a comparison of mantle convection models featuring plates oceanic plates are an integral part of the earth s mantle and thus play an important role in its dynamics and evolution to allow plate behavior to arise naturally in numerical mantle convection models self consistent plate generation methods apply a fully rheological approach featuring a temperature pressure and stress dependent viscosity however due to the extreme local viscosity changes that the self generation of model plates entails their computational requirements are demanding alternative plate modeling methods specify the existence of plates explicitly but can also obtain dynamically determined velocities e g by employing a force balance method here we present modifications to a force balance model by utilizing a rheology dependent viscosity profile accordingly plate viscosity and plate thickness are no longer prescribed by the modeler but now follow as a dynamic consequence of the temperature and stress dependence of the viscosity and the model s evolution we describe the new method and present benchmark results for a rheologically self consistent mantle convection model and the modified force balance plate model our results show that both plate modeling methods lead to the same system behavior for a wide range of system parameters making the new method a powerful tool to also achieve plate like surface motion naturally

chemical heterogeneity in diffusion dominated aquitards 1 high resolution measurements of the three dimensional 3 d variability in pore water electrical conductivity ec were obtained at a clay rich aquitard research site the king site 120 m 70 m area in southern saskatchewan canada using direct push probing technology n 35 boreholes these measurements revealed complex lateral heterogeneity in the subsurface salt distributions which raised questions on the validity of the commonly used approach of transposing chemical data from distributed piezometers onto 1 d depth profiles for interpretation and solute transport modeling two dimensional numerical modeling simulations demonstrated that distributed nonuniform salt inputs have existed at the top of the unoxidized till aquitard for the past 3 5 kyr the source of the nonuniform salt inputs is unclear but may be due to either variations in microtopography 0 5 m or variable penetration depth of surface fractures a suite of generic solutions was developed to estimate the critical depth required to obtain chemical homogeneity in aquitards with nonuniform solute inputs and diffusion as the dominant transport mechanism these solutions are presented for a range of sinusoidal input functions with varying amplitudes magnitudes and wavelengths the solutions indicate ec distributions at the king site should be laterally uniform below a depth of around 20 m bg which is consistent with observed ec values at the site this study shows that aquitards with nonuniform salt inputs at their upper boundary are chemically heterogeneous above some critical depth and thus require careful attention when modeling solute transport processes within them

mobility of arsenic and trace element inventories in sediment cores from masuda city southwestern japan the vertical distribution of arsenic and other trace and major elements has been studied in four sediment cores from masuda city nagashima and okite in the shimane prefecture of southwestern japan the sediment cores were also subjected to leaching techniques and 14c dating the stratigraphic sequences in the cores consist of silt and sandy silt at top passing downward into gray to black clays elevated values of as pb zn cu ni cr and v are present in several horizons while abundances of these elements tend to be higher in the black and gray clays probably due to adsorption onto clay sediments higher concentrations of fe and total sulfur ts occur in black clays the correlations of the trace metals with iron suggest their adsorption onto fe oxy hydroxides whereas correlations with sulfur in some cores indicate that they were precipitated as fe sulfides age determinations suggest that clay horizons at 5 m depth were deposited at around 5 000 and 6 000 years bp 14c ages during the transgressive phase of sea level change the results of the leaching techniques in the core samples show that higher amounts of as were extracted with deionized water even at neutral ph as can be released from sediments to groundwater and therefore groundwater pollution is a concern in masuda city and the surrounding area

pollution assessment of arsenic and other selected elements in the groundwater and soil of the gulf of aqaba saudi arabia forty six soil and groundwater samples were collected from the agricultural farms of the gulf of aqaba coast additionally 24 granitic and marine sedimentary rock samples were collected from the study area the collected samples were analyzed for as al au b ba be fe sb se sn ti and v using inductively coupled plasma mass spectrometry levels of the studied metals in the groundwater samples lie within the acceptable limits of the world health organization who the rock samples exhibit a significant variation in mean metal content from one rock type to another concentrations of as and b in the soil samples were determined to be higher than those of canadian soil quality guidelines csqg and were primarily due to agricultural and seawater inputs chemical weathering of various rock units also plays a significant role the calculations of geoaccumulation index are found to be more reliable than of those of enrichment factor for arsenic contamination levels assessment the study area is not significantly affected by as contamination the correlation coefficient analysis results for the soil and groundwater data reveal a variable degree of correlations between as and other metals in the study area

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

vertical distribution of as iii and as v in a coastal sandy aquifer factors controlling the concentration and speciation of arsenic in the stuarts point groundwater system northern new south wales australia arsenic species were measured in a bundled piezometer installed in the holocene barrier of the stuarts point coastal sands aquifer northern new south wales australia vertical distribution shows two peaks of elevated as concentration at a depth of 10 11 m concentrations of as tot as v and as iii are in the range of 52 85 38 67 and 14 18 mg l respectively and the ratio of as v as iii is well above 1 at 3 7 2 7 the second peak at a depth of 25 m shows the highest concentrations of as tot as v and as iii with values reaching 337 125 and 212 mg l respectively the as v as iii ratio is below 1 at 0 6 0 7 high as tot and as v concentrations at shallower depths are associated with acidic conditions and very low concentrations of all ions desorption of as from al hydroxides and as enriched feoxyhydroxides are plausible mechanisms releasing as into the groundwater system the elevated concentration of as tot and as iii at 25 m is potentially related to the leaching of the clay surfaces elevated hco3 and alkaline ph conditions at this depth cause desorption of as which is later present as as iii species in the reducing environment the high concentrations of hco3 further reduce the possible extent of as sorption on fe and mn oxyhydroxides the identification of as in a groundwater system associated with the coastal barrier sand dune environment raises serious questions of the suitability of human consumption of untreated groundwater drawn from these aquifer types further investigation both in australia and globally are needed to classified the extent of this hydrogeochemical occurrence near coastal communities that rely on groundwater 2003 elsevier science ltd all rights reserved

plate motions and the viscosity structure of the mantle insights from numerical modelling abstract the viscosity structure of the earth s mantle is likely to play an important role with respect to the motion of the lithospheric plates a zone of low viscosity in the upper mantle as postulated by seismological studies and also by investigations of postglacial uplift has been proposed to facilitate plate motion another inferred feature of the viscosity profile in the mantle is a significant increase at greater depth we have employed a three dimensional mantle convection model to explore the relation between the appearance of plate motion and the viscosity structure of the mantle beneath the model allows for a complex fluid rheology strong temperature pressure and stress dependence of the viscosity and can so account for the self consistent formation of plates at the surface of the convecting mantle plate like motion appears only in a limited parameter range and we demonstrate that this appearance is closely linked to a specific viscosity structure the viscosity is characterized by a local minimum at shallow depth low viscosity zone and by a local maximum at greater depth self consistent plate motion is only observed if such a viscosity structure exists in order to determine the functional relationship between the viscosity structure and plate motion we also employ an approach in which a certain viscosity profile is prescribed these studies again reveal the importance of the local extrema in the viscosity profile taking into account an increased interior viscosity also obtained by the pressure dependence we find stable plate motion as a general result we observe that the balance of the temperature and stress dependence of the viscosity is necessary to obtain plate motion at the same time we find a viscosity profile showing a minimum at shallow depth and a maximum at greater depth

the origin of large varioles in flow banded pillow lava from the hooggenoeg complex barberton greenstone belt south africa exceptionally well preserved pillowed and massive phenocryst free metabasaltic lava flows in the uppermost part of the palaeoarchaean hooggenoeg complex of the barberton greenstone belt exhibit both flow banding and large leucocratic varioles the flow banding is defined by blebs and bands of pale and dark green metabasalt and was the result of mingling of two types of basalt robins et al in bull volcanol 72 579 592 2010a varioles occur exclusively in the dark chlorite mgo and feo rich metabasalt varioles are absent in the outermost rinds of pillows and increase in both abundance and size towards the centres of pillows in the central parts of some pillows they impinge to form homogeneous pale patches bands or almost homogenous cores individual varioles consist essentially of radially orientated or outwardly branching dendritic crystals of albite many varioles exhibit concentric zones and finer grained rims some varioles seem to have grown around tiny vesicles and vesicles appear to have been trapped in others between a core and a finer grained rim the matrix surrounding the ocelli contains acicular pseudomorphs of actinolite and chlorite after chain like skeletal ca rich pyroxenes that are partly overgrown by the margins of varioles varioles are enriched in the chemical constituents of feldspar but contain concentrations of immobile tio2 cr zr and ree that are similar to the host metabasalts the shape distribution texture and composition of the varioles exclude liquid immiscibility and support an origin by spherulitic crystallisation of plagioclase from severely undercooled basalt melt and glass nucleation of plagioclase was strongly inhibited and took place on vesicles on the bases of drainage cavities and along early fractures eruption in deep water and retention of relatively high concentrations of volatiles in the melt may be the principal cause of spherulitic crystallisation in the interiors of pillows rather than only in their margins as in younger submarine flows

trace element geochemistry of a thick till and clay rich aquitard sequence saskatchewan canada abstract samples collected from a 170 m borehole penetrating till bentonitic clay and sand at the birsay site saskatchewan canada has been studied as a first step for understanding the controls on dissolution and transport of trace elements including rees in porewaters of a clay rich aquitard the ree budgets of the till clay and sand are dominated by apatite and monazite with sporadic zircon and y phosphate apatites contain 0 3 to 1 7 wt ree and monazites 12 to 55 wt all three units are characterized by unusual bulk sample ree patterns with maxima at eu gd convex down patterns from la sm with minima at ce and variably fractionated hree apatites feature maxima at eu gd consequently the complex patterns arise from variable modal proportions of apatite monazite and plagioclase feldspar relative to post archean average shale paas the till is sio2 and cao rich with about half the contents of al2o3 tio2 and fe2o3 these compositional relationship stem from relatively high proportions of quartz and carbonate but low modal abundances of ferromagnesian and ti oxide minerals similarly the till has cs co ni sc v cu pb and zn contents at about half paas the bentonite clay has slightly higher sio2 and cao contents than paas with lower abundances of most other trace elements including ree which plot at 0 7 to 0 9 paas the major and trace element composition is similar to continental arc andesites and the clay is interpreted to have a mixed provenance of arc andesite and eroded underlying sands the composition of the till appears to reflect mixed provenance erosion of the underlying cretaceous clay and sand paleozoic carbonate rocks and the precambrian shield the silicious ardkenneth sand has lower concentrations of ree and most trace elements compared to the clay but the most fractionated ree patterns the clay and till have inherited more subdued ree patterns in part from the underlying sand unit compositional variations between the units are marked by an upturn of nb th u and ree concentrations at the till clay interface 80 2 m and decrease of these elements at the clay sand boundary 156 2 m ce and eu anomalies vary erratically with depth the anomalies appear to reflect the degree of sedimentary sorting which in turn controls the modal abundances of detrital monazite apatite zircon and plagioclase feldspar zr zr hf hf and zr hf ratios are higher in the till than the clay and sand indicating an additional zr hf bearing phase with a distinctively high zr hf ratio the observed small compositional variations in the till and clay are not reflected in variations of geotechnical properties given that the ree budget of the till and clay are dominated by highly insoluble trace minerals it is unlikely that the ree patterns of porewaters will be controlled by bulk sample ree concentrations weathering of the upper 3 m of the till has oxidized fe2 bearing silicates oxides and sulphides and dissolved carbonates relative to the underlying unoxidized till weathering has not disturbed major ree or other trace element compositions

controls on the distribution of major ions in pore waters of a thick surficial aquitard transport and geochemical processes controlling the chemical and isotopic composition of pore waters in the upper 45 m of a thick surficial clay rich till battleford formation were studied the upper 3 m of the till is oxidized and fractured the remaining 77 m of the till the aquitard is nonfractured and unoxidized concentrations of total dissolved solids so42 na mg2 and k decrease with depth through the unoxidized zone to a depth of about 15 m below ground below which the concentrations remain constant a similar trend was observed for alkalinity however the decrease in concentration occurs over 20 m in contrast to these ions ca2 concentrations increase with depth through the upper 20 m unoxidized zone below which the concentrations remain relatively constant the distribution of dissolved ions shows the presence of two end members elevated solute concentrations in the oxidized zone attributed to geochemical weathering that occurred since the start of the holocene and connate pleistocene age water at depths between 20 and 45 m abiotic and biotic geochemical reaction rates in the aquitard were shown to be extremely slow or not occurring

implementation and application of adaptive mesh refinement for thermochemical mantle convection studies numerical modeling of mantle convection is challenging owing to the multiscale nature of mantle dynamics high resolution is often required in localized regions with coarser resolution being sufficient elsewhere when investigating thermochemical mantle convection high resolution is required to resolve sharp and often discontinuous boundaries between distinct chemical components in this paper we present a 2 d finite element code with adaptive mesh refinement techniques for simulating compressible thermochemical mantle convection by comparing model predictions with a range of analytical and previously published benchmark solutions we demonstrate the accuracy of our code by refining and coarsening the mesh according to certain criteria and dynamically adjusting the number of particles in each element our code can simulate such problems efficiently dramatically reducing the computational requirements in terms of memory and cpu time when compared to a fixed uniform mesh simulation the resolving capabilities of the technique are further highlighted by examining plume induced entrainment in a thermochemical mantle convection simulation

release of arsenic to deep groundwater in the mekong delta vietnam linked to pumping induced land subsidence deep aquifers in south and southeast asia are increasingly exploited as presumed sources of pathogen and arsenic free water although little is known of the processes that may compromise their long term viability we analyze a large area 1 000 km2 of the mekong delta vietnam in which arsenic is found pervasively in deep pliocene miocene age aquifers where nearly 900 wells at depths of 200 500 m are contaminated there intensive groundwater extraction is causing land subsidence of up to 3 cm y as measured using satellite based radar images from 2007 to 2010 and consistent with transient 3d aquifer simulations showing similar subsidence rates and total subsidence of up to 27 cm since 1988 we propose a previously unrecognized mechanism in which deep groundwater extraction is causing interbedded clays to compact and expel water containing dissolved arsenic or arsenic mobilizing solutes e g dissolved organic carbon and competing ions to deep aquifers over decades the implication for the broader mekong delta region and potentially others like it across asia is that deep untreated groundwater will not necessarily remain a safe source of drinking water

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 1 presented here are self consistent three dimensional simulations of mantle convection some of which display an approximation of plate tectonic behavior that is continuous in space and time plate behavior arises through a reasonable material description of silicate deformation with a simple yield stress being sufficient to give first order plate like behavior however the required yield strength or fault frictional coefficient is much less than experimentally determined values toroidal poloidal ratios are within geologically observed limits the sensitivity of the system to yield strength and the form of strength envelope is systematically investigated optimum plate character is obtained in a narrow range of yield strength below which diffuse boundaries and above which episodic behavior and eventually a rigid lid are observed models with mobile lids develop very long wavelength horizontal structure the longest wavelength possible in the domain two dimensional models display much greater time dependence than three dimensional models

sequential leachates of multiple grain size fractions from a clay rich till saskatchewan canada implications for controls on the rare earth element geochemistry of porewaters in an aquitard porewater ree concentrations vary by an order of magnitude over 45 vertical m in a thick clay rich till aquitard in southern saskatchewan to address controls on aqueous ree the till was disaggregated into seven size fractions 850 425 850 295 425 180 295 150 180 75 150 and 75 m each fraction was sequentially leached with water l1 1 m naoac ph 5 l2 0 25 m nh2oh hcl l3 1 m nh2oh hcl l4 12 m hcl kclo3 4 m hno3 l5 and the residue digested in na2o2 l6 aqueous leachate of the 75 m size fraction has a near flat ree pattern at 0 1 paas and carries 99 of ree relative to the total from all l1 leached size fractions aqueous leachates of the coarser size fractions have flat patterns 1 to 2 orders of magnitude lower leachate l2 extracts metals held electrostatically on inorganic or organic material the ree also plot at 0 1 paas with a convex up pattern and a maximum at gd leachate l3 of amorphous fe and mn oxyhydroxides carries more ree than l1 l2 or l4 and like l2 has a mildly convex up pattern there are lower ree abundances associated with crystalline fe and mn oxides l4 than with amorphous counterparts and hree are fractionated and depleted leaching of organic matter l5 produces ree patterns comparable to those of l4 the residue l6 has convex down ree patterns at 0 1 to 0 4 paas with prominent positive eu anomalies from plagioclase feldspar excepting the 75 m size fraction natural porewaters have mildly fractionated patterns with lree and mree depletion versus convex up patterns for leachates l1 l2 and l3 lree and mree may readily sorb onto clays or clay coatings in the till and be desorbed during leaching

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

geochemistry and mineralogy of shallow alluvial aquifers in daudkandi upazila in the meghna flood plain bangladesh the shallow alluvial aquifers of the delta plains and flood plains of bangladesh comprises about 70 of total land area are mostly affected by elevated concentrations of arsenic as in groundwater exposing a population of more than 35 million to as toxicity geochemical studies of shallow alluvial aquifer in the meghna flood plain show that the uppermost yellowish grey sediment is low in as 1 03 mg kg compared to the lower dark grey to black sediment 5 24 mg kg rich in mica and organic matter sequential extraction data show that solid phase as bound to poorly crystalline and amorphous metal fe mn al oxyhydroxides is dominant in the grey to dark grey sediment and reaches its maximum level 3 05 mg kg in the mica rich layers amount of as bound to sulphides and organic matter also peaks in the dark grey to black sediment vertical distributions of major elements determined by x ray fluorescence xrf show that iron fe2o3 aluminum al2o3 and manganese mno follow the general trend of distribution of as in the sediments concentrations of as mn fe hco3 so42 and no3 in groundwater reflect the redox status of the aquifer and are consistent with solid phase geochemistry mineralogical analysis by x ray diffraction xrd and scanning electron microscopy sem fitted with energy dispersive x ray spectrometer eds revealed dominance of crystalline iron oxides and hydroxides like magnetite hematite and goethite in the oxidised yellowish grey sediment amorphous fe oxyhydroxides identified as grain coating in the mica and organic matter rich sediment suggests weathering of biotite is playing a critical role as the source of fe iii oxyhydroxides which in turn act as sink for as presence of authigenic pyrite in the dark grey sediment indicates active reduction in the aquifer

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

application of arsenic in barite as a redox indicator for suboxic anoxic redox condition abstract redox condition is an important factor that controls the behavior of various elements in an aquatic environment incorporation of redox sensitive trace elements in minerals describes redox conditions in water and provides information about past environmental changes unless it is dissolved or recrystallized during sediment burial in the present study we explore the application of arsenate arsenite ratios as v as iii in barite as a new geochemical proxy for paleo environmental reconstruction laboratory experiments showed that both as iii and as v could be incorporated into barite as as iii and as v coprecipitated barite respectively which could more or less retain the information of the as v as iii ratio in coexistent water if under equilibrium in terms of the redox reactions furthermore the as v as iii ratios in natural barite collected in i tamagawa hot spring and ii okinawa hydrothermal vent were determined by micro x ray fluorescence and micro x ray absorption fine structure analysis to a evaluate the reliability of the as v as iii ratio in barite as a redox indicator in natural system and b estimate the depositional redox condition in water whether barite precipitated below or above the redox boundary of as v as iii or the suboxic anoxic boundary our previous study showed that barite selenium oxyanion system could also be used as a proxy for the oxic suboxic boundary because the selenate selenite se vi se iv ratio in barite is primarily correlated with the ratio in water given that the redox boundary of the se vi se iv ratio has higher redox potential than the as v as iii boundary the combination of these two systems can extend the redox condition which can be specified by the analysis of one single barite grain to a wide region thus one barite particle can provide information on the as v as iii and se vi se iv ratios in water and whether barite precipitated under oxic suboxic or anoxic redox environments

source and distribution of naturally occurring arsenic in groundwater from alberta s southern oil sands regions abstract arsenic as concentrations as high as 179 g l have been observed in shallow groundwater in the alberta s southern oil sand regions the geology of this area of alberta includes a thick cover up to 200 m of unconsolidated glacial deposits with a number of regional interglacial sand and gravel aquifers underlain by marine shale arsenic concentrations observed in 216 unconsolidated sediment samples ranged from 1 and 17 ppm a survey of over 800 water wells sampled for as in the area found that 50 of the wells contained as concentrations exceeding drinking water guidelines of 10 g l higher as concentrations in groundwater were associated with reducing conditions measurements of as speciation from 175 groundwater samples indicate that as iii was the dominant species in 74 of the wells speciation model calculations showed that the majority of groundwater samples were undersaturated with respect to ferrihydrite suggesting that reductive dissolution of fe oxyhydroxides may be the source of some as in groundwater detailed mineralogical characterization of sediment samples collected from two formations revealed the presence of fresh framboidal pyrite in the deeper unoxidized sediments electron microprobe analysis employing wavelength dispersive spectrometry indicated that the framboidal pyrite had variable as content with an average as concentration of 530 ppm reaching up to 1840 ppm in contrast the oxidized sediments did not contain framboidal pyrite but exhibited spheroidal fe oxyhydroxide grains with elevated as concentrations the habit and composition suggest that these fe oxyhydroxide grains in the oxidized sediment were an alteration product of former framboidal pyrite grains x ray absorption near edge spectroscopy xanes indicated that the oxidized sediments are dominated by as v species having spectral features similar to those of goethite or ferrihydrite with adsorbed as suggesting that fe oxyhydroxides are the dominant as carriers xanes spectra collected on unoxidized sediment samples in contrast indicated the presence of a reduced as species as i characteristic of arsenopyrite and arsenian pyrite the results of the mineralogical analyses indicate that the oxidation of framboidal pyrite during weathering may be the source of as released to shallow aquifers in this region

time domain parallelization for computational geodynamics 1 i present a time domain parallelization approach for geodynamic modeling this algorithm named parareal is based on the use of coarse sequential and fine parallel propagators to predict and to iteratively correct the solution of the governing equations over a given time interal although the method has been successfully used to solve differential equations in various scientific areas it has not been applied to model solid state convective motions relevant to the earth and other planetary mantles in that case the time dependence of the velocity is only implicit which requires modifications to the original algorithm the performances of this adapted version of the parareal algorithm were investigated using theoretical model predictions in good agreement with numerical experiments i show that under optimum conditions the parallel speedup increases linearly with the number of processors and speedups close to 10 were measured using only few tens of cpus this parareal approach can be used alone or combined with any spatial parallel algorithm allowing significant additional increase in speedup with increasing number of processors

distribution of arsenic and other trace elements in the holocene sediments of the meghna river delta bangladesh geochemical study of the holocene sediments of the meghna river delta chandpur bangladesh was conducted to investigate the distribution of arsenic and related trace and major elements the work carried out includes analyses of core sediments and provenance study by rare earth element ree analysis results showed that the cores pass downward from silty clays and clays into fine to medium sands the uppermost 3 m of the core sediments are oxidized average oxidation reduction potential orp 230 mv and the orp values gradually become negative with depths 45 to 170 mv indicating anoxic conditions prevail in the meghna sediments the ree patterns of all lithotypes in the study areas are similar and are comparable to the average upper continental crust arsenic and other trace elements pb zn cu ni and cr have greater concentrations in the silts and clays compared to those in the sands positive correlation between as and fe was found in the sediments indicating as may be adsorbed on fe oxides in aquifer sediments

geochemical study of arsenic and other trace elements in groundwater and sediments of the old brahmaputra river plain bangladesh the geochemical study of groundwaters and core sediments from the old brahmaputra plain of bangladesh was conducted to investigate the distribution of arsenic and related trace elements groundwaters from tube wells are characterized by ph of 6 4 7 4 dissolved oxygen do of 0 8 1 8 mg l ca contents of 5 50 mg l and fe contents of 0 2 12 9 mg l arsenic concentrations ranged from 8 to 251 g l with an average value of 63 g l a strong positive correlation exists between as and fe r2 0 802 p 0 001 concentrations in groundwater the stratigraphic sequences in the cores consist of yellowish silty clays at top passing downward into grayish to yellowish clays and sands the uppermost 3 m and lower parts from 13 to 31 m of the core sediments are oxidized average oxidation reduction potential orp 170 and 220 mv respectively and the orp values gradually become negative from 3 to 13 m depths 35 to 180 mv indicating that anoxic conditions prevail in the shallow aquifers of the brahmaputra plain age determinations suggest that clay horizons at 10 m depth were deposited at around 2 000 and 5 000 years bp 14c ages during the transgressive phase of sea level change elevated concentrations of as pb zn cu ni cr and v are present in the silts and clays probably due to adsorption onto clay particles significant concentrations of as occur in black peat and peaty sediments at depths between 9 and 13 m a strong positive correlation between as and fe was found in the sediments indicating as may be adsorbed onto fe oxides in aquifer sediments

present environment of dam lake sambe southwestern japan a geochemical study of bottom sediments geochemical analyses of lakebed and core sediments from lake sambe on the outskirts of oda city in shimane prefecture in southwestern japan were carried out in order to assess the water quality and the concentration and distribution patterns of sixteen elements the lake water showed a stratified condition with respect to dissolved o2 and as fe and mn concentrations in the bottom layers which increased in the summer the chemical composition of the sediments as measured by x ray fluorescence included major and trace elements p ca sc ti v cr fe ni cu zn as sr zr pb and th and total sulfur ts elevated values of as zn v fe p and ts were present in several layers of the upper cores from 0 to 5 cm and other surface sediments increases in the abundances of these metals in lake sediments are probably related to the reducing condition of the sediments fine grained organic rich sediments and post depositional diagenetic remobilization moreover correlations between the concentrations of trace metals and iron in the sediments suggest their adsorption onto fe oxy hydroxides whereas correlations with sulfur indicate that they were precipitated as fe sulfides the average abundances of as pb zn and cu exceeded the lowest effect level and interim sediment quality guideline values that the new york state department of environmental conservation and the canadian council of ministers of the environment determined to have moderate impact on aquatic organisms in addition concentrations of as and zn exceeded the coastal ocean sediment database threshold value indicating potentially toxic levels therefore the presence of trace metals in the lake sediments may result in adverse effects on biota health

role of aquitards in hydrogeochemical systems a synopsis abstract aquitards exert significant influence on the hydrogeochemistry of aquifer systems this influence is manifested somewhat differently depending on the relative position of aquitards within a system in the deeper regimes they are influential in the origin and distribution of brines and the development of geopressured zones in intermediate regimes they form multi layered aquifer systems and provide a source of reactive minerals and exchangeable ions in shallow regimes aquitards can influence the topography and drainage patterns this influences the relationship between the water table and the potentiometric surface of confined aquifers controls the rates of infiltration and discharge and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas in coastal areas aquitards can determine the depth of the saltwater freshwater interface its distance from the shoreline and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity

chloride and chlorine isotopes 36cl and δ37cl as tracers of solute migration in a thick clay rich aquitard system detailed vertical profiles of dissolved cl and its isotopes 36cl and 37cl provided new information on the origin and systematics of this conservative tracer in pore waters of a thick aquitard system the aquitard system consists of surficial quaternary clay rich till 80 m thick deposited 30 20 kyr b p overlying cretaceous marine clay 76 m thick deposited 71 ma the distribution of cl 37cl and br showed the presence of five distinct end members for cl the top of the unoxidized till a regional aquifer underlying the cretaceous clay two localized geological heterogeneities sand streaks in the till and glacial meltwater emplaced with the till and still present at depths of between 36 and 60 m numerical simulations of the transport of cl from the sand streaks indicated that this geochemical profile has been developing throughout most of the holocene the 36cl measurements showed that the age of the dissolved cl in the upper cretaceous clay is likely between 0 75 and 1 9 myr the 36cl measurements further suggested that the dissolved cl in the till was not directly derived from the underlying cretaceous clay finally it was not possible to quantify the effects of isotopic fractionation of 37cl relative to 35cl because of diffusion in this aquitard system

effects of salinity and an ions on arsenic behavior in sediment of bosten lake northwest china bosten lake the largest inland lake in china has been contaminated with arsenic as due to anthropogenic activities salinity of the lake water has been increasing due to the extremely dry climate and salt drainage from the farmland effects of the increasing salinity on the geochemical behavior of arsenic in sediment are still unclear in this study vertical variation of geochemical fractions of as in sediment and arsenic concentration in porewater along the salinity gradient of bosten lake was studied effects of salinity and ions and anions in porewater on arsenic concentration were also examined arsenic content in the five tessier fractions followed the decreasing order of residual exchangeable fe and mn oxides organic carbonate the ecological risk of arsenic in the sediment was at low or medium levels indicated by the low percentage of exchangeable and carbonate bound fractions contrary to the decreasing electrical conductivity of porewater with depth arsenic concentration in porewater showed an increasing trend with depth arsenic concentration in porewater increased from around zero at the surface layer of sediment to over 600 g l 1 in deeper layers of some sites arsenic concentration inversely correlated with electrical conductivity in porewater arsenic concentration generally decreased with increasing concentrations of so4 2 ca2 mg2 and na but showed no clear relation with concentrations of k and cl this study implies that increase of salinity may decrease the release of arsenic from sediment and its ecological risk

the source and transport of arsenic in a bedrock aquifer new hampshire usa the geographic distribution of elevated groundwater as concentrations in a fractured silicate bedrock aquifer in central new hampshire correlates with the presence of pegmatites which border late devonian granites and intrude metasedimentary rocks as concentrations in the pegmatites averaged 9 6 mg kg which is much higher than the associated granites 0 24 mg kg and metasedimentary rocks 0 8 mg kg as was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low as concentrations and pegmatites with high as concentrations arsenic behaves similarly to b which was also concentrated into these late stage fluids arsenopyrite feass with an oxidation reaction rim of scorodite feaso4 2h2o was observed in aquifer materials elevated as concentrations have been observed to occur in other new england locations in pelitic metasediments intruded by anatectic plutons it is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating as in crystalline aquifer materials which can then cause local as enrichment of groundwaters groundwater as concentrations ranged from 26 nmol l to 5300 nmol l with a median value 210 nmol l that is more than 30 times higher than the median for groundwaters from all of new hampshire 6 5 nmol l high chloride concentrations 1 mmol l resulting from road salt contamination of recharge waters suggest that groundwaters are most likely young 670 nmol l or 50 g l have very low fe concentrations 7 samples with low as 7 fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb as at ph 7 fe oxyhydroxide formation is slow and depends on dissolved o2 availability however the resultant fe oxyhydroxides have a positive net surface charge and appear to adsorb as readily

an alternative view on the origin of chemical and isotopic patterns in groundwater from the milk river aquifer canada the milk river artesian aquifer underlies 15 000 km2 of southern alberta canada it consists of thin 30 75 m thick sandstone and is confined above by the pakowki shale typically 120 m thick and below by the colorado shale the aquifer subcrops in southern alberta and northern montana groundwater movement is to the north west and east from the outcrop area dominant recharge area cl and i concentrations increase in the direction of flow from less than 0 05 and 0 001 mmol l respectively near the recharge area to more than 140 and 0 15 mmol l at the northern edge of the aquifer similarly waters become more enriched in oxygen 18 and deuterium from less than 21 0 and 167 near the recharge area to values approaching 8 0 and 70 0 in the north isotope values in the recharge area plot on the global meteoric water line and indicate that the recharge waters are isotopically unaltered meteoric waters downgradient from the recharge area the data deviate from the meteoric water line slope of 6 3 instead of 8 0 three mechanisms have been advanced to explain the origin of the chemical and isotopic patterns the introduction of connate formation water through the colorado shale and subsequent mixing with infiltrating meteoric water a finite source of meteoric recharge mixing with more saline water in the aquifer and chemical and isotopic enrichment due to ion filtration hydraulic conductivity and hydraulic head data cl concentrations of drill cuttings and isotopic values from groundwater from the colorado shale provide arguments for reevaluating these ideas an alternative mechanism based on diffusion between saline isotopically enriched water in the colorado shale and fresher isotopically depleted water in the aquifer is tested using an analytical mass transport model although uncertainty exists in the model parameters results showed that the patterns in cl i 18o and 2h in the aquifer can be explained by aquitard diffusion further aquitard diffusion must be considered when using chlorine 36 dating methods in aquifer aquitard systems such as the milk river aquifer

distribution of the rare earth elements in porewaters from a clay rich aquitard sequence saskatchewan canada a 91 4 m thick porewater profile of the rare earth elements rees is presented from a thick clay rich pleistocene till 0 80 m and cretaceous clay 80 156 m aquitard sequence saskatchewan canada the upper 3 4 m of till is weathered absolute aqueous concentrations of the rees vary by more than one order of magnitude aqueous ree concentrations decrease with depth through the weathered till to a depth of about 15 m below ground below which the concentrations in the unweathered till remain relatively consistent the only porewater sample from the clay 91 4 m has the highest total ree concentration elevated ree concentrations in the oxidized zone attributed to geochemical weathering there is no significant seasonal variation in aqueous ree concentrations within the water columns in the piezometers redox conditions and solution complexation reactions exert predominant controls on the aqueous ree concentrations the majority of ree patterns are coherent characterized by overall negatively fractionated ree with convex down profiles from la to pr or nd and convex up from nd to tb in the porewater sample from the clay the lrees are an exception with a flat normalized profile hree lree fractionations in the porewaters are depth dependent and are mainly controlled by solution complexation with dissolved sulfate phosphate free carbonate ions and doc porewaters in the unweathered till 5 76 2 m are characterized by negative ce anomalies which may reflect in part negative ce anomalies in the host till geochemistry the negative ce anomalies are magnified relative to the host tills during water rock interaction and ree transportation porewaters from the weathered till exhibit strong positive ce anomalies relative to paas the host rocks and their leachates the ce anomalies are predominantly controlled by the redox conditions and solution complexation the large positive ce anomalies in these porewaters are probably caused by the oxidation of ce iii to ce iv that was stabilized in solution through complexation with high contents of inorganic and organic ligands positive eu anomalies relative to paas the host rocks and leachates of the host rocks result from hydrolysis of plagioclase feldspar the limited depth profile for total dissolved rees across the weathered unweathered till boundary is retarded with respect to that previously established for na k mg2 so42 and tds suggesting that chemical reactions control the migration of dissolved rees

the spiral grid a new approach to discretize the sphere and its application to mantle convection 1 this paper presents a new method to generate a three dimensional spherical grid using natural neighbor voronoi cells distributed by spiral functions a unique property of this grid is the complete removal of symmetries with arbitrary selectable lateral and radial resolution which are not restricted to discrete radial levels or geometrical constraints as compared to the commonly used grids based on projected triangulated platonic solids such as a cube a rhomboid or an icosahedron the spiral grid can be refined in certain areas of interest and makes it possible to have a very small inner radius to outer radius ratio cell volumes can be made almost constant throughout the computational domain analysis statistics and computation methods are described in detail as well as a possible domain decomposition suitable for parallel computing conductive temperature profiles were numerically calculated in the spherical shell and directly compared with the analytic solution as verification the grid is applied to numerical simulations of mantle convection using a finite volume scheme the model is validated by a comparison of steady state cubic and tetrahedral convection patterns with other published models

distribution of arsenic iii arsenic v and total inorganic arsenic in porewaters from a thick till and clay rich aquitard sequence saskatchewan canada abstract inorganic arsenic species were measured in the porewaters collected from eighteen piezometers installed between 3 and 91 4 m below ground in a thick till and clay rich aquitard sequence located in southern saskatchewan to investigate the distribution of and controls on arsenic speciation in the sequence aqueous concentrations of as v as iii and total as are in the range of 0 31 97 0 71 21 and 3 2 98 ppb respectively profiles of as iii and as v concentration distribution with depth broadly track that of total as erratic increases to 15 2 m then more uniform concentrations to 88 m aqueous arsenic is accumulated at the upper redox transition zone 6 14 m the alkaline porewater at 91 4 m contains the highest concentrations of as v and total as which might result from the facilitated desorption of arsenate from the host solid due to decrease of positive surface charge of the oxides in alkaline solution the ratio of as v as iii is greater than unity in the uppermost oxidized porewater 3 m less than unity from 4 6 to 71 6 m and greater than unity in the lowest four porewaters 76 2 to 91 4 m in the 3 m porewater low as iii but high as v as iii is due to the oxidized nature of the near surface weathered till the high as v as iii in the deepest porewater at 91 4 m likely results from the enhanced and heterogeneous oxidation of as iii to as v on clay mineral surfaces in the alkaline solution total as and arsenic speciation may not be controlled by as fe or mn concentrations in the host till or clay dissolved as v and total as positively covary with aqueous chloride whereas dissolved as iii is independent of aqueous chloride aqueous as iii and to a less extent as v and total as are positively correlated with dissolved mn in the till in the clay aqueous as v and total as show strong negative covariation with mn however aqueous as iii as v and total as exhibit almost no correlation with total dissolved fe in the till the as v as iii ratio has strong negative correlation with dissolved mn but positive covaration with dissolved chloride generally good agreement between the redox potentials eh calculated from aqueous as v and as iii concentrations and those measured by a pt electrode throughout most of the unoxidized till suggests the suitability of using as v as iii redox couple as a redox indicator for the studied aquitard system however large negative bias of the calculated eh from the measured eh in the oxidized till upper unoxidized till and the clay is attributed to errors associated with the field measurements of eh

dynamical consequences in the lower mantle with the post perovskite phase change and strongly depth dependent thermodynamic and transport properties article i nfo we have carried out numerical simulations of large aspect ratio 2 d mantle convection with the deep phase change from perovskite pv to post perovskite ppv using the extended boussinesq approximation for a fluid with temperature and pressure dependent viscosity we have investigated the effects of various ppv phase parameters on the convective planform heat transport and mean temperature and viscosity profiles since ppv is expected to have a relatively weak rheology with respect to pv and a large thermal conductivity we have assumed that the transition from pv to ppv is accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an increase in thermal conductivity by a factor of 2 furthermore we have analyzed the combined effects of a strongly decreasing thermal expansivity in pv and steeply increasing thermal conductivity according to recent evidence from high pressure experiments and first principle calculations as long as the thermal expansivity and conductivity are constant ppv exerts a small but noticeable effect on mantle convection it destabilizes the d layer causes focusing of the heat flux peaks and an increase of the average mantle temperature and of the temporal and spatial frequency of upwellings when the latest depth dependent thermal expansivity and conductivity models are introduced the effects of ppv are dramatic on the one hand without ppv we obtain a very sluggish convective regime characterized by a relatively cool mantle dominated by large downwellings that tend to stagnate beneath the transition zone with ppv on the other hand we observe an extremely significant increase of the average mantle temperature due to the formation of large sized and vigorous upwellings that in some cases tend to cluster thus forming superplumes if a very large thermal conductivity at the core mantle boundary is assumed k 20 wk 1 m 1 we obtain a quasi steady state regime characterized by large and stable plumes with long lifetimes the combination of strongly depth dependent expansivity and conductivity is a viable mechanism for the formation of long wavelength long lived thermal anomalies in the deep mantle even if a low viscosity ppv atop the core mantle boundary is included

a systematic 2 d investigation into the mantle wedge s transient flow regime and thermal structure complexities arising from a hydrated rheology and thermal buoyancy 1 arc volcanism at subduction zones is likely regulated by the mantle wedge s flow regime and thermal structure and hence numerous studies have attempted to quantify the principal controls on mantle wedge conditions in this paper we build on these previous studies by undertaking a systematic 2 d numerical investigation into how a hydrated rheology and thermal buoyancy influence the wedge s flow regime and associated thermal structure we quantify the role of a range of plausible i water contents 0 5000 h 106si ii subduction velocities 2 10 cm yr and iii upper plate ages 50 120 myr finding that small scale convection ssc resulting from rayleigh taylor instabilities or drips off the base of the overriding lithosphere is a typical occurrence the morphology of ssc varies with viscosity and subduction parameters with drips at their most prominent when subduction velocities and wedge viscosities are low our results confirm that high subduction velocities and wedge viscosities promote a dominantly corner flow regime and strong upper plate erosion below the arc region by contrast we find that back arc upper plate erosion by ssc is largely controlled by wedge viscosity occurring when i viscosities are 5 1018 pa s and ii the length of the upper plate available for destabilization exceeds the characteristic wavelength of instabilities thus if hydrous weakening of wedge rheology extends at least 100 150 km from the trench our 2 d models predict an unstable flow regime resulting in temperature fluctuations of 50 100 k which are sufficient to influence melting and the stability of hydrous minerals

a community benchmark for viscoplastic thermal convection in a 2 d square box numerical simulations of thermal convection in the earth s mantle often employ a pseudoplastic rheology in order to mimic the plate like behavior of the lithosphere yet the benchmark tests available in the literature are largely based on simple linear rheologies in which the viscosity is either assumed to be constant or weakly dependent on temperature here we present a suite of simple tests based on nonlinear rheologies featuring temperature pressure and strain rate dependent viscosity eleven different codes based on the finite volume finite element or spectral methods have been used to run five benchmark cases leading to stagnant lid mobile lid and periodic convection in a 2 d square box for two of these cases we also show resolution tests from all contributing codes in addition we present a bifurcation analysis describing the transition from a mobile lid regime to a periodic regime and from a periodic regime to a stagnant lid regime as a function of the yield stress at a resolution of around 100 cells or elements in both vertical and horizontal directions all codes reproduce the required diagnostic quantities with a discrepancy of at most 3 in the presence of both linear and nonlinear rheologies furthermore they consistently predict the critical value of the yield stress at which the transition between different regimes occurs as the most recent mantle convection codes can handle a number of different geometries within a single solution framework this benchmark will also prove useful when validating viscoplastic thermal convection simula tions in such geometries

natural arsenic contamination of holocene alluvial aquifers by linked tectonic weathering and microbial processes 1 linked tectonic geochemical and biologic processes lead to natural arsenic contamination of groundwater in holocene alluvial aquifers which are the main threat to human health around the world these groundwaters are commonly found a long distance from their ultimate source of arsenic where chemical weathering of as bearing minerals occurs we propose a gbh as model that ties together all of the important tectonic biologic and hydrologic processes that cause natural as release in groundwater in holocene terrestrial deposits processes highlighted by the gbh as model can explain the movement of arsenic from lithosphere to hydrosphere however we propose a factor that has increased dissolved as concentrations in holocene aquifers to levels where human health is threatened mechanical weathering associated with pleistocene glaciation our model invokes erosion of mountain belts aided by glaciers transport of arsenic by surface waters adsorption of as by stream sediments and deposition of stream sediments and organic matter in alluvial deposits subsequently fe iii reducing bacteria present in alluvial aquifers cause the release of sorbed as to groundwater under moderately reducing conditions

planforms of self consistently generated plates in 3d spherical geometry 1 in the past decade several studies have documented the effectiveness of plastic yielding in causing a basic approximation of plate tectonic behavior in mantle convection models with strongly temperature dependent viscosity strong enough to form a rigid lid in the absence of yielding the vast majority of such research to date has been in either two dimensional or three dimensional cartesian geometry in the present study mantle convection calculations are performed to investigate the planform of self consistent tectonic plates in three dimensional spherical geometry the results are compared to those of similar calculations where a three dimensional cartesian geometry is used we found that when yield stress of the lithosphere is low 20 mpa a great circle subduction zone forms at low intermediate yield stresses 100 mpa plates spreading centers and subduction zones formed and were destroyed over time at high intermediate yield stresses 200 mpa two plates form separated by a great circle boundary that is a spreading centre on one side and a subduction zone on the other side at high yield stresses 400 mpa a rigid lid was observed the great circle subduction zone and the rigid lid are stable over time while at intermediate yield stresses some episodic behavior is observed the spherical cases showed a higher more earth like toroidal poloidal ratio of the surface velocity field than the cartesian cases

contamination risk assessment of fresh groundwater using the distribution and chemical speciation of some potentially toxic elements in calabar southern nigeria the computer program phreeqc was used to determined the distribution chemical speciation and mineral saturation indices in a fresh groundwater environment with limited mining activities in the adjoining areas the aim was mainly to determine the potential risk of a coastal plain aquifer contamination by some potentially toxic elements the results show that the elements ba cd cu fe mn ni rb sr and zn are distributed as free metal ions arsenic is in the neutral form of h3aso3 o while three species of aluminium al3 aloh2 al oh 2 dominate the major species of uranium include uo2co3 uo22 uo2 and uo2oh respectively in order of abundance the groundwater is saturated with respect to alunite kal3 so4 2 oh 6 basaluminite al4 oh 10 so4 boehmite al oh cu metal cu cuprous ferrite cufeo2 diaspore alo oh gibbsite al oh 3 goethite feooh hematite fe2o3 magnetite fe3o4 and uraninite uo2 most of the species are not mobile under the prevailing ph 3 3 to 5 9 and eh 7 to 158 mv conditions the mobile ones are very low in concentration and will be immobilized by precipitation of mineral phases the study concludes that presently these species do not pose any risk to the aquifer

hydrogeology of a thick clay till and cretaceous clay sequence saskatchewan canada clay rich glacial till and cretaceous clay are common throughout the interior plains of north america quantifying groundwater flow through these aquitards has implications for solute transport in aquitards and protection of underlying aquifers groundwater flow through a two tiered aquitard system was investigated using laboratory and field methods at a test site in saskatchewan canada the aquitard system consists of 80 m of uniform plastic clay rich battleford till deposited 12 18 ka bp disconformably overlying 77 m of late cretaceous plastic marine clay snakebite member deposited 70 72 ma bp the upper 3 4 m of till is oxidized and fractured whereas the remainder is unoxidized for the scales investigated results suggested that hydraulic conductivity k is independent of scale for relatively thick till and clay bedrock deposits analysis of slug tests in the unoxidized till and laboratory tests on cores of unoxidized till yielded geometric mean k values of 5 4 10 11 and 2 7 10 11 m s res

contamination of shallow aquifers by arsenic in upper reaches of tista river at siliguri jalpaiguri area of west bengal india arsenic is present in groundwater at siliguri jalpaiguri area west bengal india this is the place where tista river descending from the himalayas meets the alluvial plain the area represents alluvial fan and floodplains of tista mahananda balasan jaladhaka and its tributaries in the river sediment samples para and ferro magnetic minerals within 0 3 0 05 mm fraction contain 9 80 ppm of arsenic the study indicates that iron bearing minerals viz biotite hornblende as well as iron coated grains of the sediment are major contributors towards arsenic budget though magnetite as a mineral shows maximum arsenic content 22 ppm it is volumetrically not of much significance measurement of groundwater collected from tube wells shows up to 0 05 ppm of arsenic these arsenic contaminated tube wells occur in a linear fashion along the course of the rivers moreover localization of contaminated tube wells coincides with the change of channel gradient as observed in longitudinal section the study enumerates a cause effect relationship of arsenic occurrence with river gradient and fluvial sedimentation

fluidity a fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics we present a new computational modeling framework fluidity for application to a range of two and three dimensional geodynamic problems with the focus here on mantle convection the approach centers upon a finite element discretization on unstructured simplex meshes which represent complex geometries in a straightforward manner throughout a simulation the mesh is dynamically adapted to optimize the representation of evolving solution structures the adaptive algorithm makes use of anisotropic measures of solution complexity to vary resolution and allow long thin elements to align with features such as boundary layers the modeling framework presented differs from the majority of current mantle convection codes which are typically based upon fixed structured grids this necessitates a thorough and detailed validation which is a focus of this paper benchmark comparisons are undertaken with a range of two and three dimensional isoviscous and variable viscosity cases in addition model predictions are compared to experimental results such comparisons highlight not only the robustness and accuracy of fluidity but also the advantages of anisotropic adaptive unstructured meshes significantly reducing computational requirements when compared to a fixed mesh simulation

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s

geochemistry of dissolved inorganic carbon in a coastal plain aquifer 1 sulfate from confining beds as an oxidant in microbial co2 production a primary source of dissolved inorganic carbon dic in the black creek aquifer of south carolina is carbon dioxide produced by microbially mediated oxidation of sedimentary organic matter groundwater chemistry data indicate however that the available mass of inorganic electron acceptors oxygen fe iii and sulfate and observed methane production is inadequate to account for observed co2 production although sulfate concentrations are low approximately 0 05 0 10 mm in aquifer water throughout the flow system sulfate concentrations are greater in confining bed pore water 0 4 20 mm the distribution of culturable sulfate reducing bacteria in these sediments suggests that this concentration gradient is maintained by greater sulfate reducing activity in sands than in clays calculations based on fick s law indicate that possible rates of sulfate diffusion to aquifer sediments are sufficient to explain observed rates of co2 production about 10 5 mmoll 1 year 1 thus eliminating the apparent electron acceptor deficit furthermore concentrations of dissolved hydrogen in aquifer water are in the range characteristic of sulfate reduction 2 6 nm which provides independent evidence that sulfate reduction is the predominant terminal electron accepting process in this system the observed accumulation of pyrite and calcite cemented sandstones at sand clay interfaces is direct physical evidence that these processes have been continuing over the history of these sediments