speciation of arsenic in greek travertines co precipitation of arsenate with calcite abstract the western part of the chalkidiki peninsula in northern greece is a geothermally active area that contains high levels of naturally derived arsenic in its alkaline groundwaters up to 3760 g l near wells equilibration of these groundwaters with atmospheric carbon dioxide leads to the precipitation of travertines that contain very high levels of arsenic up to 913 mg kg to determine the mechanism of arsenic uptake in these travertines we analyzed two different types of travertine from this region using both bulk and micro focused x ray absorption spectroscopy xas and xas and micro focused x ray fluorescence spectroscopy xrf bulk xas showed that in all of the studied samples arsenic is present in the pentavalent oxidation state arsenate xrf analyses indicated that arsenic is closely associated with the calcite matrix and that it generally does not correlate well with iron the arsenic k edge xas spectra of all samples closely matched each other and closely resembled a reference spectrum for arsenate coprecipitated with calcite rather than adsorbed or pure calcium arsenate iron on the other hand was found to be mainly present as a constituent of clay minerals of presumably detrital origin suggesting that iron hydr oxides were not sufficiently abundant to act as major scavengers for arsenic in the chalkidiki travertines we estimated that calcite in these travertines could sequester at least 25 of aqueous arsenic in the form of as v and thus immobilize a substantial part of arsenic present in the geothermal groundwaters these results may also be relevant for other areas where geothermal groundwaters carry arsenic to the surface and possibly as well for arsenic geochemistry in other environments with co2 enriched water

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

determination of shale stiffness tensor from standard logs a technique allowing inversion of the shale stiffness tensor from standard logging data sonic velocities density porosity and clay content is developed the inversion is based on the effective medium theory the testing of the technique on laboratory measurements of the elastic wave velocities in shale samples shows that the inversion makes it possible to predict the elastic wave velocities vp vs1 and vs2 in any direction within an error of a few per cent the technique has been applied for the stiffness tensor inversion along a well penetrating a shale formation of the mississippian age altered by thin layers of limestone it is demonstrated that the symmetry of a stiffness tensor inverted at the sonic frequency 2 khz is slightly orthorhombic and taking into account the experimental errors can be related to the vertical transverse isotropy symmetry for the productive interval of the shale formation the thomsen parameters and average respectively 0 32 0 25 and 0 21 which indicate anelliptic behaviour of the velocities in this shale the coefficients of anisotropy of this shale interval are around 24 and 20 for the compressional and shear waves respectively the values of the inverted velocities in the bedding plane for this interval are in good agreement with the laboratory measurements the technique also allows inversion of the water saturation of the formation sw and the inverted values are in agreement with the sw values available for this formation a backus like upscaling of the inverted stiffness tensors is carried out for the lower and upper bounds of the frequency band used in the crosswell tomography 100 hz and 500 hz these results can serve as an initial velocity model for the microearthquake location during hydrofracking of the shale formation

external validation of the grace ggm01c gravity field using gps and doris positioning results 1 we have used the new gravity field model ggm01c derived from grace data to reanalyze doris data from 1993 0 to 2003 2 using the gipsy oasis software and a free network approach we have estimated the position and velocity of each doris station in itrf2000 in order to test the accuracy of these results we have compared them to the positions and velocities of 43 collocated gps stations using local ties and covariance information doris results computed using the ggm01c gravity field instead of the egm96 gravity field show a significantly improved external agreement with gps position agreement of 12 26 mm was reduced to 10 13 mm and velocity agreement of 3 3 3 7 mm yr was reduced to 2 4 3 3 mm yr this can be interpreted as an external test of the accuracy of the new ggm01c gravity field

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

the role of the subcutaneous zone in karst hydrology abstract the subcutaneous zone is the upper weathered layer of rock beneath the soil but above the permanently saturated phreatic zone it is of particular hydrological importance in karst because of its high secondary permeability arising from the considerable chemical solution in this zone however corrosional enlargement of fissures diminishes with depth thus permeability decreases in the same direction with the result that percolation is inhibited except down widened master joints and faults storage of water consequently occurs in this zone particularly after storms the upper surface of this suspended saturated layer in the subcutaneous zone is defined by a perched water table which slopes towards points of rapid vertical percolation the potential induces lateral water movement converging on the most permeable areas such as beneath dolines leakage from the subcutaneous store sustains slow percolation in the vadose zone cross correlation of rainfall with percolation rates in caves in new mexico u s a and new zealand reveal response lags of 2 14 weeks with no apparent relationship to depth below the surface other percolation sites show no correlation with rainfall interpreted as being a consequence of considerable friction in tight fissure networks the recognition of storage and rapid as well as very slow percolation from the subcutaneous zone requires re interpretation of the components of hydrographs from karst springs and of some conceptual models of karst aquifers the importance of subcutaneous storage in sustaining baseflow discharge at some sites must be recognised as must the contribution of subcutaneous water to flood hydrographs methods of estimating the volumes of subcutaneous and phreatic components of karst spring flood hydrographs are presented the paper concludes with a discussion of the significance of subcutaneous hydrologic processes for an understanding of karst geomorphology the desirability of explaining karst landform evolution in terms of hydrologic processes is stressed

a classification for the pressure sensitivity properties of a sandstone rock frame a three parameter formula is proposed to describe the sensitivity of the bulk modulus or shear rigidity for a sandstone rock frame under applied isotropic loading the theoretical basis for this relation is the use of excess compliance as a pseudo function to describe all internal weaknesses in the rock regardless of their origin the law is fitted to 179 sets of laboratory measurements on unsaturated reservoir core and outcrop sandstones that have low to moderate porosity and a range of clay fractions and cementation a stress index defined to quantitatively rank the reservoir sands indicates that laboratory derived pressure sensitivity is highest 10 per mpa maximum for the clean moderate porosity paleocene sands from the forties formation and west of shetlands whereas the lower porosity cemented and more consolidated permian sands rank as least pressure sensitive 1 per mpa maximum higher porosity sands are stress sensitive because of the overall rock frame compressibility all of t

estimation of effective anisotropy simultaneously with locations of microseismic events passive seismic tomography in which the event locations and the velocity model are inferred simultaneously is seldom used to process microseismic surveys acquired in the oil and gas industry we discuss advantages of applying tomographic ideas to typical microseismic data recorded in a single nearly vertical well to monitor hydraulic stimulation of a shale gas reservoir microseismic events are conventionally located in the energy industry applications using a velocity model derived from sonic logs and perforation shots instead of fixing the model as is normally done we alter it while locating the events this added flexibility not only makes it possible to accurately predict traveltimes of the recorded p and s waves but also provides a convincing evidence for anisotropy of the examined shale formation while we find that velocity heterogeneity does not need to be introduced to explain the data acquired at each stage of hydraulic fracturing the obtained models are suggestive of possible time laps

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

unroofing history of the eastern himalaya and the indo burman ranges heavy mineral study of cenozoic sediments from the bengal basin bangladesh abstract upper eocene to neogene fill of the bengal basin provides an earlier unroofing history of the eastern himalaya and indo burman ranges than that provided by drilling of the bengal fan and analysis of heavy minerals in these sequences provides useful provenance constraints quartzose sandstones of the eocene kopili and oligocene barail formations contain only 0 2 heavy minerals comprising abundant opaque minerals and stable minerals such as tourmaline garnet rutile and zircon this assemblage suggests sediment sources dominated by low to intermediate grade metamorphic silicic igneous and metasedimentary rocks the low heavy mineral content and abundance of stable minerals suggest intense chemical weathering these sediments may have been derived in part from incipient uplifts of the proto himalaya and or the indo burman ranges but a more likely source is the indian craton immediately to the west miocene sandstones of the surma group contain more abundant and more diverse heavy mineral assemblages than oligocene sandstones these include abundant opaque minerals and garnet mostly almandine and moderate to minor amounts of tourmaline kyanite zircon calcic amphibole rutile chlorite zoisite staurolite epidote sillimanite and clinopyroxene indicating a broad range of mostly metamorphic source rocks upper strata of the surma group also contain abundant blue green amphibole orthopyroxene and sparse chromite suggesting exhumation of arc and ophiolitic rocks sands and sandstones from the upper miocene to pliocene tipam group and the pliocene to pleistocene dupi tila formation contain assemblages similar to those of the underlying miocene sandstones along with more abundant orthopyroxene and sillimanite in dupi tila sands temporal variations in heavy mineral assemblages in the bengal basin suggest that orogenic detritus first appears in the eastern himalayan foredeep much later early miocene than in the western part of the foredeep eocene the appearance of high pressure phases in upper miocene strata reflects continued unroofing of deeper crustal metamorphic and silicic to ultramafic plutonic rocks

4d imaging of fracturing in organic rich shales during heating to better understand the mechanisms of fracture pattern development and fluid escape in low permeability rocks we performed time resolved in situ x ray tomography imaging to investigate the processes that occur during the slow heating from 60 to 400 c of organic rich green river shale at about 350 c cracks nucleated in the sample and as the temperature continued to increase these cracks propagated parallel to shale bedding and coalesced thus cutting across the sample thermogravimetry and gas chromatography revealed that the fracturing occurring at approx 350 c was associated with significant mass loss and release of light hydrocarbons generated by the decomposition of immature organic matter kerogen decomposition is thought to cause an internal pressure build up sufficient to form cracks in the shale thus providing pathways for the outgoing hydrocarbons we show that a 2d numerical model based on this idea qualitatively reproduces the experimentally observed dynamics of crack nucleation growth and coalescence as well as the irregular outlines of the cracks our results provide a new description of fracture pattern formation in low permeability shales

impact of irrigation with as rich groundwater on soil and crops a geochemical case study in west bengal delta plain india abstract the distribution of as was explored in soils and crops in order to investigate the influence of irrigation with as rich groundwater on the soil plant system and to determine its impact on the environment and human health the study was carried out in an intensively cultivated agricultural area of the bengal delta plain west bengal india soils plants and irrigation water from adjacent rice and wheat fields were analysed for as and other elements irrigation water has concentrations of up to 780 g l 1 as in the study area rice and wheat grains are not contaminated by as about 0 3 and 0 7 mg kg 1 respectively but concentrations in rice roots were found to be 169 178 mg kg 1 as which is more than 20 times higher than value of 7 7 mg kg 1 measured at the uncontaminated reference site this high content is due to an fe rich plaque which coats the rice roots a significant increase of as concentration was registered also in the stems of the rice plants irrigated with as rich groundwater 6 55 7 06 relative to 0 36 mg kg 1 as in the reference plant arsenic concentration in the uppermost soil layers of the rice paddy field 38 mg kg 1 was found to be roughly twice as high as in the soil of the less intensively watered wheat field 18 mg kg 1 and more than 5 times higher than in the soil of a rice paddy irrigated with uncontaminated water 7 mg kg 1 in both soil sections as contents decreased downwards to 11 mg kg 1 at 100 110 cm depth approaching the background value of 5 10 mg kg 1 measured in an unaffected reference area sequential extraction experiments show that most of the mobile as in soils is bound to fe oxides though no tight correlation was found between fe and as in the bulk soil samples these experiments coupled with synchrotron radiation xrf analysis of single soil particles from the rice paddy also indicates that additional to the mobile fraction a substantial part of as is immobilized in chiefly fe bearing silicates in rice soil some as was also found in the sulphide bearing phase of the extraction

review of arsenic geochemical characteristics and its significance on arsenic pollution studies in karst groundwater southwest china abstract as a metalloid element arsenic as is widely distributed in the natural environment its ingestion can cause cancer deformities and mutations therefore it has become an important environmental problem in recent years there are large amounts of arsenic ore and coal with high arsenic content in southwest china a karst area there arsenic occurs in wastewater from mines contaminate soil vegetation and surface water karst underground aquifers are also contaminated through pipes cracks scuttles and sinkholes leading to a more serious arsenic pollution problem than in non karst areas due to the unique karst hydrogeological conditions to prevent and curb karst underground water contamination and guarantee water resource security and public health a review on the arsenic contamination in the karst area is necessary this paper discusses the progress of geochemical studies on arsenic through an analysis of the hydrogeology of karst areas this paper proposes that studies on arsenic pollution in karst regions should be combined with the spatial distribution and redox characteristics of groundwater more attention should be paid to chemical compositions of water soil and rocks as well as adsorption desorption processes between water and sediment when conducting arsenic geochemical research in karst groundwater

arsenic release from biotite into a holocene groundwater aquifer in bangladesh abstract continuous core sediments to a depth of 90 1 m taken at a transitional area of holocene and pleistocene deposits in sonargaon bangladesh were characterized for their mineralogy and chemistry among the sediments of the lower part of the holocene aquifer depth 18 29 m where most domestic wells are installed as is mostly fixed in biotite and organic phases a positive correlation of as concentration with those of al and fe but not that of total organic c clearly suggests that biotite is a primary source of as although microbial reduction dissolution of as containing fe oxyhydroxides is thought to cause as enriched groundwater in the ganges brahmaputra meghna delta plain the authors conclude that chemical weathering of biotite is the primary formation mechanism and prevailing reducing conditions contribute to the expansion of as enriched groundwater in the study area

effect of acid mine drainage on a karst basin a case study on the high as coal mining area in guizhou province china acid mine drainage amd is a common pollution in mining areas due to the oxidation of pyrite and associated sulfide minerals at mines tailings and mine dumps elevated metals fe mn al and metalloids as hg in amd would deteriorate the local aquatic environment and influence the water supply a carbonate basin with deposits of high arsenic coal in xingren county southwestern china was chosen to study the behavior of as and other chemical constituents along a river receiving amd heavy metals fe mn and major ions such as ca2 mg2 cl so42 in surface water and as in sediment and surface water were analyzed it was found that high concentrations of so42 1 324 7 560 mg l and fe 369 1 472 mg l in surface water were mainly controlled by the interactions between water and rocks such as the oxidation of pyrite in the local coal seams precipitation and adsorption of iron minerals although ubiquitous carbonate minerals in the bedrock and the riverbeds low ph 3 water was maintained until 2 km downstream from the amd source due to the fe hydro oxide minerals coating on the surface of carbonate minerals to restrain the neutralization of acidic water moreover the formation of fe hydro oxide precipitations absorbed as was dominated the attenuation of as from water to sediment whereas the dilution also played an important role in decrease of as in river water

ultrasonic velocities in cretaceous shales from the williston basin compressional and shear wave velocities were measured in the laboratory from 1 bar to 4 kbar confining pressure for wet undrained samples of cretaceous shales from depths of 3200 and 5000 ft in the williston basin north dakota these shales behave as transversely isotropic elastic media the plane of circular symmetry coinciding with the bedding plane for compressional waves the velocity is higher for propagation in the bedding plane than at right angles to it and the anisotropy is greater for the 5000 ft shale for shear waves the sh wave perpendicular to bedding and the sv wave parallel to bedding propagate with the same speed which is about 25 percent lower than that for the sh wave parallel to bedding in general compressional and shear velocities are higher for the indurated 5000 ft shale than for the friable 3200 ft shale all velocities increase with in increasing confining pressure to 4 kbar the 3200 ft shale exhibits velocity hysteresis as a function of pressure whereas this effect is a

elastic anisotropy of source rocks implications for hydrocarbon generation and primary migration we combined acoustic velocities and anisotropy measurements with x ray diffraction mineralogy rock eval organic geochemical analyses vitrinite reflectance and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins the velocity data indicate the lamellar distribution of solid organic matter kerogen in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness the experimental results are consistent with the two component anisotropic model of black shales with a continuous kerogen network in both bedding parallel and bedding normal directions beginning from relatively low toc values the strong elastic anisotropy of tight thermally mature source rocks i further enhanced by overpressure induced bedding subparallel microcrack development during the main stage of hydrocarbon hc generation and primary migration at r0 greater than 0 75 in agreement with published petrographic observations these microcracks significantly aid primary hc migration particularly horizontally elastic anisotropy of medium to high porosity illite smectite shales is weaker than in lower porosity predominantly illite shales however the overall positive trend of elastic anisotropy with kerogen content remains unchanged

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

on the evolution of the elastic properties of organic rich shale upon pyrolysis induced thermal maturation abstractthe evolution of the elastic properties of organic rich shale as a function of thermal maturity remains poorly constrained this understanding is pivotal to the characterization of source rocks and unconventional reservoirs to better constrain the evolution of the elastic properties and microstructure of organic rich shale we have studied the acoustic velocities and elastic anisotropy of samples from two microstructurally different organic rich shales before and after pyrolysis induced thermal maturation to more physically imitate in situ thermal maturation we performed the pyrolysis experiments on intact core plugs under applied reservoir magnitude confining pressures iterative characterization of the elastic properties of a clay rich laminar barnett shale sample documents the development of subparallel to bedding cracks by an increase in velocity sensitivity to pressure perpendicular to the bedding these cracks however are not visible through time lapse scanning electron microscope imag

hydrogeochemical characteristics of streams with and without acid mine drainage impacts a paired catchment study in karst geology sw china summary a paired catchment study was used to assess hydrochemical characteristics of streams in amd acid mine drainage impacted and non amd impacted catchments in a karstic region sw china the catchments shared similar geology slope aspect area and climate but different land use since one encompassed mining activities as a result hydrogeochemical characteristics of the amd impacted water were dominantly controlled by oxidation of pyrite while that of non amd impacted were influenced by water rock reactions it is also coincident with the source of 34 s so4 in both waters upstream unit ion fluxes of the amd impacted catchment for ca mg h fe and so 4 were 17 g m 2 yr 2 g m 2 yr 0 0002 g m 2 yr 17 g m 2 yr and 136 g m 2 yr respectively while unit ion fluxes of the non amd impacted catchment were much lower than that of the amd impacted catchment along the flow fluxes of h ca mg fe and so 4 decreased in the amd impacted water but increased in the non amd impacted water the observation was coincident with variations of ion concentrations in both waters along the flow supported by phreeqc modeling of minerals such as goethite jarosite ss melanterite scorodite calcite and dolomite it suggests that the transport of constituents of interest except for ca and mg is dramatically different in these catchments results provide important information on karst system response to amd together with the baseline data to understand the degree and type of impairment

hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from la pampa argentina groundwaters from quaternary loess aquifers in northern la pampa province of central argentina have significant quality problems due to high concentrations of potentially harmful elements such as as f no3 n b mo se and u and high salinity the extent of the problems is not well defined but is believed to cover large parts of the argentine chaco pampean plain over an area of perhaps 106 km2 groundwaters from la pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km dissolved as spans over 4 orders of magnitude 4 5300 g l 1 and concentrations of f have a range of 0 03 29 mg l 1 b of 0 5 14 mg l l v of 0 02 5 4 mg l 1 no3 n of 0 2 140 mg l 1 mo of 2 7 990 g l 1 and u of 6 2 250 g l 1 of the groundwaters investigated 95 exceed 10 g as l 1 the who guideline value and 73 exceed 50 g as l 1 the argentine national standard in addition 83 exceed the who guideline value for f 1 5 mg l 1 99 for b 0 5 mg l 1 47 for no3 n 11 3 mg l 1 39 for mo 70 g l 1 32 for se 10 g l 1 and 100 for u 2 g l 1 total dissolved solids range between 730 and 11400 mg l 1 the high values resulting mainly from evaporation under ambient semi arid climatic conditions the groundwaters are universally oxidising with high dissolved o2 concentrations groundwater phs are neutral to alkaline 7 0 8 7 arsenic is present in solution predominantly as as v groundwater as correlates positively with ph alkalinity hco3 f and v weaker correlations are also observed with b mo u and be desorption of these elements from metal oxides especially fe and mn oxides under the high ph conditions is considered an important control on their mobilisation mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved as and other trace elements concentrations of as and other anions and oxyanions appear to be particularly high in groundwaters close to low lying depressions which act as localised groundwater discharge zones concentrations up to 7500 g l 1 were found in saturated zone porewaters extracted from a cored borehole adjacent to one such depression concentrations are also relatively high where groundwater is abstracted from close to the water table presumably because this zone is a location of more active weathering reactions the development of groundwaters with high ph and alkalinity results from silicate and carbonate reactions facilitated by the arid climatic conditions these factors together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of as and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal

emerging methodologies to characterize the rock physics properties of organic rich shales the high price of conventional oil and the ensuing focus on the security of hydrocarbon supplies is a clear signal for broader uses of unconventional sources of oil and gas organic rich shales represent an enormous potential unconventional resource the u s energy information administration eia estimates the world supply of oil shale at 2 6 trillion barrels of recoverable oil the u s owns by far the largest amount 1 1 2 trillion barrels mainly near the common borders of wyoming utah and colorado where much of the resource occurs at a saturation of about 10 by weight in layers that are 30 300 m thick national petroleum council 2007

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

microcrack induced versus intrinsic elastic anisotropy in mature hc source shales recent experimental studies of ultrasonic velocity anisotropy in kerogen rich shales indicate that these rocks are characterized by a very strong anisotropic response related to a very fine bedding parallel lamination of organic matter and preferred orientation of clay particles in the rock matrix vernik and nur 1992 this intrinsic anisotropy is further enhanced in thermally mature shales by bedding parallel microcracks caused by the processes of hydrocarbon generation vernik paper in preparation however the potential of recognizing mature source rocks in situ using downhole sonic crosshole seismic or vsp data clearly depends on our ability to discriminate between these two major causes of elastic anisotropy remove the effect of the intrinsic anisotropy and estimate crack density and crack porosity from velocity measurements

spatial variation in arsenic and fluoride concentrations of shallow groundwater from the town of shahai in the hetao basin inner mongolia abstract twenty nine wells were selected for groundwater sampling in the town of shahai in the hetao basin inner mongolia four multilevel samplers were installed for monitoring groundwater chemistry at depths of 2 5 20 m results show that groundwater as exhibits a large spatial variation ranging between 0 96 and 720 g l with 71 of samples exceeding the who drinking water guideline value 10 g l fluoride concentrations range between 0 30 and 2 57 mg l there is no significant correlation between as and f concentrations greater as concentrations were found with increasing well depth however f concentrations do not show a consistent trend with depth groundwater with relatively low eh has high as concentrations indicating that the reducing environment is the major factor controlling as mobilization low as concentrations 2 into the aquifers and lead to reducing conditions that favor as release sediment composition is an additional factor in determining dissolved as concentrations in aquifers composed of yellowish brown fine sands at depths around 10 m groundwater generally has low as concentrations which is attributed to the high as adsorption capacity of the yellow brown fe oxyhydroxide coatings fluoride concentration is positively correlated with ph and negatively correlated with ca 2 concentration all groundwater samples are over saturated with respect to calcite and under saturated with respect to fluorite dissolution and precipitation of ca minerals such as fluorite and calcite and f adsorption desorption are likely controlling the concentration of f in groundwater

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

arsenic associations in sediments from shallow aquifers of northwestern hetao basin inner mongolia understanding the mechanism of arsenic mobilization from sediments to groundwater is important for water quality management in areas of endemic arsenic poisoning such as the hetao basin in inner mongolia northern china aquifer geochemistry was characterized at three field sites sh hf tys in hangjinhouqi county of northwestern hetao basin the results of bulk geochemistry analysis of sediment samples indicated that total as concentrations have a range of 6 8 58 5 mg kg with a median of 14 4 mg kg the highest as concentrations were found at 15 25 m depth in the meanwhile the range of as concentration in the sediments from background borehole is 3 21 8 mg kg with a median value of 9 mg kg the as sediments concentrations with depth from the sh borehole were correlated with the contents of fe sb b v total c and total s generally the abundance of elements varied with grain size with higher concentrations in finer fractions of the sediments distinct lithology profile and different geochemical characteristics of aquifer sediments indicate the sediments are associated with different sources and diverse sedimentary environments up to one third of arsenic in the sediments could be extracted by ammonium oxalate suggesting that fe oxyhydroxides may be the major sink of as in the aquifer sequential extraction results indicate that arsenic occurs as strongly adsorbed on and or co precipitated with amorphous fe oxyhydroxides in sediments accounting for 35 and 20 respectively of the total contents of arsenic the release of as into groundwater may occur by desorption from the mineral surface driven by reductive dissolution of the fe oxide minerals furthermore small proportions of as associated with iron sulfides occur in the reductive sediments

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

mobilisation of arsenic and other trace elements in fluviolacustrine aquifers of the huhhot basin inner mongolia observed as concentrations in groundwater from boreholes and wells in the huhhot basin of inner mongolia northern china range between 1 g l 1 and 1480 g l 1 the aquifers are composed of quaternary largely holocene lacustrine and fluvial sediments high concentrations are found in groundwater from both shallow and deep boreholes as well as from some dug wells well depths ranging between 10 m and 400 m populations from the affected areas experience a number of as related health problems the most notable of which are skin lesions keratosis melanosis skin cancer but with internal cancers lung and bladder cancer also having been reported in both the shallow and deep aquifers groundwaters evolve down the flow gradient from oxidising conditions along the basin margins to reducing conditions in the low lying central part of the basin high as concentrations occur in anaerobic groundwaters from this low lying area and are associated with moderately high dissolved fe as well as high mn nh4 dissolved organic c doc hco3 and p concentrations many of the deep groundwaters have particularly enriched doc concentrations up to 30 mg l 1 and are often brown as a result of the high concentrations of organic acid in the reducing groundwaters inorganic as iii constitutes typically more than 60 of the total dissolved as the highest as concentrations tend to be found in groundwater with low so4 concentrations and indicate that as mobilisation occurs under strongly reducing conditions where so4 reduction has been an active process high concentrations of fe mn nh4 hco3 and p are a common feature of reducing high as groundwater provinces e g bangladesh west bengal high concentrations of organic acid humic fulvic acid are not a universal feature of such aquifers but have been found in groundwaters from taiwan and hungary for example the observed range of total as concentrations in sediments is 3 29 mg kg 1 n 12 and the concentrations correlate positively with total fe up to 30 of the as is oxalate extractable and taken to be associated largely with fe oxides the release of as into solution under the reducing conditions is believed to be by desorption coupled with reductive dissolution of the fe oxide minerals the association of dissolved as with constituents such as hco3 doc and p may be a coincidence related to the prevalent reducing conditions and slow groundwater flow but they may also be directly involved because of their competition with as for binding sites on the fe oxides the huhhot groundwaters also have some high concentrations of dissolved u up to 53 g l 1 and f up to 6 8 mg l 1 in contrast to as u occurs predominantly under the more oxidising conditions along the basin margins fluoride occurs dominantly in the shallow groundwaters which have na and hco3 as the dominant ions the combination of slow flow of groundwater and the young age of the aquifer sediments are also considered potentially important causes of the high dissolved as concentrations observed as the sediments are likely to contain newly formed and reactive minerals and have not been well flushed since burial

experimental laboratory determination of the dynamic elastic properties of wet drained shales the ultrasonic anisotropic elastic properties of drained saturated shales were measured as a function of confining pressure two shales were characterized in this study a jurassic outcrop shale retrieved under the sea in a saturated state and a kimmeridge clay shale cut from a north sea borehole both shales were highly anisotropic both texturally as revealed by scanning electron microscopy analysis and elastically as measured ultrasonically in the laboratory the strongest anisotropy was seen in the kimmeridge clay shale where up to 38 compressional wave anisotropy thomsen s parameter e and up to 58 shear wave anisotropy thomsen s parameter were observed in addition for both shales e and were found to decrease as a function of increasing confining pressure with the pore pressure drained to atmosphere while the anellipticity deviation of the slowness surfaces from ellipses was found to be positive and decreased as a function of increasing confining pressure accompanying these changes in elastic properties was a decrease in porosity with increasing confining pressure from 10 5 to 8 5 for the jurassic shale the decrease in overall anisotropy of the shales with increasing confining pressure was found to be consistent with theoretical modeling of shale properties where the shale anisotropy and anellipticity were predicted to decrease as a function of decreasing fluid filled porosity

arsenic associations in sediments from the loess aquifer of la pampa argentina groundwater from the quaternary loess aquifer of la pampa central argentina has significant problems with high concentrations of as up to 5300 g l 1 as well as other potentially toxic trace elements such as f b mo u se and v total as concentrations in 45 loess samples collected from the aquifer have a range of 3 18 mg kg 1 with a mean of 8 mg kg 1 these values are comparable to world average sediment as concentrations five samples of rhyolitic ash from the area have as concentrations of 7 12 mg kg 1 chemical analysis included loess sediments and extracted porewaters from two specially cored boreholes results reveal a large range of porewater as concentrations being generally higher in the horizons with highest sediment as concentrations the displaced porewaters have as concentrations ranging up to 7500 g l 1 as well as exceptionally high concentrations of some other oxyanion species including v up to 12 mg l 1 the highest concentrations are found in a borehole located in a topographic depression which is a zone of likely groundwater discharge and enhanced residence time comparison of sediment and porewater data does not reveal unequivocally the sources of the as but selective extract data acid ammonium oxalate and hydroxylamine hydrochloride suggest that much of the as and v is associated with fe oxides primary oxides such as magnetite and ilmenite may be partial sources but given the weathered nature of many of the sediments secondary oxide minerals are probably more important extract compositions also suggest that mn oxide may be an as source the groundwaters of the region are oxidising with dissolved o2 no3 and so4 normally present and as v usually the dominant dissolved as species under such conditions the solubility of fe and mn oxides is low and as mobilisation is strongly controlled by sorption desorption reactions desorption may be facilitated by the relatively high ph conditions of the groundwaters in the region 7 0 8 8 and high concentrations of potential competitors e g v p hco3 phreeqc modelling suggests that the presence of v at the concentrations observed in the pampean porewaters can suppress the sorption of as to hydrous fe iii oxide hfo by up to an order of magnitude bicarbonate had a comparatively small competitive effect oxalate extract concentrations have been used to provide an upper estimate of the amount of labile as in the sediments a near linear correlation between oxalate extractable and porewater as in one of the cored boreholes investigated has been used to estimate an approximate kd value for the sediments of 0 94 l kg 1 this low value indicates that the sediments have an unusually low affinity for as