hydrochemical assessment of freshening saline groundwater using multiple end members mixing modeling a study of red river delta aquifer vietnam abstract in this study we evaluated the water quality status especially salinity problems and hydrogeochemical processes of an alluvial aquifer in a floodplain of the red river delta vietnam based on the hydrochemical and isotopic data of groundwater samples n 23 from the kien xuong district of the thai binh province following the historical inundation by paleo seawater during coastal progradation the aquifer has been undergone progressive freshening and land reclamation to enable settlements and farming the hydrochemical data of water samples showed a broad hydrochemical change from na cl through na hco 3 to ca hco 3 types suggesting that groundwater was overall evolved through the freshening process accompanying cation exchange the principal component analysis pca of the hydrochemical data indicates the occurrence of three major hydrogeochemical processes occurring in an aquifer namely 1 progressive freshening of remaining paleo seawater 2 water rock interaction i e dissolution of silicates and 3 redox process including sulfate reduction as indicated by heavy sulfur and oxygen isotope compositions of sulfate to quantitatively assess the hydrogeochemical processes the end member mixing analysis emma and the forward mixing modeling using phreeqc code were conducted the emma results show that the hydrochemical model with the two dimensional mixing space composed of pc 1 and pc 2 best explains the mixing in the study area therefore we consider that the groundwater chemistry mainly evolved by mixing among three end members i e paleo seawater infiltrating rain and the k rich groundwater the distinct depletion of sulfate in groundwater likely due to bacterial sulfate reduction can also be explained by emma the evaluation of mass balances using geochemical modeling supports the explanation that the freshening process accompanying direct cation exchange occurs through mixing among three end members involving the k rich groundwater this study shows that the multiple end members mixing model is useful to more successfully assess complex hydrogeochemical processes occurring in a salinized aquifer under freshening as compared to the conventional interpretation using the theoretical mixing line based on only two end members i e seawater and rainwater

cation exchange and caco3 dissolution during artificial recharge of effluent to a calcareous sandstone aquifer summary this research describes a field study and laboratory simulations of the geochemical evolution of groundwater following a recharge of effluent into aquifers the study was conducted in the soil aquifer treatment sat system of the shafdan sewage reclamation plant israel the sat system recharges secondary effluent into the calcareous sandstone sediments of the israeli coastal aquifer as a tertiary treatment the reclaimed effluent is recovered ca 500 m off the recharge basin and is used for unlimited irrigation the laboratory simulations in which effluent was pumped through experimental columns packed with pristine shafdan sediment showed that the chemical composition of the outflowing water was controlled mainly by cation exchange and caco 3 dissolution na k and mg 2 were adsorbed and ca 2 was desorbed during the initial stage of recharge the equilibrium distribution of the adsorbed cations was ca 2 60 mg 2 20 and na and k 10 each the ca 2 in the shafdan production wells and in the experimental columns outflow 5 meq l 1 was always higher than the ca 2 in the recharged effluent 3 5 meq l 1 indicating continuous caco 3 dissolution this study demonstrates that besides mixing a suite of geochemical processes should be considered when assessing groundwater quality following artificial recharge of aquifers

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

hydrogeochemical processes in an arid region of europe almeria se spain abstract in the lower andarax river basin 3 aquifer units have been defined namely the carbonate aquifer the deep aquifer and the detrital aquifer which between them contain a wide variety of water types identification of hydrogeochemical processes has been performed by studying a series of ionic ratios comprising the principal constituents together with b and li cl vs so4 cl vs mg cl vs li b vs li among the processes detected the circulation of groundwaters with high concentrations of so4 was found to have significant effects moreover in the coastal region naturally occurring processes related to the flushing of saline waters from sediments of marine origin occur in conjunction with others clearly of human origin that are related to saltwater intrusion a further factor is the entry from overlying deposits of waters with a high saline content this salinity is related to the flushing of sediments of marine origin the use of b and li together enables waters in which salinity is related to seawater to be distinguished from others in which salinity is related to evaporitic layers or to thermal areas the concentration of li is directly related to water temperature while that of b is greater in the most saline sectors of gypsiferous and or seawater origin

application of galdit index to assess the intrinsic vulnerability to seawater intrusion of coastal granular aquifers groundwater within coastal aquifers is considered a very sensitive natural resource since its quantity and quality is threatened by anthropogenic activities as well as global and climate changes the major environmental hazard with respect to coastal aquifers is the phenomenon of seawater intrusion and is more pronounced in the mediterranean basin in the light of smart management of such hydrogeological systems galdit index was introduced in the form of a numerical ranking system that aims to assess the vulnerability of such aquifers to seawater intrusion this paper provides an application of galdit vulnerability index for a typical mediterranean alluvial aquifer under variable hydrogeological conditions in different research periods 1992 and 2004 the results proved the significance of the piezometric conditions of the aquifer with respect to its vulnerability to seawater intrusion

isostatic rebound due to tectonic denudation a viscous flow model of a layered lithosphere a four layer model of the upper 150 km of the earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation the model comprises a strong upper crustal layer a weak lower crustal layer a very strong mantle lithosphere layer and a weak mantle asthenosphere layer which is in accord with experimental constraints on strength depth profiles for continental lithosphere the strength of each layer is represented by its effective viscosity flow in the crust and mantle is driven by buoyancy forces which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements earthquakes or rapid creep events numerical solutions obtained by using a finite element technique predict footwall uplift moho deflection and surface topography that are consistent with observations from the basin and range province of the western united states the calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault such bending need not be elastically controlled hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust the upward deflection of the moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities by matching observations from the basin and range province which indicate no moho deflection and low magnitude of surface topography 3 5 km we estimate the upper crustal lower crustal and mantle lithospheric viscosities in the ranges 1021 1023 pa s 1019 1021 pa s and 1021 1023 pa s respectively

holocene dynamics of the salt fresh groundwater interface under a sand island inhaca mozambique abstract the configuration of coastal groundwater systems in southeast africa was strongly controlled by the holocene sea level changes with an early holocene transgression 15 m 10 000 5000 cal bp and two assumed high stand events in the middle and late holocene with levels higher than the present the fluctuation of the salt fresh groundwater interface under inhaca island in mozambique during the holocene has been studied using an adapted version of the numerical code sutra s aturated u nsaturated tra nsport in this study small scale variations such as tidal effects have not been considered a number of transient simulations were run with constant boundary conditions until the steady state condition was reached in order to study the sensitivity of response time salt fresh interface position and thickness of the transition zone to different parameters such as hydraulic conductivity porosity recharge and dispersivity a 50 increase in horizontal hydraulic conductivity yields a rise in the location of the interface of 15 m while an increase in recharge from 8 to 20 of mean annual precipitation map causes a downward shift in the interface position of 40 m a full transient simulation of the holocene dynamics of the salt fresh groundwater interface showed a response time of several hundred years with a duration sensitive to porosity hydraulic conductivity and recharge and a position determined by the recharge rate and the hydraulic conductivity dispersivity controls the thickness of the transition zone in this non tidal model physical processes such as changes in recharge and or the sea level may cause rapid shifts in the interface position and affect the thickness of the transition zone

analysis of salinization processes in the coastal carbonate aquifer of porto torres nw sardinia italy the porto torres nw sardinia coastal aquifer is hosted in the carbonate rocks of the miocene succession which fills a nnw oriented half graben this aquifer is an important contributor to the fresh water supply of the local area it is increasingly being degraded by salinization processes mainly associated with intense groundwater over exploitation a combined hydrochemical and hydrogeological study has been carried out to identify the origin of the salinity in the groundwater the behavior of the major ions showed that the changes in the chemical composition of the groundwater have been mainly controlled by the encroachment of seawater accompanied by cation exchange reactions since seawater intrusion is a dynamic process an hydrochemical facies evolution diagram hfe d gimenez forcada 2010 approach was taken to establish whether the aquifer was in the seawater intrusion or in the freshening phase the assessment model developed provides regional authorities with a basic tools for sustainable water management in the framework of multi disciplinary research activities aiming to combat and or mitigate desertification and land degradation processes

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

dynamics of periodic instabilities during stretching of the continental lithosphere view from centrifuge models and comparison with natural examples 1 analogue models are used to investigate the development of periodic instabilities during a wide rifting mode of continental extension the results of modeling support that unstable lithospheric extension is accompanied by development of regularly spaced structures at two characteristics wavelengths a crustal shorter wavelength and mantle longer wavelength both wavelengths are dependent on the brittle ductile decoupling i e dominance of the brittle rheology of the upper crust over the ductile behavior as an increase in the brittle layer thickness and a decrease in the extension velocity decreasing the ductile resistance lead to an increase in both crustal and mantle wavelengths the characteristic wavelengths also increase with increasing the amount of total extension whereas variations in the thickness of the ductile lower crust have minor effects on the spacing of structures these results are similar to previous analytical models and match well the wavelength of structures in regions undergoing wide rifting as the basin and range the aegean the west antarctic rift system and examples from the extensional systems of the china mongolia regions

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

seismic anisotropy of the french massif central and predisposition of cenozoic rifting and volcanism by variscan suture hidden in the mantle lithosphere 1 we model the structure of the lithosphere its thickness and seismic anisotropy caused mainly by systematic orientation of olivine crystals from spatial variations of p wave delay times and the shear wave splitting observed at a dense network of mobile and permanent stations within the french massif central mc three major lithosphere domains with different seismic anisotropy are distinguished a suture in the deep lithosphere limits the thick lithosphere of the limousin western mc characterized by a consistent anisotropic pattern with high velocities dipping to the west the eastern mc comprising two different domains with a weakened mantle fabric in the south shows the high velocities dipping systematically to the east the suture between the western and eastern mc parallels the major crustal boundary the late variscan sillon houiller transfer fault in the south and the tauve aigueperse fault in the north with an offset of 10 20 km to the east the offset of the crustal and mantle parts of the same suture indicates that the rigid upper crust might be detached from the mantle lithosphere we suggest that the mantle suture hidden beneath an allochthonous crust was reactivated during the cenozoic extension of the weakened and thinned lithosphere in the south of the eastern mc and predestined a space for the major volcanism mont dore cantal the rigid northern domain characterized by a thicker lithosphere with a well preserved mantle fabric reacted to the extension predominantly by rifting e g limagne and roanne grabens

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

symmetric alternative to asymmetric rifting models we describe the first numerical simulation of continental rifting that reproduces the three major structures observed at magma poor margins such as the galicia margin west of the iberian peninsula or the apulia margin in the alps 1 distal continental margins consist of fault bounded blocks separated by oceanward dipping normal faults 2 at the tip of the continent lower crust is scarcely preserved or absent and upper crust directly overlies exhumed mantle 3 the base of the rotated crustal blocks is a prominent seismic reflector and represents a high strain zone with a top to the ocean sense of shear in our model these structures do not reflect asymmetric rift geometry at a lithospheric scale instead they derive from upper crustal collapse over a mid crustal shear zone into the rift center and are present on both sides of the rift axis the model has a horizontal weak zone in the middle crust on top of strong lower crust and a localized vertical zone of thermal weakness in the rift center we hypothesize that the development of a thermal perturbation and associated strain localization in the deeper lithosphere may cause the transition from widely distributed faulting and crustal thinning to constricted faulting directed toward a well defined rift center

fe and mn levels regulated by agricultural activities in alluvial groundwaters underneath a flooded paddy field abstract iron and mn concentrations in fresh groundwaters of alluvial aquifers are generally high in reducing conditions reflecting low so4 concentrations the mass balance and isotopic approaches of this study demonstrate that reduction of so4 supplied from agricultural activities such as fertilization and irrigation is important in lowering fe and mn levels in alluvial groundwaters underneath a paddy field this study was performed to investigate the processes regulating fe and mn levels in groundwaters of a point bar area which has been intensively used for flood cultivation four multilevel groundwater samplers were installed to examine the relationship between geology and the vertical changes in water chemistry the results show that fe and mn levels are regulated by the presence of no3 at shallow depths and by so4 reduction at the greater depths isotopic and mass balance analyses revealed that no3 and so4 in groundwater are mostly supplied from the paddy field suggesting that the fe and mn rich zone of the study area is confined by the agricultural activities for this reason the geologic conditions controlling the infiltration of agrochemicals are also important for the occurrence of fe mn rich groundwaters in the paddy field area

characteristics and genesis of mineral water from changbai mountain northeast china mineral water from changbai mountain is the main source of drinking water and a major component of the economy in jingyu county jilin province northeast china in this study the hydrochemical characteristics of 18 mineral springs are examined mineral water samples from each of these springs were collected and analysed for major anions and cations electrical conductivity and ph results showed that the ion concentrations were low suggesting that there is minimal influence from anthropogenic activities silica and strontium were the dominant components of the water the mineral water is classified as a silicate type the type of mineral water is ca2 mg2 hco3 mineral water ages ranged from 11 to 61 years which indicate that the mineral water circulation is local rocks in the area mainly consist of olivine albite anorthite pyroxene and potassium feldspar geochemical modelling showed that they are undersaturated with respect to olivine albite anorthite pyroxene fluorite and strontianite olivine albite anorthite pyroxene and potassium feldspar dissolved in the water rock interaction experiments hydrochemical elements are produced by water rock interactions silica is produced from olivine albite anorthite and pyroxene and strontium is produced by strontianite co2 is beneficial for silicate and strontium formation

the lincolnshire limestone hydrogeochemical evolution over a ten year period abstract in the lincolnshire jurassic limestone of britain a sequence of hydrogeochemical processes along a 28 km flow line was defined in 1969 these processes include solution redox and ion exchange reactions sulphate reduction and mixing with saline formation water resampling of the aquifer profile after 10 years has confirmed the overall sequence although downgradient increases in both sulphate and calcium together with changes in redox system can be recognised these changes are related to the migration of agrichemical pollutants and the results demonstrate that the aquifer must have a significant capacity for in situ nitrate reduction

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

mode of crustal extension determined by rheological layering the results of numerical modelling show that the mechanical stratification of the crust provides the fundamental control on fault spacing and ultimately the mode of extension macroscopic pre existing structures and weaknesses are often thought to govern the behaviour of continental crust under extension and many prior studies have focussed on the effect of heterogeneities in triggering faulting and exhumation of lower crustal material the role played by such features is in fact subordinate to that exerted by the rheological contrast from upper to lower crust in our numerical model the temperature gradient dictates the transition from a strong brittle upper crust to a weaker ductile lower crust we see two distinct extension modes that depend on this vertical rheological contrast the distributed faulting mode and the metamorphic core complex mode the ratio of the integrated strength of the upper to lower crust is an indicator of the resulting mode of extension when this strength ratio is small i e the lower crust is relatively strong the result is distributed densely spaced faulting with limited slip on each fault and no exposure of lower crustal rocks an example could be faulting in the north sea a large strength ratio hence a weak lower crust that flows easily leads to stretching being strongly localised onto relatively few normal fault zones each fault accommodates large displacements eventually dissecting the upper crust and resulting in exhumation of the lower crust this is representative of the metamorphic core complexes of the western u s a and the aegean the actual critical strength ratio for the transition between modes will depend upon secondary factors such as the relative thickness of the lower crust with respect to the upper crust and the degree of fault weakening d 2005 elsevier b v all rights reserved

space time development of seawater intrusion a study case in vinaroz coastal plain eastern spain using hfe diagram and spatial distribution of hydrochemical facies summary a new method has been developed to recognize and understand the temporal and spatial evolution of seawater intrusion in a coastal alluvial aquifer the study takes into account that seawater intrusion is a dynamic process and that seasonal and inter annual variations in the balance of the aquifer cause changes in groundwater chemistry analysis of the main processes by means of the hydrochemical facies evolution diagram hfe diagram provides essential knowledge about the main hydrochemical processes subsequently analysis of the spatial distribution of hydrochemical facies using heatmaps helps to identify the general state of the aquifer with respect to seawater intrusion during different sampling periods this methodology has been applied to the pilot area of the vinaroz plain on the mediterranean coast of spain the results appear to be very successful for differentiating variations through time in the salinization processes caused by seawater intrusion into the aquifer distinguishing the phase of seawater intrusion from the phase of recovery and their respective evolutions the method shows that hydrochemical variations can be read in terms of the pattern of seawater intrusion groundwater quality status aquifer behaviour and hydrodynamic conditions this leads to a better general understanding of the aquifers and a potential for improvement in the way they are managed

role of weak zone orientation in continental lithosphere extension 1 lithosphere extension and continental breakup axes are often sub parallel to orogenic belts and suture zones in an attempt to understand the relation between weak zone orientation and extension direction a 3 d numerical model is developed with which the following aspects are studied the relation between weak zone axis and axis of maximum crustal deformation the amount and distribution of crustal thinning and subsequent implications for rift a symmetry the results suggest that upon oblique extension rifts develop within the weak zone that individually cross the inherited structure while as a group they follow the weak trend this results in an alternating rift asymmetry the pattern of crustal thinning topography and thermal structure are not symmetric around the rift axes of the major rift zones and change along axis over such a rift this is a possible explanation for alternating asymmetric rift structures such as observed in the east african rift system

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

multivariate statistical analysis and hydrogeochemical modelling of seawater freshwater mixing along selected flow paths case of korba coastal aquifer tunisia abstract groundwater sampling and piezometric measurements were carried out along two flow paths corresponding to two transects in korba coastal plain northeast of tunisia the study aims to identify hydrochemical processes occurring when seawater and freshwater mix those processes can be used as indicators of seawater intrusion progression and freshwater flushing into seawater accompanying submarine groundwater discharge sgd seawater fractions in the groundwater were calculated using the chloride concentration hierarchical cluster analysis hca was applied to isolate wells potentially affected by seawater in addition phreeqc was used to simulate the theoretical mixing between two end members seawater and a fresh brackish groundwater sample geochemical conventional diagrams showed that the groundwater chemistry is explained by a mixing process between two end members results also revealed the presence of other geochemical processes correlated to the hydrodynamic flow paths direct cation exchange was linked to seawater intrusion and reverse cation exchange was associated to the freshwater flushing into seawater the presence of these processes indicated that seawater intrusion was in progress an excess of ca that could not be explained by only cation exchange processes was observed in both transects dedolomitization combined to gypsum leaching is the possible explanation of the groundwater ca enrichment finally redox processes were also found to contribute to the groundwater composition along flow paths

geochemical calculations and observations on salt water intrusions i a combined geochemical minxing cell model abstract a combined geochemical mixing cell flow model has been developed the geochemical model is based on the computer program eq 3 6 wolery 1982 has been modified to include cation exchange reactions and is incorporated in a one dimensional mixing cell model for stationary one dimensional transport dispersion or diffusion is calculated by mixing the cell contents with neighbouring cells relationships between mixing factors and dispersivity and diffusion coefficients are derived the model is used to simulate concentration changes during salt water intrusions fundamental differences are shown to exist between diffusion of salt water from a stagnant water body and dispersive intrusions during marine transgressions or flooding events with dispersive flow characteristic water compositions develop in space and time as a result of cation exchange with diffusion these steep concentration gradients are smoothed out and mixed compositions occur the concept that characteristic concentrations occur during dispersive flow is used to obtain paleohydrological information in a dutch polder the depth of the fresh salt water interface before polder development is deduced from a simulation of observed exchange reactions in the polder groundwater

geochemical tracing and hydrogeochemical modelling of water rock interactions during salinization of alluvial groundwater upper rhine valley france in the southern upper rhine valley groundwater has undergone intensive saline pollution caused by the infiltration of mining brines a consequence of potash extraction carried out during the 20th century major and trace elements along with sr and u isotopic ratios show that groundwater geochemical characteristics along the saline plumes cannot reflect conservative mixing between saline waters resulting from the dissolution of waste heaps and one or more unpolluted end members the results imply the occurrence of interactions between host rocks and polluted waters and they suggest that cationic exchange mechanisms are the primary controlling process a coupled hydrogeochemical model has been developed with the numerical code kirmat which demonstrates that cationic exchange between alkalis from polluted waters and alkaline earth elements from montmorillonite present in the host rock of the aquifer is the primary process controlling the geochemical evolution of the groundwater the model requires only a small amount of montmorillonite between 0 75 and 2 25 which is in agreement with the observed mineralogical composition of the aquifer the model also proves that a small contribution of calcite precipitation dissolution takes places whereas other secondary mineral precipitation or host rock mineral dissolution do not play a significant role in the geochemical signature of the studied groundwater samples application of the model demonstrates that it is necessary to consider the pollution history to explain the important cl na and ca concentration modifications in groundwater samples taken over 2 km downstream of waste heaps additionally the model shows that the rapidity of the cationic exchange reactions insures a reversibility of the cation fixation on clays in the aquifer

impacts of simultaneous action of drought and overpumping on quaternary aquifers of glafkos basin patras region western greece the glafkos river basin extends southeast of patras and is the city s main groundwater reservoir in the last few years a steady decline of the groundwater level has been observed in 19 wells due to overpumping and prolonged drought periods the present paper assesses the adverse simultaneous effects of drought and overpumping on the groundwater regime analysis of the rainfall data of the last 60 years shows a decrease of precipitation during the last decade runoff measurements show a decrease in the outflow of the glafkos river the increased water pumping and the decrease of recharge during the last decades have resulted in depletion of the aquifers chemical analyses showed a constant reduction of the groundwater quality comparison of piezometric maps of two periods with a 19 year interval shows a clear decline of the general piezometry as a result of overpumping and drought

geochemical modeling of the madison aquifer in parts of montana wyoming and south dakota stable isotope data for dissolved carbonate sulfate and sulfide are combined with water compo sition data to construct geochemical reaction models along eight flow paths in the madison aquifer in parts of wyoming montana and south dakota the sulfur isotope data are treated as an isotope dilution problem whereas the carbon isotope data are treated as rayleigh distillations all reaction models reproduce the observed chemical and carbon and sulfur isotopic composition of the final waters and are partially validated by predicting the observed carbon and sulfur isotopic compositions of dolomite and anhydrite from the madison limestone the geochemical reaction models indicate that the dominant groundwater reaction in the madison aquifer is dedolomitization calcite precipita tion and dolomite dissolution driven by anhydrite dissolution sulfate reduction ca mg2 ina cation exchange and halite dissolution are locally important particularly in central montana the groundwater system is treated as closed to c02 gas from external sources such as the soil zone or cross formational leakage but open to c02 from oxidation of organic matter coupled with sulfate reduction and other redox processes occurring within the aquifer the computed mineral mass transfers and modeled sulfur isotopic composition of madison anhydrites are mapped throughout the study area carbon 14 groundwater ages adjusted for the modeled carbon mass transfer range from modem to about 23 000 years b p and indicate flow velocities of 7 7 ftlyr 2 1 26 5 rnlyr most horizontal hydraulic conductivities calculated from darcy s law using the average 14 c flow velocities are within a factor of 5 of those based on digital simulation the calculated mineral mass transfer and adjusted 14 c groundwater ages permit determination of apparent rates of reaction in the aquifer the apparent rate of organic matter oxidation is typically 0 12 jlidol uyr sulfate and to a lesser extent ferric iron are the predominant electron acceptors the kinetic biochemical fractionation of 34 s between sulfate and hydrogen sulfide is approximately 44 o at 25 c with a temperature variation of

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

iapetus suture located beneath the north sea by birps deep seismic reflection profiling the iapetus suture one of the more fundamental geologic boundaries of the british isles has been located beneath the north sea by deep seismic reflection profiling carried out by the british institutions reflection profiling syndicate birps reflection profiling and magnetotelluric sounding have previously been used to infer the position of the iapetus suture beneath the irish sea and the southern uplands but the present study provides the first direct evidence for the location of the iapetus suture east of britain a comparison of the seismic reflection profiles over the iapetus suture east and west of britain shows variations in the dip of the suture which steepens to the northeast beneath the north sea the comparison also reveals unexpected changes in lower crustal reflectivity along the strike of the suture and suggests that there may be no straightforward relation in this area between the crustal reflectivity and the crustal age

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

squeeze box in a centrifuge abstract a squeeze box which operates in a centrifuge is presented in the experimental set up the centrifugal body force mimics the role of gravity in nature and at the same time drives a hydraulic press which simulates lateral forces the rate of lateral shortening is controlled by varying the loads placed on the hydraulic press utilization of a squeeze box in a centrifuge enhances the range of scaling and enables the use of coherent model materials without violating similitude requirements in the traditional squeeze box which operates under normal gravity similitude requirements severely limit the choice of model materials as a test of the apparatus simple experiments were carried out comparing models shortened in a centrifuge with similar models shortened under normal gravity the realistic form of decollement structures e g folds faults and diapirs shortened in a centrifuge in these tests as well as in other tests which will be reported later show the potential of this new technique for rigorous modelling of geodynamic systems which evolve under the influence of both gravity and a push from the rear the set up can also be used as a simple shear box in a centrifuge after a slight adjustment

a dynamic model of extension in the baltimore canyon trough region a dynamic model of extension in the baltimore canyon trough region of the central north atlantic indicates that rifting proceeded in two stages and was controlled by laterally offset preexisting weaknesses within the middle crust and upper mantle beneath the appalachian orogen the crustal weakness is hypothesized to result from thin skinned thrust faulting in the appalachian foreland and the upper mantle weakness results from crustal thickening beneath the appalachian hinterland during the early stages of rifting 225 185 ma the loci of extension in the crust and mantle lie within the respective preweakened regions and are connected by a shear zone in the lower crust in a manner similar to whole lithosphere simple shear extension and thinning of the upper crust at this time results in the formation of late triassic early jurassic rift basins in the appalachian piedmont and in the development of a horizontal pressure gradient which drives widespread ductile flow in the lower crust the pressure gradient may also have influenced magma transport moving melt generated beneath the eastern baltimore canyon trough into the more westerly triassic basin province during the late stages of rifting 185 175 ma extension in the crust shifts to a position overlying the preexisting upper mantle weakness as a result of thermal weakening by the upwelling asthenosphere in this region the loci of extension in the crust and mantle are then coincident and the style of rifting becomes similar to whole lithosphere pure shear the shift in the locus of crustal extension is accompanied by the cessation of subsidence and volcanism in the triassic basins the onset of rapid subsidence in the baltimore canyon trough and the formation of a basement hinge zone separating the highly thinned crust beneath the baltimore canyon trough from moderately thinned crust beneath the triassic basin province extension becomes increasingly focused beneath the eastern edge of the baltimore canyon trough until the onset of seafloor spreading approximately 50 m y after rifting began the resulting conjugate margins are asymmetric with most of the region encompassing the triassic basins stranded on the north american plate

modelling of colloidal particle and heavy metal transfer behaviours during seawater intrusion and refreshing processes the proper management of coastal aquifers commonly requires an understanding of regional mass flow and complete seawater freshwater circulation in this study time series observations of seawater intrusion and refreshing were conducted using a column experiment based on natural flow conditions in coastal groundwater and a sampled medium from a coastal sandy aquifer without chemical treatment ranges of hydrodynamic and hydrochemical variables were tested and analysed the results showed that the zeta potential of suspended colloids in aqueous solution in an aquifer polluted with 0 5 g kg of heavy metals exhibited an isoelectric point for ph values ranging from 5 70 to 6 07 when freshwater or seawater completely occupied the aquifer pores which is representative of natural hydrochemical conditions in this scenario a high background concentration of heavy metals induced colloidal immobilization otherwise seawater freshwater circulation enabled colloid mobilization due to ionic strength and ph fluctuations the migration of multiple heavy metals occurred at a characteristic time of approximately 1 pore volume after each intrusion stage began and when the peak rate of colloid release was reached at these times the colloid behaviour determined the quantity and pathway of heavy metal transport on the basis of the influences of seawater and freshwater interactions the quantity of mobilized particles generally decreased and was uniformly distributed in each fraction due to particle loss and decreased porous connectivity we speculate that the decrease in the total surface area of the migratory colloids may cause colloid associated heavy metal transport to decrease the experimental results provide a useful basis for testing coastal groundwater flow and mass transport models because these phenomena require full characterization to precisely evaluate the associated fluxes from the field scale to the microscopic dimension

development of volcanic passive margins two dimensional laboratory models 1 continental breakup above an anomalously hot mantle may lead to the development of volcanic margins volcanic margins are characterized by 1 thick seaward dipping lava flow sequences 2 central intrusive complexes associated with dyke swarms parallel to the coast and 3 high seismic velocity bodies in the lower crust attributable to magma underplating a conceptual model for volcanic margins development has recently been proposed based on onshore studies of the greenland margins and the british tertiary igneous province it is proposed that the long lived central intrusions are genetically linked to underlying persistent zones of mantle fusion these localized melting domains or soft spots equivalent to small mantle diapirs may locally soften the extending continental lithosphere the low viscosity diapirs would 1 localize tectonic strain and 2 feed the volcanic margin with magma thus such soft spots can control the along strike magmatic and tectonic segmentation of volcanic margins recent geophysical investigations appear to show that the along strike structure of volcanic passive margins is compatible with such a segmentation process here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub moho mantle may have a mechanical effect on continental breakup four layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle the soft spots are simulated by low viscosity silicone putty emplaced within the brittle material at the scale of the entire breakup zone the soft spots display an oceanic type strength profile defining low strength zones where continental breakup is initiated the rift orientation and segmentation are strongly controlled by the distribution of the low viscosity heterogeneities rather than by the direction of regional extension the experiments are compared with the geometry and segmentation of the onshore part of the greenland volcanic margins

dynamical models of the role of crustal shear zones in asymmetric continental extension abstract this paper presents a simple quantitative model for the transfer of lithospheric extension along deep crustal shear zones during rifting of continents these shear zones are interpreted to connect laterally offset pre existing weaknesses in the crust to the centre of mantle lithospheric thinning the rift axis for quartz and quartz over feldspar dominated crustal models we predict the formation of major crustal shear zones which transfer extension to offset crustal weak zones as much as several hundreds of kilometres from the rift axis the length scale of this transfer depends on the initial thermal regime of the lithosphere the composition and thickness of the crust and the relative strengths of the crust at the rift axis and in the offset weak zones when rifting results from a combination of in plane tensile stress and active heating of the mantle lithosphere from below the lateral transfer of strain occurs for a period of time early in the rifting process later crustal thinning switches to the rift axis because heat diffusing from below weakens the crust at this location and it is no longer energetically preferable to transfer strain to offset weak zones for the parameter values used in our models the sedimentary basins formed by extension at the offset weak zones remain tectonically active for no more than 10 my transfer of strain to the offset or rift flank basins is most efficient when the rate of active heating of the lithosphere is similar to the rate of deformation along the crustal shear zone results from a more complete plane strain finite element model agree with the basic predictions of the simple quantitative model they do however indicate that effects ignored in the simple model will reduce our estimates of the maximum distance of shear transfer this is particularly true when the deformation is large we interpret sedimentary basins located on the flanks of rifts or atlantic type margins to be the products of this process of strain partitioning in an inhomogeneous lithosphere the models provide an explanation for flank or offset basins that were active for a short interval early in a rift phase that have no thermal subsidence phase and that cannot be attributed to the gravitationally driven extension and collapse of orogenically overthickened crust

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

multicomponent heterovalent chromatography in aquifers modelling salinization and freshening phenomena in field conditions abstract analysis of hydrochemical data using diagrams and the geochemical transport model phreeqc confirmed that for the argos plain aquifer and for other heterogeneous confined alluvial coastal aquifers in greece the variation of water qualities along a flow path has basically a chromatographic origin due to seawater intrusion and freshening processes additionally calcite precipitation dolomite and gypsum dissolution take place intense fertilization and overexploitation of the area resulted in degradation of the groundwater quality by high amounts of nitrate ions and increased salinization the main physical factor affecting groundwater quality in this area is the cation exchange capacity that is calculated to be 4 3 meq 100 gr 1 good adaptation of the transport model to the field data showed that the model can be used to predict the freshening process after an artificial recharge hence in a first stage for cl there is a continuous decrease to the initial fresh water concentrations corresponding to about 1 1 5 pores volumes that should have been shifted through the aquifer while for both cations ca 2 and mg 2 this decrease is even lower and leads to concentrations lower than that of the initial fresh water complete restoration of the aquifer requires a great amount of recharge water that should be transported through the aquifer and corresponds to 32 pores volumes

modeling heterogeneous stretching during episodic or steady rifting of the continental lithosphere the geometry of brittle ductile extension and modes of normal faulting during steady or episodic rifting of the continental lithosphere are investigated by analogue modeling using the centrifuge technique we use a frictional material to simulate the brittle behavior and various nonlinear bouncing putties to represent the ductile rheology of the stratified lithosphere steady stretching of the continental lithosphere supports rapid differential subsidence and flexure of the brittle crust with formation of marginal grabens and a panel of domino style fault blocks in contrast episodic stretching of the lithosphere progressing in a series of pulses separated by longer periods of quiescence supports formation of spaced grabens and horsts with less subsidence this is due to a counterflow of ductile material from the margins to the central rift zone we show here that the isostatic adjustment in the lithosphere can be related to the steady or episodic mode of heterogeneous stretching we apply the model results to the southern red sea for which the inferred opening history involved continuous or stepwise extension

measurements of attenuation from vertical seismic profiles values of attenuation have been extracted from five detailed velocity surveys significant amounts of attenuation are observed in all five wells measured values of attenuation vary by a factor of 10 ranging from less than 0 1 to 0 9 db wavelength correlation of attenuation with lithology is good considering the resolution limits of the surveys one of the surveys was performed in west texas and the other four were carried out in the gulf coast area the total number of recording depths in each survey varied from 31 to 208 with downhole recordings taken every 10 or 20 ft over key stratigraphic zones in all five surveys a monitor geophone was placed near the well to check for source consistency our procedure for measuring attenuation involved constructing the spectral ratio between each downhole pulse and one particular reference pulse a value of cumulative attenuation ca was then extracted from each spectral ratio and plotted as a function of geophone depth this method allowed us to use many m

seawater intrusion and associated processes in a small coastal complex aquifer castell de ferro spain an attempt to identify the hydrogeochemical processes that accompany current or past intrusion of seawater in the castell de ferro coastal aquifer se spain was made using ionic deltas ionic ratios and saturation indices alluvial and coastal quaternary sediments and triassic marbles constitute this complex aquifer cation exchange dolomitization and calcite dissolution were identified as operating in the aquifer the cation exchange processes in the detrital materials are different from those in the marbles in addition it was determined that while the processes of marine intrusion associated with periods of low groundwater levels still persist in the detrital aquifer in the carbonate sector the flushing process characteristic of wetter conditions has begun