96 research outputs found
Se Isotopes as Groundwater Redox Indicators:Detecting Natural Attenuation of Se at an in Situ Recovery U Mine
One
of the major ecological concerns associated with the in situ
recovery (ISR) of uranium (U) is the environmental release of soluble,
toxic selenium (Se) oxyanions generated by mining. Post-mining natural
attenuation by the residual reductants in the ore body and reduced
down-gradient sediments should mitigate the risk of Se contamination
in groundwater. In this work, we investigate the Se concentrations
and Se isotope systematics of groundwater and of U ore bearing sediments
from an ISR site at Rosita, TX, USA. Our results show that selenate
(SeÂ(VI)) is the dominant Se species in Rosita groundwater, and while
several up-gradient wells have elevated SeÂ(VI), the majority of the
ore zone and down-gradient wells have little or no Se oxyanions. In
addition, the ÎŽ<sup>82</sup>Se<sub>VI</sub> of Rosita groundwater
is generally elevated relative to the U ore up to +6.14â°, with
the most enriched values observed in the ore-zone wells. Increasing
ÎŽ<sup>82</sup>Se with decreasing SeÂ(VI) conforms to a Rayleigh
type distillation model with an Δ of â2.25Ⱐ±
0.61â°, suggesting natural SeÂ(VI) reduction occurring along
the hydraulic gradient at the Rosita ISR site. Furthermore, our results
show that Se isotopes are excellent sensors for detecting and monitoring
post-mining natural attenuation of Se oxyanions at ISR sites
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Chlorine-36 abundance in natural and synthetic perchlorate
Perchlorate (ClO{sub 4}{sup -}) is ubiquitous in the environment. It occurs naturally as a product of atmospheric photochemical reactions, and is synthesized for military, aerospace, and industrial applications. Nitrate-enriched soils of the Atacama Desert (Chile) contain high concentrations of natural ClO{sub 4}{sup -}; nitrate produced from these soils has been exported worldwide since the mid-1800's for use in agriculture. The widespread introduction of synthetic and agricultural ClO{sub 4}{sup -} into the environment has complicated attempts to understand the geochemical cycle of ClO{sub 4}{sup -}. Natural ClO{sub 4}{sup -} samples from the southwestern United States have relatively high {sup 36}Cl abundances ({sup 36}Cl/Cl = 3,100 x 10{sup -15} to 28,800 x 10{sup -15}), compared with samples of synthetic ({sup 36}Cl/Cl = 0.0 x 10{sup -15} to 40 x 10{sup -15}) and Atacama Desert ({sup 36}Cl/Cl = 0.9 x 10{sup -15} to 590 x 10{sup -15}) ClO{sub 4}{sup -}. These data give a lower limit for the initial {sup 36}Cl abundance of natural ClO{sub 4}{sup -} and provide temporal and other constraints on its geochemical cycle
Environmental controls on observed spatial variability of soil pore water geochemistry in small headwater catchments underlain with permafrost
Soil pore water (SPW) chemistry can vary substantially across
multiple scales in Arctic permafrost landscapes. The magnitude of these
variations and their relationship to scale are critical considerations for
understanding current controls on geochemical cycling and for predicting
future changes. These aspects are especially important for Arctic change
modeling where accurate representation of sub-grid variability may be
necessary to predict watershed-scale behaviors. Our research goal is to
characterize intra- and inter-watershed soil water geochemical variations at
two contrasting locations in the Seward Peninsula of Alaska, USA. We then
attempt to identify the key factors controlling concentrations of important
pore water solutes in these systems. The SPW geochemistry of 18 locations
spanning two small Arctic catchments was examined for spatial variability
and its dominant environmental controls. The primary environmental controls
considered were vegetation, soil moisture and/or redox condition, waterâsoil
interactions and hydrologic transport, and mineral solubility. The sampling
locations varied in terms of vegetation type and canopy height, presence or
absence of near-surface permafrost, soil moisture, and hillslope position.
Vegetation was found to have a significant impact on SPW NO3-
concentrations, associated with the localized presence of nitrogen-fixing
alders and mineralization and nitrification of leaf litter from tall willow
shrubs. The elevated NO3- concentrations were, however, frequently
equipoised by increased microbial denitrification in regions with sufficient
moisture to support it. Vegetation also had an observable impact on soil-moisture-sensitive constituents, but the effect was less significant. The
redox conditions in both catchments were generally limited by Fe reduction,
seemingly well-buffered by a cache of amorphous Fe hydroxides, with the most
reducing conditions found at sampling locations with the highest soil
moisture content. Non-redox-sensitive cations were affected by a wide
variety of waterâsoil interactions that affect mineral solubility and
transport. Identification of the dominant controls on current SPW
hydrogeochemistry allows for qualitative prediction of future geochemical
trends in small Arctic catchments that are likely to experience warming and
permafrost thaw. As source areas for geochemical fluxes to the broader
Arctic hydrologic system, geochemical processes occurring in these
environments are particularly important to understand and predict with
regards to such environmental changes.</p
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The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern1. Multi-year droughts during the instrumental period2 and decadal-length droughts of the past two millennia1, 3 were shorter and climatically different from the future permanent, âdust-bowl-likeâ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming4. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies5 to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ~2â°C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4â6â°C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase
Global Boundary Stratotype Section and Point (GSSP) for the Anthropocene Series: Where and how to look for potential candidates
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