H41K-01 INVITED
Widespread Occurrence of Plant Perchlorate
Perchlorate is a water soluble oxyanion containing four oxygens bonded to a single chlorine atom. High concentration of perchlorate can competitively block the uptake of iodide by the sodium iodide symporter and disrupt thyroid function. Due to this ability to potentially impair thyroid function, perchlorate in environmental exposure pathways has been of concern for more than a decade. Our knowledge of the spatial and temporal aspects of environmental perchlorate has increased dramatically in the past few years. To date, perchlorate has been found in numerous different environmental media, including water, soils and sediments, and plants, from many parts of the world. Perchlorate can be found in marine alage, food and plant samples from Asia, Africa, Europe, North and South America. It is becoming increasingly apparent that perchlorate in low levels is ubiquitous. Perchlorate has been found in several different carbon age-dated water and midden samples that pre-date the industrial age and agricultural use of Chilean nitrate fertilizers by thousands of years. While anthropogenic sources of perchlorate exist, the accumulating spatial and temporal evidence suggests that perchlorate must have a significant natural source. This natural source of perchlorate under the appropriate geochemical and climatic conditions is contributing a natural background level of perchlorate. Concentrations of perchlorate in soils appears to be influenced by soil geochemistry. Soils with low organic content usually have higher levels of perchlorate then soils with abundant organic matter. High levels of perchlorate have been found in remotely located xerophytes growing in aridosols and in deciduous phreatophytes growing in humid densely populated areas. Often the amount of perchlorate in a plant cannot be explained by the amount of perchlorate in either the soil or precipitation. Investigations into the relative source contribution of lithogenic, atmospheric and other sources and mechanisms that lead to plant perchlorate accumulation need to be conducted.
H41K-02 INVITED
The Microbiology of Perchlorate in the Environment
In the last decade perchlorate has been identified as an important groundwater component that poses potential health threat. Although primarily sourced anthropogenically, many recent studies have identified significant natural pools throughout the US and the natural mechanisms of its synthesis remain a mystery. As such, the true perchlorate concentrations naturally present in the environment are still unknown making its regulation problematic. Because of its solubility and non-reactivity the fate and transport of perchlorate in the environment is primarily a function of microbial activity. In the last seven years more than forty specialized perchlorate respiring organisms have been identified and characterized. These dissimilatory perchlorate reducing bacteria (DPRB) are metabolically diverse and environmental populations tend to be dominated by two primary genotypes, the Dechloromonas and the Azospira species. As such, the majority of our understanding of this metabolism is based on these organisms. These organisms are readily found in soil and sedimentary environments and often associate with the rhizosphere. Recent research has demonstrated an accumulation of these organisms along plant roots suggesting their catabolism of root exudates and molecular studies has demonstrated their existence as endophytic infections of the stem and leaves of actively growing Brachypodium grass plants although their exact role under these conditions is unknown. These microorganisms are generally not nutritionally fastidious and vitamin supplementation is unnecessary for growth although molybdenum is a required trace element for perchlorate reduction. The Dechloromonas and Azospira species generally grow optimally at pH values near neutrality in freshwater environments. Even so, recent field studies have shown that related deep-branching members of these genera often predominate in sites of adverse pH or salinity with some species being capable of growth and perchlorate respiration at pH values as low as pH 5. Although studies have demonstrated microbial perchlorate reduction in salt brines as concentrated as 11% NaCl, to date no microorganism isolated has been demonstrated to grow by perchlorate respiration in salinities greater than 2%. The metabolism is negatively regulated by oxygen and nitrate. Preference for oxygen is observed even at low oxygen partial pressures while preference for nitrate is observed regardless of the nitrate to perchlorate ratio. Even when the DPRB are pre-grown anaerobically in perchlorate, nitrate is still preferentially reduced prior to perchlorate. Molecular studies reveal that electron acceptor utilization is regulated at the genetic level and is not simply a matter of chemical kinetics. Whole genome sequencing of Dechlormonas aromatica revealed a large number of signaling proteins, a high proportion of which were two component histidine kinase systems suggesting that this organism has exquisite sensitivity to its environment. This supported by the observed ability of this organism to sense, distinguish, and chemotax towards perchlorate or nitrate depending on the growth conditions. These studies demonstrate how the concerted efforts over the last decade have resulted in significant advances in our understanding of the geobiology of microorganisms capable of reductively transforming perchlorate into innocuous chloride. Several in-situ and ex-situ bioremediative processes have been engineered and many monitoring tools based on immunology, molecular biology, and stable isotope content are now available. As such, the rapid scientific response to this emerging contaminant offers great hope for its successful elimination from contaminated environments in the future. http://pmb.berkeley.edu/~coates/
H41K-03
Wet Deposition of Perchlorate Over the Continental United States
Natural perchlorate (ClO4-) has been detected in soil, vegetation, food products, and ground and drinking water supplies at various concentrations across the world. For almost a century natural perchlorate has been known to exist in Chilean nitrate deposits that are up to 16 million years old, and recent isotopic evidence has confirmed its source to be predominantly atmospheric. Although the source of natural perchlorate has been attributed to atmospheric deposition, there is almost no data available concerning the deposition rate of perchlorate from precipitation. This research effort, supported by SERDP, was designed to investigate the range of concentrations, and temporal and spatial variations in perchlorate deposition. Sub-samples of precipitation collected through the National Atmospheric Deposition program over a two year period were analyzed for perchlorate. Sample locations included 14 continental states, and Puerto Rico. Perchlorate has been detected (DL= 5 ng/L) in over 65 % of all samples tested with a mean value of 12.60 ± 13.60 ng/L and ranged from < 5 ng/L to 75 ng/L with higher concentrations predominantly in summer months and inland regions. The Cl- /ClO4- molar ratio varied over three orders of magnitude with few thousands inland to few millions near coastal regions. There were no significant correlations (r > 0.5) between ClO4- and other ions (Cl-, NO3-, SO4-2, Na+, K+, Ca+2, Mg+2, and NH4+). Results from this study will have important implications to the national perchlorate issue and may aid in explaining the occurrence of non-anthropogenic perchlorate being reported in arid and semi-arid areas.
H41K-04
Natural Perchlorate and Climate-An Imperfect Relationship
The distribution of natural perchlorate has a strong climatic link; this is not unexpected given that perchlorate forms in the upper atmosphere and is transported to the earth's surface through wet and dry precipitation. Occurrences of high levels of natural perchlorate in plants, soils, sediments, and some water tend to be correlated with varying degrees of aridity. The processes that are known to concentrate perchlorate are restricted to, or work more efficiently, in dry climates. Processes in more temperate climates, such as dissolution and mobilization, work against perchlorate accumulations. Studies to date have shown that plants and soils that occur in both arid and more temperate environments tend to contain more perchlorate in the arid settings. The association of natural perchlorate with dry climates can be found in both modern settings and in pre-industrial materials such as rodent middens and sediments that are thousands to millions of years old. Nevertheless, there are exceptions to this relationship; perchlorate has been found in unexpectedly high concentrations in plants, soils/sediments, and water where the climate would indicate relatively low concentrations and there is no identifiable source of anthropogenic perchlorate. Other natural processes seem to contribute to accumulations of perchlorate. In some places, unexpectedly high perchlorate levels are spatially associated with contemporaneous volcanism that may have contributed perchlorate. Other indirect evidence indicates that some plants may manufacture their own perchlorate under conditions that are not yet fully understood. As with most phenomena in nature, the occurrence and concentration of perchlorate in the environment is complex and that complexity offers many avenues for future research into areas as diverse as processes of formation and concentration and human exposure pathways.
H41K-05 INVITED
Isotopic Discrimination of Perchlorate Sources in Ground Water
Perchlorate has been detected in ground water and drinking water in many areas of the U.S. during the past decade. Sources of potential perchlorate enrichment in ground water include releases from past military activities, fireworks manufacture and display, fertilizer applications, discarded road flares, and local atmospheric deposition. Here we present analyses of stable isotopes (δ37Cl, δ18O, and Δ17O) of dissolved perchlorate, along with other supporting environmental tracer data, from selected occurrences in ground water in the U.S. The isotope data indicate that both synthetic and natural perchlorate are present in ground water, and that multiple sources are present locally in some areas. The sampled ground waters generally were oxic and the perchlorate isotopes generally were not affected substantially by biodegradation. In some areas, natural perchlorate, with Δ17O = +7 to +10 ‰, can be attributed to agricultural applications of atmospherically derived natural nitrate fertilizer imported from South America (Atacama Desert, Chile). In at least one agricultural area in New York, concentrations of perchlorate increase with depth and ground-water age, possibly because of decreasing application rates of Atacama nitrate fertilizer and(or) decreasing perchlorate concentrations in the imported fertilizer products in recent years.
H41K-06
Trace Perchlorate in Background Ground Water and Local Precipitation, Northern Rio Grande Basin, New Mexico
Perchlorate occurs at detectable concentrations of 0.07 to 0.45 parts per billion (ppb) in ground water of background quality within the northern Rio Grande basin, New Mexico. Ground-water samples were collected from 47 wells and springs near Los Alamos, Santa Fe, and Taos, New Mexico. Analytical methods consisted of liquid and ion chromatography-mass spectrometry mass spectrometry (LC/MS/MS and IC/MS/MS). An upper tolerance limit (mean plus two standard deviations) of 0.40 ppb was calculated from 184 analytical results for the background samples. Six distinguishable ground-water zones were sampled based on location, age, and hydrochemistry. In the Los Alamos area, ground water within the mountain-front and mountain-block region is mostly young or modern (less than 50 years). The regional aquifer including the White Rock Canyon springs are of sub-modern age (greater than 50 years). Tritium data from springs north of Taos indicate ground water of modern and sub-modern ages. Background perchlorate concentrations within the Los Alamos area were consistently higher than those measured in the Taos area. Ground water from the Taos area contains less perchlorate and has lower δ18O and δ2H values than ground water from the Los Alamos area. The elevation at which precipitation occurs with respect to recharge and/or the amount of evapotranspiration may play a role in perchlorate concentration in ground water. Natural variability, hydrogeology, and atmospheric inputs may also affect perchlorate concentration in ground water. A linear regression through perchlorate and chloride concentrations for all stations resulted in an r2 = 0. However, the r2 value of the Los Alamos regional aquifer for perchlorate versus chloride was 0.66. Thirteen precipitation samples were collected in the Los Alamos area. Results from eleven of these samples showed no perchlorate greater than 0.05 and 0.009 ppb, the method detection limit (MDL). Two precipitation samples analyzed using the IC/MS/MS method detected perchlorate at 0.0210 and 0.0099 ppb with an MDL of 0.0012 ppb. These results have application for establishing site-specific background values, delineating anthropogenic impacts to ground water, measuring natural attenuation and contaminant gradients, and may aid in the process of setting drinking-water standards.
H41K-07
Perchlorate in Turfgrass Systems, Suffolk County, Long Island, NY
Perchlorate concentrations in precipitation, grass clippings, and soil water were analyzed at nine turfgrass sites in Suffolk County, NY. The samples were collected monthly between June, 2006 and January, 2007. The soil water was collected from suction lysimeters at 100 cm depth. Four of these sites were treated with chemical fertilizer, three with organic fertilizer and two were not fertilized. Concentrations of ClO4 in grass clippings and soil water, at the sites treated with chemical fertilizer or not treated with fertilizer, are found to increase when spikes of ClO4 concentrations in precipitation are observed. We believe that the spikes in perchlorate in precipitation collected shortly after the Fourth of July are due to firework displays. The concentration of ClO4 in soil water are 1 to 3 times higher than the maximum perchlorate concentrations in precipitation, with maximum soil water concentrations ranging from 0.5 to 3.0 ppb. At the sites treated with organic fertilizer, grass clippings and soil water ClO4 concentrations increase after the fertilizer application in May. The organic fertilizer that was applied has nine mg ClO4 per kg (9,000 ppb). Soil water concentrations at the sites treated with organic fertilizer increase 100 to 300 times the maximum ClO4 concentration observed in precipitation, with maximum soil water concentrations ranging from 120 to 625 ppb. The increase in ClO4 concentrations in the soil water cannot be explained by evaporation alone since the Cl to ClO4 ratios decrease in the soil water relative to precipitation. This decrease in the Cl to ClO4 ratio suggests another source of perchlorate besides precipitation. We postulate that this additional source is associated with the decomposition of mulched grass left after mowing. Grass takes only a few weeks to decompose after mulching, thus providing a continuous source of perchlorate throughout the mowing season. The Cl to ClO4 ratio of the grass is unknown.
H41K-08
Perchlorate and Superfund Response to Uncertainty and the Geochemical Cycle
Perchlorate, a chemical that had been known both in nature and through synthesis since the nineteenth century, only emerged into the limelight as an environmental contaminant in 1997. US EPA's Superfund Program became involved in perchlorate issues in the late 1980s and early 1990s due to the chemical's presence mixed with other contaminants at cleanup sites. Relying largely on pharmaceutical studies primarily from the 1950s and 1960s, EPA scientists in 1992 made a provisional estimate of toxicity and estimated that about 4 micrograms per liter (parts per billion or ppb) in drinking water would be protective. "Uncertainty factors" were incorporated to address for several identified information gaps. Results of new animal and human studies funded by the Defense Department and industry in the late 1990s shifted the concern from affects on adults with unhealthy thyroids to the potential developmental health risks to infants and children. EPA's January, 2002, draft toxicity assessment was referred to a committee of the National Research Council. In January, 2005, this committee recommended a "reference dose" based primarily on human clinical data. Many decisions remain on interpretation of the scientific recommendations for regulatory applications. After California's 1997 development of an analytical method to detect perchlorate in water to 4 ppb, EPA and state officials quickly discovered this chemical at 10 Superfund sites in the Pacific Southwest Region and at more than 30 other locations in California, Arizona and Nevada. Even before current research on the potential for natural sources of this anion, reported detections of perchlorate were investigated with reasonable care and appropriate skepticism. A brief overview of the search for likely sources of perchlorate detected in California water supplies is presented from a regional Superfund perspective. Some are clearly anthropogenic and others may be unrelated to industrial or disposal practices. Currently, there is no Federal standard for perchlorate. In March, 2004, California established Public Health Goal of 6 ppb in drinking water and a drinking water standard may be promulgated by November 2007. Seven other states have advisory levels ranging from 1 to 51 ppb. http://www.epa.gov/fedfac/documents/perchlorate.htm