subsurface seawater intrusion barrier analysis seawater intrusion barriers are currently being constructed in the coastal aquifers of japan based on the results of a computer model employing finite difference numerical techniques this model provides a practical design and management tool for predicting the likelihood of seawater intrusion through a semipervious subsurface barrier in an unconfined coastal aquifer the analysis assumes an abrupt interface the dupuit approximations and various simplifying boundary conditions the movements of both the water table and the freshwater saltwater interface are numerically simulated in the unsteady state the computed results are validated with observations obtained from a vertical two dimensional sand box model the application of the computer model is illustrated by a site specific analysis for a proposed subsurface barrier location in a limestone coastal aquifer on okinawa jima island in the western pacific ocean from this analysis it is found that the subsurface barrier is able to delay seawater intrusion for about 2 mo under critical conditions of continual pumping of the aquifer without recharge the subsurface barrier is technically feasible and is a viable solution to the problem of seawater intrusion in coastal aquifers

transience of seawater intrusion in response to sea level rise 1 understanding seawater intrusion swi induced by sea level rise slr is important for the future management of many coastal aquifers only simplified steady state sharp interface analyses of generalized slr swi exist in the literature and the important issue of associated time scales has been neglected we employ numerical modeling in order to explore the transience of dispersive slr swi in common unconfined coastal aquifer settings an instantaneous slr is adopted to compare with the instantaneous sea level drop sld case of a previous sld swi analysis temporal asymmetry between the swi responses to slr and sld is observed a slr swi simulation series indicates that toe representative response times time to reach 95 of new steady state range from decades to centuries for a 1 m slr significant discrepancies between the representative response times of various swi quantitative indicators e g toe position wedge center of mass are observed this demonstrates that the indication of steady state swi conditions depends upon the monitoring approach and thus holds implications for studies reporting that swi steady state has been attained we adopt 100 years as a typical planning time frame and compare 100 year and steady state slr swi as expected the simplified steady state sharp interface solution overpredicts the 100 year landward toe shift in most cases however some simulations exhibit temporary overshoot of the steady state interface position this is in contradiction to the presumption that steady state swi is the worst case steady state sharp interface estimates appear to be at best useful as initial approximations of slr swi given that they span 40 250 of 100 year dispersive interface results for the cases considered

variable density flow and solute transport simulation of regional aquifers containing a narrow freshwater saltwater transition zone variable density flow and solute transport simulation of aquifer systems containing narrow transition zones between freshwater and saltwater requires particular attention to certain aspects of the numerical method and its application to be successful typically only cases involving wide transition zones have been simulated with variable density transport models possibly because of inaccuracies in the modeling approaches used the major components of a successful approach are threefold first functionally consistent approximation of terms involved in fluid velocity calculations is necessary in the case of galerkin finite element methodology a significant modification is required to the standard approach in order to achieve consistency second the simulator must be verified in a particular series of tests the usual tests using henry s problem for verification of density dependent transport simulators are inadequate to check for consistency of the velocity approximations and for the accuracy of simulating flow driven by bouyancy forces third adequately fine spatial discretization is required when applying the simulator for spatial stability of the numerical transport solution and to allow accurate representation of narrow transition zones and the effects of low transverse dispersivity the effectiveness of this approach is demonstrated through simulation of the flow of fresh and saline groundwater in the layered basalt aquifer of southern oahu hawaii the transition zone in this regional flow system is narrow except near the discharge area where it is broadly dispersed simulation of this common situation with an inconsistent approximation gives grossly incorrect results while simulation with a consistent model provides a robust tool for analysis of system hydrology

a comparison of several methods for the determination of iron hydroxides and associated orthophosphates in estuarine particulate matter abstract five different methods commonly used for the extraction and the determination of iron hydroxides and or the associated phosphorus held in marine seston or in sediments have been tested on single phase minerals goethite and apatite on reference minerals so1 and so2 and on suspended matter samples from the st lawrence estuary and from the estuary of the eastmain river a major james bay affluent in northern quebec canada the citrate dithionite bicarbonate cdb procedure generally seemed to be more reproducible and more specific the validity of the other methods respectively using as extracting reagents ammonium oxalate oxalic acid hydroxylamine hydrochloride calcium nitrilotriacetic acid and acetate tartrate is discussed the cdb extraction is included in an analytical scheme which allows to distinguish between three fractions of phosphorus this chemical speciation is applied to the study of p retention on fe hydroxides and to its potential desorption

the phosphorus cycle in coastal marine sediments the concentration of dissolved phosphate in sediment box cores from the laurentian trough in the gulf of st lawrence increased sharply across the sediment water interface from 2 0 mol po4 1 in the bottom water to 6 3 mot po4 1 in the top cm remained almost constant at this value down to 5 15 cm depth and then increased rapidly with further depth in the region of constant concentration phosphate is buffered by sorption equilibria with the sediment the production rate of phosphate the sorption capacity of the sediment and the thickness of the diffusive boundary layer at the sediment water interface appear to control the shape of the pore water profile even though the buffering places an upper limit on the concentration gradient across the sediment water interface and hence on the flux the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment a model is proposed to relate sediment chemistry to phosphorus fluxes

chemodynamics of trace metal fractions in surface sediments of the pandoh lake lesser himalaya india the seasonal variation in the trace metals concentrations cd co cu fe mn ni pb and zn were investigated in surface sediments of the pandoh lake the horizontal distribution of tc tn and tp reflects spatial and temporal differences in sedimentary organic production the chemical sequential extraction of heavy metals was carried out by seven step fractionation scheme leleyter and probst in int j environ chem 73 109 128 1999 the significant concentrations of ni and cd were associated with water soluble eua fraction in the monsoon and winter respectively while exchangeable exch and carbonate bound carb fractions for ni and cd were abundant in winter and summer the cd cu and pb associated with exch fraction in the summer season support their availability on exchange sites due to oxidized nature of surface sediments enrichment of co fe mn and zn in afeo fraction showed poor bioavailability while cd cu and mn in the monsoon co in the winter and summer and zn in the winter season showed significant organically bound org fraction the anova was significant for chemical fractions of trace elements except carb fraction of pb and zn and cfeo fraction of pb factor analysis revealed that the eua exch and carb fractions together control the metal enrichment of mno afeo and cfeo fractions in the summer season

phosphorus speciation and availability in intertidal sediments of the yangtze estuary china abstract in order to better understand p cycling and bioavailability in the intertidal system of the yangtze estuary both surface 0 5 cm and core 30 cm long sediments were collected and sequentially extracted to analyze the solid phase reservoirs of sedimentary p loosely sorbed p fe bound p authigenic p detrital p and organic p the total sedimentary p in surface and core sediments ranged from 14 58 36 81 mol g 1 and 17 11 24 55 mol g 1 respectively and was dominated by inorganic p the average percentage of each fraction of p in surface sediments followed the sequence detrital p 54 9 fe bound p 23 7 organic p 14 3 authigenic p 6 3 loosely sorbed p 0 8 whereas in core sediments it followed the sequence detrital p 61 7 fe bound p 17 0 authigenic p 13 1 organic p 7 5 loosely sorbed p 0 7 post depositional reorganization of p was observed in both surface and core sediments converting organic p and fe bound p to authigenic p the accumulation rates and burial efficiencies of the total p in the intertidal area ranged from 118 70 904 98 mol cm 2 a 1 and 80 29 88 11 respectively high burial efficiency of the total p is likely related to the high percentage of detrital p and the high sediment accumulation rate in addition the bioavailable p represented a significant proportion of the sedimentary p pool which on average accounted for 37 4 and 25 1 of the total p in surface and core sediments respectively this result indicates that the tidal sediment is a potential internal source of p for this p limiting estuarine ecosystem

transition zone width in ground wateron ocean atolls an analytical solution for the thickness of the transition zone between fresh and seawater in aquifers on ocean islands is presented the interface is treated as a mixing layer similar to a laminar boundary layer between fluids moving at different velocities the method requires assumptions of homogeneous isotropic dispersion uniform hydraulic conductivity and steady flow all water withdrawal is assumed to occur at the water table the slope of the sharp interface is used in the solution for the mixing layer and the sharp interface is determined using dupuit forchheimer assumptions of horizontal flow the method presented efficiently estimates the transition zone thickness thus providing information not available from sharp interface models the method is illustrated by application to the freshwater lens on an island of the tarawa atoll in the pacific ocean and is used to show the effects of different pumping arrangements on the width of the mixing layer

reply to comments by n sultan on sedimentation controls on methane hydrate dynamics across glacial interglacial stages an example from international ocean discovery program site u1517 hikurangi margin

mixed convection and density dependent seawater circulation in coastal aquifers 1 density dependent circulation of seawater in coastal aquifers results in submarine groundwater discharge sgd across the seabed that is a mixture of terrestrial groundwater and former marine water in this study the controls of the relative amount of seawater to freshwater in sgd were investigated numerically using the feflow and sutra codes it was found that the key controls could be expressed in the form of a single nondimensional recirculation number that incorporates the combined effects of free convection forced convection and hydrodynamic dispersion on convective overturn within the coastal salt wedge anisotropy effects were incorporated into the recirculation number with limited success based on the principle of equivalent isotropic hydraulic conductivity the type of boundary condition employed along the seabed was shown to be important convective overturn was substantially increased if backward dispersion of salt into the aquifer from along the outflowing portion of the seabed boundary was prevented overall the results demonstrated a strong dependence of convective overturn on the aquifer dispersivities suggesting that results from numerical simulations are problematic to apply to real aquifer systems that typically exhibit uncertain scale dependent dispersion properties

tidal effects on sea water intrusion in unconfined aquifers abstract a variable density groundwater model is used to analyse the effects of tidal fluctuations on sea water intrusion in an unconfined aquifer it is shown that the tidal activity forces the sea water to intrude further inland and it also creates a thicker interface than would occur without tidal effects moreover the configuration of the interface is radically changed when the tidal fluctuations are included this is because of the dramatic changes in the flow pattern and velocity of the groundwater near the shoreline for aquifer depths much larger than tidal amplitudes the tidal fluctuation does not have much effect on how far the sea water intrudes into the aquifer nevertheless a significant change in the configuration of concentration contours because of the effect of tidal fluctuations is observed this change is more noticeable at the top of the aquifer near the water table than at the bottom of the aquifer and is caused by the infiltration of salt water into the top of the aquifer at higher tidal levels a flatter beach slope therefore intensifies this phenomenon the interface configurations do not change noticeably over the course of a tidal cycle neglecting tidal fluctuation effects results in an inaccurate evaluation of the water table elevation at the land end of the aquifer although no distinguishable difference is seen between the water tables near the shoreline where the landward boundary condition is a constant head the effects of tidal fluctuations on sea water intrusion are more pronounced than for cases where the landward boundary condition is a specified flux also it is shown that the effects of tidal fluctuations are more significant for a sloping beach than for a vertical shoreline and the salt water intrudes further inland for the sloping case

an individual particle analysis and budget study of onondaga lake sediments1 the sedimentation of particles in onondaga lake n y was examined by scanning electron microscopy with automated image analysis and x ray energy spectroscopy sax a technique of individual particle characterization the composition of the particles coming into the lake and those present in the sediments was analyzed to calculate an annual sediment budget an allochthonous sediment input of 21 800 tonnesmyr and a sediment output of 3 300 tonnesmyr i were calculated from flow rates sediment loads and concentrations a sediment mass budget yielded an autochthonous sediment formation of 15 900 tonneseyr l and a net sedimentation of 34 500 tonnessyr l calcite particles comprised a majority of these autochthonous sediments and were found in the epilimnion in addition measurable amounts of iron sulfide were formed in the hypolimnion such budgets provide information based on different particle classes for studies of particle formation transport and deposition processes in limnetic environments

seawater groundwater mixing in and fluxes from coastal sediment overlying discrete fresh seepage zones a modeling study submarine fresh groundwater discharging from discrete zones such as conduits overlain by thick sediment remains poorly documented and understood despite common anecdotal accounts of it we analyzed this phenomenon by a suite of variable density variably saturated groundwater flow simulations guided by field studies the field observations included point flux and salinity measurements using seepage meters spread across the intertidal and the subtidal zones the discretely measured seepage fluxes in the subtidal region were as high and the salinity was as fresh as those from the intertidal zone both fluxes and salinities varied with tides two dimensional simulations of the idealized beach ocean vertical section with discrete freshwater springs at the base showed the development of seawater recirculation cells along the edge of freshwater discharge plumes emanating from the outlets causing mixing brackish water discharges at the sediment water interface above where the plume mixes with the recirculating seawater sensitivity analyses showed that stronger terrestrial fresh groundwater flow increases the freshwater plume size but does not affect the mixing zone width tidal oscillations and a dynamic fresh groundwater flow widened the mixing zones and increased porewater flux across the sediment water interface sediment thickness had a minor effect on the width of both the freshwater plume and its surrounding mixing zone the subtidal freshwater plume and mixing zone likely represent a unique but potentially ubiquitous setting which hosts dramatic ecological and chemical gradients their local biogeochemical niche and coastal scale ecosystem impacts will need to be considered in investigations of coastlines

shallow methane hydrate system controls ongoing downslope sediment transport in a low velocity active submarine landslide complex hikurangi margin new zealand morphological and seismic data from a submarine landslide complex east of new zealand indicate flow like deformation within gas hydrate bearing sediment this creeping deformation occurs immediately downslope of where the base of gas hydrate stability reaches the seafloor suggesting involvement of gas hydrates we present evidence that contrary to conventional views gas hydrates can directly destabilize the seafloor three mechanisms could explain how the shallow gas hydrate system could control these landslides 1 gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide 2 overpressure below low permeability gas hydrate bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body 3 gas hydrate bearing sediment could exhibit time dependent plastic deformation enabling glacial style deformation we favor the final hypothesis that the landslides are actually creeping seafloor glaciers the viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time dependent deformation behavior of gas hydrate bearing sands the controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which given the widespread occurrence of gas hydrates in the submarine environment may require a reevaluation of slope stability following future climate forced variation in bottom water temperature

phosphorus fractionation in surficial sediments of pandoh lake lesser himalaya himachal pradesh india abstract the fractionation of p in pandoh lake surface sediments has been investigated for the first time in order to understand its environmental availability and sources and the eutrophication status of this lake inorganic p is present mainly as authigenic p step iii the authigenic p concentration is higher in winter relative to the summer and monsoon seasons and ranged from 35 9 to 46 9 g g the loosely sorbed or exchangeable p step i fe iii bound p step ii and detrital inorganic p step iv were higher in the monsoon season and varied from 3 70 to 11 1 g g 16 9 to 32 0 g g and 9 89 to 17 0 g g respectively organic p reached a maximum in the summer season and ranged from 8 00 to 14 9 g g authigenic p and detrital inorganic p show seasonal changes as ph influences the interaction between p and caco 3 in the water column in the winter season phosphate is precipitated out of the water column and fixed in the sediments as a result of an increase in ph calcite bound p in the sediments may be redissolved by decreasing ph in the summer season relatively high rates of mineralization during the monsoon results in the seasonal pattern of organic p fractionation to sediment as follows monsoon winter

erosion of the seafloor at the top of the gas hydrate stability zone on the hikurangi margin new zealand 1 the dissociation of gas hydrates in sediment pores is thought to decrease seafloor strength potentially facilitating submarine slides because of the generation of overpressured gas and the melting of load bearing or cementing solid hydrate here we present findings that suggest gas hydrates may lead to a previously unknown mechanism of seafloor erosion gas hydrate bearing sub sea ridges on the hikurangi margin are eroded close to the top of the hydrate stability field in the ocean we hypothesize that gas hydrate instability may lead to ridge erosion by a combination of two processes hydrate destabilization caused by depressurization during ridge uplift and repeated pore volume expansion and contraction from hydrate dissociation and formation triggered by fluctuating water temperatures

changes in extent of phosphorus release in a shallow lake lake groβer muggelsee germany berlin due to climatic factors and load abstract based upon biweekly investigations of interstitial and pelagial water parameters as well as sediment phosphorus p speciation the mechanisms important for the summer p release in eutrophic lake gro er muggelsee germany are described microbial processes were the driving force of mainly anaerobic and also aerobic p release from iron reductant soluble and organic p rich sediments for 1992 and 1993 two years of very different runoff characteristics the extent of p release was indirectly controlled by external factors including the supply of dissolved oxygen and nitrate via inflow high runoff changes in land use e g decreasing nitrogen fertilization and climatic factors and it was facilitated by wind induced rapid changes of stratification and mixing events

comment on sedimentation controls on methane hydrate dynamics across glacial interglacial stages an example from international ocean discovery program site u1517 hikurangi margin by e j screaton et al

adsorption capacity of phosphorus in baltic sea sediments abstract nutrient budgets of the baltic sea indicate that an increased loading of phosphorus to a great extent is counteracted by adsorption to the sediments resulting in a comparatively small net increase of the phosphorus concentration in the water column the parmanent salinity stratification and increasing eutrophication of this sea has caused almost permanent anoxic conditions which limits the phosphorus storage capacities in the deep basins below the halocline the phosphorus loading has increased about eight fold during this century and does not show any sign of declination the questions then arise what type of sediments store the phosphorus what are the binding mechanisms and is the maximim storage capacity approached in an attempt to partly answer these questions the phosphorus adsorption capacities of three different baltic sea sediments have been investigated the greatest amount of adsorbed phosphorus measured as mol g 1 dry substance is found for an accumulation bottom from the deeper part of the baltic sea and the lowest for a transportation bottom above the halocline however if the rates are recalculated as phosphorus adsorption per area which is more pertinent from an ecological point of view i e most phosphorus is adsorbed in the shallow sediments a comparison of the experimental data and ambient conditions in the water and sediments indicate that adsorption of phosphate from the water column plays a minor role in the phosphorus budget of the baltic sea only the assimilation of phosphate and subsequent sedimentation and decomposition of organic matter at the sediment surface will result in a sufficiently high sediment pore water concentration to explain the large sink of phosphorus to the baltic sea sediments calculated from input and output data

offshore continuation of coastal groundwater systems predictions using sharp interface approximations and variable density flow modelling analytical sharp interface solutions and numerical variable density flow and transport modelling are used to study how far into the offshore the influence of meteoric continental groundwater systems extends under steady state flow and transport conditions results show that factors which favour the development of an extensive fresh water wedge in the offshore are 1 a high head or fresh water discharge at the coastline 2 a thick semi confined high permeability submarine aquifer 3 optimum but not necessarily maximum and minimum values for the thickness and permeability of the confining layer near the seafloor respectively sharp interface models tend to overpredict the seaward extent of fresh water considerably because in distal parts upward seepage rates through the overlying confining layer are too small to prevent salinisation by diffusion from the overlying sea and ensuing convective mixing by density differences within underlying aquifers the overprediction is particularly large for relatively low heads at the coastline and low permeabilities of the confining layer under favourable conditions fresh water connected to continental flow systems is predicted to occur up to several tens of kilometers offshore comparison of model predictions with observational data demonstrates that relatively fresh submarine pore waters off the coasts of new jersey us and suriname south america occur too far offshore to be explained by active flow systems indicating that they are paleo waters which formed during the pleistocene when sea levels were lower than at present

an assessment of the importance of some parameters for seawater intrusion in aquifers and a comparison of dispersive and sharp interface modelling approaches abstract the problem considered is that of determining the location and shape of the steady interface in a coastal aquifer discharging fresh water to the sea for homogeneous isotropic aquifers the location of the interface is obtained by the boundary integral method for an abrupt interface model these results are compared with the distribution of salt in the dispersion zone where the two fluids meet as determined by a finite difference solution of the flow and convective dispersion equations considerations are restricted to a rectangular section of isotropic homogeneous aquifer and examples are limited to those showing the effects of different convection to dispersion ratios including anisotropic dispersion different fresh water flow gradients and different relative densities of seawater these examples give an indication of the relative accuracy of the abrupt interface model for different ratios of convection to dispersion and enable the influence of various parameters on the shape and extent of the dispersion zone to be studied

modes of seawater intrusion during transgressions analytical methods and numerical experiments are used to study salinization of groundwater in response to sea level rise the system that is studied involves a saturated porous medium with an inclined upper surface the upper surface is progressively inundated during sea level rise to simulate transgression the landward migration of the shoreline four modes of seawater intrusion are distinguished 1 horizontal intrusion for slow transgression and a relatively high permeability substrate 2 vertical intrusion by seawater fingering for fast transgression and a sand silt substrate 3 vertical intrusion by diffusion for fast transgression and a low permeability clay substrate 4 vertical intrusion by combined diffusion and low salinity fingering for fast transgression and a clay layer at the seafloor overlying an aquifer these four modes are characterized by the development of very distinctive transition zones between the fresh and salt groundwater domains an analytical expression is derived for the critical transgression rate which separates horizontal mode 1 from dominantly vertical modes 2 4 intrusion for modes 3 and 4 salinization significantly lags behind sea level rise the results are consistent with observations of fossil fresh brackish groundwater beneath many continental shelves and shallow seas

tracing phosphorous distributions in the surficial sediments of two eastern himalayan high altitude lakes through sequential extraction multivariate and hysplit back trajectory analyses phosphorous p fractionation of the surficial sediment of high altitude lakes hal i e p t tso and sella lakes of arunachal pradesh state india were studied with an objective to investigate the phosphorus availability in the environment nutrient status of the lake and the role of climate change moreover the variation in phosphorous fraction was investigated through mineralogical studies like grain size analysis and x ray diffraction the findings revealed that the surficial sediment of lakes are least influenced by anthropogenic sources since a slight increase in each of the p fractions was observed from year 2011 to 2012 except organic p loosely sorbed exchangeable phosphorous was found to be slightly high which was due to difference in atmospheric precipitation which is one of the strong factors in high altitude regions decrease in organic fraction of p indicated that lakes became more oligotrophic in nature mineralogical investigation shows that illite montmorillonite and montmorillonite are the main minerals in sella lake whereas montmorillonite and graphite are main minerals in p t tso lake factor analysis of seasonal data of p fractionation in surface sediments along with physical parameters of sella lake water indicates three factors for the 2011 data principle factors are ph ec oxide bound and organic p factor analysis of p t tso lake water indicates three factors for the 2011 data with principle factors are adsorbed p and authigenic p ph oxide bound p this study shows that except organic fractions all other fractions are increasing in the following year it implies that even at such remote high altitude locations p characteristics are changing year to year which sought for further investigation and enables hals to show the early impacts of climate change related changes a clear influence of long range transported air pollutants has been found through hysplit back trajectory models which needs to be substantiated through long term monitoring

comment on excess pore pressure resulting from methane hydrate dissociation in marine sediments a theoretical approach by wenyue xu and leonid n germanovich while it is well accepted that gas hydrate dissociation at the base of the gas hydrate stability zone ghsz can generate high excess pore pressure and leads to sediment deformation the consequence in terms of pore pressure of the dissolution of the gas hydrate at the top of the gas hydrate occurrence zone ghoz remains neglected the purpose of this comment on xu and germanovich 2006 article is to demonstrate that gas hydrate dissolution in the ghsz may generate excess pore pressure and to point out the risk related to hydrate dissolution at the top of the ghoz

sedimentation controls on methane hydrate dynamics across glacial interglacial stages an example from international ocean discovery program site u1517 hikurangi margin