl were used to trace perchlorate sources and behavior in the Laurentian Great Lakes. These lakes were selected for study as a likely repository of recent atmospheric perchlorate deposition. Perchlorate concentrations in the Great Lakes range from 0.05 to 0.13 μg per liter. δ(37)Cl values of perchlorate from the Great Lakes range from +3.0‰ (Lake Ontario) to +4.0‰ (Lake Superior), whereas δ(18)O values range from -4.1‰ (Lake Superior) to +4.0‰ (Lake Erie). Great Lakes perchlorate has mass-independent oxygen isotopic variations with positive Δ(17)O values (+1.6‰ to +2.7‰) divided into two distinct groups: Lake Superior (+2.7‰) and the other four lakes (∼+1.7‰). The stable isotopic results indicate that perchlorate in the Great Lakes is dominantly of natural origin, having isotopic composition resembling that measured for indigenous perchlorate from preindustrial groundwaters of the western USA. The (36)Cl/Cl ratio of perchlorate varies widely from 7.4 × 10(-12) (Lake Ontario) to 6.7 × 10(-11) (Lake Superior). These (36)ClO4(-) abundances are consistent with an atmospheric origin of perchlorate in the Great Lakes. The relatively high (36)ClO4(-) abundances in the larger lakes (Lakes Superior and Michigan) could be explained by the presence of (36)Cl-enriched perchlorate deposited during the period of elevated atmospheric (36)Cl activity following thermonuclear bomb tests in the Pacific Ocean.

Isotopic mapping of groundwater perchlorate plumes

Analyses of stable isotope ratios of chlorine and oxygen in perchlorate can, in some cases, be used for mapping and source identification of groundwater perchlorate plumes. This is demonstrated here for large, intersecting perchlorate plumes in groundwater from a region having extensive groundwater perchlorate contamination and a large population dependent on groundwater resources. The region contains both synthetic perchlorate derived from rocket fuel manufacturing and testing activities and agricultural perchlorate derived predominantly from imported Chilean (Atacama) nitrate fertilizer, along with a likely component of indigenous natural background perchlorate from local wet and dry atmospheric deposition. Most samples within each plume reflect either a predominantly synthetic or a predominantly agricultural perchlorate source and there is apparently a minor contribution from the indigenous natural background perchlorate. The existence of isotopically distinct perchlorate plumes in this area is consistent with other lines of evidence, including groundwater levels and flow paths as well as the historical land use and areal distribution of potential perchlorate sources.

Isotopic composition and origin of indigenous natural perchlorate and co-occurring nitrate in the southwestern United States

Perchlorate (ClO(4)(-)) has been detected widely in groundwater and soils of the southwestern United States. Much of this ClO(4)(-) appears to be natural, and it may have accumulated largely through wet and dry atmospheric deposition. This study evaluates the isotopic composition of natural ClO(4)(-) indigenous to the southwestern U.S. Stable isotope ratios were measured in ClO(4)(-) (delta(18)O, Delta(17)O, delta(37)Cl) and associated NO(3)(-) (delta(18)O, Delta(17)O, delta(15)N) in groundwater from the southern High Plains (SHP) of Texas and New Mexico and the Middle Rio Grande Basin (MRGB) in New Mexico, from unsaturated subsoil in the SHP, and from NO(3)(-)-rich surface caliche deposits near Death Valley, California. The data indicate natural ClO(4)(-) in the southwestern U.S. has a wide range of isotopic compositions that are distinct from those reported previously for natural ClO(4)(-) from the Atacama Desert of Chile as well as all known synthetic ClO(4)(-). ClO(4)(-) in Death Valley caliche has a range of high Delta(17)O values (+8.6 to +18.4 per thousand), overlapping and extending the Atacama range, indicating at least partial atmospheric formation via reaction with ozone (O(3)). However, the Death Valley delta(37)Cl values (-3.1 to -0.8 per thousand) and delta(18)O values (+2.9 to +26.1 per thousand) are higher than those of Atacama ClO(4)(-). In contrast, ClO(4)(-) from western Texas and New Mexico has much lower Delta(17)O (+0.3 to +1.3 per thousand), with relatively high delta(37)Cl (+3.4 to +5.1 per thousand) and delta(18)O (+0.5 to +4.8 per thousand), indicating either that this material was not primarily generated with O(3) as a reactant or that the ClO(4)(-) was affected by postdepositional O isotope exchange. High Delta(17)O values in ClO(4)(-) (Atacama and Death Valley) are associated with high Delta(17)O values in NO(3)(-), indicating that both compounds preserve characteristics of O(3)-related atmospheric production in hyper-arid settings, whereas both compounds have low Delta(17)O values in less arid settings. Although Delta(17)O variations in terrestrial NO(3)(-) can be attributed to mixing of atmospheric (high Delta(17)O) and biogenic (low Delta(17)O) NO(3)(-), variations in Delta(17)O of terrestrial ClO(4)(-) are not readily explained in the same way. This study provides important new constraints for identifying natural sources of ClO(4)(-) in different environments by multicomponent isotopic characteristics, while presenting the possibilities of divergent ClO(4)(-) formation mechanisms and(or) ClO(4)(-) isotopic exchange in biologically active environments.

Discovery of natural perchlorate in the Antarctic Dry Valleys and its global implications

In the past few years, it has become increasingly apparent that perchlorate (ClO(4)(-)) is present on all continents, except the polar regions where it had not yet been assessed, and that it may have a significant natural source. Here, we report on the discovery of perchlorate in soil and ice from several Antarctic Dry Valleys (ADVs) where concentrations reach up to 1100 microg/kg. In the driest ADV, perchlorate correlates with atmospherically deposited nitrate. Far from anthropogenic activity, ADV perchlorate provides unambiguous evidence that natural perchlorate is ubiquitous on Earth. The discovery has significant implications for the origin of perchlorate, its global biogeochemical interactions, and possible interactions with the polar ice sheets. The results support the hypotheses that perchlorate is produced globally and continuously in the Earth's atmosphere, that it typically accumulates in hyperarid areas, and that it does not build up in oceans or other wet environments most likely because of microbial reduction on a global scale.