geochemical evolution of groundwater beneath mexico city abstract the geochemical evolution of groundwaters along a 24 km flow beneath central mexico city path from the sierra de las cruces towards lake texcoco has been investigated using stable isotopes radiocarbon and major and trace elements to determine the natural baseline conditions the extent of any contamination and the effectiveness of the overlying aquitard seal modern groundwaters of low salinity 1 are found up to 11 km from the outcrop area and groundwater ages of up to 6000 yr bp occur in the middle part of the section groundwater stable isotope ratios 18 o and 2 h lie close to the global meteoric water line indicating that the groundwater originates from local rainfall the groundwater chemistry may be interpreted as the result of inputs from the source area with progressive water rock interaction down the horizontal flow gradient a redox boundary is found at 9 km along the line of section coincident with the start of the confined section relatively low nitrate concentrations below 9 mg l 1 no 3 n are found in the aerobic waters low concentrations of no 3 in the aerobic waters and low cl reflect inputs prior to the modern development some elements cr u as se sb increase their concentration with distance time as far as the redox boundary but low concentrations occur in the reducing aquifer section the chemistry of several major ions mg na cl k as well as trace elements such as li rb ba reflect the weathering of the basaltic mineral assemblage feldspars and mafic minerals and their increases are generally proportional to residence time phosphate f and i concentrations indicate a probable source from apatite in the basaltic or rhyolitic rocks a borehole in the east of the city some 17 km downgradient intercepted thermal water si geothermometry indicates 163 c at depth this water gives a distinctive composition indicating possible addition of metamorphic co 2 which has then reacted with the igneous rocks increases in b and cl are derived from volatiles trapped in the glass or vesicular basalt a thermal anomaly found in the middle section of the heavily pumped aquifer is interpreted as the up coming of warmer water from medium to greater depth mainly from basalts rhyolites and possibly limestones the geochemistry indicates that groundwater beneath mexico city is of good quality and there is no obvious evidence of leakage of inorganic compounds from surface sources of contamination through the aquitard the younger groundwater drawn from the western outcrop area is generally of good inorganic quality increased drawdowns in the confined aquifer have induced flow of warmer water with higher cl from depth the resources in the aquifer represent an important reserve of good quality water which need to be properly managed as a high quality resource as part of integrated plans for the city s future supplies

oxygen isotope calibration of the onset of ice rafting and history of glaciation in the north atlantic region we report here that dsdp site 552a cored with the hydraulic piston corer on the west flank of rockall bank recovered an undisturbed sequence of alternating white deep sea carbonate oozes and dark coloured layers that are rich in glacial debris oxygen isotope analysis of the sequence together with detailed nannofossil and palaeomagnetic stratigraphy shows that the first major horizon of ice rafting occurred at about 2 4 myr and was preceded by a minor pulse of ice rafting at about 2 5 myr the carbon isotope record shows that the site has been bathed by a water mass of similar characteristics to present day north atlantic deep water at least since 3 5 myr

seismic evidence for blind thrusting of the northwestern flank of the venezuelan andes surface geology and seismic and well data from the northwestern flank of the venezuelan andes indicate overthrusting of andean basement rocks toward the adjacent maracaibo basin along a blind thrust fault the frontal monocline is interpreted as the forelimb of a northwestward verging fault related fold deformed over a crustal scale ramp the andean block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20o 30 o southeastward the thrust fault ramps up through crystalline basement rocks to a decollement horizon within the shaly units of the cretaceous colon mito juan formations backthrusts in the monocline produce a wedge geometry and reduce the amount of blind slip required on the decollement northwest of the andes the rigid andean uplift was caused by northwest southeast compressive tectonic forces related to the convergence of the caribbean plate the panama volcanic arc and northwestern south america the thick up to 6 km molasse deposits accumulated in the foredeep basin indicate that the venezuelan andes started to rise as early as the early miocene however a late miocene intramolasse unconformity marks the beginning of the formation of the monocline and the greatest uplift the crustal scale fault related fold model may explain structural features seen in other areas of basement involved foreland deformation the regional geologic setting of the area can be described as a mountain front adjacent to a foredeep basin the mountain front represents a precambrian paleozoic igneous metamorphic complex partially covered with sediments of mesozoic and cenozoic ages the foredeep is an asymmetric structural basin that contains up to 12 km of sedimentary section the early miocene to pleistocene molasse deposits up to 6 km in thickness dominate the sedimentary section the oldest sedimentary rocks that crop out in the mountain front are jurassic red beds which are unconformably overlain by a cretaceous sequence up to 2 km in thickness from early cretaceous to middle eocene sediments were deposited in an extensive passive continental margin setting figure 3 coarse clastic sediments of the rio negro formation bioclastic carbonates of the apon formation and siliciclastic sediments of the aguardiente formation were deposited first in the initial stage of the cretaceous transgressive event limestones calcareous shales and black shales of the capacho and la luna formations represent anoxic facies deposited during the maximum marine transgressiolof late cretaceous cenomanian campanian age regression began in the latest cretaceous maestrichtian with marine deposition of the shaly colon formation and the sandier mito juan formation this

the development of antarctic glaciation and the neogene paleoenvironment of the maurice ewing bank abstract a micropaleontologic magnetostratigraphic and sedimentologic analysis of 56 piston cores provides the basis of a geologic study of the late miocene to holocene depositional and erosional history of the intermediate depth maurice ewing bank meb located at the eastern extremity of the falkland malvinas plateau southwest atlantic ocean sedimentation on the meb is controlled presently by the position of the polar front and strong bottom currents beneath the axis of the antarctic circumpolar current acc fluctuations in the position of the polar front and in the intensity of the acc are inferred to have largely controlled deposition on the meb since the initiation of acc flow over the bank in the miocene the sedimentary record of the meb is used to infer the late miocene to holocene oceanographic and climatic conditions in the south atlantic sector of the southern ocean the meb suffered major erosion by intense acc flow during late miocene time which exposed cretaceous miocene sediment near the surface and shaped the present configuration of the bank this erosional event is inferred to have occurred during late miocene to early pliocene time 7 2 4 7 m y b p with the major phase of erosion occurring between middle magnetostratigraphic chron 7 and late chron 6 7 2 6 2 m y b p we suggest that the miocene sedimentary record of the meb and other paleoenvironmental evidence from the circum antarctic region precludes the existence of a west antarctic ice sheet or extensive ice shelves along the antarctic margin prior to the late miocene during the late miocene a west antarctic ice shelf formed in the former west antarctic sea and rapidly thickened until it grounded below sea level to form the west antarctic ice sheet formation of this ice sheet with floating and partially grounded extensions ice shelves in the ross and weddell embayments led to the first major production of true antarctic bottom water formed in a manner similar to today which permanently altered global abyssal circulation late miocene changes in oceanic ccd levels and a permanent shift in the oceanic 13 c 12 c composition may be attributed to this major change in abyssal circulation a decrease in acc velocity during an early pliocene amelioration of climate led to resumption of deposition on the bank from 4 7 to 3 9 m y b p intensification of the acc during the late gilbert and early gauss chrons again resulted in limited deposition and widespread erosion and or non deposition over most of the meb from 4 0 to 3 2 m y b p acc velocity apparently began to wane during the middle of the gauss chron and allowed widespread deposition on the bank during late gauss time 2 9 2 48 m y b p and over more limited areas during earliest matuyama time 2 48 to about 2 2 m y b p during middle to late matuyama time bottom current intensity reached its greatest post miocene level forming a regional disconformity between sediments of about 2 0 and 1 0 m y in age this increased circumpolar circulation and associated erosion is inferred to have peaked during the late matuyama chron 1 2 1 0 m y b p and is approximately the same age as the tentatively dated greatest patagonian glaciation in nearby southern argentina between 1 0 and 0 7 m y b p the bank was blanketed with a coarse erosion resistant layer of ice rafted detritus which armored the older sediment thereby protecting it from major subsequent erosion sedimentation on the meb during the bruhnes chron 720 000 yr b p present was intermittent and finally culminated with the deposition of a veneer of carbonate ooze during or since latest pleistocene time

climate and bioproductivity control on carbonate turbidite sedimentation paleocene to earliest eocene gulf of biscay zumaia spain abstract the paleocene to lowermost eocene zumaia series situated along the basque coast northern spain consists of pelagic limestones and marlstones with interbedded mixed calcarenite turbidites we have compared the composition and frequency of turbidite beds in time as well as the clay mineral and bulk rock carbon isotope composition of the pelagic beds to discern the factors controlling turbidite deposition the pelagic beds are coccolithic mudstones with mainly planktic foraminifera the sources of detrital material to turbidites were the carbonate producing shelves bearing foramol associations around the evolving bay of biscay and to a minor extent the incipient pyrenean chain for siliciclastic components we have found close relationships between variations of turbidite facies and depositional rates with clay mineral assemblages and carbon isotopes in the pelagic beds the latter are in line with global trends it appears that in the early bay of biscay climate and nutrient availability controlled turbidite sedimentation the bulk rock late paleocene np 5 np 9 positive carbon isotope excursion coincides with a period of relatively cool and dry climate characterized by the highest frequency of turbidity flow we propose that during this period globally enhanced latitudinal temperature gradients and strong thermohaline ocean circulation increased bioproductivity in surface waters by upwelling and or by wind driven ekman pumping in contrast warm perennially wet periods such as the early paleocene np 1 np 4 and the paleocene eocene transition upper np 9 np 10 are characterized by low frequency of turbidity flow in the basin this may have been caused by sluggish thermohaline circulation and low carbonate productivity during this time period

post depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east atlantic abstract two cores from a ne atlantic pelagic clay area of low accumulation rate each contain a single turbidite with different age thickness and composition diagenesis following introduction of exotic turbidite material into the pelagic clay sequence has resulted in distinctive colour changes the diagenetic process is thought to be driven by bacterially mediated organic oxidation and compositional differences in the turbidites and clays allow examination of the effects on metal concentrations of this process in one core a long turbidite section emplaced 330 000 years ago is overlain by clay organic oxidation has apparently proceeded from the turbidite top downwards and has maintained a u concentration peak below the oxidation front marked by a colour change the u source is believed to be u initially associated with the organic matter in the oxidised section of the turbidite vanadium and cu behave similarly but upward migration of the redox sensitive metals mn and ni is also seen in the second core an 8 cm turbidite section was emplaced about 170 000 years ago in a clay column in this case organic consumption now appears complete but evidence for diagenetic effects is found in a 16 cm compositional alteration halo of the underlying pelagic clay fe ii has been enriched and the hydrogenous component of mn co ni and cu removed from the pelagic clay to form the halo

the role of biomineralization in the origin of sepiolite and dolomite sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear sepiolite is frequently found with dolomite mg smectites or palygorskite in the intermediate unit of miocene sediments of the madrid basin herein we describe mineralogy texture and mineral chemistry of sepiolite rich layers up to 90 concentration in marl clay sediments either with gypsum chert micro crystalline quartz or dolomite that are up to 2 to 3 m thick the materials studied were formed in shallow lake environments or mudflats these sediments contain dolomite aggregates resembling mineralized microorganisms biomorphs with ovoid morphologies and spherical to tubular internal voids the regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms the paper describes the presence of organic debris silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers potassium depletion as a nutrient in detrital components and the occurrence of organic and reduced sediment u and sulfide concentration at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite in addition the occurrence of dolomite with carbon that has 13c pdb values of

the behaviour of manganese in atlantic carbonate sediments abstract manganese porewater and solid phase data are considered from two different atlantic carbonate sediments both with good geochemical characterisation the first case is a continuously accumulated alternating pelagic marl carbonate ooze where there is a correlation in post oxic conditions between mn ii porewater levels and an acetate leachable mn fraction of the sediment this leachable mn appears to have an association with calcite and in this example the leached residue mn content approximates to that of shale in the second case a very recent compositionally homogeneous calcareous turbidite is underlain by mn oxyhydroxide at the former sediment surface which is now acting as a source of mn ii by reduction sorption of mn by the turbidite sediment and shale like leached residue mn levels in anoxic conditions are again observed as mn ii diffuses towards present oxic conditions although no unique equilibrium is achieved similar older buried turbidites have concentration decreases in solid phase mn content preserved from bottom to top as relict evidence that this process has occurred in these turbidites 54 64 of the total mn is leachable by the acetate method and therefore presumed to be sorbed by calcite the oldest example has persisted for 125 kyr it is suggested that the slow breakdown of metastable mn oxyhydroxide and sorption of mn ii to calcite surfaces are important controls on the behaviour of mn in pelagic carbonates in mildly reducing conditions where accumulation fluctuations occur these processes act together with accumulation to limit overall mn migration and to prevent the large mn enrichments seen near the sediment surface in more reducing environments

major ion chemistry of the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal the ganga brahmaputra one of the world s largest river systems is first in terms of sediment transport and fourth in terms of water discharge a detailed and systematic study of the major ion chemistry of these rivers and their tributaries as well as the clay mineral composition of the bed sediments has been conducted the chemistry of the highland rivers upper reaches of the ganga the yamuna the brahmaputra the gandak and the ghaghra are all dominated by carbonate weathering ca mg and hco3 account for about 80 of the cations and anions in the lowland rivers the chambal the betwa and the ken hco3 excess over ca mg and a relatively high contribution of na k to the total cations indicate that silicate weathering and or contributions from alkaline saline soils and groundwaters could be important sources of major ions to these waters the chemistry of the ganga and the yamuna in the lower reaches is by and large dictated by the chemistry of their tributaries and their mixing proportions illite is the dominant clay mineral about 80 in the bedload sediments of the highland rivers kaolinite and chlorite together constitute the remaining 20 of the clays in the chambal betwa and ken smectite accounts for about 80 of the clays this difference in the clay mineral composition of the bed sediments is a reflection of the differences in the geology of their drainage basins the highland rivers weather acidic rocks whereas the others flow initially through basic effusives the ganga brahmaputra river system transports about 130 million tons of dissolved salts to the bay of bengal which is nearly 3 of the global river flux to the oceans the chemical denudation rates for the ganga and the brahmaputra basins are about 72 and 105 tons km yr 1 respectively which are factors of 2 to 3 higher than the global average the high denudation rate particularly in the brahmaputra is attributable to high relief and heavy rainfall

magnetic measurements used to identify paleoxidation fronts in deep sea sediments from the madeira abyssal plain we describe the results of rock magnetic measurements of a sediment core from the madeira abyssal plain which contains multiple turbidites previous studies of trace element e g u concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts we show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements low concentrations of magnetic minerals occur in turbidites with a high organic content reflecting the destruction of magnetic material via the process of reductive diagenesis in addition fossil redox fronts are readily identified by rock magnetic properties

spatial assessment of groundwater quality in the jangseong region south korea a total of 129 groundwater samples were collected in the jangseong region of south korea to characterize and evaluate groundwater quality and its suitability for irrigation and domestic uses samples were chemically analyzed for major ions ph electrical conductivity and total dissolved solids following standard methods the aquachem 2014 2 model linked with phreeqc was used for the statistical analysis and characterization of the hydrochemistry of the groundwater the analysis showed that in all samples ca hco3 was the leading water type and that the abundance of major cations was in the order ca na mg k and of anions in the order hco3 cl so4 f according to the correlation analysis ca showed strong interdependence with hco3 suggesting that these parameters may have originated from common sources saturation index calculations indicated that all samples were undersaturated with respect to aragonite calcite dolomite fluorite gypsum halite and siderite and oversaturated with respect to goethite and hematite the irrigation suitability analysis revealed that groundwater in the jangseong area can be used for irrigation without any restrictions based on ec sodium adsorption ratio percent sodium residual sodium carbonate kelley ratio permeability index and the us salinity laboratory diagram analysis the drinking water suitability analysis made for major parameters by comparison with the who guidelines indicates that the groundwater in the area is suitable for drinking except in some samples with high nitrate n concentrations the elevated nitrate concentrations in the groundwater are likely an indicator of agricultural pollution

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

18o studies on recharge of phreatic aquifers and groundwater flow paths of mixing in the delhi area widespread decline in the groundwater level and growth of salinization are the critical factors controlling future groundwater quality and availability in the delhi area s180 and salinity data have been integrated for studying recharge conditions of phreatic aquifers and tracing the flowpaths of groundwater mixing wide range 2 8 0 to 8 6 and spatial inhomogeneity of 6180 in groundwater indicate that the aquifer is characterized by a slow process of natural mixing of groundwater in its lateral extent there is infiltration of river water to the groundwater in addition to the recharge from rainfall groundwater recharge in the area also occurs through stagnant water pools in low elevation areas where water surface run off collects chloride concentration in groundwater has a wide range from 15 ppm to 5070 ppm with a log normal frequency distribution indicating that the salt deposits in the area are not homogeneously distributed the s180 c1 relationship clearly suggests mixing of multiple sources of high salinity groundwater with low salinity groundwater or river water along specific pathways the data also indicates leaching of salts present in the soil during vertical recharge from rainfall and irrigation large lateral variation in chloride concentration indicates changes in evapotranspiration rate during recharge detailed investigations on identifying the potential recharging zones flow paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization

faulting in madeira abyssal plain sediments abstract interpretation of high resolution seismic data revealed the occurrence of faults in the sediments of two north atlantic abyssal plain environments the madeira abyssal plain map and the nares abyssal plain nap most of the data are from the map where faulting appears to be confined to particular locations from our data individual faults cannot be traced from one seismic section to another a positive correlation exists between the concentration of faults and the maximum sediment thickness although the mechanism that caused the faulting is still not understood the faults apparently are related to processes which have been active for a long period of time

geochemical modelling ionic ratio and gis based mapping of groundwater salinity and assessment of governing processes in northern gujarat india in semi arid arid regions groundwater is the major source of irrigation drinking and industrial requirements water salinity and shortage are major problems of concern north gujarat india is one such area where highly saline groundwater is generally ascribed to rapid increase of population agriculture and industries induced decline in water table by unplanned abstraction of groundwater however no effort has been made to discriminate the natural and anthropogenic influences on groundwater salinity in this brief background the present study attempts to identify the factors and processes controlling the groundwater salinity in the area based on ionic ratios in integration with various graphical methods saturation indices and geographical information system na ca2 1 indicates the deficiency of ca2 possibly due to caco3 precipitation or ion exchange process na cl 1 and text so 4 2 text cl gg 0 05 suggest salinization is mainly due to wastewater infiltration and or due to irrigation water return flow sea water intrusion in coastal parts vertical and lateral mixing of water and anthropogenic inputs are also responsible for salinization of groundwater ussl diagram na sodium adsorption ratio residual sodium carbonate and magnesium hazard indicate unsuitability of groundwater for irrigation purposes to prevent groundwater salinization appropriate measures need to be taken to control further indiscriminate exploitation of groundwater for irrigation

calcium carbonate preservation patterns in the central north atlantic during the last 150 000 years abstract the dissolution record of the canary and eastern north american basins was examined in 15 cores spanning the last 150 000 years stratigraphic control was based on fluctuations in percent carbonate and 18 0 16 0 dissolution changes were determined based on variations in percent planktonic foraminiferal fragments and benthonic foraminifera results indicate three main dissolution events coincident with 18 0 stages 2 4 and 6 the lysocline apparently shallowed about 300 m in the canary basin at these times there is no simple linear relationship between dissolution intensity and ice volume dissolution was more intense during stage 4 than during stage 2 even though 18 0 records indicate the reverse is true for ice volume also carbonate preservation during the lower part of stage 3 is comparable to stages 1 and 5e a comparison of atlantic pacific carbonate dissolution patterns indicates a tendency for out of phase preservation patterns in 18 0 stages 1 and 2 and an in phase dissolution event during stage 4 the out of phase pattern may partially reflect variations in the production rate of north atlantic deep water the in phase dissolution peak correlates with a major cooling regression transport of organic carbon from transient shallow reservoirs into the deep sea could have contributed to formation of such a pattern however additional evidence from longer atlantic pacific records is needed before the shelf basin model can be more fully evaluated

the controls on spatial and temporal variation of hydrochemical facies and major ion chemistry in groundwater of south west district delhi india the existence of freshwater resource is a critical pre requisite for any growing urban center the south west district of delhi is among the most rapidly developing urban areas of delhi region the freshwater requirement of this district is heavily dependent on groundwater due to absence of other sources groundwater availability as well as its quality is controlled primarily by geological features such as older alluvial in the western to central parts and quartzite exposure in the eastern part of the district the groundwater flow direction and the presence of the najafgarh drain in the district impact the groundwater quality as well as hydrochemical facies evolution this article establishes spatial and temporal hydrochemical facies variation in groundwater of the district and its linkages with the geologic and anthropogenic factors

identification and evolution of hydrogeochemical processes in the groundwater environment in an area of the palar and cheyyar river basins southern india the palar and cheyyar river basins in tamil nadu state of southern india are characterised by different geological formations and groundwater is the major source for domestic agricultural and other water related activities hydrogeochemical studies were carried out in this area with the objective of identifying the geochemical processes and their relation to groundwater quality groundwater samples were collected once a month from 43 groundwater wells in this area from january 1998 to july 1999 sampling procedures and chemical analysis were carried out as per the standard methods chemical data are used for mathematical calculations and graphical plots to understand the chemical process and its relation to the groundwater quality the chemical composition of groundwater in the central part of the study area mainly depends on the recharge from lakes and the river which is explained by a mixing mechanism in addition weathering of silicate minerals controls the concentration of major ions such as sodium calcium magnesium and potassium in the groundwater of this area further the activity ratios indicate that the groundwater is in equilibrium with kaolinite smectite and montmorrillonite the reverse ion exchange process controls the concentration of calcium magnesium and sodium in hard rock formations and dissolution of carbonate minerals and accessory minerals is the source of ca and mg in addition to cation exchange in the sedimentary formations in general the chemical composition of the groundwater in this area is influenced by rock water interaction dissolution and deposition of carbonate and silicate minerals ion exchange and surface water interactions

assessment of hydrogeochemistry and the quality of groundwater in 24 parganas districts west bengal hydrogeochemistry of an area helps in understanding the geological processes which control the chemistry of water and play an important role in determining the suitability of groundwater for various purposes in the present study an attempt has been made to understand the geological processes controlling the quality of water in a part of north and south 24 parganas districts of west bengal 39 representative groundwater samples were collected from the study area and physico chemical parameters were analyzed for all the samples schoeller and durov diagram were used to understand the hydrochemical nature of water results obtained from water chemistry were used in the interpretation of controlling processes using different conventional graphs and determining the quality of groundwater silicate weathering and ion exchange are the dominant processes controlling the chemistry of groundwater in the study area where calcium and magnesium in the water are replaced by the sodium and potassium in the minerals from the host rock as chloro alkaline indices are negative at most of the places saturation index was calculated to understand the mineralogy of the subsurface the groundwater is oversaturated with iron containing minerals like fe oh 3 goethite and hematite while undersaturated with anhydrite and gypsum the groundwater suitability was determined by calculating water quality index for drinking purpose while sar and residual sodium carbonate indices for the agricultural purpose the groundwater in the study area is not suitable for drinking but can be used for other household use and in irrigation for agriculture

hydrogeological processes controlling the release of arsenic in parts of 24 parganas district west bengal a study was conducted to understand the hydrogeological processes dominating in the north 24 parganas and south 24 parganas based on representative 39 groundwater samples collected from selected area the abundance of major ions was in the order of ca2 na mg2 k fe2 for cations and hco3 po43 cl so42 no3 for anions piper trilinear diagram was plotted to understand the hydrochemical facies most of the samples are of ca hco3 type based on conventional graphical plots for ca mg vs so4 hco3 and na k vs cl it is interpreted that silicate weathering and ion exchange are the dominant processes within the study area previous studies have reported quartz feldspar illite and chlorite clay minerals as the major mineral components obtained by the xrd analysis of sediments mineralogical investigations by sem and edx of aquifer materials have shown the occurrence of arsenic as coating on mineral grains in the silty clay as well as in the sandy layers excessive withdrawal of groundwater for irrigation and drinking purposes is responsible for fluctuation of the water table in the west bengal aeration beneath the ground surface caused by fluctuation of the water table may lead to the formation of carbonic acid carbonic acid is responsible for the weathering of silicate minerals and due to the formation of clay as a product of weathering ion exchange also dominates in the area these hydrogeological processes may be responsible for the release of arsenic into the groundwater of the study area which is a part of north 24 parganas and south 24 parganas

seasonal variation of groundwater quality in a part of guntur district andhra pradesh india the area in guntur district andhra pradesh india is selected to discuss the impact of seasonal variation of groundwater quality on irrigation and human health where the agriculture is the main livelihood of rural people and the groundwater is the main source for irrigation and drinking granite gneisses associated with schists and charnockites of the precambrian eastern ghats underlie the area groundwater samples collected seasonally pre and post monsoons during three years from forty wells in the area were analyzed for ph ec tds ta th ca2 mg2 na k co 3 2 hco 3 cl so 4 2 no 3 and f the chemical relationships in piper s diagram chebotarev s genetic classification and gibbs s diagram suggest that the groundwaters mainly belong to non carbonate alkali type and cl group and are controlled by evaporation dominance respectively due to the influence of semi arid climate gentle slope sluggish drainage conditions greater water rock interaction and anthropogenic activities a comparison of the groundwater quality in relation to drinking water quality standards proves that most of the water samples are not suitable for drinking especially in post monsoon period us salinity laboratory s and wilcox s diagrams and na used for evaluating the water quality for irrigation suggest that the majority of the groundwater samples are not good for irrigation in post monsoon compared to that in pre monsoon these conditions are caused due to leaching of salts from the overlying materials by infiltrating recharge waters a management plan is suggested for sustainable development of the area

climatic control of turbidite deposition on the madeira abyssal plain studies of sedimentation in the southwestern part of the madeira abyssal plain have revealed sequences of turbidites correlatable over thousands of square kilometres turbidites are sediments transported by turbidity currents and represent rapidly accumulated sedimentary units1 in this case in the distal part of the flow they are very fine grained and homogeneous occasionally with silty bases individual turbidites are separated by thin pelagic units which alternate from oozes to pelagic clays a stratigraphy for these pelagic units has been produced from combining coccolith distributions and lithological changes which enables comparison to cores containing only pelagic sediments from outside the turbidite province these comparisons show that turbidity currents cause no significant erosion in the area the major turbidites occur between lithological alternations which reflect global climate changes associated with glaciations and related eustatic sea level fluctuations turbidites have been linked with eustacy on previous occasions but only with low stands of sea level2 3 our work shows that turbidites are more likely to occur during regressions and transgressions rather than in the quiet periods between

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

early organic diagenesis the significance of progressive subsurface oxidation fronts in pelagic sediments abstract porewater and solid phase geochemical data at two contrasting ne atlantic stations are reported station 10552 on the cape verde abyssal plain is a site of slow pelagic accumulation ca 0 4 cm kyr 1 molecular oxygen is present in the sediment column to at least 2 m and probably much deeper labile organic carbon is almost totally consumed in the upper few centimetres of the sediment by contrast at station 10554 on the madeira abyssal plain the pelagic sequence has been interrupted by the occasional deposition of organic rich turbidites porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column this metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time this phenomenon exerts a controlling influence on the present nutrient profile and redox succession in a less extreme form substrate distributions of this latter type are not uncommon in atlantic sediments a model has been developed which is controlled by both oxygen and nitrate data this model permits a vertical profile of metabolic activity to be derived and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations about 30 of the total activity at station 10554 is located within the turbidite at the deepening reaction front this is a non steady state condition in fact it is found that the integrated metabolic activity at the two stations is not dissimilar ca 1 2 10 13 moles cm 2 sec 1 the striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column

carbonate dissolution and abyssal hydrography in the atlantic ocean abstract surface sediment samples from two regions in the eastern atlantic angola basin and cape verde basin and two regions in the western atlantic brazil basin and vema channel were examined to evaluate the relationship among carbonate dissolution abyssal hydrography and carbonate chemistry of the water column at each of these locations the position of the lysocline was determined carbonate saturation 1 was calculated and major deep water mass boundaries were identified as previously observed the correlation among these variables is strikingly different between the eastern and western atlantic in the vema channel and the brazil basin there is a first order correlation among the position of the lysocline the benthic thermocline aabw nadw boundary and the transition from saturation to undersaturation with respect to carbonate this relationship breaks down in the eastern atlantic where there is no noticeable benthic thermocline and the lysocline occurs significantly deeper than the saturation undersaturation transition