W. Andrew Jackson

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.

Natural Chlorate in the Environment: Application of a New IC-ESI/MS/MS Method with a Cl18O3- Internal Standard

A new ion chromatography electrospray tandem mass spectrometry (IC-ESI/MS/MS) method has been developed for quantification and confirmation of chlorate (ClO₃⁻) in environmental samples. The method involves the electrochemical generation of isotopically labeled chlorate internal standard (Cl¹⁸O₃⁻) using ¹⁸O water (H₂¹⁸O) he standard was added to all samples prior to analysis thereby minimizing the matrix effects that are associated with common ions without the need for expensive sample pretreatments. The method detection limit (MDL) for ClO₃⁻ was 2 ng L⁻¹ for a 1 mL volume sample injection. The proposed method was successfully applied to analyze ClO₃⁻ in difficult environmental samples including soil and plant leachates. The IC-ESI/MS/MS method described here was also compared to established EPA method 317.0 for ClO₃⁻ analysis. Samples collected from a variety of environments previously shown to contain natural perchlorate (ClO₄⁻) occurrence were analyzed using the proposed method and ClO₃⁻ was found to co-occur with ClO₄⁻ at concentrations ranging from < 2 ng L⁻¹ in precipitation from Texas and Puerto Rico to >500 mg kg⁻¹ in caliche salt deposits from the Atacama Desert in Chile. Relatively low concentrations of ClO₃⁻ in some natural groundwater samples (0.1 µg L⁻¹) analyzed in this work may indicate lower stability when compared to ClO₄⁻ in the subsurface. The high concentrations ClO₃⁻ in caliches and soils (3-6 orders of magnitude greater) as compared to precipitation samples indicate that ClO₃⁻, like ClO₄⁻, may be atmospherically produced and deposited, then concentrated in dry soils, and is possibly a minor component in the biogeochemical cycle of chlorine.

Perchlorate formation by ozone oxidation of aqueous chlorine/oxy-chlorine species: role of ClxOy radicals.

The environmental occurrence of perchlorate (ClO4(-)) can be related to either natural or anthropogenic sources. Recent studies highlighted the ubiquitous occurrence of natural ClO4(-) in the environment including wet deposition in the United States. Limited studies have investigated potential mechanisms responsible for natural ClO4(-) production in the environment. These studies have neither addressed the influence of relevant reaction conditions nor have they evaluated the rates of ClO4(-) production. The purpose of this study was to determine the comparative yields and rates of ClO4(-) production from O3 mediated oxidation of Cl(-), OCl(-), ClO2(-), ClO3(-), and ClO2. The influence of reactant (O3 and ClOx(-)) concentration and pH were evaluated. The comparative rate and efficiency of ClO4(-) production is generally greater for higher oxidation states of Cl (2.7 to 0.5% for ClO2(-)/ClO2 and 0.02 to 0.005% for OCl(-)/HOCl oxidation) with the notable exception of ClO3(-) which does not react with O3. The very slow rate of ClO4(-) production from Cl(-) ( approximately 20 x 10(-9) mM min(-1)) even at elevated O3 and Cl(-) concentrations implies negligible potential for anthropogenic ClO4(-) formation in process units of water/wastewater systems that use O3 for treatment. Based on results of ClO4(-) formation from tested Cl species and available literature, we propose a potential formation pathway for ClO4(-) from Cl(-) with emphasis on the role of ClO2 and higher oxy-chlorine radicals/intermediates (e.g., Cl2O6) in its formation.

Perchlorate, Nitrate, and Iodide Intake through Tap Water

Perchlorate is ubiquitous in the environment, leading to human exposure and potential impact on thyroid function. Nitrate can also competitively inhibit iodide uptake at the sodium-iodide symporter and thus reduce thyroid hormone production. This study investigates the intake of perchlorate, nitrate, and iodide attributable to direct and indirect tap water consumption. The National Health and Nutrition Examination Survey collected tap water samples and consumption data from 3262 U.S. residents during the years 2005-2006. The median perchlorate, nitrate, and iodide levels measured in tap water were 1.16, 758, and 4.55 μg/L, respectively. Measured perchlorate levels were below the United States Environmental Protection Agency (U.S. EPA) drinking water equivalent level for perchlorate (24.5 μg/L). Significant correlations were found between iodide and nitrate levels (r = 0.17, p < 0.0001) and perchlorate and nitrate levels (r = 0.25, p < 0.0001). On the basis of 24 h recall, 47% of the study participants reported drinking tap water; 89% reported either direct or indirect consumption of tap water. For the adult population (age ≥ 20 yrs) the median tap water consumption rate was 11.6 mL/kg-day. Using individual tap water consumption data and body weight, we estimated the median perchlorate, nitrate, and iodide dose attributable to tap water as 9.11, 11300, and 43.3 ng/kg-day, respectively, for U.S. adults. This perchlorate exposure dose from tap water is relatively small compared to the total perchlorate exposure dose previously characterized for the U.S. adults (median 64 ng/kg-day) and the U.S. EPA reference dose (700 ng/kg-day).

Perchlorate production by ozone oxidation of chloride in aqueous and dry systems

Overwhelming evidence now exists that perchlorate is produced through natural processes and can be ubiquitously found at environmentally relevant concentrations in arid and semi-arid locations. A number of potential production mechanisms have been hypothesized and ClO(4)(-) production by ozone oxidation of surface bound Cl(-) was demonstrated. However, no information concerning the impact of concentration, final reaction products distribution, impact of reaction phase, or oxidation of important oxychlorine intermediates has been reported. Using IC-MS-MS analysis and replicate oxidation experiments, we show that exposing aqueous solutions or Cl(-) coated sand or glass surfaces to O(3) (0.96%) generated ClO(4)(-) with molar yields of 0.007 and 0.01% for aqueous Cl(-) solutions and 0.025 and 0.42% for Cl(-) coated sand and glass, respectively. Aqueous solutions of Cl(-) produced less ClO(4)(-) than Cl(-) coated sand or glass as well as a higher ratio of ClO(3)(-) to ClO(4)(-). Reduction of the initial Cl(-) mass resulted in substantially higher molar yields of ClO(4)(-) and ClO(3)(-). In addition, alkaline absorbers that captured gaseous products contained substantial quantities of Cl(-), ClO(3)(-), and ClO(4)(-). Solutions of possible oxychlorine intermediates (OCl(-) and ClO(3)(-)) exposed to O(3) produced only scant amounts of ClO(4)(-) while a ClO(2)(-) solution exposed to O(3) produced substantial molar yields of ClO(4)(-) (4% molar yield). Scanning electron microscopy coupled with energy energy-dispersive X-ray analysis demonstrated a significant loss of Cl(-) and an increase in oxygen on the Cl(-) coated silica sand exposed to O(3). While the experimental conditions are not reflective of natural conditions this work clearly demonstrates the relative potential of Cl(-) precursors in perchlorate production and the likely importance of dry aerosol oxidation over solution phase reactions. It also suggests that ClO(2)(-) may be a key intermediate while ClO(3)(-) and OCl(-) are unlikely to play a significant role.

Degradation Kinetics of Perchlorate in Sediments and Soils

This study investigated the intrinsic perchlorate (ClO4-)degradation kinetics of sediments and soils from multiple sites in microcosm studies, including the influence of varying nitrate concentration (NO3--N from 1 to 22.8 ppm) and up to 300 ppm sulfate. The first-order degradation rates and lag times of both ClO4- and NO3- degradation were site-specific and dependent on environmental conditions such as organic substrate availability, nitrate, initial ClO4- concentration, and prior ClO4- exposure. At an initial ClO4- concentration of 5 ppm, ClO4- degradation rates ranged from 0.13 to 0.46 day-1, and lag times of ClO4- degradation ranged from 0 to 60.0 days; while NO3- degradation occurred at rates ranging from 0.03 to 1.42 day-1, with lag times ranging from 0 to 29.7 days. Under the same treatment conditions, NO3- degradation rates were relatively higher than that of ClO4-. Perchlorate degradation rates remained constant at both lower (0.5 ppm) and higher (5 ppm) ClO4- concentrations. Generally, ClO4- rates were affected by the availability of organic substrate, which was represented here by Total Volatile Solids (TVS) of sediments and soils, and not by NO3-. Nitrate did increase the lag time of ClO4- degradation, which may account for the persistence of ClO4- in the environment, especially when ClO4- is typically ppb levels in the environment compared to ppm levels of NO3-. This study showed rapid intrinsic ClO4- degradation in sediments and soils of contaminated sites, and highlighted the potential for natural attenuation of ClO4- in the environment.

Distribution of depth to ice-cemented soils in the high-elevation Quartermain Mountains, McMurdo Dry Valleys, Antarctica

Abstract We report on 475 measurements of depth to ice-cemented ground in four high-elevation valleys of the Quartermain Mountains, McMurdo Dry Valleys, Antarctica. These valleys have pervasive ice-cemented ground, and the depth to ice-cemented ground and the ice composition may be indicators of climate change. In University Valley, the measured depth to ice-cemented ground ranges from 0–98 cm. There is an overall trend of increasing depth to ice-cemented ground with distance from a small glacier at the head of the valley, with a slope of 32 cm depth per kilometre along the valley floor. For Farnell Valley, the depth to ice-cemented ground is roughly constant (c. 30 cm) in the upper and central parts of the valley, but increases sharply as the valley descends into Beacon Valley. The two valleys north of University Valley also have extensive ice-cemented ground, with depths of 20–40 cm, but exhibit no clear patterns of ice depth with location. For all valleys there is a tendency for the variability in depth to ice-cemented ground at a site to increase with increasing depth to ice. Snow recurrence, solar insolation, and surface albedo may all be factors that cause site to site variations in these valleys.

Characteristics of perchlorate formation via photodissociation of aqueous chlorite

Environmental context. Perchlorate, a well-known thyroid disruptor with both man-made and natural sources represents a major environmental problem in the United States but little information is available concerning the source of natural perchlorate. Previous research has demonstrated that perchlorate can be produced from exposure of some chlorine compounds to ultraviolet radiation, but no information was available how quickly or comparatively how much perchlorate was formed. The results of the present study can be used to evaluate the potential impact of ultraviolet processes on the overall occurrence of perchlorate in the environment. Abstract. The present study provides new and important information on perchlorate (ClO4–) formation through ultraviolet (UV) photodissociation of unbuffered chlorite (ClO2–) solutions from the standpoint of kinetics under three different wavelength regimes having maximum emissions, λe,max, at 235.7, 300 and 350 nm. ClO4– production rates and yields were in general found to be inversely related, with higher yields and lower rates at higher wavelengths, and vice versa. A simple kinetic model for ClO4– production as a function of the ClO2– first-order decay constant and starting concentration was fitted to the experimental data, resulting in the calculation of a rate constant, k2, which is a function of light-source characteristics. Further, a conceptual scheme for ClO4– formation via photochemical reactions between oxychlorine species was proposed based on the experimental results and available literature. The present study is a further step towards understanding the formation of ClO4– from the photolysis of its precursors.

Perchlorate, nitrate, and iodine uptake and distribution in lettuce (Lactuca sativa L.) and potential impact on background levels in humans.

Much focus has been placed on the impact of exposure to perchlorate (ClO4(-)) from drinking water. Recently, it has become more apparent that a significant percentage of the total ClO4(-) exposure may be due to ingestion of food. Most studies have only evaluated the uptake and distribution of ClO4(-) by plants without considering the potential for uptake of iodine (I) by the plant and the subsequent impacts on ClO4(-) uptake and distribution on human health. The objectives of this research effort were to evaluate the relative uptake of ClO4(-) and I supplied as either KI or KIO3, the two major environmental forms of I in a standard hydroponic nutrient solution using butter head lettuce. No interaction of ClO4(-) uptake and distribution was found in the presence of I(-) or IO3(-) relative to previous studies evaluating ClO4(-) alone. Bioconcentration factors for ClO4(-) and total I in butter head lettuce when coexposed to both anions were similar for outer (292 ± 17 and 294 ± 12 L kg(-1) of dry weight, respectively) and inner (76 ± 18 and 60 ± 8 L kg(-1) of dry weight, respectively) leaves but not for roots (23 ± 3.7 and 359 ± 1.7 L kg(-1) of dry weight, respectively) when the iodine was supplied as I(-). The uptake of iodine was lower (BCF = 47 ± 3.8, 19 ± 0.6, and 189 ± 16, L kg(-1) of dry weight for the outer and inner leaves and roots, respectively) for all tissues when iodine was supplied as IO3(-), with the greatest accumulation by the roots. These results suggest that if lettuce is grown using fertilizers containing both ClO4(-) and I(-), then the final ratio of IT/ClO4 in the leaves will be essentially equal to the ratio in the fertilizer but lower if the I is supplied as IO3(-). Therefore, the impact of the consumption of lettuce containing ClO4(-) may be mitigated if the lettuce is grown using fertilizer with an appropriate amount of I to maintain the existing ratio of serum I to total goitrogen load (TGL). Nevertheless, the TGL in lettuce appeared to be almost completely dominated by NO3(-) with only a minor contribution of ClO4(-), even for the highest exposure to ClO4(-).

Stable Isotopic Composition of Chlorine and Oxygen in Synthetic and Natural Perchlorate

Neil C. Sturchio, J. K. Bohlke, Baohua Gu, Juske Horita, Gilbert M. Brown, Abelardo D. Beloso, Jr., Leslie J. Patterson, Paul B. Hatzinger, W. Andrew Jackson, and Jacimaria Batista 1 University of Illinois at Chicago, Chicago, IL 60607 2 U. S. Geological Survey, Reston, VA 20192 3 Oak Ridge National Laboratory, Oak Ridge, TN 37831 4 Shaw Environmental, Inc., Lawrenceville NJ 08648 5 Texas Tech University, Lubbock, TX 79410 6 University of Nevada, Las Vegas, NV 89154