chemical and isotopical characterisation of fluid manifestations of ischia island italy abstract between 1994 and 1995 gas samples from fumaroles and thermal waters were collected on ischia island the chemical composition of the shallow and deep natural hydrothermal fluids discharged is related to the main hydrological and lithological characteristics of the rock formations present in the reservoir a biphase reservoir liquid gas is identified where the dominant liquid has a temperature of about 280 c on the basis of 13ctdc values it was possible to hypothesize a deep source characterised by carbon isotopic values varying from 0 to 3 these values are noticeably more positive with respect to those attributed to magmatic co2 13 c co 2 ranging from 5 to 8 thus suggesting a magmatic source modified by crustal contamination this hypothesis is supported by the carbon isotopic composition of co2 in sampled gases which varied from 0 to 5 the inferred isotopic value of carbon of magmatic co2 would then be about 2 the observed differences in c isotopic composition between fumarolic and magmatic gases would be caused by kinetic and or equilibrium fractionation processes these processes would cause a fractionation of 13c of deep co2 towards more negative values down to 5 actually co2 removal or addition processes caused by the interaction between deep gases and shallow hydrothermal waters are likely to be responsible for the different chemical and isotopic compositions of gaseous emissions for these reasons and on the basis of the homogeneity of geothermometric values the existence of a single large reservoir that feeds all of the fluids discharged at ischia island can be hypothesised based on acquired data a new geochemical model of the geothermal system of ischia island is proposed

origin and availability of organic matter leading to arsenic mobilisation in aquifers of the red river delta vietnam abstract groundwater arsenic as concentrations in the red river delta vietnam are often patchy and related to the microbially induced reduction of fe oxy hydroxides in this study we explored the influence of the origin composition and availability of natural organic matter on the hydrochemical variability in the aquifers of van phuc carbon isotope signatures 13corg and c n ratios were assessed in combination with lithology geochemistry hydrochemistry hydrology and the distribution of specific biomarkers the elationship of c n ratios and 13corg distinguished four groups of sediment types that differ in their organic carbon sources this includes organic carbon originating predominantly from vascular c3 plants c n 15 4 21 0 13corg 28 6 to 26 7 c4 plants c n 10 6 13corg 14 8 freshwater derived particulate organic carbon c n 8 13corg 24 as well as mixtures incorporating both sources at the high as sites we found particulate organic carbon poc being 1 2 less depleted in 13corg than at low as sites more importantly however our assessment shows that the availability of organic matter has to be considered decisive with regard to groundwater as contamination fine grained clayey sediments overlaying sands generally protect organic matter from substantial degradation and its leaching into an adjacent aquifer however at the sites that are high in dissolved as in van phuc sediment layers rich in organic matter are hydraulically connected to the underlying aquifer here soluble organic matter seeping into the aquifer can induce and or enhance reducing conditions thereby mobilising as from fe oxy hydroxides our study shows that both the clay content as well as the origin of organic matter are largely controlled by the depositional environment of the sediments

comparison of arsenic concentrations in simultaneously collected groundwater and aquifer particles from bangladesh india vietnam and nepal one of the reasons the processes resulting in as release to groundwater in southern asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers in an attempt to overcome this difficulty a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in bangladesh recently published results from bangladesh and india relying on the needle sampler are augmented here with new data from 37 intervals of grey aquifer material of likely holocene age in vietnam and nepal a total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for as 1 1000 lg l fe 0 01 40 mg l mn 0 2 4 mg l and s 0 04 14 mg l are compared the p extractable 0 01 36 mg kg and hcl extractable as 0 04 36 mg kg content of the particulate phase was determined in the same suite of samples in addition to fe ii fe ratios 0 2 1 0 in the acid leachable fraction of the particulate phase needle sampler data from bangladesh indicated a relationship between dissolved as in groundwater and p extractable as in the particulate phase that was interpreted as an indication of adsorptive equilibrium under sufficiently reducing conditions across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 ml g the more recent observations from india vietnam and nepal show groundwater as concentrations that are often an order of magnitude lower at a given level of p extractable as compared to bangladesh even if only the subset of particularly reducing intervals characterized by leachable fe ii fe 0 5 and dissolved fe 0 2 mg l are considered without attempting to explain why as appears to be particularly mobile in reducing aquifers of bangladesh compared to the other regions the consequences of increasing the distribution coefficient for as between the particulate and dissolved phase to 40 ml g for the flushing of shallow aquifers of their initial as content are explored

fumarolic and diffuse soil degassing west of mount epomeo ischia italy abstract fumarolic fluid compositions and diffuse soil emissions of hydrothermal fluids of the donna rachele area 0 86 km 2 western flank of mt epomeo ischia island have been studied in order to develop a conceptual geochemical model of the hydrothermal system the degassing area was mapped and the total release of hydrothermal gas and heat associated with the diffuse emission of hydrothermal fluids was estimated a mesostructural study was carried out in order to investigate the relations between the brittle structures and the main pathways of the uprising vapor the fumarolic compositions are typical of hydrothermal fluids and water 99 represents the major component all gas species in the h 2 o h 2 co ch 4 co 2 h 2 s system are close to equilibrium concentrations at temperatures of 300 c and at redox conditions slightly more oxidizing than expected the compositions of the donna rachele fumarolic gas approach the pure liquid equilibrium composition this indicates a high fraction of separated vapor and suggests the presence of a highly energetic hydrothermal system at depth the pure liquid equilibrium compositions of the donna rachele fumaroles along with the historical records of shallow seismicity the explosion of a well in 1995 the occurrence of intense acoustic phenomena and of shallow wells discharging vapor indicate that the internal pressure of the hydrothermal system is occasionally larger than the hydrostatic pressure to quantify the energy dissipated in the donna rachele area by the emission of fumarolic fluids the hydrothermal diffuse degassing was studied by means of 336 soil co 2 flux measurements the highest co 2 fluxes were measured in hydrothermally altered areas along the faults that border mt epomeo structural data indicate that the vapor rises up along nw se striking normal faults related to gravity induced stresses and affecting highly fractured lavas the older faults which are related to the mt epomeo resurgence act as a permeability barrier and bound the donna rachele diffuse degassing structure the total hydrothermal co 2 output was estimated to be 9 t d 1 assuming that the h 2 o co 2 ratio of the fluids that feed the diffuse degassing is the same as that of fumarolic effluents the calculated heat flux is 40 mw this value which represents an important energy release is only a part of the total thermal energy release of ischia where other fumarolic areas occur the results obtained at ischia indicate the importance of thermal energy released by diffuse degassing structures in the energy balance of quiescent volcanoes values of the thermal energy release from the ischia hydrothermal system are comparable with those estimated on other quiescent volcanoes

gravitational slope deformation of a resurgent caldera new insights from the mechanical behaviour of mt nuovo tuffs ischia island italy abstract ischia island italy is an impressive example of the rare phenomenon of caldera resurgence the emplacement and replenishment of magmas at shallow depth resulted in a vertical uplift of about 900 m concentrated in the western portion of mt epomeo 789 m a s l as a consequence of this uplift the island has experienced several slope instabilities at different scales since the holocene from shallow mass movements to large rock and debris avalanches these mass wasting events which mobilised large volumes of greenish alkali trachytic tuff the mt epomeo green tuff megt were strictly related to volcano tectonic activity and the interaction between the volcanic slopes and the hydrothermal system beneath the island deep seated gravitational slope deformation dsgsd at mt nuovo located adjacent to densely populated coastal villages is an ongoing process that covers an area of 1 6 km 2 the mt nuovo dsgsd involves a rock mass volume of 190 mm 3 and is accommodated by a main shear zone and a series of sub vertical fault zones associated with high angle joint sets to improve our understanding of this gravity induced process we performed a physical porosity and permeability and mechanical uniaxial and triaxial deformation experiments characterisation of two ignimbrite deposits both from the megt that form a significant component of the nw sector of mt epomeo the main conclusions drawn from our experiments are twofold first the presence of water dramatically reduces the strength of the tuffs suggesting that the movement of fluids within the hydrothermal system could greatly impact slope stability second the transition from brittle dilatant to ductile compactant behaviour in the tuffs of the megt occurs at a very low effective pressure analogous to a depth of a couple of hundred metres and that this transition is likely moved closer to the surface in the presence of water we hypothesise that compactant porosity decreasing behaviour at the base of the layer could therefore facilitate slope instability although our results show that transient exposure to 300 c does not influence the short term strength of the tuff we speculate that the high in situ temperature could increase the efficiency of brittle and compactant creep and therefore increase the rate of slope deformation taken together our experimental data highlight a potentially important role for the hydrothermal system that reaches a minimum depth of 1 km in dictating the dsgsd at mt nuovo an understanding of this deformation process is not only important for the proximal coastal villages at risk of engulfment by a large debris avalanche but also for the towns and cities along the coast of the gulf of naples that are at risk to a secondary consequence of such an avalanche a tsunami wave

inverse and forward modelling of groundwater circulation in a seismically active area monferrato piedmont nw italy insights into stress induced variations in water chemistry municipality of nizza monferrato asti italy and the regional administration of regione piemonte italy

redox control of arsenic mobilization in bangladesh groundwater abstract detailed hydrochemical measurements 34sso4 and 3h analyses were performed on 37 groundwater samples collected during february 1999 january and march 2000 from 6 locations in eastern and southeastern bangladesh to examine redox processes that lead to as mobilization in groundwater the study sites were chosen based on available nation wide as surveys to span the entire spectrum of as concentrations in bangladesh groundwater and to represent 3 of 5 major geological units of the ganges brahmaputra delta uplifted pleistocene terrace fluvial flood plain and delta plain arsenic was found to be mobilized under fe reducing conditions in shallow aquifers 50 g l or 0 7 m were always accompanied by high dissolved hco3 4 mm and were close to saturation with respect to calcite groundwater enriched in as 200 800 g l or 2 7 10 7 m and phosphate 30 100 m but relatively low in dissolved fe 5 40 m probably resulted from re oxidation of reducing as and fe enriched water this history was deduced from isotopic signatures of 34sso4 and 3h2o 3h to delineate the nature of redox changes for some of the reducing groundwaters in contrast as is not mobilized in presumed pleistocene aquifers both shallow 30 60 m and deep 150 270 m because conditions were not reducing enough due to lack of sufficient o2 demand

hydrologic control of temporal variability in groundwater arsenic on the ganges floodplain of nepal abstract elevated arsenic in groundwater affects some 100 million people in south asia yet mitigation efforts are hindered by persistent uncertainty about the proximal source of arsenic and mechanisms for its mobilization at the core of this uncertainty are the relative roles of surficial organic clays vs deeper aquifer matrix iron oxyhydroxides temporal variations in groundwater chemistry can serve to distinguish the contributions of these two sources and such variation is especially pronounced in headwater areas of the ganges floodplain immediately adjacent to the himalayan foothills e g the terai of nepal tubewells down to 50 m in the terai commonly exhibit cyclical temporally correlated variation in dissolved arsenic iron and other species in nawalparasi the most arsenic affected district these wells tap thin 2 m gray sand aquifers embedded in a thick 50 m sequence of organic clays monsoon recharge refreshes these aquifers temporarily minimizing arsenic concentrations post monsoon average groundwater compositions exhibit increasing trends in water rock interaction higher tds with cation exchange to form increasingly na hco 3 waters arsenic and iron this cycle can be repeated during dry season precipitation events as well revealing direct correlation between trends in degree of clay interaction sodium fraction of major cations and arsenic concentrations during the year reversals in vertical head gradient yield reversals in arsenic temporal trend and downward gradients in the dry season correlate with increases in arsenic collectively these observations strongly support a model of reductive mobilization of arsenic from adjacent clays into aquifers tempered by repeated flushing during periods of appreciable rainfall

chemical and isotopic assessment in volcanic thermal waters cases of ischia italy and sao miguel azores portugal isotopic fractionation of 10b 11b provides a sound tool for identifying hydrogeochemical processes in complex areas owing to its ability to discriminate between various scenarios in addition the occurrence of boron as a minor element in areas of active volcanism allows its use in comparison with concentrations of other conservative or non conservative ions this allows the detection of water mixtures of diverse origin and temperature deep or shallow including fresh water seawater and even brines this tool was applied in studies of the volcanic islands of ischia and sao miguel across widely differing geographical and climatic contexts five groups of waters have been identified in ischia island marine transition hot carbonated cold carbonated and fresh waters for sao miguel island the identified groups are cold carbonic hot carbonic boiling and acidic boiling waters copyright 2008 john wiley sons ltd

geological controls on groundwater chemistry and arsenic mobilization hydrogeochemical study along an e w transect in the meghna basin bangladesh summary hydrogeochemical investigations along an e w transect in the middle meghna basin show groundwater chemistry and redox condition vary considerably with the change in geology groundwater in the holocene shallow 10 g l and salinity on the other hand groundwater from the pliocene dupi tila sandy aquifer in the eastern part is fresh and low in as hco 3 fe and dissolved organic carbon doc but generally low in so 4 2 and no 3 high hco 3 concentrations 250 716 mg l together with high doc concentrations 1 4 21 7 mg l in these aquifers reflect active sources of degradable natural organic matter that drives the biogeochemical process there is generally de coupling of as from other redox sensitive elements in contrast the pliocene aquifers are low in as hco 3 and doc molar ratio of hco 3 h4sio4 suggests that silicate weathering is dominant in the deeper holocene aquifers and in the pliocene aquifers molar ratios of cl hco 3 and na cl suggest mixing of relict seawater with the fresh water as the origin of groundwater salinity speciation calculations show that saturation indices for siderite and rhodochrosite vary significantly between the holocene and pliocene aquifers stable isotopes 2h and 18o in groundwater indicate rapid infiltration without significant effects of evaporation the isotopic data also indicates groundwater recharge from monsoonal precipitation with some impact of altitude effect at the base of the tripura hills in the east the results of the study clearly indicate geological control i e change in lithofacies on groundwater chemistry and distribution of redox sensitive elements such as as along the transect

water rock interaction in the magmatic hydrothermal system of nisyros island greece abstract in this work we investigated the water rock interaction processes taking place in the hydrothermal reservoir of nisyros through both 1 a review of the hydrothermal mineralogy encountered in the deep geothermal borehole nisyros 2 and 2 a comparison of the analytically derived redox potentials and acidities of fumarolic related liquids with those controlled by redox buffers and ph buffers involving hydrothermal mineral phases the propylitic zone met in the deep geothermal borehole nisyros 2 from 950 to 1547 m total depth is characterised by abundant well crystallised epidote adularia albite quartz pyrite chlorite and sericite muscovite accompanied by less abundant anhydrite stilpnomelane wairakite garnet tremolite and pyroxene these hydrothermal minerals were produced in a comparatively wide temperature range from 230 to 300 c approximately hydrothermal assemblages are well developed from 950 to 1360 m whereas they are less developed below this depth probably due to low permeability based on the r h values calculated for fumarolic gases and for the deep geothermal fluids of nisyros 1 and nisyros 2 wells redox equilibrium with the feo feo 1 5 rock buffer appears to be closely attained throughout the hydrothermal reservoir of nisyros this conclusion may be easily reconciled with the nearly ubiquitous occurrence of anhydrite and pyrite since r h values controlled by coexistence of anhydrite and pyrite can be achieved by gas separation the ph of the liquids feeding the fumarolic vents of stephanos and polybote micros craters was computed by means of the eq3 code based on the cl d relationship which is constrained by the seawater magmatic water mixing occurring at depth in the hydrothermal magmatic system of nisyros the temperature dependence of analytically derived ph values for the reservoir liquids feeding the fumarolic vents of stephanos and polybote micros craters suggests that some unspecified ph buffer fixes the acidity of these reservoir liquids at values of 4 72 4 85 and 4 88 5 23 respectively many of these ph values are lower than those expected for the full equilibrium condition although they are close to those of the reservoir liquids of nisyros 1 5 16 and nisyros 2 4 87 it is likely that this excess of acidity producing species chiefly co 2 promotes release of fe ii and fe iii to the reservoir liquids through rock dissolution permitting the attainment of redox equilibrium with the feo feo 1 5 rock buffer as already suggested by the late werner giggenbach

tracing organic matter composition and distribution and its role on arsenic release in shallow cambodian groundwaters biogeochemical processes that utilize dissolved organic carbon are widely thought to be responsible for the liberation of arsenic from sediments to shallow groundwater in south and southeast asia the accumulation of this known carcinogen to hazardously high concentrations has occurred in the primary source of drinking water in large parts of densely populated countries in this region both surface and sedimentary sources of organic matter have been suggested to contribute dissolved organic carbon in these aquifers however identification of the source of organic carbon responsible for driving arsenic release remains enigmatic and even controversial here we provide the most extensive interrogation to date of the isotopic signature of ground and surface waters at a known arsenic hotspot in cambodia we present tritium and radiocarbon data that demonstrates that recharge through ponds and or clay windows can transport young surface derived organic matter into groundwater to depths of 44 m under natural flow conditions young organic matter dominates the dissolved organic carbon pool in groundwater that is in close proximity to these surface water sources and we suggest this is likely a regional relationship in locations distal to surface water contact dissolved organic carbon represents a mixture of both young surface and older sedimentary derived organic matter ground surface water interaction therefore strongly influences the average dissolved organic carbon age and how this is distributed spatially across the field site arsenic mobilization rates appear to be controlled by the age of dissolved organic matter present in these groundwaters arsenic concentrations in shallow groundwaters 20 m groundwaters we suggest that while the rate of arsenic release is greatest in shallow aquifer sediments arsenic release also occurs in deeper aquifer sediments and as such remains an important process in controlling the spatial distribution of arsenic in the groundwaters of se asia our findings suggest that any anthropogenic activities that alter the source of groundwater recharge or the timescales over which recharge takes place may also drive changes in the natural composition of dissolved organic carbon in these groundwaters such changes have the potential to influence both the spatial and temporal evolution of the current groundwater arsenic hazard in this region

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

a model for the evolution in water chemistry of an arsenic contaminated aquifer over the last 6000 years red river floodplain vietnam aquifers on the red river flood plain with burial ages ranging from 500 to 6000 years show with increasing age the following changes in solute concentrations a decrease in arsenic increase in fe ii and decreases in both ph ca and bicarbonate these changes were interpreted in terms of a reaction network comprising the kinetics of organic carbon degradation the reduction kinetics of as containing fe oxides the sorption of arsenic the kinetics of siderite precipitation and dissolution as well as of the dissolution of caco3 the arsenic released from the fe oxide is preferentially partitioned into the water phase and partially sorbed while the released fe ii is precipitated as siderite the reaction network involved in arsenic mobilization was analyzed by 1 d reactive transport modeling the results reveal complex interactions between the kinetics of organic matter degradation and the kinetics and thermodynamic energy released by fe oxide reduction the energy released by fe oxide reduction is strongly ph dependent and both methanogenesis and carbonate precipitation and dissolution have important influences on the ph overall it is the rate of organic carbon degradation that determines the total electron flow however the kinetics of fe oxide reduction determines the distribution of this flow of electrons between methanogenesis which is by far the main pathway and fe oxide reduction modeling the groundwater arsenic content over a 6000 year period in a 20 m thick aquifer shows an increase in as during the first 1200 years where it reaches a maximum of about 600 g l during this initial period the release of arsenic from fe oxides actually decreases but the adsorption of arsenic onto the sediment delays the build up in the groundwater arsenic concentration after 1200 years the groundwater arsenic content slowly decreases controlled both by desorption and continued further but diminishing release from fe oxide being reduced after 6000 years the arsenic content has decreased to 33 g l the modeling enables a quantitative description of how the aquifer properties the reactivity of organic carbon and fe oxides the number of sorption sites and the buffering mechanisms change over a 6000 year period and how the combined effect of these interacting processes controls the groundwater arsenic content

spatial vertical and temporal variation of arsenic in shallow aquifers of the bengal basin controlling geochemical processes a detailed understanding of the geochemical processes that regulate the spatial temporal and vertical variation of dissolved arsenic as in shallow aquifers 50m is a prerequisite for sustainable drinking water management in the bengal basin the present study conducted at chakdaha block of the nadia district west bengal india combined a high resolution hydrogeochemical monitoring study over 20months from two sets of piezometers 2 5 to the sediment geochemistry at areas with high average 146 g l n 5 and relatively low average 53 3 g l n 10 dissolved as concentrations in groundwater the determination of the isotopic composition of 2h and 18o in groundwater of the two sites indicated the recharge of evaporative surface water to the aquifer the concentrations of major aqueous solutes ca2 mg2 na k hco3 and cl and electrical conductivity were considerably higher in wells at the high as site compared to the low as site additionally at the high as site the major ions fe so4 2 electrical conductivity 2h and 18o showed markedly greater enrichment in the shallowest part 24m of the aquifer compared to the deeper part reflecting vertical layering of groundwater composition within the aquifer the oxidation of pyrites has been attributed to the high rate of mineral dissolution resulting in such greater enrichments in this part of the aquifer in addition the anthropogenic input with recharge water possibly increased the concentrations of cl in this part of the aquifer the vertical layering of groundwater was absent in the aquifer at the low as site the absence of such layering and relatively low major ion concentrations and electrical conductivity could be linked to the enhanced aquifer flushing and decreased water sediment interactions influenced by local scale groundwater abstraction the seasonal variations of as concentrations in groundwater were observed only in the shallowest part of the aquifers 30m furthermore the as concentrations in groundwater at the uppermost part of the shallow aquifers 21m increased continuously over the monitoring period at both sites this study supports the view that the reductive dissolution of fe oxyhydroxides coupled with competitive po4 3 sorption reactions in the aquifer sediment enriches as in groundwater of the bengal basin however the additional fe released by the weathering of silicate minerals especially biotite or the precipitation of fe as secondary mineral phases such as siderite vivianite and acid volatile sulfides may result in the decoupling of as and fe enrichment in groundwater the redox zonation within the aquifer possibly regulates the vertical distribution of as in the groundwater

exhumation of an active magmatic hydrothermal system in a resurgent caldera environment the example of ischia italy abstract ischia is a rare case of a well exposed caldera system that has experienced rapid recent resurgence which can be used to dissect the anatomy of an otherwise inaccessible active hydrothermal system developed in a caldera forming environment integrated analyses of melt and fluid inclusions mineralogy and stable isotopic compositions of pumices tuffs and syenitic xenoliths of the ischia volcanic system provide strong physico chemical constraints on the shallow magmatic reservoir and the hydrothermal system the hydrothermal system has been exposed by the rapid uplift of the mt epomeo resurgent block the engine of the hydrothermal system of ischia can be identified in the shallow magmatic system at around 2 km depth that hosts hot c 1000 c trachytic magma the hydrothermal system developed principally within thick intracaldera ignimbrite deposits and extended to a depth of at least 1 km defining a series of facies characterized by associations of alteration mineral assemblages typical of a seawater dominated high temperature geothermal system supplementary material sample descriptions and analytical methods are available at http www geolsoc org uk sup18371

arsenic in groundwater of the red river floodplain vietnam controlling geochemical processes and reactive transport modeling abstract the mobilization of arsenic as to the groundwater was studied in a shallow holocene aquifer on the red river flood plain near hanoi vietnam the groundwater chemistry was investigated in a transect of 100 piezometers results show an anoxic aquifer featuring organic carbon decomposition with redox zonation dominated by the reduction of fe oxides and methanogenesis enhanced pco2 pressure causes carbonate dissolution to take place but mainly in the soil and unsaturated zone the concentration of as increases over depth to a concentration of up to 550 g l most as is present as as iii but some as v is always found arsenic correlates well with nh4 relating its release to organic matter decomposition and the source of as appears to be the fe oxides being reduced part of the produced fe ii is apparently reprecipitated as siderite containing less as results from sediment extraction indicate most as to be related to the fe oxide fractions the measured amount of sorbed as is low in agreement speciation calculations for a fe oxide surface suggest as iii to constitute only 3 of the surface sites while the remainder is occupied by carbonate and silica species the evolution in water chemistry over depth is homogeneous and a reactive transport model was constructed to quantify the geochemical processes along the vertical groundwater flow component a redox zonation model was constructed using the partial equilibrium approach with organic carbon degradation in the sediment as the only rate controlling parameter apart from the upper meter a constant degradation rate of 0 15 c mmol l yr could explain the redox zonation throughout the aquifer modeling also indicates that the fe oxide being reduced is of a stable type like goethite or hematite arsenic is contained in the fe oxides and is first released during their dissolution our model further suggests that part of the released as is adsorbed on the surface of the remaining fe oxides and in this way may be retarded

simple shearing block resurgence in caldera depressions a model from pantelleria and ischia abstract similar resurgent blocks occur in caldera depressions at the islands of pantelleria and ischia the two blocks which are tilted show similar deformation patterns and similar distributions of volcanic vents related to resurgence the remaining parts of the floors of the two calderas are not deformed similarities between the two features are so striking that the resurgence mechanism must have been alike a dynamic model for resurgence is presented and called simple shearing block resurgence the triggering mechanism is an increase of the magmatic pressure in the upper part of a shallow magma chamber which results in a vertical direction of the maximum stress applied on the roof rocks the block is first defined by high angle inland dipping marginal detachments and then deformed through a simple shearing mechanism reverse faults on its mostly uplifted side determine an horizontal shortening of the block which is balanced on the opposite side by normal faults oblique movements take place along the other two sides due to this distribution of the stress around the block magmas can reach the surface only through the extensional faults on the less uplifted side

geothermal surface alteration of basalts krýsuvik iceland alteration mineralogy water chemistry and the effects of acid supply on the alteration process abstract the geothermal surface alteration of basalts and associated water chemistry at kr suvik sw iceland were studied the geothermal area was characterised with zones of intensive surface alteration steam vents mud pots and hot springs the steam heated geothermal surface waters had ph values between 1 69 and 7 67 and total dissolved solid tds concentrations between 154 and 6660 ppm with cl and so 4 concentration decreasing and increasing with decreasing ph respectively alteration mineral assemblages observed were strongly associated with the surface intensity in areas of most intensive activity the basaltic rocks were altered to amorphous silica anatase and pyrite with a crust of native sulphur at the surface with decreased activity kaolinite became important as well as iron oxyhydroxides and oxides on the flanks of the area montmorillonite was the predominant alteration product based on these observations the surface geothermal activity was divided into three groups 1 high activity areas with active steam vents and mud pots and intensive acid leaching 2 medium activity areas where the ground is hot steam vents and mud pots are uncommon and the surface alteration is less intensive and 3 low activity areas on the margins of the surface geothermal activity the primary factors influencing the steam heated acid sulphate alteration of basalts included the redox state oxidation front supply of acids and ph and the extent of reaction the formation of iron and sulphur containing minerals and the respective elemental mobility depended on the redox conditions with pyrite formation under reduced conditions and goethite and or hematite under oxidised conditions at low ph ca mg k and na were mobile and leached out whereas fe ti and al and to a large degree si were retained in the alteration product at higher ph values 5 the mobility of ca mg k and na was reduced due to the formation of clays

dissolved potassium in the shallow groundwaters circulating in the volcanic rocks of central southern italy abstract the water rock interaction processes occurring in the low temperature shallow volcanic aquifers of central southern italy were simulated by means of the eq3 6 software package in reaction progress mode in the investigation both the roman magmatic province and monte amiata which belongs to the tuscan magmatic province were included the average k2o content of the volcanic rocks from the two areas is similar but the main k bearing phases are leucite in the roman magmatic province and sanidine at monte amiata computed concentrations of major dissolved components and sio2 are consistent with analytical data suggesting that the results of the eq3 6 runs are reliable simulations of the natural water rock interaction processes hence eq3 6 results can be used to investigate the transfer of k from rocks to water it turns out that the attainment of saturation with sanidine during early stages of rock dissolution limits k concentration in the monte amiata groundwaters to around an average of 2 9 1 3 mg l 1 1sd in the silica undersaturated roman magmatic province saturation with leucite is not attained during the evolution of groundwaters and consequently k concentration reaches comparatively high levels with an average of 24 6 27 1 mg l 1 1sd most of these unusually high k concentrations are due to natural weathering of silicates without any anthropogenic influence

geochemical and hydrogeological contrasts between shallow and deeper aquifers in two villages of araihazar bangladesh implications for deeper aquifers as drinking water sources sediment and groundwater profiles were compared in two villages of bangladesh to understand the geochemical and hydrogeological factors that regulate dissolved as concentrations in groundwater in both villages fine grained sediment layers separate shallow aquifers 28 m high in as from deeper aquifers 40 90 m containing 10 g l as in one village dari radiocarbon dating indicates deposition of the deeper aquifer sediments 50 ka ago and a residence time of groundwater of thousands of years in the other village bay the sediment is 20 ka old down to 90 m and the deeper aquifer groundwater is younger with a residence time of hundreds of years the shallow aquifers in both villages that are high in as contain bomb 3 h and bomb 14 c indicating recent recharge the major and minor ion compositions of the shallow and deeper aquifers also differ significantly deeper aquifer water is of the na hco3 type with relatively little dissolved nh4 76 192 mol l fe 27 43 mol l and mn 3 2 mol l in contrast shallow aquifer water is of the ca mg hco3 type with elevated concentrations of dissolved nh4 306 355 mol l fe 191 73 mol l and mn 27 43 mol l in both villages the quantity of as extractable from deeper aquifer sands with a 1 mol l phosphate solution 0 2 0 3 mg kg n 12 0 1 0 1 mg kg n 5 is 1 order of magnitude lower than p extractable as from shallow deposits 1 7 1 2 mg kg n 9 1 4 2 0 mg kg n 11 the differences suggest that the concentration of p extractable as in the sediment is a factor controlling the concentration of as in groundwater low p extractable as levels are observed in both deeper aquifers that are low in as even though there is a large difference in the time of deposition of these aquifers in the two villages the geochemical data and hydrographs presented in this study suggest that both holocene and pleistocene deeper aquifers that are low in as should be a viable source of drinking water as long as withdrawals do not exceed recharge rates of 1 cm yr copyright 2005 elsevier ltd

reply to the comment on arsenic release from biotite into a holocene groundwater aquifer in bangladesh by hossain m anawar and martin mihaljevic

stress induced changes in plate density vail sequences epeirogeny and short lived global sea level fluctuations the stratigraphic record contains clear evidence that broad regions have experienced rapid changes in sea level unrelated to changes in glacial ice volume e g third and higher order vail sea level variations continental foundering we propose that many of these changes can be caused by stress induced changes in plate density stress changes produce significant changes in the density of the crust and lithosphere a point missed in previous investigations and propagate across even the largest plates in less than 30 000 years lithospheric plates interacting at existing boundaries can produce stress related density changes sufficient to cause several meters change in plate elevation these may account for many of the regressions and transgressions seen in the stratigraphic record the creation of new rifts could increase plate compression enough to cause 50 m of plate subsidence plate elevation changes of up to 200 m could result from increased plate compression during continental collisions a particularly enigmatic kind of rapid nonglacial global ngg sea level change is a coupled 50 m sea level fall regression and roughly equal rise transgression occurring in less than 106 years these couplets associated with black shales and marine extinctions can be explained by the elastic snapback attending the rapid formation of a new rift isostatic disequilibria along the new rift depresses the seafloor sufficiently to cause a 50 m fall in sea level mantle flow restores isostatic equilibrium along the rift axis and erases the fall in 60 000 years unusually intense hydrothermal circulation along the new rift during the snapback promotes anoxic bottom water conditions and deposition of black shales a rapid drop in global sea levels reduces the area of ocean bottom within the photic zone causing overpopulation food exhaustion and extinctions the connections between changes in plate stress and density have many implications perhaps the most far reaching is that tectonic events on the 3 4 of the globe that is covered by oceans as well as the 1 4 that is subaerial are recorded in sedimentary strata

tectonic episodes of cratons conflicting north american concepts over the past one hundred years north american geologists have been influenced by concepts of earth history based on periodic events involving marked neocatastrophic changes and competing concepts involving steady state uniformitanan behaviour t c chamberlin a late 19th early 20th century exponent of a nonuniformitarian scenario of earth history proclaimed diastrophism as the ultimate basis of correlation meaning that geological history is punctuated by orogenic paroxysms causing retreat of epicontinental waters to oceanic basins creating segmentation of the stratigraphic and chronological record at mid century james gilluly gained wide support for the proposition that orogeny has followed a uniformitarian near steady state course accordingly episodic diastrophism is a myth and cannot be the basis for periodic eustatic sea level change interregional unconformities etc in more recent decades two powerful new influences on geological thought have emerged the global tectonic paradigm has forced upon us the realization that rearrangements of lithospheric plates are capable of changing the freeboard of continents at the same time technological advances in the acquisition and processing of seismic reflection data have been seized by receptive minds to organize the newly available stratigraphic imagery for application to the discipline of seismic stratigraphy which emphasizes the identification and correlation of unconformity bounded successions of strata expansion of the methodology to the outcrop beyond the limitations of seismic resolution has brought about the flowering of the new sequence stratigraphy doctrine that has deservedlyachieved wide application and acclaim practitioners of sequence stratigraphy impressed by the distances over which unconformity bounded stratal units can be correlated hew to the belief that eustatic commonly glacially engendered sea level change is the control this paper examines evidence of tectonic episodes affecting cratons and their margins these offer promise of a more satisfactory understanding of stratigraphic sequences and of the earth history they represent consideration of these matters makes it inescapable that the elevation and subsidence of cratons and their component basins and arches reflect tectonic phenomena of the continental lithosphere quite independent of eustatic events

variation in the total dissolved carbon isotope composition of thermal waters of the island of ischia italy and its implications for volcanic surveillance abstract during surveys conducted in 1997 30 thermal waters for a total of 39 analysis were collected from shallow wells utilised for resort spas on the island of ischia well temperatures ranged from 25 to 82 c and their depths from 30 to 150 m pools with temperatures of 26 to 67 c were also sampled on the northern flank of the mt epomeo the samples were analysed for bulk chemical composition as well as h o and c isotope composition isotopic analyses of water samples showed wide variations in both deuterium and oxygen 18 which ranged from 37 4 to 8 1 and 6 8 to 1 1 respectively overall 13 c in total dissolved carbon tdc ranged from 11 5 to 2 3 and computed 13 c of co 2 g results ranged from 18 5 to 1 72 water samples with co 2 content of up to about 1 bar partial pressure exhibited 13 c co 2 ranging from 4 to 1 water samples varied chemically from low salinity hco 3 well waters 1 4 g l of essentially meteoric origin to high salinity cl waters up to 40 g l mixing processes from local groundwater seawater and evaporated waters have also been identified models for the chemical and isotopic data on the thermal manifestations have been developed for low and high pressure conditions in order to assess the isotopic composition of deep co 2 assuming that this component is continuously added to the solutions until degassing occurs at shallow depths or at the surface constraints due to the reaction of water with volcanic glass have been taken into account for in the computations in addition to those relative to the carbon components participating in the carbonate equilibria results from such simulations indicate that deep co 2 added to the shallow waters probably has a carbon isotope composition 2 which most likely stems from a mixed magmatic sedimentary origin

the tectonic factor in sea level change a countervailing view it is now demonstrable that many unconformity bounded assemblages of strata the sequences and groupings of sequences of seismic stratigraphy can be correlated subglobally the facies tracts that are identifiable within sequences clearly represent depositional environments that bear a systematic relationship to the strandline and to water depths and thus to sea levels lacking a more suitable explanation for the intercontinental correlation of sequences it has become widely accepted that eustatic changes of sea level are responsible for the interruptions in sedimentation that separate sequences and for the shifts of coastal onlap and of facies tracts implicit in sequence analysis students of cratonic basins are convinced that continental interiors are subject to a tectonic evolution of positive and negative vertical movements and that these control relative sea level and the distribution of sedimentary environments proof of this control is buried hi the sedimentary fills of cratonic basins but it has been difficult to produce such proof in a form digestible by neo neptunist doubters digitization of isopach maps of successive stratal units produces thickness data amenable to numerical study for example the logarithms of thicknesses of a stratal unit commonly exhibit a uniform areal distribution the hallmark of a bivariate normal distribution this revelation makes available a number of gaussian statistical measures as descriptors of time variable basin geometry extraction and analysis of geometric parameters lead to the realization that basin evolution necessarily including relative sea levels and the transgression and progradation of strandlines and sedimentary environments has a strong tectonic component that gives evidence of synchrony of widely separated basins on separate cratons secular variation in radial heat flow leading to episodic inflation and deflation of continents is suggested as a possible globally effective mechanism

geochemical processes underlying a sharp contrast in groundwater arsenic concentrations in a village on the red river delta vietnam abstract the spatial variability of as concentrations in aquifers of the red river delta vietnam was studied in the vicinity of hanoi two sites only 700 m apart but with very different as concentrations in groundwater site l nh 4 10 mg l hco 3 500 mg l and dissolved p 600 mg l in addition to elevated as at site h are consistent with a release coupled to microbially induced reductive dissolution of fe oxyhydroxides other processes such as precipitation of siderite and vivianite which are strongly supersaturated at site h or the formation of amorphous fe ii as iii phases and fe sulfides may also influence the partitioning of as between groundwater and aquifer sands the origin of the redox contrast between the two sites is presently unclear peat was observed at site l but it was embedded within a thick clayey silt layer at site h instead organic rich layers were only separated from the underlying aquifer by thin silt layers leaching of organic matter from this source could cause reducing conditions and therefore potentially be related to particularly high concentrations of dissolved nh 4 hco 3 p and doc in the portion of the aquifer where groundwater as concentrations are also elevated

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

geothermal assessment of the island of ischia southern italy from isotopic and chemical composition of the delivered fluids abstract the ischia geothermal system is hosted by silicic rocks of the quaternary potassic roman province in southern italy exploration drilling down to 1156 m depth in the mid 1950s provided information on boiling profiles up to 250 c and on the depth and permeability of the potential reservoirs discharge fluid samples were collected and analyzed to define the inflow of surrounding seawater c1 ranges from 2 5 to 20 g kg into the system analyses of samples from surface manifestations and shallow wells collected during 1983 and 1988 point to the existence of three distinct mixing regimes involving three water components a dishomogeneous body of diluted water cl less than 2 5 g kg that occurs at depths 700 m and reequilibrates at 240 c at least is overlain by an aquifer of groundwater variably mixed with variably seawater cl from 4 to 10 g kg which tends to reequilibrate at 160 c steam heated waters locally develop and act as dilutants of the rising geothermal fluids dilution mixing and evaporation of the ascending chloride fluids are supported by oxygen and hydrogen isotopic data the thermal waters being enriched in 18 o and d with respect to local meteoric water by up to 7 and 30 respectively the relative composition of the major cations in thermal solutions was used to discriminate the two main groups of thermal waters the reservoir temperatures of which are estimated from the na k gethermometer k mg geothermometer indicates reequilibration in near surface conditions the isotopic composition of the fumarolic steam varies from 7 to 12 in 8 o and from 35 to 70 in d in agreement with a deep mixed fluid that boils adiabatically from 240 to 80 c the deuterium content of the h 2 o h 2 pair gives enrichment factor of about 830 corresponding to equilibrium temperature conditions slightly higher than the surface boiling temperatures the 13 c of co 2 is almost constant at 4 5 1 0 4 suggesting an important magmatic contribution and the 18 o values of co 2 appears to in equilibrium with accompanying steam at the measured temperatures the co 2 ar and h 2 ar chemical ratios have been used to derive aquifer temperatures the values obtained being consistent with those of solute geothermometers

new insights into late pleistocene explosive volcanic activity and caldera formation on ischia southern italy a new pyroclastic stratigraphy is presented for the island of ischia italy for the period 75 50 ka bp the data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought numerous vents were probably active during this period the deposits of at least 10 explosive phonolite to basaltic trachyandesite eruptions are described and interpreted they record a diverse range of explosive volcanic activity including voluminous fountain fed ignimbrite eruptions fallout from sustained eruption columns block and ash flows and phreatomagmatic eruptions previously unknown eruptions have been recognised for the first time on the island several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of procida and parts of the mainland the morphology of ischia was significantly different to that seen today with edifices to the south and west and a submerged depression in the centre the largest volcanic event the monte epomeo green tuff megt resulted in caldera collapse across all or part of the island it is shown to comprise at least two thick intracaldera ignimbrite flow units separated by volcaniclastic sediments that were deposited during a pause in the eruption extracaldera deposits of the megt include a pumice fall deposit emplaced during the opening phases of the eruption a widespread lithic lag breccia outcropping across much of ischia and procida and a distal ignimbrite in south west campi flegrei during this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber which was partially destroyed during the megt eruption this contrasts with the small volume holocene and historical effusive and explosive activity on ischia the timing and distribution of which has been controlled by the resurgence of the monte epomeo block the new data contribute to a clearer understanding of the long term volcanic and magmatic evolution of ischia

palaeogeological hiatus surface mapping a tool to visualize vertical motion of the continents dynamic topography is a well established consequence of global geodynamic models of mantle convection with horizontal dimensions of 1000 km and amplitudes up to 2 km such physical models guide the interpretation of geological records on equal dimensions continent scale geological maps therefore serve as reference frames of choice to visualize erosion non deposition as a proxy for long wavelength low amplitude vertical surface motion at a resolution of systems or series such maps display conformable and unconformable time boundaries traceable over hundreds to thousands of kilometres unconformable contact surfaces define the shape and size of time gap hiatus in millions of years based on the duration of time represented by the missing systems or series hiatus for a single system or series base datum diminishes laterally to locations anchor points where it is conformable at the mapped resolution it is highly dependent upon scale a comparison of hiatus area between two successive system or series boundaries yields changes in location shape size and duration indicative of the transient nature of vertical surface motion as a single step technique it serves as a quantitative proxy for palaeotopography that can be calibrated using other geological data the tool magnifies the need for geological mapping at the temporal resolution of stages matching process rates the method has no resolving power within conformable regions basins but connects around them when applied to marine seismic sections that relate to rock record not to time biostratigraphic and radiometric data from deep wells are needed before hiatus areas that relate to time can be mapped

chemical and isotopic investigations of fumarolic gases from ischia island southern italy evidences of magmatic and crustal contribution abstract chemical and isotopic investigations have been carried out on subaerial and submarine fumarolic fluids on the island of ischia gulf of napoli southern italy chemical analyses of gases from the hottest fumarole 117 c located on mount epomeo show a typical geothermal composition thermodynamic computations indicate a feeding source temperature of mt epomeo ranging from 300 to 500 c isotopic compositions suggest that the island is divided into two parts east and west fumarole gases on the west side of the island indicate a magmatic source with small crustal contamination and are characterized by carbon isotopic data similar to morb 13 c ranging from 3 5 to 5 11 with about 50 of the helium having a magmatic origin 3 6 3 7 r r a fumaroles located on the east side of the island have carbon isotopic data ranging from 0 45 to 0 76 and a lower helium isotopic ratio from 2 09 to 2 89 r r a the east fumaroles seem to be fed by fluids deriving from a reservoir located in or close to a carbonatic source in the crust

linking organic matter deposition and iron mineral transformations to groundwater arsenic levels in the mekong delta cambodia enriched as in drinking water wells in south and southeast asia has increased the risk of cancer for nearly 100 million people this enrichment is generally attributed to the reductive dissolution of fe oxides however the complex expression of as enrichment in these areas is not yet well understood here the coupled sedimentological and geochemical factors that contribute to the extent and spatial distribution of groundwater as concentrations in the mekong river delta cambodia in an avulsed scroll bar sequence are examined x ray absorption spectroscopy xas was used to determine fe and as speciation in redox preserved sediment collected from drilled cores dissolved as fe and s solution concentrations in existing and newly drilled wells cores differed considerably depending on their source sedimentology the rapid burial of organic matter in the scroll bar sequence facilitated the development of extensive fe reducing conditions and as release into the aquifer in older features organic c levels are high enough to sustain extensive fe reduction and provide ample so4 which is reduced to sulfide this s reduction impacts as levels as is sequestered in sulfide minerals outside of the scrollbar sequence decreasing pore water concentrations in contrast as is depleted in sediments from the scroll sequence and associated with elevated pore water aqueous concentrations the concentration and form of organic c in the scrollbar sequence is related to depositional environment and can facilitate fe and s mineral transformations distinct sedimentary environments explain a portion of the inherent heterogeneity of aquifer as concentrations

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

the pantelleria caldera geothermal system data from the hydrothermal minerals abstract this paper proposes on the basis of petrographic and mineralogic data on cutting and cores from two deep wells pantelleria 1 and pantelleria 2 the first model of the active hydrothermal system of the island of pantelleria phyllosilicates were studied in detail because they are considered key minerals in the identification of hydrothermal processes the results of these studies emphasize differences between the intracaldera and pericaldera areas of the island within the 45 ka caldera there is a high temperature 240 260 c at 600 800 m depth active hydrothermal system with five zones of characteristic alteration minerals with increasing depth rocks are unaltered to a depth of 200 m contain smectite and mixed layer chlorite smectite c s between 200 and 380 m chlorite illite chalcedony and quartz from 380 to 500 m albite adularia and saponite from 500 to 680 m mixed layer biotite vermiculite from 680 m to the depth drilled 1100 outside the caldera but near the rim a low temperature and low permeability

water rock interaction in the active geothermal system of pantelleria italy abstract the geothermal fluid sampled in a deep 990 m b s l well drilled into the peralkaline volcano of pantelleria consists for the most part of seawater the fluid is depleted in ca and mg and enriched in k rb cs and f due to interaction with rocks of trachytic composition a forward geochemical model has been used to simulate the water rock interaction processes occurring within the geothermal system in the southern part of the island the simulation assumes different mixtures of seawater with groundwater and volcanic gas the system model that emerges from the simulation is the following the seawater flows through fractures and faults reaching temperatures of over 300 c in the southern part of the island where an upflow zone of volcanic gas is present volcanic gases increase the amount of c and s in the system lower the ph and enhance the dissolution of trachyte after reaction with trachyte the ph increases and the fluid reaches saturation conditions with respect to albite quartz saponite k feldspar and muscovite in agreement with the natural hydrothermal mineral assemblages the model confirms that the reservoir fluid is a mix between seawater and meteoric water more saline fluids possibly existing in deeper levels the fluid chemistry and the hydrothermal minerals are similar to those found in the icelandic geothermal systems of reykjanes and svartsengi

quantitative models of hydrothermal fluid mineral reaction the ischia case abstract the intricate pathways of fluid mineral reactions occurring underneath active hydrothermal systems are explored in this study by applying reaction path modelling to the ischia case study ischia island in southern italy hosts a well developed and structurally complex hydrothermal system which because of its heterogeneity in chemical and physical properties is an ideal test sites for evaluating potentialities limitations of quantitative geochemical models of hydrothermal reactions we used the eq3 6 software package version 7 2b to model reaction of infiltrating waters mixtures of meteoric water and seawater in variable proportions with ischia s reservoir rocks the mount epomeo green tuff units megt the mineral assemblage and composition of such megt units were initially characterised by ad hoc designed optical microscopy and electron microprobe analysis showing that phenocrysts dominantly alkali feldspars and plagioclase are set in a pervasively altered with abundant clay minerals and zeolites groundmass reaction of infiltrating waters with megt minerals was simulated over a range of realistic for ischia temperatures 95 260 c and co 2 fugacities 10 0 2 to 10 0 5 bar during the model runs a set of secondary minerals selected based on independent information from alteration minerals studies was allowed to precipitate from model solutions when saturation was achieved the compositional evolution of model solutions obtained in the 95 260 c runs were finally compared with compositions of ischia s thermal groundwaters demonstrating an overall agreement our simulations in particular well reproduce the mg depleting maturation path of hydrothermal solutions and have end of run model solutions whose na k mg compositions well reflect attainment of full equilibrium conditions at run temperature high temperature 180 260 c model runs are those best matching the na k mg compositions of ischia s most chemically mature water samples supporting quenching of deep reservoir conditions for these surface manifestations whilst fe sio 2 and to a lesser extent so 4 contents of natural samples are better reproduced in low temperature 95 c runs suggesting that these species reflect conditions of water rock interaction in the shallow hydrothermal environment the ability of model runs to reproduce the compositional features of ischia s thermal manifestations demonstrated here adds supplementary confidence on reaction path modelling as a realistic and insightful representation of mineral fluid hydrothermal reactions our results in particular demonstrate the significant impact of host rock minerals assemblage in governing the paths and trends of hydrothermal fluids maturation

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

natural organic matter in sedimentary basins and its relation to arsenic in anoxic ground water the example of west bengal and its worldwide implications in order to investigate the mechanism of as release to anoxic ground water in alluvial aquifers the authors sampled ground waters from 3 piezometer nests 79 shallow 80 m wells in an area 750 m by 450 m just north of barasat near kolkata calcutta in southern west bengal high concentrations of as 200 1180 mug l 1 are accompanied by high concentrations of fe 3 13 7 mgl 1 and po4 1 6 5 mg l 1 ground water that is rich in mn 1 5 3 mg l 1 contains 50 mug l 1 of as the composition of shallow ground water varies at the 100 m scale laterally and the metre scale vertically with vertical gradients in as concentration reaching 200 mug l 1 m 1 the as is supplied by reductive dissolution of feooh and release of the sorbed as to solution the process is driven by natural organic matter in peaty strata both within the aquifer sands and in the overlying confining unit in well waters thermotolerant coliforms a proxy for faecal contamination are not present in high numbers 10 cfu 100 ml in 85 of wells showing that faecally derived organic matter does not enter the aquifer does not drive reduction of feooh and so does not release as to ground water arsenic concentrations are high much greater than50 mug l 1 where reduction of feooh is complete and its entire load of sorbed as is released to solution at which point the aquifer sediments become grey in colour as feooh vanishes where reduction is incomplete the sediments are brown in colour and resorption of as to residual feooh keeps as concentrations below 10 mug l 1 in the presence of dissolved fe sorbed as released by reduction of mn oxides does not increase as in ground water because the as resorbs to feooh high concentrations of as are common in alluvial aquifers of the bengal basin arise because himalayan erosion supplies immature sediments with low surface loadings of feooh on mineral grains to a depositional environment that is rich in organic mater so that complete reduction of feooh is common c 2004 published by elsevier ltd

arsenic incorporation into authigenic pyrite bengal basin sediment bangladesh abstract sediment from two deep boreholes 400 m approximately 90 km apart in southern bangladesh was analyzed by x ray absorption spectroscopy xas total chemical analyses chemical extractions and electron probe microanalysis to establish the importance of authigenic pyrite as a sink for arsenic in the bengal basin authigenic framboidal and massive pyrite median values 1500 and 3200 ppm as respectively is the principal arsenic residence in sediment from both boreholes although pyrite is dominant ferric oxyhydroxides and secondary iron phases contain a large fraction of the sediment bound arsenic between approximately 20 and 100 m which is the depth range of wells containing the greatest amount of dissolved arsenic the lack of pyrite in this interval is attributed to rapid sediment deposition and a low sulfur flux from riverine and atmospheric sources the ability of deeper aquifers 150 m to produce ground water with low dissolved arsenic in southern bangladesh reflects adequate sulfur supplies and sufficient time to redistribute the arsenic into pyrite during diagenesis

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

the structure of a hydrothermal system from an integrated geochemical geophysical and geological approach the ischia island case study the complexity of volcano hosted hydrothermal systems is such that thorough characterization requires extensive and interdisciplinary work we use here an integrated multidisciplinary approach combining geological investigations with hydrogeochemical and soil degassing prospecting and resistivity surveys to provide a comprehensive characterization of the shallow structure of the southwestern ischia s hydrothermal system we show that the investigated area is characterized by a structural setting that although very complex can be schematized in three sectors namely the extra caldera sector ecs caldera floor sector cfs and resurgent caldera sector rcs this contrasted structural setting governs fluid circulation geochemical prospecting shows in fact that the caldera floor sector a structural and topographic low is the area where co2 rich 40 cm3 l hydrothermally mature log mg na ratios 150 g m 2 d 1 is clearly captured by electrical resistivity tomography ert and transient electromagnetic tem surveys as a highly conductive resistivity 10 000 mg l and poorly conductive meteoric derived tds 4 000 mg l waters are observed respectively we finally integrate our observations to build a general model for fluid circulation in the shallowest 0 5 km part of ischia s hydrothermal system

chemical and isotopic signatures of the basement rocks from the campi flegrei geothermal field naples southern italy inferences about the origin and evolution of its hydrothermal fluids abstract the campi flegrei naples campanian plain southern italy geothermal system is hosted by quaternary volcanic rocks erupted before during and after the formation of the caldera that represents one of the major structural features in the neapolitan area the volcanic products rest on a mesozoic carbonate basement cropping out north east and south of the area chemical major minor and trace elements and stable isotope c h o analyses were conducted on drill core samples recovered from geothermal wells mf 1 mf 5 sv 1 and sv 3 at depths of 1100 to 2900 m the study was complemented by petrographic and sem examination of thin sections the water which feeds the system is both marine and meteoric in origin mineral zonation typical of a high temperature geothermal system exists in all the geothermal wells measured temperatures in wells are as high as 400 c the chemical composition of the waters suggests the existence of two reservoirs a shallow reservoir depth 2000 m of hypersaline water the drill cores are mainly hydrothermally altered volcanics of trachy latitic affinity but some altered pelites and limestones are also present published na mg and k concentrations of selected geothermal waters indicate that the hydrothermal fluids are in equilibrium with their host rocks with respect to k feldspar albite sericite and chlorite the measured 18 o smow values of rocks range from 4 3 to 16 5 the measured d smow values range from 79 to 46 the calculated isotopic composition of the fluids at equilibrium with the samples vary from 1 to 8 3 18 o and from 52 to 1 d the estimated isotopic composition of the waters at equilibrium with the studied samples confirmed the existence of two distinct fluid types circulating in the geothermal system the shallower has a marine water signature while the deeper water has a signature consistent both with magmatic and meteoric origins in the latter case the recharge of this aquifer likely occurs at the outcrop of the mesozoic limestones surrounding the campanian plain after infiltration the water percolates through evaporitic layers becoming hypersaline and d depleted

redox zonation and oscillation in the hyporheic zone of the ganges brahmaputra meghna delta implications for the fate of groundwater arsenic during discharge abstract riverbank sediment cores and pore waters shallow well waters seepage waters and river waters were collected along the meghna riverbank in gazaria upazila bangladesh in jan 2006 and oct nov 2007 to investigate hydrogeochemical processes controlling the fate of groundwater as during discharge redox transition zones from suboxic 0 2 m depth to reducing 2 5 m depth then suboxic conditions 5 7 m depth exist at sites with sandy surficial deposits as evidenced by depth profiles of pore water n 7 and sediment n 11 diffuse reflectance fe iii fe ratios and fe iii concentrations the sediment as enrichment zone up to 700 mg kg 1 is associated with the suboxic zones mostly between 0 and 2 m depth and less frequently between 5 and 7 m depth the as enriched zones consist of several 5 10 cm thick dispersed layers and span a length of 5 15 m horizontally from the river shore depth profiles of riverbank pore water deployed along a 32 m transect perpendicular to the river shore show elevated levels of dissolved fe 11 6 11 7 mg l 1 and as 118 91 g l 1 mostly as arsenite between 2 and 5 m depth but lower concentrations between 0 and 2 m depth 0 13 0 19 mg l 1 fe 1 1 g l 1 as and between 5 and 6 m depth 1 14 0 45 mg l 1 fe 28 17 g l 1 as because it would take more than a few hundred years of steady groundwater discharge 10 m yr 1 to accumulate hundreds of mg kg 1 of as in the riverbank sediment it is concluded that groundwater as must have been naturally elevated prior to anthropogenic pumping of the aquifer since the 1970s not only does this lend unequivocal support to the argument that as occurrence in the ganges brahmaputra meghna delta groundwater is of geogenic origin it also calls attention to the fate of this as enriched sediment as it may recycle as into the aquifer

geochemical evolution of groundwater flowing through arsenic source sediments in an aquifer system of west bengal india the source of geogenic arsenic as contaminating a shallow aquifer in west bengal was traced to fine grained sediments deposited in an abandoned river channel along with as bearing phases these sediments contain 0 46 co deposited organic carbon the release of as and the geochemistry of groundwater within the channel fill deposits is investigated using a detailed mass balance model supported by aqueous solid phase and mineralogical data the model describes the evolution of groundwater chemistry along a flow path extending from its recharge in an abandoned channel pond through the channel fill to the underlying aquifer variations in groundwater composition are explained in terms of mineral weathering of host sediments driven by organic carbon decay arsenic is released through the reductive dissolution of goethite and the weathering of chlorite concomitantly some as is sequestered in precipitating vivianite these competing processes reach equilibrium deeper in the channel fill sequence as groundwater as concentrations stabilize the model yields estimates of mineral reaction or precipitation rates including rates of organic carbon oxidation 1 15 mmol c l 1 a 1 and net as release 4 57 10 4 mmol l 1 a 1 fine grained slightly permeable deposits such as channel fill containing reactive organic carbon and as bearing goethite and phyllosilicates are centers of intense chemical weathering conducive to as mobilization

unusually large subsidence and sea level events during middle paleozoic time new evidence supporting mantle convection models for supercontinent assembly new evidence from basin modeling documents a nearly synchronous episode of rapid subsidence between middle devonian and earliest mississippian time in almost all preexisting margins and interior basins of north america the subsidence events appear to have been accompanied by a large sea level rise relative to the stable cratonic platform herein we suggest that the timing and magnitude of basin subsidence and sea level change are compatible with predictions by recent mantle convection models for the assembly of supercontinents if so the sea level and subsidence events were consequences of the onset of accretion of pangea above a major zone of mantle downwelling

a transect of groundwater and sediment properties in araihazar bangladesh further evidence of decoupling between as and fe mobilization a new sampling device was used to obtain 8 detailed profiles of groundwater and associated sediment properties to 30 m depth in a 4 km 2 area of bangladesh that is characterized by high spatial variability in groundwater as concentrations of dissolved as fe and mn ranged from b0 1 to 600 g l b0 1 to 18 mg l and b0 1 to 4 mg l respectively voltammetric measurements conducted in the field indicate that n95 of the dissolved arsenic was as iii the proportion of fe ii in the hot leachable iron fraction of the sediment ranged from 0 2 to 0 9 the concentration of phosphate extractable as in aquifer sands ranged from b0 1 to 1 1 mg kg although the highest dissolved as concentration was measured at the site where the highest groundwater fe and mn concentrations were also observed the dissolved as maximum was located several meters below the dissolved fe and mn maxima fe ii fe ratios and p extractable as concentrations in the solid phase were particularly elevated at the same site though again the profiles of these properties did not consistently match that of groundwater as concentrations a 500 m section composed of 5 vertical profiles confirms that whereas low as concentrations are broadly associated with low dissolved fe and mn concentrations as well as low fe ii fe ratios in the solid phase the spatial distributions of these properties in the subsurface do not follow a simple relationship in combination with other observations from the area the results suggest that elevated local recharge in areas where the permeability of surface soils is high prevents as from accumulating in groundwater conversely dissolved as concentrations tend to be high in areas where local recharge is restricted by a surface cover of low permeability the data demonstrate that further investigation of the mechanisms underlying as mobilization is likely to require sampling at lateral scales b1 km 2006 elsevier b v all rights reserved

naturally occurring arsenic mobilization at a landfill in maine and implications for remediation abstract elevated levels of dissolved arsenic 300 g l 1 have been detected beneath and in groundwater plumes extending away from a closed landfill in southern maine this study sought to determine the source of arsenic to the aquifer the processes responsible for arsenic mobilization and to evaluate the effectiveness of remediation efforts that have occurred at this site the as appears to originate in the natural glacial aquifer solids which contain 5 mg kg 1 as on a dry weight basis this conclusion is supported by the relatively uniform distribution of as in sediment samples results of laboratory batch incubation experiments and comparisons with groundwaters in nearby wetlands which also have high levels of dissolved as that do not appear to originate within the landfill the as is mobilized in the subsurface by strongly reducing conditions beneath the landfill and in nearby wetlands in the aquifer beneath the landfill the average oxidation reduction potential orp is 95 mv eh 105 mv and these reducing conditions were primarily induced by landfill leachate remediation efforts at this site have included installation of a low permeability clay cap groundwater extraction oxidation and re injection and subsurface oxidation by injection of magnesium peroxide the natural source of arsenic within the aquifer solids coupled with widespread reducing conditions has severely limited the effectiveness of these interventions on groundwater arsenic concentrations

a model for ischia hydrothermal system evidences from the chemistry of thermal groundwaters abstract ischia volcano in central italy has long been known for its copious surface hydrothermal manifestations signs of a pervasive circulation of hot fluids in the subsurface because of the significant chemical heterogeneity of fumarolic gas discharges and hot spring discharges evidences of a complex hydrothermal setting a definite model of fluid circulation at depth is currently unavailable in spite of the several previous efforts here we report on the chemical and isotopic composition of 120 groundwater samples collected during several sampling surveys from 2002 to 2007 the acquired data suggest that the composition of surface manifestations reflect contributions from meteoric water sea water and thermal fluids rising from two distinct hydrothermal reservoir with equilibrium temperatures of respectively 150 c and 270 c and depths of 150 300 m and 300 m but possibly 1000 m we also make use of an isotopic characterization of the dissolved gas phase in thermal waters to demonstrate that the ischia hydrothermal system is currently supplied by a deep rising gas component dgc characterized by co2 97 7 1 2 vol on a water free basis 13cco2 3 51 0 9 and helium isotopic ratio of about 3 5 ra 3he 4he ratio normalized to the air ratio ra likely magmatic in origin an assessment of the thermal budget for ischia hydrothermal system is also presented in the attempt to derive a first estimate of the size and rate of degassing of the magmatic reservoir feeding the gas emissions we calculate that a heat flow of about 153 222 mw presently drives hydrothermal circulation on the island which we suggest is supplied in convective form e g by the ascent of a high t magmatic vapour phase by complete degassing of 2 2 3 3 107 m3 yr 1 of trachytic magma with 2 1 wt dissolved h2o content if extrapolated to entire period of quiescence lasting since the arso eruption in 1302 a d this volume corresponds to 1 6 2 3 1010 m3 of magma degassed in about 700 years of quiescent activity

phase equilibria constraints on the chemical and physical evolution of the campanian ignimbrite the campanian ignimbrite is a 4200 km trachyte phonolite pyroclastic deposit that erupted at 39 3 0 1ka within the campi flegrei west of naples italy here we test the hypothesis that campanian ignimbrite magma was derived by isobaric crystal fractionation of a parental basaltic trachyandesitic melt that reacted and came into local equilibrium with small amounts 5 10 wt of crustal rock skarns and foid syenites during crystallization comparison of observed crystal and magma compositions with results of phase equilibria assimilation fractionation simulations melts is generally very good oxygen fugacity was approximately buffered along qfm 1 where qfm is the quartz fayalite magnetite buffer during isobaric fractionation at 0 15 gpa 6 km depth the parental melt reconstructed from melt inclusion and host clinopyroxene compositions is found to be basaltic trachyandesite liquid 51 1 wt sio2 9 3 wt mgo 3 wt h2o a significant feature of phase equilibria simulations is the existence of a pseudo invariant temperature 883 8c at which the fraction of melt remaining in the system decreases abruptly from 0 5 to50 1 crystallization at the pseudo invariant point leads to abrupt changes in the composition properties density dissolved water content and physical state viscosity volume fraction fluid of melt and magma a dramatic decrease in melt viscosity from 1700 pa s to 200 pa s coupled with a change in the volume fraction of water in magma from 0 1 to 0 8 and a dramatic decrease in melt and magma density acted as a destabilizing eruption trigger thermal models suggest a timescale of 200 kyr from the beginning of fractionation until eruption leading to an apparent rate of evolved magma generation of about 10 3 km year in situ crystallization and crystal settling in density stratified regions as well as in convectively mixed less evolved subjacent magma operate rapidly enough to match this apparent volumetric rate of evolved magma production

stratigraphic framework for the plume mode of mantle convection and the analysis of interregional unconformities on geological maps abstract mantle convection is a fundamental planetary process its plate mode is established and expressed by plate tectonics its plume mode also is established and expressed by interregional geological patterns we developed both an event based stratigraphic framework to illustrate the surface effects predicted by the plume model of griffiths et al 1989 and griffiths and campbell 1990 and a methodology to analyze continent scale geological maps based on unconformities and hiatuses the surface expression of ascending plumes lasts for tens of millions of years and rates vary over a few million years as the plume ascends its surface expression narrows but increases in amplitude leaving distinct geological and stratigraphic patterns in the geologic record not only above the plume head center but also above its margins and in distal regions a few thousands of kilometers from the center to visualize these patterns we constructed sequential geological maps chronostratigraphic sections and hiatus diagrams dome uplift with erosion engor 2001 and the flood basalts duncan and richards 1991 ernst and buchan 2001a are diagnostic starting points for plume stratigraphic analyses mechanical collapse of the dome results in narrow rifting burke and dewey 1973 drainage network reorganization cox 1989 and flood basalt eruption in the marginal region patterns of vertical movement deformation and surface response are transient and complex at first the plume margin is uplifted together with the central region but then it subsides as the plume ascents farther with plume head flattening the plume margin experiences renewed outward migrating surface uplift erosion broad crustal faulting and drainage reorganization knickpoint migration occurs first inward directed at the rate of plume ascent and later outward directed at the rate of asthenospheric flow interregional scale unconformity bounded stratigraphic successions document the two inversions the distal regions which did not experience any plume related uplift yield complete sedimentary records of the event event related time gaps hiatuses in the sedimentary record increase towards the center but the event horizon is best preserved in the distal region it may be recognized by tracing its contacts from the center outwards we extracted system and series hiatuses from interregional geological maps and built hiatus maps as proxies for paleo dynamic topography and as a basis for comparison with results from numerical models interregional scale geological maps are well suited to visualize plume related geological records of dynamic topography in continental regions however geological records and hiatus information at the resolution of stages will be needed at interregional scales the plume stratigraphic framework is event based interregional but not global with time dependent amplitudes that are significantly larger than those of global eustatic sea level fluctuations global stratigraphic syntheses require integration of plate and plume stratigraphic frameworks before eustatic contributions may be assessed

controls on facies distribution and stratigraphic preservation in the ganges brahmaputra delta sequence abundant sediment supply and accommodation space in the bengal basin have led to the development of a major late quaternary delta sequence this sequence has formed in a tectonically active setting and represents an important example of a high energy marine and fluvial high yield continental margin deposit recent studies have detailed the delta s stratigraphy and development noting that tectonics and sediment supply control the ganges brahmaputra more significantly than in many other delta systems these ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment supply have had on deltaic processes and sequence character unique and differing stratigraphies are found within thedeltasystem suchthatfine grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding block faulting and subsidence coarse grained deposits dominate the stratigraphy under the control of high energy fluvial processes and mixed fine coarse stratigraphies are found in areas dominantly influenced by eustatic sea level change overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events e g earthquakes and rivers avulsions and long termchangesinclimate andsediment supply modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character results show that the timing and magnitude of sediment input relative to sea level rise is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence d 2002 elsevier science b v all rights reserved

neotectonics of the owen fracture zone nw indian ocean structural evolution of an oceanic strike slip plate boundary the owen fracture zone is a 800 km long fault system that accommodates the dextral strike slip motion between india and arabia plates because of slow pelagic sedimentation rates that preserve the seafloor expression of the fault since the early pliocene the fault is clearly observed on bathymetric data it is made up of a series of fault segments separated by releasing and restraining bends including a major pull apart basin at latitude 20 n some distal turbiditic channels from the indus deep sea fan overlap the fault system and are disturbed by its activity thus providing landmarks to date successive stages of fault activity and structural evolution of the owen fracture zone from pliocene to present we determine the durability of relay structures and the timing of their evolution along the principal displacement zone from their inception to their extinction we observe subsidence migration in the 20 n basin and alternate activation of fault splays in the vicinity of the qalhat seamount the present day owen fracture zone is the latest stage of structural evolution of the 20 myr old strike slip fault system buried under indus turbiditic deposits whose activity started at the eastern foot of the owen ridge when the gulf of aden opened the evolution of the owen fracture zone since 3 6 myr reflects a steady state plate motion between arabia and india such as inferred by kinematics for the last 20 myr period the structural evolution of the owen fracture zone since 20 myr including fault segments propagation and migration pull apart basin opening and extinction seems to be characterized by a progressive reorganisation of the fault system and does not require any major kinematics change

arsenic in groundwater testing pollution mechanisms for sedimentary aquifers in bangladesh in the deltaic plain of the ganges meghna brahmaputra rivers arsenic concentrations in groundwater commonly exceed regulatory limits 50 m gl 21 because feooh is microbially reduced and releases its sorbed load of arsenic to groundwater neither pyrite oxidation nor competitive exchange with fertilizer phosphate contribute to arsenic pollution the most intense reduction and so severest pollution is driven by microbial degradation of buried deposits of peat concentrations of ammonium up to 23 mg l 21 come from microbial fermentation of buried peat and organic waste in latrines concentrations of phosphorus of up to 5 mg l 21 come from the release of sorbed phosphorus when feooh is reductively dissolved and from degradation of peat and organic waste from latrines calcium and barium in groundwater come from dissolution of detrital and possibly pedogenic carbonate while magnesium is supplied by both carbonate dissolution and weathering of mica the 87 sr 86 sr values of dissolved strontium define a two component mixing trend between monsoonal rainfall 0 711 6 0 001 and detrital carbonate 0 735

mobility of arsenic in a bangladesh aquifer inferences from geochemical profiles leaching data and mineralogical characterization aquifer geochemistry was characterized at a field site in the munshiganj district of bangladesh where the groundwater is severely contaminated by as vertical profiles of aqueous and solid phase parameters were measured in a sandy deep aquifer depth 150 m below a thick confining clay 119 to 150 m a sandy upper aquifer 3 5 to 119 m above this confining layer and a surficial clay layer 3 5 m in the deep aquifer and near the top of the upper aquifer aqueous as levels are low 10 g l but aqueous as approaches a maximum of 640 g l at a depth of 30 to 40 m and falls to 58 g l near the base 107 m of the upper aquifer in contrast solid phase as concentrations are uniformly low rarely exceeding 2 g g in the two sandy aquifers and never exceeding 10 g g in the clay layers solid phase as is also similarly distributed among a variety of reservoirs in the deep and upper aquifer including adsorbed as as coprecipitated in solids leachable by mild acids and reductants and as incorporated in silicates and other more recalcitrant phases one notable difference among depths is that sorbed as loads considered with respect to solid phase fe extractable with 1 n hcl 0 2 m oxalic acid and a 0 5 m ti iii citrate edta solution appear to be at capacity at depths where aqueous as is highest this suggests that sorption limitations may in part explain the aqueous as depth profile at this site competition for sorption sites by silicate phosphate and carbonate oxyanions appear to sustain elevated aqueous as levels in the upper aquifer furthermore geochemical profiles are consistent with the hypothesis that past or ongoing reductive dissolution of fe iii oxyhydroxides acts synergistically with competitive sorption to maintain elevated dissolved as levels in the upper aquifer microprobe data indicate substantial spatial comapping between as and fe in both the upper and deep aquifer sediments and microscopic observations reveal ubiquitous fe coatings on most solid phases including quartz feldspars and aluminosilicates extraction results and xrd analysis of density magnetic separates suggest that these coatings may comprise predominantly fe ii and mixed valence fe solids although the presence of fe iii oxyhydroxides can not be ruled out these data suggest as release may continue to be linked to dissolution processes targeting fe or fe rich phases in these aquifers copyright 2004 elsevier ltd

mineralogy and geochemistry of shallow sediments of sonargaon bangladesh and implications for arsenic dynamics focusing on the role of organic matter abstract mineralogy and geochemistry of modern shallow sediments up to 5 m thick within the zone of water table fluctuations were studied to determine the likely sources and processes responsible for releasing as into groundwater samples were collected from different geological settings with varying groundwater as concentrations during dry december 2005 and wet september 2006 seasons at sonargaon bangladesh stratigraphic sequences of the studied sediments showed three distinct lithofacies viz clayey silt silty clay and silty very fine sand corresponding to fine grained overbank associations total as concentrations of shallow sediments ranged from 80 of the as was fixed in insoluble and organic phases while the amount of as in reducible and acid soluble phases was very low

bacterial sulfate reduction limits natural arsenic contamination in groundwater natural arsenic contamination of groundwater increasingly recognized as a threat to human health worldwide is characterized by arsenic concentrations that vary sharply over short distances variation in arsenic levels in the mahomet aquifer system a regional glacial aquifer in central illinois appears to arise from variable rates of bacterial sulfate reduction in the subsurface not differences in arsenic supply where sulfate reducing bacteria are active the sulfide produced reacts to precipitate arsenic or coprecipitate it with iron leaving little in solution in the absence of sulfate reduction methanogenesis is the dominant type of microbial metabolism and arsenic accumulates to high levels

14c geochronological constraints for the volcanic history of the island of ischia italy over the last 5000 years abstract the chronology of the most significant eruptions at the island of ischia in the last 5000 years has been studied by means of accelerator mass spectrometry to obtain 14 c data the results are in good agreement with stratigraphical and archeological constraints this allows interpretation of the measured dates as ages of the eruptions which produced the deposits overlying or incorporating the sampled paleosols and charcoal fragments respectively these ages define the timing of volcanism in the last 5000 years suggesting that the recent volcanic activity at ischia is characterized by periods of very intense volcanism alternating to periods of quiescence this timing of the volcanism is correlated with the behaviour of the magmatic system and dynamics of resurgence of the mt epomeo block

crystal chemistry of ca rich pyroxenes from undersaturated to oversaturated trachytic rocks and their relationships with pyroxenes from basalts abstract a study of 23 ca rich pyroxenes from silica undersaturated and oversaturated rocks shows that all the pyroxenes from trachytes are characterized by a relatively large m1 site related to its high fe 2 occupancy the volume of the m1 site is inversely related to that of the tetrahedral site the high si content of the pyroxene from trachytes causes significant increase in the longest m2 o3c2 d2 distances the most striking difference between the pyroxenes from trachytes and those from basalts is found in the volumes of the m1 and t sites and in the degree of distortion of the m2 polyhedron m2 the relationship between the angle and m2 provides an estimate of 1 the degree of alkalinity in trachytic melts and 2 indications on the nature of their possible parental basaltic magmas

hydrological control of as concentrations in bangladesh groundwater 1 the elevated arsenic as content of groundwater from wells across bangladesh and several other south asian countries is estimated to slowly poison at least 100 million people the heterogeneous distribution of dissolved arsenic in the subsurface complicates understanding of its release from the sediment matrix into the groundwater as well as the design of mitigation strategies using the tritium helium 3h 3he groundwater dating technique we show that there is a linear correlation between groundwater age at depths 20 m and dissolved as concentration with an average slope of 19 g l 1 yr 1 monitoring wells only we propose that either the kinetics of as mobilization or the removal of as by groundwater flushing is the mechanism underlying this relationship in either case the spatial variability of as concentrations in the top 20 m of the shallow aquifers can to a large extent be attributed to groundwater age controlled by the hydrogeological heterogeneity in the local groundwater flow system

contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in araihazar bangladesh extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments here we examine patterns of arsenic heterogeneity in the shallow 1 10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size thickness and distribution of fine grained 63 m sediments that overlie buried aquifer sands the overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface whereas higher arsenic levels typically underlie or are adjacent to thicker fine grained deposits furthermore chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution and consequently the patterning of dissolved arsenic is readily explained in the context of local river history and flood plain development within the past 1000 yr an important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history this finding is especially relevant because the village and tube well locations are closely linked with surface landforms such as former levees and bars an additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine grained stratigraphy commonly associated with high concentrations of arsenic

pre quaternary sea level changes while the paramount importance of global sea level changes has been widely accepted by quaternary geologists throughout this century it is only within the last decade that such changes in the rest of the phanerozoic have received much attention there are two principal reasons for this arousal of interest first increased exploration of the stratigraphic record across the earth both on land and under the sea and significant improvement in biostratigraphic correlation have drawn attention to what appear to have been globally synchronous events second oceanographic research has demonstrated the importance of tectonically controlled changes in the cubic capacity of the ocean basins notably variations in ridge volume which provide a ready mechanism for sea level changes even at nonglacial times the adjective eustatic was first proposed for global changes of sea level by suess 1906 in his great treatise the face of the earth the corresponding noun is eustasy not eustacy as it is commonly misspelt the unconvinced reader need only think of ecstasy eustatic sea level changes can be inferred as suess recognized by a plotting the temporal spread of marine sediments over the continents and b estimating the depositional depth variations in marine stratal sequences which can be correlated across and between continents to these methods can be added seismic stratigraphy in which biostratigraphically age determined seismic sequences on the con tinental margins are analyzed in terms of cycles of coastal onlap and offiap when the results of these methods are broadly in accord confidence in a eustatic interpretation is increased but the range of confidence can vary considerably it is widely accepted that there was an exceptionally high sea

decoupling of as and fe release to bangladesh groundwater under reducing conditions part i evidence from sediment profiles this study reexamines the notion that extensive as mobilization in anoxic groundwater of bangladesh is intimately linked to the dissolution of fe oxyhydroxides on the basis of analyses performed on a suite of freshly collected samples of aquifer material detailed sediment profiles extending to 40 to 70 m depth below the surface were obtained at six sites where local groundwater as concentrations were known to span a wide range the sediment properties that were measured include 1 the proportion of fe ii in the fe fraction leached in hot 1 2 n hcl 2 diffuse spectral reflectance and 3 magnetic susceptibility in parallel with local concentrations of dissolved as ranging from 5t o 600g l fe ii fe ratios in shallow gray holocene sands tended to gradually increase with depth from values of 0 3 to 0 5 to up to 0 9 in deeper orange aquifers of presumed pleistocene age that were separated from shallow sands by a clay layer and contained 5 g l dissolved as leachable fe ii fe ratios averaged 0 2 there was no consistent relation between sediment fe ii fe and dissolved fe concentrations in groundwater in nearby wells the reflectance measurements indicate a systematic linear relation r 2 of 0 66 n 151 between the first derivative transform of the reflectance at 520 nm and fe ii fe the magnetic susceptibility of the shallow aquifer sands ranged from 200 to 3600 x 10 9 m 3 kg si and was linearly related r 2 of 0 75 n 29 to the concentrations of minerals that could be magnetically separated 0 03 to 0 79 dry weight no systematic depth trends in magnetic susceptibility were observed within the shallow sands although the susceptibility of deeper low as aquifers was low up to 200 10 9 m 3 kg si

effect of irrigation on fe iii so42 redox cycling and arsenic mobilization in shallow groundwater from the datong basin china evidence from hydrochemical monitoring and modeling summary seasonal hydrochemical monitoring has been conducted at a well known arsenic as contaminated site to understand the critical controlling processes on as mobilization and enrichment in groundwater the hydrochemical results indicate that the water chemistry at the studied site is mainly controlled by evaporates dissolution and redox reactions during irrigation and non irrigation periods respectively the measured redox sensitive components indicate that the groundwater experienced periodic redox changes from irrigation to non irrigation periods and that an enhanced reducing environment prevailed during the non irrigation period the observed high as concentration and the strong positive correlation between as and fe support the conclusion that the fe oxides hydroxides reduction and fe sulfide oxidation releases as during the irrigation period however the relatively low as concentration and the close inverse correlation between as fe and so 4 2 show that fe iii and so 4 2 reduction accompanied by the formation of fe sulfide precipitates retains as during the non irrigation periods the geochemical inverse modeling performed for the selected wells confirms that fe oxide hydroxide and fe sulfide dissolution as well as siderite precipitation are the dominant processes related to the mobilization of as during the water evolution from the pre irrigation to irrigation period while fe sulfide precipitation immobilize as during the groundwater evolution from the irrigation to the post irrigation period in general the combined results of the hydrochemical monitoring and the geochemical inverse modeling support the conclusion that the fe iii so 4 2 redox cycling induced by periodic irrigation practices controls the mobilization of as in groundwater at this site

mineral control of arsenic content in thermal waters from volcano hosted hydrothermal systems insights from island of ischia and phlegrean fields campanian volcanic province italy this paper documents arsenic concentrations in 157 groundwater samples from the island of ischia and the phlegrean fields two of the most active volcano hosted hydrothermal systems from the campanian volcanic province southern italy in an attempt to identify the environmental conditions and mineral solution reactions governing arsenic aqueous cycling on ischia and in the phlegrean fields groundwaters range in composition from nacl brines which we interpret as the surface discharge of deep reservoir fluids to shallow depth circulating fluids the latter ranging from acid sulphate steam heated to hypothermal cold bicarbonate groundwaters arsenic concentrations range from 1 6 to 6900 g l 1 and from 2 6 to 3800 g l 1 in the phlegrean fields and on ischia respectively they increase with increasing water temperature and chlorine contents and in the sequence bicarbonate groundwatersbsteam heated groundwatersbnacl brines according to thermochemical modeling we propose that high as concentrations in nacl brines form after prolonged water rock interactions at reservoir t fo2 and fh2s conditions and under the buffering action of an arsenopyrite pyrite pyrrhotite rock assemblage on their ascent toward the surface nacl brines become diluted by as depleted meteoric derived bicarbonate groundwaters giving rise to hybrid water types with intermediate to low as contents steam heated groundwaters give their intermediate to high as concentrations to extensive rock leaching promoted by interaction with as bearing hydrothermal steam 2005 elsevier b v all rights reserved

the whisper of the hydrothermal seismic noise at ischia island abstract this work is devoted to the study of the background seismic noise at ischia island we highlight two persistent independent signals with dominant frequency peaks in the range 1 2 hz and 3 4 hz respectively the first signal is the most energetic and persistent and its polarization properties are well defined revealing a shallow propagation likely controlled by both tectonic volcanic structures and morphological features this signal is well correlated at all the stations in the casamicciola area and its group velocity is estimated as 500 600 m s the second signal shows similar polarization properties but the temporal pattern of its amplitude is strongly affected by anthropogenic noise revealing a modulation on a diurnal scale on the investigated spatial scales any clearly correlated signal is not evidenced we interpret the 1 2 hz correlated wave packets as the persistent whisper of the shallow circulation of fluids which are a mixing of sea and meteoric water and thermal fluids of the hydrothermal reservoir in the conceptual scheme of the self sustained musical instruments we hypothesize that the solid structure of the shallow hydrothermal system of ischia is formed by a network of channels continuously excited by the circulating hydrothermal fluids which produce a whispered sound i e the seismic noise this mechanism is able to produce persistent self sustained oscillations that we observe as the 1 2 hz seismic signal and eventually higher modes

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