1,027 research outputs found
Palaeosol Control of Arsenic Pollution: The Bengal Basin in West Bengal, India
Groundwater in the Bengal Basin is badly polluted by arsenic (As) which adversely affects human health. To provide low-As groundwater for As mitigation, it was sought across 235 km2 of central West Bengal, in the western part of the basin. By drilling 76 boreholes and chemical analysis of 535 water wells, groundwater with <10 µg/L As in shallow aquifers was found under one-third of a study area. The groundwater is in late Pleistocene palaeo-interfluvial aquifers of weathered brown sand that are capped by a palaeosol of red clay. The aquifers form two N-S trending lineaments that are bounded on the east by an As-polluted deep palaeo-channel aquifer and separated by a shallower palaeo-channel aquifer. The depth to the top of the palaeo-interfluvial aquifers is mostly between 35 and 38 m below ground level (mbgl). The palaeo-interfluvial aquifers are overlain by shallow palaeo-channel aquifers of gray sand in which groundwater is usually As-polluted. The palaeosol now protects the palaeo-interfluvial aquifers from downward migration of As-polluted groundwater in overlying shallow palaeo-channel aquifers.
The depth to the palaeo-interfluvial aquifers of 35 to 38 mbgl makes the cost of their exploitation affordable to most of the rural poor of West Bengal, who can install a well cheaply to depths up to 60 mbgl. The protection against pollution afforded by the palaeosol means that the palaeo-interfluvial aquifers will provide a long-term source of low-As groundwater to mitigate As pollution of groundwater in the shallower, heavily used, palaeo-channel aquifers. This option for mitigation is cheap to employ and instantly available
Incipient Wigner Localization in Circular Quantum Dots
We study the development of electron-electron correlations in circular
quantum dots as the density is decreased. We consider a wide range of both
electron number, N<=20, and electron gas parameter, r_s<18, using the diffusion
quantum Monte Carlo technique. Features associated with correlation appear to
develop very differently in quantum dots than in bulk. The main reason is that
translational symmetry is necessarily broken in a dot, leading to density
modulation and inhomogeneity. Electron-electron interactions act to enhance
this modulation ultimately leading to localization. This process appears to be
completely smooth and occurs over a wide range of density. Thus there is a
broad regime of ``incipient'' Wigner crystallization in these quantum dots. Our
specific conclusions are: (i) The density develops sharp rings while the pair
density shows both radial and angular inhomogeneity. (ii) The spin of the
ground state is consistent with Hund's (first) rule throughout our entire range
of r_s for all 4<N<20. (iii) The addition energy curve first becomes smoother
as interactions strengthen -- the mesoscopic fluctuations are damped by
correlation -- and then starts to show features characteristic of the classical
addition energy. (iv) Localization effects are stronger for a smaller number of
electrons. (v) Finally, the gap to certain spin excitations becomes small at
the strong interaction (large r_s) side of our regime.Comment: 14 pages, 12 figure
Interaction-Induced Strong Localization in Quantum Dots
We argue that Coulomb blockade phenomena are a useful probe of the cross-over
to strong correlation in quantum dots. Through calculations at low density
using variational and diffusion quantum Monte Carlo (up to r_s ~ 55), we find
that the addition energy shows a clear progression from features associated
with shell structure to those caused by commensurability of a Wigner crystal.
This cross-over (which occurs near r_s ~ 20 for spin-polarized electrons) is,
then, a signature of interaction-driven localization. As the addition energy is
directly measurable in Coulomb blockade conductance experiments, this provides
a direct probe of localization in the low density electron gas.Comment: 4 pages, published version, revised discussio
Interaction Effects in the Mesoscopic Regime: A Quantum Monte Carlo Study of Irregular Quantum Dots
We address the issue of accurately treating interaction effects in the
mesoscopic regime by investigating the ground state properties of isolated
irregular quantum dots. Quantum Monte Carlo techniques are used to calculate
the distributions of ground state spin and addition energy. We find a reduced
probability of high spin and a somewhat larger even/odd alternation in the
addition energy from quantum Monte Carlo than in local spin density functional
theory. In both approaches, the even/odd effect gets smaller with increasing
number of electrons, contrary to the theoretical understanding of large dots.
We argue that the local spin density approximation over predicts the effects of
interactions in quantum dots.Comment: Final Version, to appear in PRB as a Rapid Com
Waste-water impacts on groundwater: Cl/Br ratios and implications for arsenic pollution of groundwater in the Bengal Basin and Red River Basin, Vietnam
Across West Bengal and Bangladesh, concentrations of Cl in much groundwater exceed the natural, upper limit of 10 mg/L. The Cl/Br mass ratios in groundwaters range up to 2500 and scatter along mixing lines between waste-water and dilute groundwater, with many falling near the mean end-member value for waste-water of 1561 at 126 mg/L Cl. Values of Cl/Br exceed the seawater ratio of 288 in uncommon NO3-bearing groundwaters, and in those containing measurable amounts of salt-corrected SO4 (SO4 corrected for marine salt). The data show that shallow groundwater tapped by tube-wells in the Bengal Basin has been widely contaminated by waste-water derived from pit latrines, septic tanks, and other methods of sanitary disposal, although reducing conditions in the aquifers have removed most evidence of NO3 additions from these sources, and much evidence of their additions of SO4. In groundwaters from wells in palaeo-channel settings, end-member modelling shows that > 25% of wells yield water that comprises ≥ 10% of waste-water. In palaeo-interfluvial settings, only wells at the margins of the palaeo-interfluvial sequence contain detectable waste water. Settings are identifiable by well-colour survey, owner information, water composition, and drilling.
Values of Cl/Br and faecal coliform counts are both inversely related to concentrations of pollutant As in groundwater, suggesting that waste-water contributions to groundwater in the near-field of septic-tanks and pit-latrines (within 30 m) suppress the mechanism of As-pollution and lessen the prevalence and severity of As pollution. In the far-field of such sources, organic matter in waste-water may increase groundwater pollution by As
Correlation Induced Inhomogeneity in Circular Quantum Dots
Properties of the "electron gas" - in which conduction electrons interact by
means of Coulomb forces but ionic potentials are neglected - change
dramatically depending on the balance between kinetic energy and Coulomb
repulsion. The limits are well understood. For very weak interactions (high
density), the system behaves as a Fermi liquid, with delocalized electrons. In
contrast, in the strongly interacting limit (low density), the electrons
localize and order into a Wigner crystal phase. The physics at intermediate
densities, however, remains a subject of fundamental research. Here, we study
the intermediate-density electron gas confined to a circular disc, where the
degree of confinement can be tuned to control the density. Using accurate
quantum Monte Carlo techniques, we show that the electron-electron correlation
induced by an increase of the interaction first smoothly causes rings, and then
angular modulation, without any signature of a sharp transition in this density
range. This suggests that inhomogeneities in a confined system, which exist
even without interactions, are significantly enhanced by correlations.Comment: final version, modified introduction and clarifications, 4 page
Lepton flavor conserving Z -> l^+ l^-$ decays in the general two Higgs doublet model
We calculate the new physics effects to the branching ratios of the lepton
flavor conserving decays Z -> l^+ l^- in the framework of the general two Higgs
Doublet model. We predict the upper limits for the couplings
|\bar{\xi}^{D}_{N,\mu\tau}| and |\bar{\xi}^{D}_{N,\tau\tau}| as 3\times 10^2
GeV and 1\times 10^2 GeV, respectively.Comment: 9 pages, 3 figure
Groundwater quality beneath an Asian megacity on a delta: Kolkata’s (Calcutta’s) disappearing arsenic and present manganese
Kolkata, the capital city of West Bengal, exploits groundwater for public water-supply. The groundwater has been reported to be widely polluted by arsenic (As). Analysis for As in 280 groundwaters from across Kolkata, failed to detect As concentrations >10 μg/L from natural processes. Arsenic concentrations between 10 and 79 μg/L found in 14 of the 280 groundwaters are remnant from a pollution-plume emanating from a single factory site where Paris Green, an arsenical pesticide, was manufactured between 1965 and 1985. In 45% of groundwaters sampled, concentrations of Mn exceed 0.4 mg/L, a putative health guideline value for drinking water. Sporadic minor hazards are posed by Pb > 10 μg/L introduced into groundwater from well-fittings, from 4% of groundwaters with F concentrations between 0.75 and 1 mg/L, and the 14% of groundwaters containing more than 500 mg/L Na, concentrations that might contribute to excessive daily intake of Na. Compounding hazards from As, F, Mn, Na, and Pb, shows that 64% of public wells and 40% of municipal wells supply groundwater of suspect quality. Groundwaters comply with WHO Guideline Values for drinking water in terms of Cr, Cu, Co, NO2, NO3, Sb, Se, and U. Aesthetic guideline values for Fe, Mn, SO4, and Cl are exceeded for many groundwaters
A texture of neutrino mass matrix in view of recent neutrino experimental results
In view of recent neutrino experimental results such as SNO, Super-Kamiokande
(SK), CHOOZ and neutrinoless double beta decay , we
consider a texture of neutrino mass matrix which contains three parameters in
order to explain those neutrino experimental results. We have first fitted
parameters in a model independent way with solar and atmospheric neutrino mass
squared differences and solar neutrino mixing angle which satisfy LMA solution.
The maximal value of atmospheric neutrino mixing angle comes out naturally in
the present texture. Most interestingly, fitted parameters of the neutrino mass
matrix considered here also marginally satisfy recent limit on effective
Majorana neutrino mass obtained from neutrinoless double beta decay experiment.
We further demonstrate an explicit model which gives rise to the texture
investigated by considering an gauge group with two
extra real scalar singlets and discrete symmetry. Majorana
neutrino masses are generated through higher dimensional operators at the scale
. We have estimated the scales at which singlets get VEV's and M by
comparing with the best fitted results obtained in the present work.Comment: Journal Ref.: Phys. Rev. D66, 053004 (2002
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