71 research outputs found
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Stable Isotopic Investigations of In-Situ Bioremediation of Chlorinated Organic Solvents
The research objectives of this program are threefold: to develop methods for measuring stable isotope ratios of carbon and chlorine in chlorinated aliphatic hydrocarbons (CAHs); to apply these methods to experimental determinations of kinetic and equilibrium isotope effects during biological, chemical, and physical transformation of CAHs; and to apply these methods to CAHs extracted from ground water at well characterized, contaminated aquifer sites. The overall objective is to develop an understanding of the environmental isotopic behavior of CAHs and to apply this understanding to better characterize, monitor, and evaluate natural and engineered bioremediation. This is an important problem because of the magnitude and frequency of ground water contamination by CAHs and the resultant health risks imposed to the population, as well as the enormous costs involved in cleaning up such contamination. This project is innovative as it represents the first systematic effort of its kind. Since its inception, a number of other scientists have also started research into the stable isotope chemistry of CAHs in both laboratory and field investigations . Complementary experimental and theoretical studies of the equilibrium stabilities and kinetics of CAH degradation by microbial and abiotic mechanisms continue to be a major field of research
Red Sea Rifting Controls on Groundwater Reservoir Distribution: Constraints from Geophysical, Isotopic, and Remote Sensing Data
Highly productive wells in the Central
Eastern Desert of Egypt are tapping groundwater
in subsided blocks of Jurassic to
Cretaceous sandstone (Taref Formation of
the Nubian Sandstone Group) and Oligocene
to Miocene sandstone (Nakheil Formation),
now occurring beneath the Red Sea coastal
plain and within the proximal basement complex.
Aquifer development is related to Red
Sea rifting: (1) rifting was accommodated by
vertical extensional displacement on preexisting
NW-SE– to N-S–trending faults forming
a complex array of half-grabens and asymmetric
horsts; and (2) subsided blocks escaped
erosion accompanying the Red Sea–related
uplift. Subsided blocks were identifi ed and
verifi ed using satellite data, geologic maps,
and fi eld and geophysical investigations. Interpretations
of very low frequency (VLF)
measurements suggest the faults acted as
conduits for ascending groundwater from the
subsided aquifers. Stable isotopic compositions
(δD: –19.3‰ to –53.9‰; δ18O: –2.7‰ to
–7.1‰) of groundwater samples from these
aquifers are interpreted as mixtures of fossil
(up to 70%) and modern (up to 65%) precipitation.
Groundwater volumes in subsided
blocks are large; within the Central Eastern
Desert basement complex alone, they are estimated
at 3 × 109 m3 and 10 × 109 m3 for the
Nakheil and Taref Formations, respectively.
Results highlight the potential for identifying
similar rift-related aquifer systems along
the Red Sea–Gulf of Suez system, and in rift
systems elsewhere. An understanding of the
distribution of Red Sea rift–related aquifers
and modern recharge contributions to these
aquifers could assist in addressing the rising
demands for fresh water supplies and water
scarcity issues in the regio
Red Sea Rifting Controls on Groundwater Reservoir Distribution: Constraints from Geophysical, Isotopic, and Remote Sensing Data
Highly productive wells in the Central
Eastern Desert of Egypt are tapping groundwater
in subsided blocks of Jurassic to
Cretaceous sandstone (Taref Formation of
the Nubian Sandstone Group) and Oligocene
to Miocene sandstone (Nakheil Formation),
now occurring beneath the Red Sea coastal
plain and within the proximal basement complex.
Aquifer development is related to Red
Sea rifting: (1) rifting was accommodated by
vertical extensional displacement on preexisting
NW-SE– to N-S–trending faults forming
a complex array of half-grabens and asymmetric
horsts; and (2) subsided blocks escaped
erosion accompanying the Red Sea–related
uplift. Subsided blocks were identifi ed and
verifi ed using satellite data, geologic maps,
and fi eld and geophysical investigations. Interpretations
of very low frequency (VLF)
measurements suggest the faults acted as
conduits for ascending groundwater from the
subsided aquifers. Stable isotopic compositions
(δD: –19.3‰ to –53.9‰; δ18O: –2.7‰ to
–7.1‰) of groundwater samples from these
aquifers are interpreted as mixtures of fossil
(up to 70%) and modern (up to 65%) precipitation.
Groundwater volumes in subsided
blocks are large; within the Central Eastern
Desert basement complex alone, they are estimated
at 3 × 109 m3 and 10 × 109 m3 for the
Nakheil and Taref Formations, respectively.
Results highlight the potential for identifying
similar rift-related aquifer systems along
the Red Sea–Gulf of Suez system, and in rift
systems elsewhere. An understanding of the
distribution of Red Sea rift–related aquifers
and modern recharge contributions to these
aquifers could assist in addressing the rising
demands for fresh water supplies and water
scarcity issues in the regio
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Precipitation of U(VI) in Low-Temperature Si-Na-H2O±CO2±Feldspar Systems
Submarine Groundwater Discharge Data at Meter Scale (223Ra, 224Ra, 226Ra, 228Ra and 222Rn) in Indian River Bay (Delaware, US)
Abstract Submarine groundwater discharge (SGD) was sampled at high-spatial resolution in Indian River Bay, DE, USA, in July 2016 to characterize the spatial variability of the activity of the radium and radon isotopes commonly used to estimate SGD. These data were part of an investigation into the methods and challenges of characterizing SGD rates and variability, especially in the coastal aquifer transition from freshwater to saltwater (Hydrogeological processes and near shore spatial variability of radium and radon isotopes for the characterization of submarine groundwater discharge (Duque et al., 2019)). Samples were collected with seepage meters and minipiezometers to obtain sufficient volumes for analytical characterization. Seepage meter samples (for 223Ra, 224Ra, 226Ra, and 228Ra) were collected at two-hour intervals over a semi-diurnal tidal cycle from 30 seepage meters. Samples for 222Rn characterization were collected with a minipiezometer from 25 cm below the bay bed at each seepage meter location. All samples were analyzed with standard and state of the art procedures
Inter-laboratory Characterisation of Apatite Reference Materials for Chlorine Isotope Analysis
Here we report on a set of six apatite reference materials (chlorapatites MGMH#133648, TUBAF#38 and fluorapatites MGMH#128441A, TUBAF#37, 40, 50) which we have characterised for their chlorine isotope ratios; these RMs span a range of Cl mass fractions within the apatite Ca-10(PO4)(6)(F,Cl,OH)(2) solid solution series. Numerous apatite specimens, obtained from mineralogical collections, were initially screened for Cl-37/Cl-35 homogeneity using SIMS followed by delta Cl-37 characterisation by gas source mass spectrometry using both dual-inlet and continuous-flow modes. We also report major and key trace element compositions as determined by EPMA. The repeatability of our SIMS results was better than +/- 0.10% (1s) for the five samples with > 0.5% m/m Cl and +/- 0.19% (1s) for the low Cl abundance material (0.27% m/m). We also observed a small, but significant crystal orientation effect of 0.38% between the mean Cl-37/Cl-35 ratios measured on three oriented apatite fragments. Furthermore, the results of GS-IRMS analyses show small but systematic offset of delta Cl-37(SMOC) values between the three laboratories. Nonetheless, all studied samples have comparable chlorine isotope compositions, with mean 10(3)delta Cl-37(SMOC) values between +0.09 and +0.42 and in all cases with 1s <= +/- 0.25
Deposition, Accumulation, and Alteration of Cl(-), NO3(-), ClO4(-) and ClO3(-) Salts in a Hyper-Arid Polar Environment: Mass Balance and Isotopic Constraints
The salt fraction in permafrost soils/sediments of the McMurdo Dry Valleys (MDV) of Antarctica can be used as a proxy for cold desert geochemical processes and paleoclimate reconstruction. Previous analyses of the salt fraction in MDV permafrost soils have largely been conducted in coastal regions where permafrost soils are variably affected by aqueous processes and mixed inputs from marine and stratospheric sources. We expand upon this work by evaluating permafrost soil/sediments in University Valley, located in the ultraxerous zone where both liquid water transport and marine influences are minimal. We determined the abundances of Cl(-), NO3(-, ClO4(-)and ClO3(-)in dry and ice-cemented soil/sediments, snow and glacier ice, and also characterized Cl(-) and NO3(-) isotopically. The data are not consistent with salt deposition in a sublimation till, nor with nuclear weapon testing fall-out, and instead point to a dominantly stratospheric source and to varying degrees of post depositional transformation depending on the substrate, from minimal alteration in bare soils to significant alteration (photodegradation and/or volatilization) in snow and glacier ice. Ionic abundances in the dry permafrost layer indicate limited vertical transport under the current climate conditions, likely due to percolation of snowmelt. Subtle changes in ClO4(-)/NO3(-) ratios and NO3(-) isotopic composition with depth and location may reflect both transport related fractionation and depositional history. Low molar ratios of ClO3(-)/ClO4(-) in surface soils compared to deposition and other arid systems suggest significant post depositional loss of ClO3(-), possibly due to reduction by iron minerals, which may have important implications for oxy-chlorine species on Mars. Salt accumulation varies with distance along the valley and apparent accumulation times based on multiple methods range from approximately 10 to 30 kyr near the glacier to 70-200 kyr near the valley mouth. The relatively young age of the salts and relatively low and homogeneous anion concentrations in the ice-cemented sediments point to either a mechanism of recent salt removal, or to relatively modern permafrost soils (less than 1 million years). Together, our results show that near surface salts in University Valley serve as an end-member of stratospheric sources not subject to biological processes or extensive remobilization
Perchlorate in The Great Lakes: Isotopic Composition and Origin
Perchlorate is a persistent and mobile contaminant in the environment with both natural and anthropogenic sources. Stable isotope ratios of oxygen (δ^(18)O, Δ^(17)O) and chlorine (δ^(37)Cl) along with the abundance of the radioactive isotope ^(36)Cl were used to trace perchlorate sources and behavior in the Laurentian Great Lakes. These lakes were selected for study as a likely repository of recent atmospheric perchlorate deposition. Perchlorate concentrations in the Great Lakes range from 0.05 to 0.13 μg per liter. Δ^(37)Cl values of perchlorate from the Great Lakes range from +3.0‰ (Lake Ontario) to +4.0‰ (Lake Superior), whereas δ^(18)O values range from −4.1‰ (Lake Superior) to +4.0‰ (Lake Erie). Great Lakes perchlorate has mass-independent oxygen isotopic variations with positive Δ^(17)O values (+1.6‰ to +2.7‰) divided into two distinct groups: Lake Superior (+2.7‰) and the other four lakes (∼+1.7‰). The stable isotopic results indicate that perchlorate in the Great Lakes is dominantly of natural origin, having isotopic composition resembling that measured for indigenous perchlorate from preindustrial groundwaters of the western USA. The ^(36)Cl/Cl ratio of perchlorate varies widely from 7.4 × 10^(–12) (Lake Ontario) to 6.7 × 10^(–11) (Lake Superior). These ^(36)ClO_4– abundances are consistent with an atmospheric origin of perchlorate in the Great Lakes. The relatively high ^(36)ClO_4– abundances in the larger lakes (Lakes Superior and Michigan) could be explained by the presence of ^(36)Cl-enriched perchlorate deposited during the period of elevated atmospheric ^(36)Cl activity following thermonuclear bomb tests in the Pacific Ocean
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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
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The impact of biogenic carbon emissions on aerosol absorption inMexico City
In order to determine the wavelength dependence of atmospheric aerosol absorption in the Mexico City area, the absorption angstrom exponents (AAEs) were calculated from aerosol absorption measurements at seven wavelengths obtained with a seven-channel aethalometer during two field campaigns, the Mexico City Metropolitan Area study in April 2003 (MCMA 2003) and the Megacity Initiative: Local and Global Research Observations in March 2006 (MILAGRO). The AAEs varied from 0.76 to 1.56 in 2003 and from 0.54 to 1.52 in 2006. The AAE values determined in the afternoon were consistently higher than the corresponding morning values, suggesting the photochemical formation of absorbing secondary organic aerosols (SOA) in the afternoon. The AAE values were compared to stable and radiocarbon isotopic measurements of aerosol samples collected at the same time to determine the sources of the aerosol carbon. The fraction of modern carbon (fM) in the aerosol samples, as determined from {sup 14}C analysis, showed that 70% of the carbonaceous aerosols in Mexico City were from modern sources, indicating a significant impact from biomass burning during both field campaigns. The {sup 13}C/{sup 12}C ratios of the aerosol samples illustrate the significant impact of Yucatan forest fires (C-3 plants) in 2003 and local grass fires (C-4 plants) at site T1 in 2006. A direct comparison of the fM values, stable carbon isotope ratios, and calculated aerosol AAEs suggested that the wavelength dependence of the aerosol absorption was controlled by the biogenically derived aerosol components
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