Sites In Arizona Research Articles

Crab Supernova Rock Art

“Star” and crescent combinations in rock art in the American Southwest were first interpreted in 1955 as eyewitness depictions of the 1054 AD supernova explosion that produced the Crab nebula. While the Crab nebula is visible only telescopically, the event that generated it was brilliant, and for a time, only the sun and moon were brighter. Additional Crab supernova candidates in California and Southwest rock art were suggested 20 years later, and they included Chaco Canyon’s Penasco Blanco pictograph panel, which became the poster child for Crab supernova rock art and is now called “Supernova” on signage at the site. By 1979, a list of 21 Crab supernova rock art sites was assembled, and the inventory has continued to expand more slowly since then. This critical review of the supernova interpretation of star/crescent rock art, the product of 35 years of fieldwork, required an independent re-examination of all of the primary sites in person. That enterprise has already demonstrated that the Tenabo, New Mexico panel does not illustrate the Crab supernova and that the two Arizona sites on which the entire supernova rock art premise is based (White Mesa and “Navaho Canyon”) are unlikely records of the event. This detailed evaluation of the primary proposed star/crescent images indicates none is a satisfactory portrayal of the striking 1054 AD supernova.

Post-fire ponderosa pine regeneration with and without planting in Arizona and New Mexico

Forest fires are increasing in size and severity globally, yet the roles of natural and artificial regeneration in promoting forest recovery are poorly understood. Post-fire regeneration of ponderosa pine (Pinus ponderosa, Lawson and C. Lawson) in the southwestern U.S. is slow, episodic, and difficult to predict. Planting of ponderosa pine after wildfire may accelerate reforestation, but little is known about survival of plantings and the amount of post-fire natural regeneration. We compared ponderosa pine regeneration between paired planted and unplanted plots at eight sites in Arizona and New Mexico that recently (2002–2005) burned severely. Two sites had no natural regeneration and no survival of planted seedlings. Seedling presence increased with number of years since burning across all plots, was positively associated with forb and litter cover on planted plots, and was positively associated with litter cover on unplanted plots. Survival of planted seedlings, measured five to eight years after planting, averaged 25% (SE=8) and varied from 0% to 70% across sites resulting in seedling densities of 0–521 trees ha−1. Based on a projected 44% survival of seedlings to mature trees and target density of mature trees determined by historical range of variability and ecological restoration principles, four of eight sites have a seedling density in planted plots (125–240ha−1) that will produce a density of mature trees (55–106ha−1) close to desired levels, whereas seedlings are currently deficient at three planted sites, and in surplus at one site, which had abundant natural regeneration. Natural regeneration in unplanted plots during the first decade after burning produced seedling densities inconsistent with desired numbers of mature trees. Natural regeneration in unplanted plots produced less than 33 seedlings ha−1 at seven of eight sites, but produced 1433 seedlings ha−1 at one high-elevation site that supported a more mesic vegetation community before burning than the other sites. Our results show that current practices for planting ponderosa pine after severe fires in Arizona and New Mexico produce desired numbers of seedlings in approximately half of all projects, whereas natural regeneration rarely does within the first decade after burning.

Mechanisms of piñon pine mortality after severe drought: a retrospective study of mature trees.

Conifers have incurred high mortality during recent global-change-type drought(s) in the western USA. Mechanisms of drought-related tree mortality need to be resolved to support predictions of the impacts of future increases in aridity on vegetation. Hydraulic failure, carbon starvation and lethal biotic agents are three potentially interrelated mechanisms of tree mortality during drought. Our study compared a suite of measurements related to these mechanisms between 49 mature piñon pine (Pinus edulis Engelm.) trees that survived severe drought in 2002 (live trees) and 49 trees that died during the drought (dead trees) over three sites in Arizona and New Mexico. Results were consistent over all sites indicating common mortality mechanisms over a wide region rather than site-specific mechanisms. We found evidence for an interactive role of hydraulic failure, carbon starvation and biotic agents in tree death. For the decade prior to the mortality event, dead trees had twofold greater sapwood cavitation based on frequency of aspirated tracheid pits observed with scanning electron microscopy (SEM), smaller inter-tracheid pit diameter measured by SEM, greater diffusional constraints to photosynthesis based on higher wood δ(13)C, smaller xylem resin ducts, lower radial growth and more bark beetle (Coleoptera: Curculionidae) attacks than live trees. Results suggest that sapwood cavitation, low carbon assimilation and low resin defense predispose piñon pine trees to bark beetle attacks and mortality during severe drought. Our novel approach is an important step forward to yield new insights into how trees die via retrospective analysis.

Geographic variation in the thermal biology of a widespread Sonoran Desert arachnid, Centruroides sculpturatus (Arachnida: Scorpiones)

Environmental temperatures can significantly influence the behavior and physiology of terrestrial ectotherms. Small-bodied terrestrial ectotherms can moderate their body temperatures behaviorally via thermoregulation; however, favorable thermal refuges may be limited across heterogeneous landscapes. In such cases, differences in the thermal environment may generate variation in preferred body temperatures among disparate populations. We tested whether geographic variation in preferred body temperatures existed for the Arizona bark scorpion Centruroides sculpturatus, an arachnid widely distributed across the Sonoran Desert. We predicted that geographic variation in thermal preference would exist between populations from a xeric, low-elevation site in western Arizona (Quartzsite) and a cooler, high-elevation site in eastern Arizona (Pinaleño Mountains). We found that scorpions from the Pinaleño Mountains were smaller in body size and exhibited significantly warmer diurnal body temperatures compared to scorpions from Quartzsite. However, no significant difference was detected in the preferred nocturnal temperatures of scorpions from either locality. Scorpions from the Pinaleño Mountains may have shifted their thermoregulatory patterns and selected warmer temperatures during the day as a way to maximize their exposure to favorable temperatures. Less time with mean temperatures within the preferred range of body temperatures may have also lowered metabolic rates of C. sculpturatus in the Pinaleño Mountains, resulting in their smaller female body sizes. The ability of C. sculpturatus to adapt to different thermal regimes may reflect variation in a life history trait which has allowed them to thrive and expand their distribution across the American Southwest.

Toxic metal(loid) speciation during weathering of iron sulfide mine tailings under semi-arid climate

Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3–2.3).Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3mmolkg−1 (3080, 2200, and 2570mgkg−1), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2-fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25–54cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0–35cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO4·nH2O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.

The Ground-Based Absolute Radiometric Calibration of Landsat 8 OLI

This paper presents the vicarious calibration results of Landsat 8 OLI that were obtained using the reflectance-based approach at test sites in Nevada, California, Arizona, and South Dakota, USA. Additional data were obtained using the Radiometric Calibration Test Site, which is a suite of instruments located at Railroad Valley, Nevada, USA. The results for the top-of-atmosphere spectral radiance show an average difference of −2.7, −0.8, 1.5, 2.0, 0.0, 3.6, 5.8, and 0.7% in OLI bands 1–8 as compared to an average of all of the ground-based measurements. The top-of-atmosphere spectral reflectance shows an average difference of 1.6, 1.3, 2.0, 1.9, 0.9, 2.1, 3.1, and 2.1% from the ground-based measurements. Except for OLI band 7, the spectral radiance results are generally within ±5% of the design specifications, and the reflectance results are generally within ±3% of the design specifications. The results from the data collected during the tandem Landsat 7 and 8 flight in March 2013 indicate that ETM+ and OLI agree to each other to within ±2% in similar bands in top-of-atmosphere spectral radiance, and to within ±4% in top-of-atmosphere spectral reflectance.

Persistence of a Groundwater Contaminant Plume after Hydraulic Source Containment at a Chlorinated-Solvent Contaminated Site.

The objective of this study was to characterize the behavior of a groundwater contaminant (trichloroethene) plume after implementation of a source-containment operation at a site in Arizona. The plume resides in a quasi three-layer system comprising a sand/gravel unit bounded on the top and bottom by relatively thick silty clayey layers. The system was monitored for 60 months beginning at start-up in 2007 to measure the change in contaminant concentrations within the plume, the change in plume area, the mass of contaminant removed, and the integrated contaminant mass discharge. Concentrations of trichloroethene in groundwater pumped from the plume extraction wells have declined significantly over the course of operation, as have concentrations for groundwater sampled from 40 monitoring wells located within the plume. The total contaminant mass discharge associated with operation of the plume extraction wells peaked at 0.23 kg/d, decreased significantly within one year, and thereafter began an asymptotic decline to a current value of approximately 0.03 kg/d. Despite an 87% reduction in contaminant mass and a comparable 87% reduction in contaminant mass discharge for the plume, the spatial area encompassed by the plume has decreased by only approximately 50%. This is much less than would be anticipated based on ideal flushing and mass-removal behavior. Simulations produced with a simplified 3-D numerical model matched reasonably well to the measured data. The results of the study suggest that permeability heterogeneity, back diffusion, hydraulic factors associated with the specific well field system, and residual discharge from the source zone are all contributing to the observed persistence of the plume, as well as the asymptotic behavior currently observed for mass removal and for the reduction in contaminant mass discharge.

Ants as a powerful biotic agent of olivine and plagioclase dissolution

The biotic enhancement of Ca-Mg silicate weathering has helped maintain Earth's habit- ability over geological time scales by assisting in the gradual drawdown of atmospheric CO 2 . 25 years of in-situ measurements of Ca-Mg silicate mineral dissolution by ants, termites, root mats, bare ground, and a control reveals ants to be one of the most powerful biotic weathering agents yet recognized. Six sites in Arizona and Texas (USA) indicate that eight different ant species enhance mineral dissolution by ~50×-300× over controls. A comparison of extracted soil at a 50 cm depth in ant colonies and adjacent bare ground shows a gradual accumulation of CaCO 3 content for all eight ant species over 25 yr. Ants, thus, have potential to provide clues on how to enhance contemporary carbon sequestration efforts to transform Ca-Mg sili- cates and CO 2 into carbonate. Given that ants underwent a great diversification and biomass expansion over the Cenozoic, a speculative implication of this research is that ant enhance- ment of Ca-Mg silicate dissolution might have been an influence on Cenozoic cooling.

Size-resolved dust and aerosol contaminants associated with copper and lead smelting emissions: Implications for emission management and human health

Mining operations, including crushing, grinding, smelting, refining, and tailings management, are a significant source of airborne metal and metalloid contaminants such as As, Pb and other potentially toxic elements. In this work, we show that size-resolved concentrations of As and Pb generally follow a bimodal distribution with the majority of contaminants in the fine size fraction (<1μm) around mining activities that include smelting operations at various sites in Australia and Arizona. This evidence suggests that contaminated fine particles (<1μm) are the result of vapor condensation and coagulation from smelting operations while coarse particles are most likely the result of windblown dust from contaminated mine tailings and fugitive emissions from crushing and grinding activities. These results on the size distribution of contaminants around mining operations are reported to demonstrate the ubiquitous nature of this phenomenon so that more effective emission management and practices that minimize health risks associated with metal extraction and processing can be developed.

Surficial weathering of iron sulfide mine tailings under semi-arid climate

Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering in a semi-arid climate at an EPA Superfund Site in central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2m of tailings. Despite nearly invariant Fe and S concentration with depth (130–140 and 100–120gkg−1, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in samples with the lowest pH, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.

Input of terrestrial invertebrates to streams during monsoon-related flash floods in the southwestern United States

Annual monsoons produce violent thunderstorms in the southwestern USA and can result in significant overland flow and flash floods in stream channels, washing many invertebrates into the streams. We identified >140 taxa of terrestrial invertebrates in samples collected from receding flows in ephemeral stream channels after monsoon-related flash floods at 14 sites in Arizona and New Mexico. Relative to the number of aquatic organisms, proportions of terrestrial individuals and terrestrial taxa were initially high but decreased over time. The initial flush of water during the flows appeared to be very destructive, such that the average organism was 40% intact 24 h after waters began to recede. On the second day, the average individual was 90% intact, but the proportion of intact organisms declined over time. These results suggest high inputs of terrestrial invertebrates to these stream ecosystems during the monsoon.

An elusive ectomycorrhizal fungus reveals itself: a new species of Geopora (Pyronemataceae) associated with Pinus edulis

Species of the genus Geopora are important ectomycorrhizal associates that can dominate the communities of some plant taxa, such as pinyon pine (Pinus edulis), a widespread tree of the western United States. Several members of the genus Geopora are known only from ectomycorrhizal root tips and thus have not been described formally. The sporocarps of some Geopora species occur infrequently because they depend on wet years for sporulation. In addition, Geopora sporocarps can be small and may be hypogeous at some developmental stage, limiting the opportunities for describing their morphology. Using molecular and morphological data, we have described a new species of fungus, Geopora pinyonensis, which produced ascocarps after unusually high precipitation at a northern Arizona site in summer 2012. Based on analysis of the ITS and nuLSU regions of the rDNA, G pinyonensis is a new species of Geopora. It has small sporocarps and ascospores relative to other members of the genus; however, these morphological features overlap with other species. Using rDNA data from sporocarps and ectomycorrhizal root tips, we show that the sporocarps correspond to an abundant species of ectomycorrhizal fungus associated with pinyon pines that is increasing in abundance in drought-affected landscapes and may promote drought tolerance.

Densities of Agrilus auroguttatus and Other Borers in California and Arizona Oaks

We investigated within-tree population density of a new invasive species in southern California, the goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), with respect to host species and the community of other borers present. We measured emergence hole densities of A. auroguttatus and other borers on the lower stem (bole) of naïve oaks at 18 sites in southern California and on co-evolved oaks at seven sites in southeastern Arizona. We sampled recently dead oaks in an effort to quantify the community of primary and secondary borers associated with mortality—species that were likely to interact with A. auroguttatus. Red oaks (Section Lobatae) produced greater densities of A. auroguttatus than white oaks (Section Quercus). On red oaks, A. auroguttatus significantly outnumbered native borers in California (mean ± SE of 9.6 ± 0.7 versus 4.5 ± 0.6 emergence holes per 0.09 m2 of bark surface), yet this was not the case in Arizona (0.9 ± 0.2 versus 1.1 ± 0.2 emergence holes per 0.09 m2). In California, a species that is taxonomically intermediate between red and white oaks, Quercus chrysolepis (Section Protobalanus), exhibited similar A. auroguttatus emergence densities compared with a co-occurring red oak, Q. kelloggii. As an invasive species in California, A. auroguttatus may affect the community of native borers (mainly Buprestidae and Cerambycidae) that feed on the lower boles of oaks, although it remains unclear whether its impact will be positive or negative.

Dryland vegetation phenology across an elevation gradient in Arizona, USA, investigated with fused MODIS and Landsat data

The patchy and heterogeneous arrangement of vegetation in dryland areas complicates the extraction of phenological signals using existing remote sensing data. This study examined whether the phenological analysis of a range of dryland land cover classes would benefit from the availability of synthetic images at Landsat spatial resolution and MODIS time intervals. We assembled a series of 500m MODIS and Landsat-5 TM datasets from April to November, 2005–2009, over a study site in central Arizona that encompasses diverse dryland vegetation classes along an elevation gradient of 2000m. We applied the spatial and temporal adaptive reflectance fusion model (STARFM) to each MODIS image to create a time series of synthetic images at 30m resolution. We subjected a subset of the synthetic imagery to a pixel-based regression analysis with temporally coincident Landsat images to analyze the effect of the underlying vegetation class on the accuracy of the STARFM results. To evaluate the usefulness of the increased spatial resolution compared to a MODIS product, we analyzed the variability of the date of peak greenness values of all 30mpixels within unmixed MODIS pixels. Finally, we examined differences in the temporal distributions of peak greenness extracted from both the Enhanced Vegetation Index (EVI) and Normalized Difference Vegetation Index (NDVI) synthetic imagery time series. Our results indicate that characteristics of the vegetation classes strongly influence STARFM algorithm performance, with Pearson correlation coefficient values ranging from 0.72 to 0.96 depending on the Landsat band and the land cover class. Responses in the near-infrared (NIR) spectrum yielded the lowest correlations, particularly for the Ponderosa Pine class. The phenological variability exhibited by each land cover class was dependent on the precipitation patterns of each growing season, but was sufficiently high to make the application of STARFM imagery at this scale uniformly beneficial. The peak greenness dates extracted from EVI and NDVI time series were temporally synchronized for the Grassland class but diverged for the classes of mixed woody and herbaceous vegetation types.

A Conceptual Framework for Clutch-Size Evolution in Songbirds

Causes of evolved differences in clutch size among songbird species remain debated. I propose a new conceptual framework that integrates aspects of traditional life-history theory while including novel elements to explain evolution of clutch size among songbirds. I review evidence that selection by nest predation on length of time that offspring develop in the nest creates a gradient in offspring characteristics at nest leaving (fledging), including flight mobility, spatial dispersion, and self-feeding rate. I postulate that this gradient has consequences for offspring mortality rates and parental energy expenditure per offspring. These consequences then determine how reproductive effort is partitioned among offspring, while reproductive effort evolves from age-specific mortality effects. Using data from a long-term site in Arizona, as well as from the literature, I provide support for hypothesized relationships. Nestling development period consistently explains fledgling mortality, energy expenditure per offspring, and clutch size while accounting for reproductive effort (i.e., total energy expenditure) to thereby support the framework. Tests in this article are not definitive, but they document previously unrecognized relationships and address diverse traits (developmental strategies, parental care strategies, energy requirements per offspring, evolution of reproductive effort, clutch size) that justify further investigations of hypotheses proposed here.

Field aging effect on chemistry and rheology of asphalt binders and rheological predictions for field aging

This work aims to investigate the field aging effect on binder rheology and structure and to establish the structure–rheological property relationships with the ultimate goal of predicting rheology of field aged binders. Three unmodified binders from different crude oil sources after varying field aging severities were examined in this work. The binders were extracted and recovered from pavement cores at varying depths from two field sites in Arizona and Virginia. The observed rheological hardening is closely correlated and attributed to the compositional changes within asphalts upon field aging, which involves the conversion of aromatics into toluene soluble asphaltenes and an increase in total pericondensed aromaticity and carbonyl and sulfoxide functional groups. Although the rheology is significantly impacted by field aging, the temperature dependence of rheological properties maintains unchanged at various aging severities. The Arrhenius temperature dependence is suggested to describe the viscoelasticity of unmodified binders rather than the Super-Arrhenius or Vogel–Fulcher–Tamman or Williams–Landell–Ferry equations due to a small dynamic fragility of 16. Both the rheological and chemical composition results suggest that the bottom slice (∼75mm from pavement surface) of the 8-year old ambient field aged core has similar aging severity to the rolling thin film oven (RTFO) followed by pressure aging vessel (PAV) aging on the control binder. This implies that the field aging on the pavement surface is far more severe than the long term laboratory RTFO PAV aging. Based on the observed structure–rheology relationships and Christensen–Anderson (CA) model, the rheological responses can be successfully predicted for field aged binders at various aging severities or at different pavement depths. The predictions are in good agreement with experimental results.

JEDDITO YELLOW WARE, MIGRATION, AND THE KAYENTA DIASPORA

In this paper, I explore the significance of Jeddito Yellow Ware found outside its production zone, the Hopi Mesas region. Using theory and method for identifying traces of migration, I evaluate case studies in which Jeddito Yellow Ware has played a role in inferences of population movement. Jeddito Yellow Ware is rare at or absent from the sites in southern Arizona that have yielded the most compelling evidence of immigrants from northern Arizona. I conclude by offering possible explanations for this pattern.

An assessment of the effect of horizontal soil moisture heterogeneity on the area-average measurement of cosmic-ray neutrons

[1] The cosmic-ray neutron probe measures soil moisture over tens of hectares, thus averaging spatially variable soil moisture fields. A previous paper described how variable soil moisture profiles affect the integrated cosmic-ray neutron signal from which depthaverage soil moisture is computed. Here, we investigate the effect of horizontal heterogeneity on the relationship between neutron counts and average soil moisture. Observations from a distributed sensor network at a site in southern Arizona indicate that the horizontal component of the total variance of the soil moisture field is less variably in time than the vertical component. Using results from neutron particle transport simulations we show that 1-D binary distributions of soil moisture may affect both the mean and variance of neutron counts of a cosmic-ray neutron detector placed arbitrarily in a soil moisture field, potentially giving rise to an underestimate of the footprint average soil moisture. Similar simulations that used 1-D and 2-D Gaussian soil moisture fields indicate consistent mean and variances of a randomly placed detector if the correlation length scales are short (less than � 30 m) and/or the soil moisture field variance is small (<0.032 m 6 m � 6 ). Taken together, these soil moisture observations and neutron transport simulations show that horizontal heterogeneity likely has a small effect on the relationship between mean neutron counts and average soil moisture for soils under natural conditions. Citation: Franz, T. E., M. Zreda, T. P. A. Ferre, and R. Rosolem (2013), An assessment of the effect of horizontal soil moisture heterogeneity on the area-average measurement of cosmic-ray neutrons, Water Resour. Res., 49, doi:10.1002/wrcr.20530.

Characterization and quantification of groundwater sulfate sources at a mining site in an arid climate: The Monument Valley site in Arizona, USA.

The Monument Valley site, a former uranium mining site located in the state of Arizona in the Southwest USA, has high concentrations of sulfate in groundwater. Stable isotope analysis of S and O for sulfate, in combination with geochemical and hydrogeological data, was used to characterize the sources and fate of sulfate. The results indicate the existence of two discrete sources of sulfate (in excess of baseline levels): sulfuric acid released during ore processing and sulfate generated via sulfide-mineral oxidation. The contributions of the sources are related to spatial distributions of sulfate in the plume through analysis of groundwater travel times. Quantification of the sources using two isotope-analysis methods yielded similar results. The results indicate that sulfuric acid served as the primary source (mean = 427 mg/L, 74%), with sulfide-mineral oxidation providing a smaller contribution (mean = 147 mg/L, 26%). It appears that the major contribution to the sulfide-mineral oxidation component originates from oxidation of sulfide minerals in exposed bedrock residing in the primary recharge zone of the local aquifer, which provides an elevated sulfate background for groundwater. Conversely, the oxidation of sulfide minerals associated with the mine tailings appears to provide a relatively minor contribution (∼8% of the overall total). Interestingly, it appears that sulfuric acid served as a sustained source of sulfate for approximately 40 years. This may be related to the accumulation of sulfate salts (formed after neutralization and disposal of the sulfuric acid) in the source zone due to the arid climate of the site. Contrary to the typical assumption applied at many mining sites that sulfide-mineral oxidation is the primary source of sulfate, these sulfate salts are hypothesized to be the primary source for this site.

Exposure to Metals in Environmental Media Near a Mine Tailings Site

Background: Residents near metal mine tailings sites may have higher exposures to certain metals. Aims: This study investigates whether there is a relationship between living near a former mine and smelter site in Arizona and exposure to metals, particularly arsenic (As) and lead (Pb), via soil, dust, and water. Methods: Metals were measured in residential soil, dust, water, urine and toenail samples from 31 children in 18 households up to 5 miles from a former mine and smelter site. Soil from the house perimeter and dust from vacuum bags were collected, sieved to &lt;63 ?m before nitric acid digestion and analysis via ICP-MS. Water and urine were analyzed without digestion, while toenails were washed, digested and analyzed. Blood lead was tested by an independent, certified laboratory. Spearman correlation coefficients were calculated between each environmental media and urine and toenails. Results: Geometric mean (GSD) As concentrations in soil and dust (&lt;63 ?m) were 29.4 (2.49) ppm and 11.0 (5.5) ppm. Concentrations in water were 8.04 (7.52) ppb. Geometric mean total urinary As was 12.7 (2.15) ?g/g creatinine. Soil and dust values for Pb were 20.3 (2.07) ppm and 15.9 (5.71) ppm. The majority of water Pb concentrations and blood Pb were below the limit of quantification. Toenail As was 0.745 (3.42) ?g/g and Pb was 1.27 (3.89) ?g/g. Correlation coefficients for As were: water-urine 0.44, soil-urine 0.11, dust-urine 0.58, urine-toenails 0.003, water-toenail 0.34, dust-toenail 0.021, soil-toenail 0.044. Conclusions: Total urinary As was above the NHANES 2009 geometric mean for ages 6-11 years with about 16% of the children above the 90th percentile, indicating a possible local source of exposure. Urine appears to have a better correlation with environmental samples than toenails. Children do not appear to be highly exposed to lead from the tailing/smelter site. Further work will examine the spatial gradient between the site and environmental and biomarker levels, as well as other potential predictors of biomarker levels.