Perchlorate in groundwater: a synoptic survey of "pristine" sites in the coterminous United States

Perchlorate is widely used as an oxidant in solid rocket propellants and energetic applications, and it has frequently been detected in groundwaters at concentrations relevant to human health. The possibility of naturally occurring perchlorate has only recently received significant attention. Relying primarily on domestic, agricultural, and recreational wells, we utilized a network of volunteers to help collect 326 groundwater samples from across the coterminous United States. Care was taken to avoid known, USEPA-documented sites of perchlorate use or release, as well as perchlorate contamination due to disinfection using hypochlorite. Using IC-ESI-MS and a Cl18O4- internal standard, we achieved a method detection limit (MDL) of 40 ng/L perchlorate and a minimum reporting level (MRL) of 120 ng/L. Of the 326 samples, 147 (45%) were below the MDL, while 42 (13%) were between the MDL and the MRL. Of the 137 samples that could be quantified, most (109) contained < 1000 ng/L perchlorate; the remaining 28 samples contained from 1000 to 10400 ng/L. Our results support the notion that perchlorate occurs naturally in many groundwaters, but the unusually high concentrations (> 10000 ng/L) previously reported for the west-central Texas area appear to be anomalous. Perchlorate concentrations were positively correlated with nitrate levels (P < 0.001) but not with chloride concentrations. Opportunities exist for follow-up studies of perchlorate's origins using isotope forensics and for further elucidation of the role of atmospheric processes in the formation or transport of perchlorate.     

Rate and extent of aqueous perchlorate removal by iron surfaces

The rate and extent of perchlorate reduction on several types of iron metal was studied in batch and column reactors. Mass balances performed on the batch experiments indicate that perchlorate is initially sorbed to the iron surface, followed by a reduction to chloride. Perchlorate removal was proportional to the iron dosage in the batch reactors, with up to 66% removal in 336 h in the highest dosage system (1.25 g mL(-1)). Surface-normalized reaction rates among three commercial sources of iron filings were similar for acid-washed samples. The most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial pH values. Surface mediation of the reaction is supported by the absence of reduction in batch experiments with soluble Fe2+ and also by the similarity in specific reaction rate constants (kSA) determined for three different iron types. Elevated soluble chloride concentrations significantly inhibited perchlorate reduction, and lower removal rates were observed for iron samples with higher amounts of background chloride contamination. Perchlorate reduction was not observed on electrolytic sources of iron or on a mixed-phase oxide (Fe3O4), suggesting that the reactive iron phase is neither pure zerovalent iron nor the mixed oxide alone. A mixed valence iron hydr(oxide) coating or a sorbed Fe2+ surface complex represent the most likely sites for the reaction. The observed reaction rates are too slow for immediate use in remediation system design, but the findings may provide a basis for future development of cost-effective abiotic perchlorate removal techniques.     

Surveillance of perchlorate level in wine, seafood, polished rice, milk, powdered milk and yogurt

Perchlorate, which may be naturally occurring or artificial in origin, inhibits iodide uptake into the thyroid gland and disturbs thyroid function. In order to investigate perchlorate contamination in foods in Japan, perchlorate levels in 28 wine samples, 20 seafood samples, 10 polished rice samples, 30 milk (include whole milk, composition modified milk, low fat milk, processed milk, milk drink) samples, 10 powdered milk samples and 10 yogurt samples were measured. Perchlorate was found in all wine, milk, powdered milk and yogurt samples tested. Perchlorate levels ranged from 0.2 ng/g to 103 ng/g in wine samples, from 2 ng/g to 11 ng/g in milk samples, from 3 ng/g to 35 ng/g in powdered milk samples, and from 2 ng/g to 11 ng/g in yogurt samples. Perchlorate levels in the seafood samples were under the LOQ (0.8 ng/g) in 8 samples and ranged from 0.8 ng/g to 72 ng/g in 12 samples. In all polished rice samples, perchlorate level was under the LOQ (1.0 ng/g).     

Perchlorate in the United States. Analysis of relative source contributions to the food chain

Perchlorate has been considered by some a potential threat to human health, especially to developing infants and children because it may inhibit iodide uptake by the sodium iodide symporter (NIS) of the thyroid. In the United States, during the last several decades, environmental perchlorate has had three recognized sources stemming from (a) its use as an oxidizer (including in rocket propellants), (b) its presence in Chilean nitrate fertilizer (CNF), and (c) natural production. An analysis of the relative source strengths and how they may influence entry into the food chain has not been conducted. Averaged over the last --60 years, we estimate that the source strengths have been (a) 10.6, (b) 0.75, and (c) 0.13-0.64 Gg/y for the United States as a whole. Of this, while (b) and (c) represent actual dispersed amounts, the figure in (a) is the amount of perchlorate produced and only a fraction (f) of it has been dispersed and often in a more localized fashion. In addition, dispersal of (b) has taken place only over agricultural land. Considering that the total land area in the United States is 5.5 x the arable land area, in terms of incorporation into the food chain,the figure cited in (b) has a proportionately greater impact. Most estimates of fwill thus suggest that over the considered period, the contribution of CNF to incorporation of perchlorate in the food chain has likely been comparable to oxidizer perchlorate, with natural production being a lesser source. Fireworks presently constitute a potentially important source of increasing importance but a quantitative impact cannotyet be assessed.     

Uptake, elimination, and relative distribution of perchlorate in various tissues of channel catfish

This study was undertaken to determine the kinetics of uptake and elimination of perchlorate in channel catfish, Ictalurus punctatus. Perchlorate--an oxidizer used in solid fuel rockets, fireworks, and illuminating munitions--has been shown to effect thyroid function, causing hormone disruption and potential perturbations of metabolic activities. For the uptake study, catfish were exposed to 100 mg/L sodium perchlorate for 12 h to 5 d in the laboratory. Perchlorate in tissues was analyzed using ion chromatography. The highest perchlorate concentrations were found in the head and fillet, indicating that these tissues are the most important tissues to analyze when determining perchlorate uptake into large fish. To calculate uptake and elimination rate constants for fillet, gills, G-I tract, liver, and head, fish were exposed to 100 ppm sodium perchlorate for 5 days, and allowed to depurate in clean water for up to 20 days. The animals rapidly eliminated the perchlorate accumulated showing the highest elimination in fillet (Ke = 1.67 day(-1)) and lowest elimination in liver (Ke = 0.79 day(-1)).     

Natural perchlorate has a unique oxygen isotope signature

Perchlorate is known to be a minor component of the hyperarid Atacama Desert salts, and its origin has long been a subject of speculation. Here we report the first measurement of the triple-oxygen isotope ratios (18O/16O and 17O/16O) for both man-made perchlorate from commercial sources and natural perchlorate extracted from Atacama soils. We found that the delta 18O values (i.e., normalized 18O/ 16O ratios) of man-made perchlorate were at -18.4+/-1.2%, whereas natural perchlorate has a variable delta 18O value, ranging from -4.5% to -24.8%. The delta 18O and delta 17O values followed the bulk Earth's oxygen isotope fractionation line for man-made perchlorate, but all Atacama perchlorates deviated from this line, with a distinctly large and positive 170 anomaly ranging from +4.2% to +9.6%. These findings provide a tool for the identification and forensics of perchlorate contamination in the environment. Additionally, they confirm an early speculation that the oxidation of volatile chlorine by 03 and the formation of HClO4 can be a sink (albeit a minor one) for atmospheric chlorine.     

Widespread presence of naturally occurring perchlorate in high plains of Texas and New Mexico

Perchlorate (CLO4-) occurrence in groundwater has previously been linked to industrial releases and the historic use of Chilean nitrate fertilizers. However, recently a number of occurrences have been identified for which there is no obvious anthropogenic source. Groundwater from an area of 155,000 km2 in 56 counties in northwest Texas and eastern New Mexico is impacted bythe presence of ClO4-. Concentrations were generally low (<4 ppb), although some areas are impacted by concentrations up to 200 ppb. ClO4- distribution is not related to well type (public water system, domestic, agricultural, or water-table monitoring) or aquifer (Ogallala, Edward Trinity High Plains, Edwards Trinity Plateau, Seymour, or Cenozoic). Results from vertically nested wells strongly indicate a surface source. The source of ClO4- appears to most likely be atmospheric deposition. Evidence supporting this hypothesis primarily relates to the presence of ClO4- in tritium-free older water, the lack of relation between land use and concentration distribution, the inability of potential anthropogenic sources to account for the estimated mass of ClO4-, and the positive relationship between conserved anions (e.g., IO3-, Cl-, SO4(-2)) and ClO4-. The ClO4- distribution appears to be mainly related to evaporative concentration and unsaturated transport. This process has led to higher ClO4- and other ion concentrations in groundwater where the water table is relatively shallow, and in areas with lower saturated thickness. Irrigation may have accelerated this process in some areas by increasing the transport of accumulated salts and by increasing the number of evaporative cycles. Results from this study highlight the potential for ClO4- to impact groundwater in arid and semi-arid areas through long-term atmospheric deposition.     

A steady-state biofilm model for simultaneous reduction of nitrate and perchlorate, part 2: parameter optimization and results and discussion

Part 1 of this work developed a steady-state, multispecies biofilm model for simultaneous reduction of nitrate and perchlorate in the H(2)-based membrane biofilm reactor (MBfR) and presented a novel method to solve it. In Part 2, the half-maximum-rate concentrations and inhibition coefficients of nitrate and perchlorate are optimized by fitting data from experiments with different combinations of influent nitrate and perchlorate concentrations. The model with optimized parameters is used to quantitatively and systematically explain how three important operating conditions (nitrate loading, perchlorate loading, and H(2) pressure) affect nitrate and perchlorate reduction and biomass distribution in these reducing biofilms. Perchlorate reduction and accumulation of perchlorate-reducing bacteria (PRB) in the biofilm are affected by four promotion or inhibition mechanisms: simultaneous use of nitrate and perchlorate by PRB and competition for H(2), the same resources in PRB, and space in a biofilm. For the hydrogen pressure evaluated experimentally, a low nitrate loading (<0.1 g N/m(2)-d) slightly promotes perchlorate removal, because of the beneficial effect from PRB using both acceptors. However, a nitrate loading of >0.6 g N/m(2)-d begins to inhibit perchlorate removal, as the competition effects become dominant.     

Perchlorate: water and infant formulae contamination in France and risk assessment in infants

Perchlorate ions ClO₄^–, known to inhibit competitively the uptake of iodine by the thyroid, have been detected in drinking water in France as well as in infant formulae. A tolerable daily intake (TDI) has been established at 0.7 µg kg^–1 bw day^–1 based on the inhibition of iodine uptake. Due to this mechanism of action, the iodine status could strongly influence the biological effect of perchlorate. Perchlorate concentrations in water and infant formulae were measured and the exposure of children under 6 months of age calculated. It appeared that the TDI could be exceeded in some children. As the iodine status is not optimal within the entire French population, there appears to be a need to clarify the sources of perchlorate ultimately to decrease exposure.     

食品中氯酸盐和高氯酸盐分析方法综述

高氯酸盐是一种新型环境污染物, 具有高扩散性和持久性, 它可以干扰人体甲状腺的正常功能, 从而影响人体生长发育。食品中氯酸盐和高氯酸盐的测定对于保证人体健康有重要意义, 也是目前国际国内研究的热点问题。目前国内外已有多种分析方法实现了对氯酸盐和高氯酸盐定量检测, 主要包括离子色谱法、离子色谱-质谱法、液相色谱-质谱法等。本文对近年来食品中氯酸盐和高氯酸盐分析检测方法的研究情况进行整理, 对各种分析技术的原理、应用现状及国内外相关标准进行探讨, 对其优缺点进行了比较, 并提出了前景展望。     

Perchlorate exposure and dose estimates in infants

Perchlorate is a naturally occurring inorganic anion used as a component of solid rocket fuel, explosives, and pyrotechnics. Sufficiently high perchlorate intakes can modify thyroid function by competitively inhibiting iodide uptake in adults; however, little is known about perchlorate exposure and health effects in infants. Food intake models predict that infants have higher perchlorate exposure doses than adults. For this reason, we measured perchlorate and related anions (nitrate, thiocyanate, and iodide) in 206 urine samples from 92 infants ages 1-377 days and calculated perchlorate intake dose for this sample of infants. The median estimated exposure dose for this sample of infants was 0.160 μg/kg/day. Of the 205 individual dose estimates, 9% exceeded the reference dose of 0.7 μg/kg/day; 6% of infants providing multiple samples had multiple perchlorate dose estimates above the reference dose. Estimated exposure dose differed by feeding method: breast-fed infants had a higher perchlorate exposure dose (geometric mean 0.220 μg/kg/day) than infants consuming cow milk-based formula (geometric mean 0.103 μg/kg/day, p < 0.0001) or soy-based formula (geometric mean 0.027 μg/kg/day, p < 0.0001), consistent with dose estimates based on dietary intake data. The ability of perchlorate to block adequate iodide uptake by the thyroid may have been reduced by the iodine-sufficient status of the infants studied (median urinary iodide 125 μg/L). Further research is needed to see whether these perchlorate intake doses lead to any health effects.     

Perchlorate and iodide in dairy and breast milk

Perchlorate inhibits iodide uptake and may impair thyroid and neurodevelopment in infants. Recently, we unambiguously identified the presence of perchlorate in all seven brands of dairy milk randomly purchased from grocery stores in Lubbock, TX. How widespread is perchlorate in milk? Perchlorate in 47 dairy milk samples from 11 states and in 36 human milk samples from 18 states were measured. Iodide was also measured in a number of the samples. Perchlorate was detectable in 81 of 82 samples. The dairy and breast milk means were, respectively, 2.0 and 10.5 microg/L with the corresponding maximum values of 11 and 92 microg/L. Perchlorate is present in virtually all milk samples, the average concentration in breast milk is five times higher than in dairy milk. Although the number of available measurements are few at this point, for breast milk samples with a perchlorate content greater than 10 microg/L, the iodide content is linearly correlated with the inverse of the perchlorate concentration with a r2 of >0.9 (n = 6). The presence of perchlorate in the milk lowers the iodide content and may impair thyroid development in infants. On the basis of limited available data, iodide levels in breast milk may be significantly lower than it was two decades ago. Recommended iodine intake by pregnant and lactating women may need to be revised upward.     

Perchlorate behavior in a municipal lake following fireworks displays

Perchlorate salts of potassium and ammonium are the primary oxidants in pyrotechnic mixtures, yet insufficient information is available regarding the relationship between fireworks displays and the environmental occurrence of perchlorate. Here we document changes in perchlorate concentrations in surface water adjacent to a site of fireworks displays from 2004 to 2006. Preceding fireworks displays, perchlorate concentrations in surface water ranged from 0.005 to 0.081 microg/L, with a mean value of 0.043 microg/L. Within 14 h after the fireworks, perchlorate concentrations spiked to values ranging from 24 to 1028x the mean baseline value. A maximum perchlorate concentration of 44.2 microg/L was determined following the July 4th event in 2006. After the fireworks displays, perchlorate concentrations decreased toward the background level within 20 to 80 days, with the rate of attenuation correlating to surface water temperature. Adsorption tests indicate that sediments underlying the water column have limited (< 100 nmol/g) capacity to remove perchlorate via chemical adsorption. Microcosms showed comparatively rapid intrinsic perchlorate degradation in the absence of nitrate consistent with the observed disappearance of perchlorate from the study site. This suggests that at sites with appropriate biogeochemical conditions, natural attenuation may be an important factor affecting the fate of perchlorate following fireworks displays.     

A bioassay for the detection of perchlorate in the ppb range

A bioassay for the determination of ppb (μg·L(-1)) concentrations of perchlorate has been developed and is described herein. The assay uses the enzyme perchlorate reductase (PR) from the perchlorate-reducing organism Dechloromonas agitata in purified and partially purified forms to detect perchlorate. The redox active dye phenazine methosulfate (PMS) is shown to efficiently shuttle electrons to PR from NADH. Perchlorate can be determined indirectly by monitoring NADH oxidization by PR. To lower the detection limit, we have shown that perchlorate can be concentrated on a solid-phase extraction (SPE) column that is pretreated with the cation decyltrimethylammonium bromide (DTAB). Perchlorate is eluted from these columns with a solution of 2 M NaCl and 200 mM morpholine propane sulfonic acid (MOPS, pH 12.5). By washing these columns with 15 mL of 2.5 mM DTAB and 15% acetone, contaminating ions, such as chlorate and nitrate, are removed without affecting the bioassay. Because of the effect of complex matrices on the SPE columns, the method of standard additions is used to analyze tap water and groundwater samples. The efficacy of the developed bioassay was demonstrated by analyzing samples from 2-17000 ppb in deionized lab water, tap water, and contaminated groundwater.     

Risk assessment of dietary exposure to perchlorate for the Austrian population

Perchlorate is frequently found as contaminant in a variety of food. Based on analytical data of perchlorate occurrence in food products from the Austrian market, this study calculated dietary perchlorate exposure of the Austrian population for the three age classes of adults, children and infants. Furthermore, a detailed risk assessment was conducted based on the tolerable daily intake (TDI) of 0.3 µg/kg body weight/day, established by the European Food Safety Authority in 2014. Calculations of a scenario of average food consumption did not indicate elevated health risks by dietary perchlorate uptake. Exposure estimates reached only 12%, 26% and 24% of the TDI for adults, children and infants, respectively. However, in a scenario of high consumption, the TDI was exceeded by all age classes with 132%, 161% and 156%. The major cause for this exceedance is the comparatively high perchlorate contamination of spinach, but also other leaf vegetables, legumes and pineapples, leading to elevated exposure of high consumers. Our calculations reveal that the current provisional intra-Union trade reference level for perchlorate in spinach of 0.2 mg/kg, advocated by the European Commission, is not sufficient to protect high consumers against possible health risks. In order to reduce health risks to a tolerable level for all consumers, lowering of the regulatory maximum perchlorate concentrations is indicated. Moreover, a generally diversified diet can also counteract excessive exposure to perchlorate as well as to other harmful food contaminants.     

Trace analysis of bromate, chlorate, iodate, and perchlorate in natural and bottled waters

A simple and rapid method has been developed to simultaneously measure sub-microg/L quantities of the oxyhalide anions bromate, chlorate, iodate, and perchlorate in water samples. Water samples (10 mL) are passed through barium and hydronium cartridges to remove sulfate and carbonate, respectively. The method utilizes the direct injection of 10 microL volumes of water samples into a liquid chromatography-tandem triple-quadrupole mass spectrometry (LC-MS/MS) system. Ionization is accomplished using electrospray ionization in negative mode. The method detection limits were 0.021 microg/L for perchlorate, 0.045 microg/L for bromate, 0.070 microg/L for iodate, and 0.045 microg/L for chlorate anions in water. The LC-MS/MS method described here was compared to established EPA methods 300.1 and 317.1 for bromate analysis and EPA method 314.0 for perchlorate analysis. Samples collected from sites with known contamination were split and sent to certified laboratories utilizing EPA methods for bromate and perchlorate analysis. At concentrations above the reporting limits for EPA methods, the method described here was always within 20% of the established methods, and generally within 10%. Twenty-one commercially available bottled waters were analyzed for oxyhalides. The majority of bottled waters contained detectable levels of oxyhalides, with perchlorate < or = 0.74 microg/L, bromate < or = 76 microg/L, iodate < or = 25 microg/ L, and chlorate < or = 5.8 microg/L. Perchlorate, iodate, and chlorate were detectable in nearly all natural waters tested, while bromate was only detected in treated waters. Perchlorate was found in several rivers and reservoirs where itwas not found previously using EPA 314.0 (reporting limit of 4 microg/L). This method was also applied to common detergents used for cleaning laboratory glassware and equipmentto evaluate the potential for sample contamination. Only chlorate appeared as a major oxyhalide in the detergents evaluated, with concentrations up to 517 microg/g. Drinking water treatment plants were also evaluated using this method. Significant formations of chlorate and bromate are demonstrated from hypochlorite generation and ozonation. From the limited data set provided here, it appears that perchlorate is a ubiquitous contaminant of natural waters at trace levels.     

Perchlorate reduction by autotrophic bacteria in the presence of zero-valent iron

A series of batch experiments were performed to study the combination of zero-valent iron (ZVI) with perchlorate-reducing microorganisms (PRMs) to remove perchlorate from groundwater. In this method, H2 produced during the process of iron corrosion by water is used by PRMs as an electron donor to reduce perchlorate to chloride. Perchlorate degradation rates followed Monod kinetics, with a normalized maximum utilization rate (rmax) of 9200 microg g(-1) (dry wt) h(-1) and a half-velocity constant (Ks) of 8900 microg L(-1). The overall rate of perchlorate reduction was affected by the biomass density within the system. An increase in the OD600 from 0.025 to 0.08 led to a corresponding 4-fold increase of perchlorate reduction rate. PRM adaptation to the local environment and initiation of perchlorate reduction was rapid under neutral pH conditions. At the initial OD600 of 0.015, perchlorate reduction followed pseudo-first-order reaction rates with constants of 0.059 and 0.033 h(-1) at initial pH 7 and 8, respectively. Once perchlorate reduction was established, the bioreductive process was insensitive to the increases of pH from near neutral to 9.0. In the presence of nitrate, perchlorate reduction rate was reduced, but not inhibited completely.     

Effects of genotype and transpiration rate on the uptake and accumulation of perchlorate (ClO4-) in lettuce

Although evidence of perchlorate accumulation in plants exists, there is a scarcity of information concerning the key factors and mechanisms involved. To ascertain whether genotypic variation in perchlorate accumulation occurs within lettuce, hydroponic plant uptake experiments were conducted with five types of lettuce (Lactuca sativa L.), which were grown to market size atthree perchlorate (ClO4-) concentrations (1, 5, or 10 microg/L). Perchlorate accumulated in the leafy tissues to varying amounts, ranging from 4 to 192 microg/kg fresh weight (FW), and the ranking of perchlorate accumulation was crisphead > butter head > romaine > red leaf > green leaf. The effect of transpiration rate on perchlorate accumulation was further examined using crisphead, butter head, and green leaf lettuce. By growing lettuce in controlled-environment chambers with two climatic regimes, "cloudy, humid, cool" (80% RH, 18/15 degrees C, 250 micromol/m2s photosynthetic photon flux density (PPFD)) and "sunny, dry, warm" (approximately 50% RH, 28/18 degrees C, 500 micromol/m2s PPFD), up to 2.7-fold differences in transpiration rates were achieved. Across all three genotypes, the plants that transpired more water accumulated more perchlorate on a whole-head basis; however, the effect of transpiration rate on perchlorate accumulation was not as great as expected. Despite 2.0-2.7-fold differences in transpiration rate, there were only 1.2-2.0-fold differences in perchlorate accumulation. In addition to whole-head analysis, plants were sectioned into inner, middle, and outer leaves and processed separately. Overall, the ranking of perchlorate accumulation was outer leaves > middle leaves > inner leaves. Transpiration rate has a clear effect on perchlorate accumulation in lettuce, but other factors are influential and deserve exploration.     

Chlorine isotope fractionation during microbial reduction of perchlorate

Perchlorate contamination of surface water and groundwater is an emerging public health problem that has adversely affected the drinking water supplies of millions of people in the western United States. Microbial reduction has shown promise as a cost-effective means for in situ bioremediation of perchlorate-contaminated water. Measurements of stable isotope ratios of light elements (H, C, N, O, S, Cl) can often be used to distinguish biodegradation of organic and inorganic molecules from abiotic loss mechanisms such as adsorption, dispersion, or volatilization because of the relatively large kinetic isotope effects accompanying biodegradation. We quantified chlorine isotope fractionation during perchlorate biodegradation by a common perchlorate-reducing bacterium, Dechlorosoma suillum, initially isolated from a perchlorate-contaminated groundwater source in southern California. The values of the chlorine isotopic fractionation factor alpha derived from two microcosm experiments were alpha = 0.9834 +/- 0.0001 (R2 = 0.9999) and alpha = 0.9871 +/- 0.0008 (R2 = 0.9832). These alpha values indicate that the rate of the 35ClO4 reduction is approximately 1.3-1.7% faster than that of the 37ClO4 reduction. This relatively large kinetic isotope effect indicates that chlorine isotope analysis provides a sensitive technique by which to document in situ bioremediation of perchlorate in groundwater.     

Thyrotoxicity of sodium arsenate, sodium perchlorate, and their mixture in zebrafish Danio rerio

Both perchlorate and arsenate are environmental contaminants. Perchlorate is a definitive thyroid disruptor, and arsenic may disrupt thyroid homeostasis via multiple pathways. To evaluate the effects of sodium perchlorate and sodium arsenate on thyroid function and possible interactions between them, zebrafish (Danio rerio) were exposed to sodium perchlorate (10 and 100 mg/L), sodium arsenate (1 and 10 mg/L), and the mixture sodium perchlorate + sodium arsenate (10 + 1 and 100 + 10 mg/ L) for up to 90 days. At day 10, 30, 60, and 90, fish were sampled and analyzed forthyroid histopathological end points including follicular cell height, follicle size, colloid size, colloid depletion, hyperplasia, and angiogenesis. Effects on epithelial cell height (hypertrophy) were seen as early as 10 days after exposure. Perchlorate induced changes in all parameters staring at 30 days of exposure. Prolonged perchlorate exposure induced angiogenesis, a relatively new marker of thyroid disruption. Sodium arsenate was less effective than sodium perchlorate in causing thyroid histopathologies, but transient responses were seen for hypertrophy, hyperplasia, and colloid depletion (% colloid). This is the first report of arsenate-induced effects on thyroid histopathology. However, because statistically significant effects were not consistently seen in all end points, evidence for arsenate as a thyroid disruptor remains equivocal. In general, the sensitivity of the following histopathological indicators for indicating thyroid perturbations is, in descending order: follicular cell height > percent of colloid area/follicle area > colloid area/follicular cell height > hyperplasia > angiogenesis > colloid area >follicle area = fish growth.