wave induced advective transport below a rippled water sediment interface the exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements in this paper we examine quantitatively the effects of modulations in the profile of the water sediment interface on this exchange and on the advective transport inside the sediment bed the flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using darcy s law the forcing is due to progressive gravity waves in the water above the results of two different models for the pressure variation imposed on the upper boundary are compared the two pressure profiles are derived from potential flow theory and from a numerical solution to the navier stokes equations for the oscillatory flow over a rippled bed from an analytic solution to the two dimensional model the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed the shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface the total exchange across the water sediment interface averaged over one wave period is significantly higher across a rippled interface than across a flat bed this difference increases with increasing ripple slope and the strength of the wave motion and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave since rippled bed forms are commonly found in coastal waters the increase in the total exchange across a rippled water sediment boundary can enhance the exchange of solutes due to wave pumping immediately below the water sediment interface circulation cells with net advective transport over a wave period are found such net advection patterns can lead to spatial in the horizontal direction inhomogeneities of the vertical concentration or temperature profiles if the overlying water column and or the sediment bed act s as source or sink this gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements

vertical sheets of dense plasma in the topside martian ionosphere 1 the low frequency radar mars advanced radar for subsurface and ionosphere sounding marsis on board the mars express spacecraft is used to sound electron densities in the topside martian ionosphere the radar records the delay times to echoes of reflected radio waves as a function of frequency yielding spectrograms with traces of radar echoes at times two traces are present in spectrograms of the martian ionosphere one of these traces corresponds to reflections from the direction to nadir the other trace originates in a localized reflector in the ionosphere the local reflectors can be associated with the cusplike regions of near vertical crustal magnetic fields the apparent nadir angle of reflection can occasionally increase to 90 this suggests that steep gradients of the altitude of the electron isodensity exist in the martian ionosphere and indicates rapid horizontal spatial variations of vertical diffusion of martian plasma such gradients may arise owing to preferential access of solar wind to the cusplike regions or to precipitation of energetic electrons from acceleration regions located on cusp magnetic field lines high above the ionosphere

the importance of submarine groundwater discharge to the near shore nutrient supply in the gulf of aqaba israel we used two short lived radium isotopes 223ra 224ra and a mass balance approach applied to the radium activities to determine the nutrient contribution of saline submarine groundwater discharge to the coastal waters of the northern gulf of aqaba israel radium isotope activities were measured along transects during two seasons at a site that lacked any obvious surficial water input an onshore well and an offshore end member were also sampled for all samples nutrients and salinity data were collected radium isotope activities generally decreased with distance offshore and exhibited significant tidal variability which is consistent with a shorederived tidally influenced source submarine groundwater contributes only 1 2 of the water along this coast but this groundwater provides 8 46 of the nutrients this saline groundwater is derived predominately from tidally pumped seawater percolating through the unconfined coastal aquifer and leaching radium and nutrients this process represents a significant source of nutrients to the oligotrophic nearshore reef

influence of porewater advection on denitrification in carbonate sands evidence from repacked sediment column experiments abstract porewater flow enhances mineralization rates in organic poor permeable sands here a series of sediment column experiments were undertaken to assess the potential effect of advective porewater transport on denitrification in permeable carbonate sands collected from heron island great barrier reef experimental conditions flow path length advection rate and temperature were manipulated to represent conditions similar to near shore tropical environments hgcl2 poisoned controls were used to assess whether reactions were microbially mediated overall significant correlations were found between oxygen consumption and n2 production the n o2 slope of 0 114 implied that about 75 of all the nitrogen mineralized was denitrified a 4 fold increase in sediment column length from 10 to 40 cm resulted in an overall increase in oxygen consumption 1 6 fold tco2 production 1 8 fold and denitrification 1 9 fold oxic respiration increased quickly until advection reached 80 l m 2 h 1 and then plateaued at higher advection rates interestingly denitrification peaked up to 336 mol n2 m 2 h 1 at intermediate advection rates 30 80 l m 2 h 1 we speculate that intermediate advection rates enhance the development of microniches i e steep oxygen gradients within porous carbonate sands perhaps providing optimum conditions for denitrification the denitrification peak fell within the broad range of advection rates often on scales of 1 100 l m 2 h 1 typically found on continental shelves implying that carbonate sands may play a major but as yet unquantified role in oceanic nitrogen budgets

the behavior of trace elements during schwertmannite precipitation and subsequent transformation into goethite and jarosite schwertmannite is a ubiquitous mineral formed from acid rock drainage ard and plays a major role in controlling the water chemistry of many acid streams the formation of schwertmannite was investigated in the acid discharge of the monte romero abandoned mine iberian pyrite belt sw spain schwertmannite precipitated from supersaturated solutions mainly owing to the oxidation of fe ii to fe iii and transformed with time into goethite and jarosite in a few hours schwertmannite precipitation removed more than half of the arsenic load from solution whereas the concentration of divalent trace metals zn cu pb cd ni and co remained almost unchanged in the laboratory natural schwertmannite was kept in contact with its coexisting acid water in a flask with a solid liquid mass ratio of 1 5 for 353 days during this time the ph of the solution dropped from 3 07 to 1 74 and the concentrations of sulfate and fe increased during the first 164 days schwertmannite transformed into goethite plus h3o jarosite but subsequently goethite was the only mineral to form some of the trace elements such as al cu pb and as were depleted in solution during the first stage as schwertmannite transformed into goethite plus h3o jarosite on the contrary the transformation of schwertmannite to goethite with no jarosite during the second stage released al cu and as to the solution despite the variation in their concentrations in solution approximately 80 of the total al and cu inventories and more than 99 as and pb remained in the solid phase throughout the entire aging process

steep transient density gradients in the martian ionosphere similar to the ionopause at venus 1 with the mars advanced radar for subsurface and ionospheric sounding marsis on the mars express mex spacecraft the electron density can be measured by two methods from the excitation of local plasma oscillations and from remote sounding a study of the local electron density versus time for 1664 orbits revealed that in 132 orbits very sharp gradients in the electron density occurred that are similar to the ionopause boundary commonly observed at venus in 40 of these cases remote sounding data have also confirmed identical locations of steep ionopause like density gradients measurements from the analyzer of space plasma and energetic atoms aspera 3 electron spectrometer and ion mass analyzer instruments also on mars express verify that these sharp decreases in the electron density occur somewhere between the end of the region where ionospheric photoelectrons are dominant and the magnetosheath combined studies of the two experiments reveal that the steep density gradients define a boundary where the magnetic fields change from open to closed this study shows that although the individual cases are from a wide range of altitudes the average altitude of the boundary as a function of solar zenith angle is almost constant the average altitude is approximately 500 km up to solar zenith angles of 60 after which it shows a slight increase the average thickness of the boundary is about 22 km according to remote sounding measurements the altitude of the steep gradients shows an increase at locations with strong crustal magnetic fields

dynamic response of surface water groundwater exchange to currents tides and waves in a shallow estuary 1 in shallow fetch limited estuaries variations in current and wave energy promote heterogeneous surface water groundwater mixing benthic exchange which influences biogeochemical activity here we characterize heterogeneity in benthic exchange within the subtidal zone of the delaware inland bays by linking hydrodynamic circulation models with mathematical solutions for benthic exchange forced by current bedform interactions tides and waves benthic fluxes oscillate over tidal cycles as fluctuating water depths alter fluid interactions with the bed maximum current driven fluxes 1 10 cm d occur in channels with strong tidal currents maximum wave driven fluxes 1 10 cm d occur in downwind shoals during high energy storms simulated wave pumping rates increase by orders of magnitude demonstrating the importance of storms in solute transfer through the benthic layer under moderate wind conditions 5 m s integrated benthic exchange rates due to wave current and tidal pumping are each 1 10 m3 s on the order of fluid contributions from runoff and fresh groundwater discharge to the estuary benthic exchange is thus a significant and dynamic component of an estuary s fluid budget that may influence estuarine geochemistry and ecology

on improving the accuracy of electron density profiles obtained at high altitudes by the ionospheric sounder on the mars express spacecraft we attempt to improve the standard inversion routine used to obtain electron density profiles from the data measured by the mars advanced radar for subsurface and ionospheric sounding marsis topside radar sounder on board the mars express spacecraft artificial ionospheric traces corresponding to how the marsis instrument would see model electron density profiles are constructed and they are inverted using the standard routine ideally this should result in the original electron density profiles however it is found that discrepancies between the original and resulting electron density profiles may be significant in particular at larger solar zenith angles and for higher spacecraft altitudes this is due to a gap of marsis time delay measurements at low sounding frequencies and a necessity to interpolate electron density profiles in this plasma frequency range although the ionospheric scale height likely increases with the altitude the standard inversion routine uses an exponential interpolation with a constant scale height we suggest a new inversion method which takes into account expected shapes of ionospheric profiles in the interpolation region we verify the performance of this method both on the model electron density profiles and on real marsis data we demonstrate that it seems to perform considerably better than the standard routine in particular for higher spacecraft altitudes finally we use the new inversion method to obtain a revised dependence of the peak altitude on the solar zenith angle

velocity measurements close to rippled beds in oscillatory flow the results are reported of velocity measurements in oscillatory flow over rippled beds velocities were measured with a laser doppler anemometer in both an oscillating tray rig and an oscillatory flow water channel both self formed and artificial ripples were examined in addition some measurements were made with an apparently plane bed with intense sediment motion the experimental results were compared with the predictions of kalkanis 1964 kajiura 1968 bakker 1973 1974 and sleath 1974 the closest agreement between theory and experiment was obtained with the method of sleath measurements of the eulerian drift velocities showed drift towards the adjacent ripple crest in the immediate vicinity of the bed and away from the crest further out

the mathematics of early diagenesis from worms to waves the changes that sediments undergo after deposition are collectively known as diagenesis diagenesis is not widely recognized as a source for mathematical ideas however the myriad processes responsible for these changes lead to a wide variety of mathematical models in fact most of the classical models and methods of applied mathematics emerge naturally from quantification of diagenesis for example small scale sediment mixing by bottom dwelling animals can be described by the diffusion equation the dissolution of biogenic opal in sediments leads to sets of coupled nonlinear ordinary differential equations and modeling organisms that eat at depth in the sediment and defecate at the surface suggests the one dimensional wave equation while the effect of waves on pore waters is governed by the two or three dimensional wave equation diagenetic modeling however is not restricted to classical methods diagenetic problems of concern to modern mathematics exist in abundance these include free boundary problems that predict the depth of biological mixing or the penetration of o2 into sediments algebraic differential equations that result from the fast reversible reactions that regulate ph in pore waters inverse calculations of input functions histories and the determination of the optimum choice in a hierarchy of possible diagenetic models this review highlights and explores these topics with the hope of encouraging further modeling and analysis of diagenetic phenomena

stratigraphic controls on fluid and solute fluxes across the sediment water interface of an estuary shallow stratigraphic features such as infilled paleovalleys modify fresh groundwater discharge to coastal waters and fluxes of saltwater and nutrients across the sediment water interface we quantify the spatial distribution of shallow surface water groundwater exchange and nitrogen fluxes near a paleovalley in indian river bay delaware using a hand resistivity probe conventional seepage meters and pore water samples in the interfluve region outside the paleovalley most nitrate rich fresh groundwater discharges rapidly near the coast with little mixing of saline pore water and nitrogen transport is largely conservative in the peat filled paleovalley fresh groundwater discharge is negligible and saltwater exchange is deep 1 m long pore water residence times and abundant sulfate and organic matter promote sulfate reduction and ammonium production in shallow sediment reducing iron rich fresh groundwater beneath paleovalley peat discharges diffusely around paleovalley margins offshore in this zone of diffuse fresh groundwater discharge saltwater exchange and dispersion are enhanced ammonium is produced in shallow sediments and fluxes of ammonium to surface water are large by modifying patterns of groundwater discharge and the nature of saltwater exchange in shallow sediments paleovalleys and other stratigraphic features influence the geochemistry of discharging groundwater redox reactions near the sediment water interface affect rates and patterns of geochemical fluxes to coastal surface waters for example at this site more than 99 of the groundwater borne nitrate flux to the delaware inland bays occurs within the interfluve portion of the coastline and more than 50 of the ammonium flux occurs at the paleovalley margin

advective flow through the upper continental shelf driven by storms buoyancy and submarine groundwater discharge data from monitoring wells on the carolina usa continental shelf require a reevaluation of pore water exchange across the seafloor temperature salinity and 226ra data from 3 offshore wells require rapid and frequent pore water exchange to depths of several meters through sands and other permeable sediments the exchange is driven by tidal pumping storms buoyancy and submarine groundwater discharge tidal pumping causes semidiurnal temperature cycles in one well that can only be explained by rapid fluid flow the effect of large storms is evident in sudden temperature changes 2 4 m below the seabed in two wells these changes require considerable pore water exchange sudden cooling of the ocean may cause pore waters to become buoyant with respect to the ocean and trigger pore water convection to several meters depth as observed in one well the wells also display frequent large factor of ten changes in 226ra activity this requires a substantial input of 226ra enriched fluids to replace 226ra that is lost during exchange events these data support a fourth flow process which is groundwater seeping from the underlying limestone the pore spaces in the overlying sand provide a temporary reservoir for these fluids we conclude that pore water residence times in 2 4 m thick sands on this continental margin are on the order of a few months or less

intercomparison of submarine groundwater discharge estimates from a sandy unconfined aquifer summary recent studies suggest that chemical loading from submarine groundwater discharge sgd may rival other major sources such as rivers in many coastal areas sgd can occur as terrestrially derived typically fresh groundwater discharge or by seawater circulation through aquifer sediments because both terrestrial and recirculated seawater sgd can be significant sources of chemical loading to coastal waters appropriate methods are needed to properly assess and quantify the rates and distribution of these different inputs although many techniques exist to quantify sgd each method samples different components of sgd e g fresh versus saline discharge over different temporal and spatial scales the result is that confusion exists about how to use the different methods in this study we investigate the applicability of a variety of techniques for estimating sgd from an unconfined sandy coastal aquifer physical methods are first used to understand the spatial variability of discharge this information is then used with physical and geochemical data to estimate sgd investigative methods include direct measurement via seepage meters hydrogeologic estimation using darcy s law and tracer based estimates using radon and radium isotopes at our field site on cape cod massachusetts nearshore landward topography appears to exert a significant control on the spatial variability of fresh groundwater discharge presumably through its effects on evapotranspiration and hence recharge rates the darcy estimate of fresh sgd 4 0 m 3 m 1 d 1 is similar to a previous estimate made using a seepage meter transect that spans the seepage face radon serves as a valuable estimator of total sgd fresh plus saline 5 6 m 3 m 1 d 1 the difference between the total and fresh sgd estimates is in good agreement with the radium based sgd estimate 0 56 m 3 m 1 d 1 which primarily measures saline circulation through coastal sediments thus we conclude that the hydrogeologic estimate and radon and radium techniques are complimentary for estimating different components of total sgd at our field site

arsenic and metal mobility from au mine tailings in rodalquilar almeria se spain in the old mining area of rodalquilar mine wastes soil and sediments were characterized and the results revealed high concentration of au ag as bi cu fe mn pb se sb and zn in tailings and sediments the contaminant of greatest environmental concern is as the mean concentration in the tailings was 679 9 and 345 mg kg in the sediments of playazo creek the groundwater samples from the alluvial aquifer showed high concentration of al as cd fe hg mn ni pb se sb and zn and very high concentration of chloride and sulfate which were above the concentration defined in the european standards for drinking water the presence of as in groundwater may be caused by the oxidation of arsenian pyrite the possible as desorption from goethite and ferrihydrite and the jarosite dissolution groundwater concentrations of cd fe mn and possibly cu were associated with low values of eh indicating the possible dissolution of oxy hydroxides of fe and mn the mobility of metals in the column experiments show the release of al fe mn cr cu ni v and zn in significant concentrations but below the detected values in groundwater however as cd sb se pb and au are generally mobilized in concentrations above the detected values in groundwater the possible mass transfer processes that could explain the presence of the contaminants in the aquifer and the leachates was simulated with the phreeqc numerical code and revealed the possible dissolution of the following mineral phases jarosite natrojarosite arsenian pyrite alunite chlorite kaolinite and calcite

hydraulic control of pore water geochemistry within the oxic suboxic zone of a permeable sediment the geochemical behavior of the top 70 cm of permeable reef flat sediments on checker reef oahu hawai i was examined using spatial and temporal changes in pore water o2 no no nh and n2o concentrations 221 324 the shallow depth of the sediment water interface relative to the height of waves propagating across the reef creates an environment in which pore water mixing and transport are dominated by wave induced mechanisms dissolved oxygen penetrated 15 50 cm in the four sites studied the depth integrated concentration of dissolved oxygen was greater under larger wave conditions and in more permeable sediments which suggests that dissolved oxygen may be a good indicator of the relationship between sediment metabolism and the physical nature of the sediment environment the absence of any subsurface n2o maxima limited temporal variability in pore water n2o concentrations and vertically coherent profiles of o2 no no and n2o suggest that suboxic processes are typically 22 32 restricted to spatial scales that are smaller than for oxic processes in these sediments the geochemical response of the sediments to a natural perturbation was monitored by examining the composition of pore waters immediately following a large wave event and for several weeks thereafter significant changes in the depth integrated concentrations of o2 no no and nh were observed following the perturbation however only changes in no 22 1

influence of sea level rise on iron diagenesis in an east florida subterranean estuary abstract subterranean estuary occupies the transition zone between hypoxic fresh groundwater and oxic seawater and between terrestrial and marine sediment deposits consequently we hypothesize in a subterranean estuary biogeochemical reactions of fe respond to submarine groundwater discharge sgd and sea level rise porewater and sediment samples were collected across a 30 m wide freshwater discharge zone of the indian river lagoon florida usa subterranean estuary and at a site 250 m offshore porewater fe concentrations range from 0 5 m at the shoreline and 250 m offshore to about 286 m at the freshwater saltwater boundary sediment sulfur and porewater sulfide maxima occur in near surface oc rich black sediments of marine origin and dissolved fe maxima occur in underlying oc poor orange sediments of terrestrial origin freshwater sgd flow rates decrease offshore from around 1 to 0 1 cm day while bioirrigation exchange deepens with distance from about 10 cm at the shoreline to about 40 cm at the freshwater saltwater boundary doc concentrations increase from around 75 m at the shoreline to as much as 700 m at the freshwater saltwater boundary as a result of labile marine carbon inputs from marine sgd this labile doc reduces fe oxides which in conjunction with slow discharge of sgd at the boundary allows dissolved fe to accumulate upward advection of fresh sgd carries dissolved fe from the fe oxide reduction zone to the sulfate reduction zone where dissolved fe precipitates as fe sulfides saturation models of fe sulfides indicate some fractions of these fe sulfides get dissolved near the sediment water interface where bioirrigation exchanges oxic surface water the estimated dissolved fe flux is approximately 0 84 m fe day per meter of shoreline to lagoon surface waters accelerated sea level rise predictions are thus likely to increase the fe flux to surface waters and local primary productivity particularly along coastlines where groundwater discharges through sediments

generalized analytical model for benthic water flux forced by surface gravity waves 1 a generalized analytical model for benthic water flux forced by linear surface gravity waves over a series of layered hydrogeologic units is developed by adapting a previous solution for a hydrogeologic unit with an infinite thickness case i to a unit with a finite thickness case ii and to a dual unit system case iii the model compares favorably with laboratory observations the amplitude of wave forced benthic water flux is shown to be directly proportional to the amplitude of the wave the permeability of the hydrogeologic unit and the wave number and inversely proportional to the kinematic viscosity of water a dimensionless amplitude parameter is introduced and shown to reach a maximum where the product of water depth and the wave number is 1 2 submarine groundwater discharge sgd is a benthic water discharge flux to a marine water body the case i model estimates an 11 5 cm d sgd forced by a wave with a 1 s period and 5 cm amplitude in water that is 0 5 m deep as this wave propagates into a region with a 0 3 m thick hydrogeologic unit with a no flow bottom boundary the case ii model estimates a 9 7 cm d wave forced sgd as this wave propagates into a region with a 0 2 m thick hydrogeologic unit over an infinitely thick more permeable unit the case iii quasi confined model estimates a 15 7 cm d wave forced sgd the quasi confined model has benthic constituent flux implications in coral reef karst and clastic regions waves may undermine tracer and seepage meter estimates of sgd at some locations

reactive transport modeling of iron diagenesis and associated organic carbon remineralization in a florida usa subterranean estuary article iron oxides are important terminal electron acceptors for organic carbon oc remineralization in subterranean estuaries particularly where oxygen and nitrate concentrations are low in indian river lagoon florida usa terrestrial fe oxides dissolve at the seaward edge of the seepage face and flow upward into overlying marine sediments where they precipitate as fe sulfides the dissolved fe concentrations vary by over three orders of magnitude but fe oxide dissolution rates are similar across the 25 m wide seepage face averaging around 0 21 mg cm 2 yr the constant dissolution rate but differing concentrations indicate fe dissolution is controlled by a combination of increasing lability of dissolved organic carbon doc and slower porewater flow velocities with distance offshore in contrast the average rate constants of fe sulfide precipitation decrease from 21 9 10 8 s 1 to 0 64 10 8 s 1 from the shoreline to the seaward edge of the seepage face as more oxygenated surface water circulates through the sediment the amount of oc

electron densities in the ionosphere of mars a comparison of marsis and radio occultation measurements radio occultation electron densities measurements from the mariner 9 and viking spacecraft which orbited mars in the 1970s have recently become available in a digital format these data are highl

a post pioneer venus reassessment of the martian dayside ionosphere as observed by radio occultation methods the dayside altitude profiles of the electron density obtained with the radio occultation experiments on mariners 4 6 7 and 9 and the viking 1 and 2 spacecraft are collectively reanalyzed to determine the global characteristics of the dayside ionosphere of mars these analyses concentrate on the comparison of the properties of both the electron density peaks and the topside profiles with the behavior expected for a chapman layer and that observed at venus with the pioneer venus orbiter radio occultation experiment as at venus the peak densities at mars behave much like chapman layer peaks with only slight departure from a cos dependence where is the solar zenith angle in contrast the peak heights depart from ideal chapman layer behavior at venus but not at mars because the dayside neutral atmosphere at venus depends on solar zenith angle the global dust storm during the mariner 9 main mission appears to have elevated the martian ionosphere as a whole by 20 30 km without otherwise notably altering its density profile these results generally corroborate the findings of earlier studies an examination of the solar zenith angle dependence of density levels on the topsides of profiles obtained both at mars and at venus near solar minimum provides a new perspective on the solar zenith angle variation of the scale heights of the two ionospheres the key contribution of this study is the improved picture of the solar zenith angle dependences of both the peaks and scale heights of the ionosphere of mars and their comparison with venus at solar minimum in particular this study illustrates that under the similar conditions where incident solar wind dynamic pressure exceeds peak ionospheric thermal pressure the martian dayside ionosphere peaks at higher altitudes in the flanks and has a greater scale height overall thus even magnetically unshielded martian and venusian ionospheres would present slightly different obstacles to the solar wind

wave driven porewater and solute circulation through rippled elastic sediment under highly transient forcing waves induce porewater flow and solute transport through permeable marine sediment however past studies have ignored high frequency pressure pulses under the assumption that the porewater flow field is adequately represented by a time averaged one or that the saturated sediment is incompressible we modeled porewater flow and solute transport inside ripples forced by instantaneous pressure profiles along the sediment water interface swi with 0 1 s temporal resolution the transient pressure profiles were taken from a field data driven large eddy simulation model of wave driven oscillatory flow the simulations suggest that in elastic permeable and saturated sediment a time averaged representation of the flow field may be inadequate and that this also leads to shortcomings in how transport is modeled bursts in fluid flushing occur when high frequency pressure fluctuations were considered leading to larger long term average fluid fluxes compared to a steady flow field driven by a time averaged pressure profile the pressure perturbations along the swi propagate within a few milliseconds to meter depths within the sediment leading to strongly transient porewater velocity fields this leads to enhanced dispersion of solutes and larger time averaged solute fluxes however enhanced solute flux across the swi diminished through time with increasing permeability the high frequency transient pressures and sediment elastic properties we considered have been largely ignored and unrecognized future observational and modeling studies should consider these processes especially since they mediate timing sensitive biogeochemical reactions

aggregation of fine particles at the sediment water interface the presence of a bottom sediment layer agitated by mechanical stirring or by resident organisms tubificid oligochaetes significantly increases the rate at which fine 1 m cohesive particles are removed from suspension in laboratory columns measured rates of particle removal are equivalent to deposition velocities ranging from 0 23 m day 1 to 0 41 m day 1 these rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls it is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed water interface characterized by loosely bound fluffy porous material hydrodynamically coupled to the water column according to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment this new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates

mobility of arsenic in groundwater in the obuasi gold mining area of ghana some implications for human health arsenic in drinking water from streams shallow wells and boreholes in the obuasi gold mining area of ghana range between 0 5 probably as a result of methylation and reduction reactions mediated by bacteria and algae concentrations of as in groundwaters reach up to 64 gl 1 being highest in deeper 40 70 m and more reducing 220 250 mv waters the as is thought to build up as a result of the longer residence times undergone by groundwaters in the deeper parts of the aquifer the proportion of as present as as iii is also higher in the deeper groundwaters deep mine exploration boreholes 70 100 m have relatively low as contents of 5 17 gl 1 possibly due to as sorption onto precipitating ferric oxyhydroxides or to localized low as concentrations of sulphide minerals median concentrations of inorganic urinary as from sample populations in two villages one a rural streamwater drinking community and the other a suburb of obuasi using groundwater for potable supply were 42 gl 1 and 18 gl 1 respectively the value for the community drinking groundwater is typical of background concentrations of urinary as the slightly higher value for the streamwater drinking community probably reflects different provenance of foodstuffs and higher as concentrations of water sources local to the village the low value obtained for the inhabitants of the obuasi suburb living close to and abstracting groundwater from the area of major mining activity suggests that groundwater can form a useful potable supply of good inorganic quality provided that deep long residence time sources are avoided

seawater circulation in sediments driven by interactions between seabed topography and fluid density 1 measurements of submarine groundwater discharge sgd in coastal areas often show that the saltwater discharge component is substantially greater than the freshwater discharge several mechanisms have been proposed to explain these high saltwater discharge values including saltwater circulation driven by wave and tidal pumping wave and tidal setup in intertidal areas currents over bedforms and density gradients resulting from mixing along the freshwater saltwater interface in this study a new mechanism for saltwater circulation and discharge is proposed and evaluated the process results from interaction between bedform topography and buoyancy forces even without flow or current over the bedform in this mechanism an inverted salinity and density profile in the presence of both a bedform on the seafloor and an upward flow of fresher groundwater from depth induces a downward flow of saline pore water under the troughs and upward flow under the adjacent crest of the bedform the magnitude and occurrence of the mechanism were tested using numerical methods the results indicate that this mechanism could drive seawater circulation under a limited range of conditions and contribute 20 30 of local sgd when and where the process is operative bedform shape hydraulic conductivity hydraulic head and salinity at depth in the porous media aquifer thickness effective porosity and hydrodynamic dispersion are among the factors that control the occurrence and magnitude of the circulation of seawater by this mechanism

impact of heterogeneity bed forms and stream curvature on subchannel hyporheic exchange 1 advection through hyporheic zones hz consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking the top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature flux calculations show that heterogeneity causes significant additional hz flux compared to an equivalent homogenous medium however the major cause of hz flux is a spatially periodic sinusoidal head distribution along the boundary representing the effect of bed forms the additional influence of heterogeneity on the total channel bed exchange and the overall hz geometry are increased when boundary head sinusoidal fluctuation is more subdued we present dimensionless numbers that summarize these relationships heterogeneity s influence is further magnified by considering the effect of channel curvature on boundary heads the simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood furthermore we show that residence times total tracking times of particles originating from the streambed follow a lognormal distribution in the presence of heterogeneity residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms hence streambed heterogeneity and stream curvature factors often neglected in previous modeling efforts combine with bed form configuration to dynamically determine hz geometry fluxes and residence time distributions

mobility and microbially mediated mobilization of gold and arsenic in soils from two gold mines in semi arid and tropical australia the mobility and microbially mediated solubilization of au and as in regolith materials from two au mines in australia i e the peak hill gold mine in semi arid new south wales and the hit or miss gold mine in tropical northern queensland was studied using a combination of geochemical and microbiological techniques gold is highly mobile in both environments the mobility of au increases with increasing degree of weathering of host materials and the resident microbiota are capable of mediating its solubilization the results of the microcosm experiments demonstrate that the activity of microorganisms needs to be taken into account when studying the mobility and solubilization of au in the australian regolith in primary unweathered mineralization material from the hit or miss mine 99 wt of au was extracted only in the strongest final step of the sequential extractions in concentrated aqua regia in alteration zone material from the peak hill gold mine 80 wt of au was associated with the operationally defined mn and fe oxides in contrast in auriferous soils overlying mineralization at both sites 90 95 wt of au was associated with the operationally defined exchangeable claybound and organic fractions microcosm experiments were incubated biologically active and inactive sterilized in 1 4 w v aqueous slurries at 25 c in the dark for up to 95 days in biologically active microcosms with soils from the peak hill and the hit or miss gold mines approximately 55 wt 907 ng g 1 d w soil and 20 wt 233 ng g 1 d w soil of the total au respectively was solubilized during the incubation in contrast no or significantly lower au concentrations were observed in biologically inactive microcosms the mobility and microbially mediated release of as was limited at both sites and appears to be mostly controlled by abiotic adsorption and desorption on mn and fe oxides arsenic has a low solubility in the more mobile fractions and is mostly associated with mn and fe oxides and the residual fraction the release of as was not elevated in biologically active compared to inactive microcosms from the peak hill gold mine in contrast in biologically active microcosms with samples from the hit or miss mine elevated concentrations of as were detected in solution compared to the biologically inactive experiments crown copyright 2006 published by elsevier inc all rights reserved

the negligible effect of bed form migration on denitrification in hyporheic zones of permeable sediments bed form celerity the migration rate of ripples along a sediment bed has previously been shown to have dramatic effects on oxygen distribution and transport within the hyporheic zone of permeable sediments this has the potential to influence denitrification rates in particular by increasing the coupling of nitrification and denitrification to further understand this we numerically modeled nitrogen cycling under migrating ripples while the simulated oxygen profiles match with expected behavior almost no effect on denitrification or coupled nitrification denitrification was observed with increasing celerity instead denitrification rates were dominantly controlled by the flow velocity of water overlying the sediment

topside of the martian ionosphere near the terminator variations with season and solar zenith angle and implications for the origin of the transient layers abstract in this paper the morphological variations of the m2 layer of the martian ionosphere with the martian seasons and solar zenith angle sza at the terminator are investigated the data used are the marsis mars advanced radar for subsurface and ionosphere sounding measurements approximately 5000 ionograms that were acquired from 2005 to 2012 which have a sza 85 and detect the topside transient layers a simple effective data inversion method is developed for the situation in which the upper portion of the height profile is non monotonic and the observed data are insufficient for adequate reduction the inverted parameters are subsequently explored using a statistical approach the results reveal that the main body of the m2 layer approximately 10 km below the first topside layer can be well characterized as a chapman layer near the terminator sza 85 98 notwithstanding the high sza and the presence of the topside layers the height of the first topside layer tends to be concentrated approximately 60 km with a standard deviation of 20 km above the main density peak the peak density and height of the first topside layer are positively correlated to the density and height of the main peak respectively the density and height of the first topside layer appear to be independent of the sza but possess seasonal variations that are similar to those of the main layer the height of the topside layer is greater by 10 km on average in the southern spring and summer than in the southern autumn and winter coinciding with the observation that in the southern spring and summer the underlying atmosphere is warmer due to dust heating e g smith m d 2004 icarus 167 148 165 the statistical regularities of the parameters suggest a possibility that the formation of the topside layers are closely related to the processes of photoionization and diffusion that occur on the topside of the m2 layer we propose that development of beam plasma instabilities in the transitional region between the lower chapman region and the upper transport dominating region is possibly a mechanism that is responsible for the occurrences of the topside layers

estimating groundwater discharge into the northeastern gulf of mexico using radon 222 abstract submarine groundwater discharge sgd may provide important chemical constituents to the ocean but the dispersed nature of this process makes locating and quantifying its input extremely difficult since groundwater contains 3 4 orders of magnitude greater radon than seawater 222rn may be a useful tracer of this process if all other sources of radon to bottom waters can be evaluated we report development of a sgd tracing tool based on radon inventories in a coastal area of the northeastern gulf of mexico we evaluated factors that influence the concentration of radon in the water column i e production decay horizontal transport and loss across the pycnocline using a linked benthic exchange horizontal transport model total 222rn benthic fluxes 2420 dpm m 2 day 1 measured with in situ chambers are of the magnitude required to support measured sub pycnocline 222rn inventories while estimates of molecular diffusion show that this input is relatively small 230 dpm m 2 day 1 using this model approach together with measurements of the radon inventory we estimated a regional subsurface fluid flow ranging from 180 to 710 m3 sec 1 into the 620 km2 study area this discharge equivalent to an upward advective velocity of approximately 2 10 cm day 1 dispersed over this entire study area is equivalent to approximately 20 first magnitude springs

increasing arsenic mobility in the fine fraction of the dry stream sediments of the semi arid san antonio gold mining district baja california peninsula mexico the geochemical mobility of arsenic and some metals in the fine fraction of 20 alluvial sediment samples collected along the thalweg of the san antonio arroyo was assessed in the abandoned gold mining district of the semi arid southeastern baja california peninsula acid digested element concentrations were determined by treating the fine fraction of the sediments with concentrated hno3 via microwave heating a separate leaching of the diluted acid soluble as and some metals from the samples was carried out using 1 m hcl the most reactive as phase was extracted from the subsamples using an ascorbic solution of na citrate buffered at ph 8 the high abundance of sulphosalts tennantite cu12as4s13 and proustite ag3ass3 in the sediments from the head of the arroyo changes downstream to the predominance of arsenic trioxide arsenolite as2o3 and arsenate scorodite feaso4 2h2o acid digested as concentrations were always found to be high 230 270 mg kg 1 from the san antonio village until the end of the arroyo the concentration of reactive as and its relative contribution increased from low values of 2 3 mg kg 1 and 1 respectively at the upstream of the arroyo to 191 220 mg kg 1 and 75 89 in the middle and lower parts and is probably a result of as release from the mineral sulfide fraction of the tailings metallic contaminants exhibited different behavior with a peak of acid digested and diluted acid leached concentrations observed in the central part of the arroyo and an almost permanent geochemical mobility

the behaviour of arsenic in muddy sediments of the bay of biscay france abstract we have studied particulate and dissolved arsenic species in sediment and porewaters at sites in the bay of biscay france ranging in depths from 150 to 2 800 m at all stations major redox species oxygen nitrate ammonia total and reactive iron and manganese sulphate and sulphur reflect early diagenetic depth sequences of redox reactions comparable to other marine environments vertical distributions of dissolved and particulate as species and major redox species are related to changes in redox conditions and their major carrier phases such as fe and mn oxides arsenic diagenesis appears strongly dependent on fe cycling a subsurface maximum of dissolved as and surface enrichment of particulate as correspond to dissolution and precipitation of fe iii phases except for the shallowest and most bioturbated site flux calculations show three different vertical diffusive as fluxes two upwards and one downwards phase changes of recycled as result in local accumulations of reactive as at different redox fronts mass balance calculations indicate that the upward as flux toward the oxidized layer can explain the enrichment of hcl extractable particulate as in this layer a portion of the upward diffusing as can escape the sediment and may be fixed onto settling fe oxides by adsorption or co precipitation and contribute to reactive particulate as input i e as is recycled across the water sediment interface

a review of the source behaviour and distribution of arsenic in natural waters the range of as concentrations found in natural waters is large ranging from less than 0 5 g l 1 to more than 5000 g l 1 typical concentrations in freshwater are less than 10 g l 1 and frequently less than 1 g l 1 rarely much higher concentrations are found particularly in groundwater in such areas more than 10 of wells may be affected defined as those exceeding 50 g l 1 and in the worst cases this figure may exceed 90 well known high as groundwater areas have been found in argentina chile mexico china and hungary and more recently in west bengal india bangladesh and vietnam the scale of the problem in terms of population exposed to high as concentrations is greatest in the bengal basin with more than 40 million people drinking water containing excessive as these large scale natural as groundwater problem areas tend to be found in two types of environment firstly inland or closed basins in arid or semi arid areas and secondly strongly reducing aquifers often derived from alluvium both environments tend to contain geologically young sediments and to be in flat low lying areas where groundwater flow is sluggish historically these are poorly flushed aquifers and any as released from the sediments following burial has been able to accumulate in the groundwater arsenic rich groundwaters are also found in geothermal areas and on a more localised scale in areas of mining activity and where oxidation of sulphide minerals has occurred the as content of the aquifer materials in major problem aquifers does not appear to be exceptionally high being normally in the range 1 20 mg kg 1 there appear to be two distinct triggers that can lead to the release of as on a large scale the first is the development of high ph 8 5 conditions in semi arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates this ph change leads either to the desorption of adsorbed as especially as v species and a range of other anion forming elements v b f mo se and u from mineral oxides especially fe oxides or it prevents them from being adsorbed the second trigger is the development of strongly reducing conditions at near neutral ph values leading to the desorption of as from mineral oxides and to the reductive dissolution of fe and mn oxides also leading to as release iron ii and as iii are relatively abundant in these groundwaters and so4 concentrations are small typically 1 mg l 1 or less large concentrations of phosphate bicarbonate silicate and possibly organic matter can enhance the desorption of as because of competition for adsorption sites a characteristic feature of high groundwater as areas is the large degree of spatial variability in as concentrations in the groundwaters this means that it may be difficult or impossible to predict reliably the likely concentration of as in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well arsenic affected aquifers are restricted to certain environments and appear to be the exception rather than the rule in most aquifers the majority of wells are likely to be unaffected even when for example they contain high concentrations of dissolved fe

the driving forces of porewater and groundwater flow in permeable coastal sediments a review abstract advective flows rapidly transport water solutes and particles into and out of permeable sand beds and significantly affects the biogeochemistry of coastal environments in this paper we reviewed the drivers of porewater and groundwater advection in permeable shelf sediments in an attempt to bridge gaps among different disciplines studying similar problems we identified the following driving forces 1 terrestrial hydraulic gradients 2 seasonal changes in the aquifer level on land moving the location of the subterranean estuary 3 wave setup and tidal pumping 4 water level differences across permeable barriers 5 flow and topography induced pressure gradients 6 wave pumping 7 ripple and other bed form migration 8 fluid shear 9 density driven convection 10 bioirrigation and bioturbation 11 gas bubble upwelling and 12 sediment compaction while these drivers occur over spatial scales ranging from mm to km and temporal scales ranging from seconds to years their ultimate biogeochemical implications are very similar i e they are often a source of new or recycled nutrients to seawater and transform organic carbon into inorganic carbon drivers 2 12 result in no net water input into the ocean taking all these mechanisms into account we conservatively estimate that a volume equivalent to that of the entire ocean is filtered by permeable sediments at time scales of about 3000 years quantifying the relative contribution of these drivers is essential to understand the contribution of sediments to the global cycles of matter

groundwater regime of a tidally influenced coastal pond abstract hydraulic gradients between the water level of little pond a small tidally influenced coastal pond located on cape cod massachusetts and 41 mini piezometers located along the shores and in the sand layer underlying soft bottom sediments indicate that the flow of groundwater to the pond is mainly concentrated along the western shore within a nearshore sandy strip similar results are obtained from the distribution of salinity in shallow bottom sediments the point measurements of groundwater head are translated to long term values by approximating tidal movements to a sinusoid and using a simple mathematical model salinity depth profiles in the shore and bottom sandy sediments indicate at least occasional outflow of salt water to the ground and suggest that a density driven convection may play an important role in determining the direction of flow and the distribution of solutes salinity depth profiles in the soft sediments compared with the results of an advection diffusion model indicate a locally strong upward flow of fresh groundwater the model is also used to estimate long term average velocities of groundwater flow within soft bottom sediments this flow makes up only about 1 of total groundwater inflow but may be important for the exchange of solutes across the sediment water interface in 1989 groundwater contributed about 60 of total freshwater input to the pond

relationship of dayside main layer ionosphere height to local solar time on mars and implications for solar wind interaction influence to understand the influence of solar wind on the daytime main layer ionosphere of mars we investigated the local solar time lst variations of three characteristic heights of the ionosphere namely the heights of the 1 mhz and 1 5 mhz reflection points i e 1 24 1010 m 3 and 2 79 1010 m 3 isodensity contours respectively and the density peak we used a total of 19 996 mars advanced radar for subsurface and ionosphere sounding observations distributed on the northern hemisphere with solar zenith angle 80 acquired from june 2005 to october 2013 we exploited the kernel partial least squares regression method to extract the nonlinear relationships of the heights to lst and a few other variables the average height of the 1 mhz reflection point decreased from 218 km at 10 00 a m to 206 km at 16 00 p m local time the height of the 1 5 mhz reflection point decreased simultaneously from 190 km to 181 km these decreasing trends are in contrast to the lst variation of the density peak height which increased from 128 km to 137 km over the same lst interval based on these findings and previous results we suggest that the solar wind may penetrate the martian ionosphere down to altitudes of about 50 km above the main density peak and may in conjunction with the asymmetric draping of the interplanetary magnetic field compress the upper part of the main ionosphere layer on the p m side 10 km more than on the a m side

radar soundings of the ionosphere of mars we report the first radar soundings of the ionosphere of mars with the marsis mars advanced radar for subsurface and ionosphere sounding instrument on board the orbiting mars express spacecraft several types of ionospheric echoes are observed ranging from vertical echoes caused by specular reflection from the horizontally stratified ionosphere to a wide variety of oblique and diffuse echoes the oblique echoes are believed to arise mainly from ionospheric structures associated with the complex crustal magnetic fields of mars echoes at the electron plasma frequency and the cyclotron period also provide measurements of the local electron density and magnetic field strength

linking regional sources and pathways for submarine groundwater discharge at a reef by electrical resistivity tomography 222rn and salinity measurements 1 submarine groundwater discharge sgd is an important component of the hydrologic cycle connecting terrestrial to marine environments sgd in fringing reefs and its consequences on biogeochemistry and ecology remain mostly unexplored the 222rn activity and salinity of seawater indicate a substantial groundwater contribution throughout most of the 20 km2 studied tropical reef in pangasinan philippines over 30 km of electrical resistivity profiles with a penetration depth of 12 5 m shows widespread zones within the reef that are much more resistive than porous reef rocks or sediment saturated with typical seawater some discrete resistive areas are located close to where seawater has 222rn peaks and where geologic lineaments are likely located suggesting that these are preferential pathways for fresher groundwater discharging to the reef sgd at the site could be a major ecological factor connecting the reef to the subsurface environment which in turn may lead to connections to land

vertical accumulation of potential toxic elements in a semiarid system that is influenced by an abandoned gold mine abstract the mining zone at el triunfo baja california sur mexico was exploited for gold extraction for 200 years this area includes more than 100 abandoned mining sites these sites contain mine tailings that are highly contaminated with potential toxic elements pte such as as cd pb sb zn and other associated elements over time these wastes have contaminated the sediments in the adjacent fluvial systems our aim was to assess the vertical pte variations in the abandoned mining zone and in the discharge of the main arroyo into a small lagoon at the pacific ocean sediments were collected from the two following locations in the mining zone near the arroyo basin tailings 1 an old alluvial terrace overbank and a test pit tp and 2 two sediment cores locations at the arroyo discharge into a hypersaline small lagoon samples were analyzed by icp ms icp oes and inaa and the methods were validated the overbank was the most contaminated and had as cd pb sb and zn concentrations of 8690 226 84 700 17 400 and 42 600 mg kg 1 respectively which decreased with depth in addition the tp contained elevated as cd pb sb and zn concentrations of 694 18 8 5001 39 2 and 4170 mg kg 1 respectively the sediment cores were less contaminated however the as cd pb sb and zn concentrations were greater than the concentrations that are generally found in the earth s crust the normalized enrichment factors nefs which were calculated from the background concentrations of these elements in the system showed that extremely severe as cd pb sb and zn nef 50 enrichment occurred at the overbank the tp was severe to very severely enriched with as cd pb sb and zn nef 10 50 the sediment cores had a severe enrichment of as pb and zn nef 10 25 their vertical profiles showed that anthropogenic influences occurred in the historic sediment deposition at the overbank and tp and in the sediment cores in addition the as pb and zn concentrations in the sediment cores were related to the deposition of fine sediments and organic carbon

magnetically controlled structures in the ionosphere of mars 1 the ionospheric sounding data obtained by the marsis mars advanced radar for subsurface and ionospheric sounding instrument on the mars express spacecraft show that the dayside ionosphere has considerable structure over regions of strong crustal magnetic fields this structure is typically seen as a hyperbola shaped trace in a display of echo intensity versus apparent altitude and time the hyperbola shapes are consistent with oblique reflections from regions of enhanced electron density that are fixed with respect to mars comparisons with the cain et al 2003 model for the crustal magnetic field of mars show that the apexes of the hyperbolas which identify the closest approach to the regions of enhanced electron density usually coincide with regions where the crustal magnetic field is strong and nearly vertical the electron density enhancements which extend as much as 50 km above the surrounding ionosphere are believed to arise from increases in the scale height of the ionosphere possibly due to heating of the ionosphere by solar wind electrons that reach the base of the ionosphere along the nearly vertical open magnetic field lines statistical analyses of the apparent altitudes of the apexes of the hyperbolas as well as analyses of repeated passes over the same region indicate that the electron density enhancements usually consist of horizontal cylinder like structures rather than isolated hemispherical structures in many cases the axes of the cylindrical density structures are aligned with the symmetry axes of adjacent cylindrical magnetic field structures with opposite polarity

breathing of a coral cay tracing tidally driven seawater recirculation in permeable coral reef sediments 1 coral reefs are characterized by high gross productivity in spite of low nutrient concentrations this apparent paradox may be partially reconciled if seawater recirculation in permeable sediments over large meters and long hours to days scales is an important source of recycled nitrogen and phosphorus to coral reefs in this paper we use radon 222rn a natural tracer to quantify tidally driven pore water or groundwater exchange between 1 an offshore coral cay island and its fringing reef lagoon and 2 a reef lagoon and the surrounding ocean as seawater infiltrates heron island at high tide it acquires a radon signal that can be detected when pore waters emerge from carbonate sands at low tide a nonsteady state model indicated that vertical pore water upwelling rates or saline submarine groundwater discharge were 40 cm d within the reef lagoon and 100 cm d outside the lagoon at low tide within the lagoon tidal pumping and temperature driven convection were the main driving forces of pore water advection at low tide the reef lagoon level is about 1 m higher than the surrounding ocean as a result a steep hydraulic gradient develops at the reef edge driving unidirectional filtration through the reef framework groundwaters were highly enriched in nitrate average of 530 mol likely influenced by bird guano relative to lagoon waters 1 9 mol rough but conservative estimates indicated that groundwater derived nitrate fluxes 7 9 mmol m2 d can replace the entire lagoon nitrate inventory every 19 days we speculate that as offshore coral islands breath inhale seawater at high tide and exhale groundwater at low tide they release nutrients that lead to sustained productivity within coral reefs

early diagenetic processes in the muddy sediments of the bay of biscay abstract in order to understand the early diagenesis processes occurring in continental margin environment modern sediments collected in six different sites from the bay of biscay have been studied these sites can be separated into two groups in the shallowest stations where sediments are highly bioturbated organic carbon levels are higher than 2 in the deepest stations sediments are much less bioturbated and organic carbon levels are lower in all sites the vertical distribution of redox sensitive species can be explained by the well established depth sequence of redox reactions based on the bacterially mediated oxidation of organic matter we have considered some alternative reaction pathways to explain the profiles of fe mn and n species these reactions deal with the ammonia oxidation by manganese oxide the aerobic denitrification and the oxidation of dissolved iron ii by nitrate or mn oxides vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the mn oxides and the oxidation of all the upward diffusing fe ii they may also be responsible for a significant part of the total dinitrogen production the relative importance of these pathways on early diagenetic processes and benthic fluxes has not been determined and must be examined with additional experimental works our study suggests however that the coupling between the benthic cycles of iron manganese and nitrogen could strongly influence the carbon cycling at the ocean floor

a wave induced transport process in marine sediments we show how surface wave action can increase the rate of transport of solutes into a sandy seabed by orders of magnitude via a mechanism known as mechanical dispersion it is most effective for large sediment permeability and thickness high surface wave amplitude and shallow water a method for setting up the appropriate transport equation valid when dispersion is well developed is given its dispersion term contains two mechanical dispersion parameters that can be estimated roughly from existing data when the sediments are well sorted the dispersion can be inhomogeneous and anisotropic in homogeneous isotropic sediments the effect of surface wave action on transport into sediments on the eastern u s shelf is shown to be significant under certain conditions the effect on thawing of subsea permafrost beneath prudhoe bay alaska seems negligible

cycling of water through the sediment water interface by passive ventilation of relict biological structures abstract the biogeochemical environment of benthic sediments is affected by seawater transport across the sediment water interface and within the sediment pore spaces water circulation through the near surface sediments is produced by passive pumping of relict biostructures such as worm tubes and burrows this passive ventilation results from hydrodynamic interactions between structures that protrude from the sediment surface and flowing water in the overlying boundary layer flow over the end of a cylindrical worm tube projecting above the sediment surface into the benthic boundary layer results in lowered pressure in the tube and in sediments around the bottom end of the tube this induces flow of surface water through the adjacent sediment and back to the surface through tube a physical mechanism for this passive circulation is presented a mathematical model of interstitial water circulation produced by passive hydrodynamic effects has been solved analytically for steady flow in homogeneous isotropic sediments modflow a finite difference groundwater flow model is used to numerically determine the hydraulic gradients induced and the resulting flow through sediments near a structure under various conditions the magnitude of passive irrigation fluxes depends on the size and position of relict structures velocity of overlying surface water hydraulic conductivity of the sediments and the spatial distribution of tubes for a typical sandy sediment the hydraulic head in sediments near the bottom of a tube can be reduced by up to several centimeters and the zone of reduced head can extend over several hundred square centimeters around the tube surface water is drawn into the sediment throughout this area with a mean influx velocity of 10 3 cm s or greater yielding discharge rates on the order of tens to hundreds of milliliters per hour these passive fluxes are of the same magnitude as active pumping rates significantly greater than molecular diffusive fluxes and may represent a major driving mechanism for cycling water through surface sediments

transport and degradation of phytoplankton in permeable sediment in flume and field experiments we demonstrate that interfacial water flows generated when bottom currents interact with sea bed topography provide a fast and efficient pathway for the transport of suspended phytoplankton into subsurface layers of permeable sandy sediments the advective transport associated with small mounds and ripples as commonly found on shelf sediments increased penetration depth of unicellular algae dunaliella spec into sandy sediment permeability k 5 4 3 10 211 m 2 up to a factor of 7 and flux up to a factor of 9 relative to a smooth control sediment the pore water flow field produced a distinct distribution pattern of particulate organic matter in the sediment with subsurface concentration maxima and zones depleted of algae flux chamber simulations of advective transport of algae into sands of different grain sizes revealed increasing fluxes algal penetration depths and degradation rates with increasing permeability of the sediment two experiments conducted in intertidal sand flats confirmed the importance of the advective interfacial transport of phytoplankton for natural settings showing permeability dependent penetration of planktonic algae into embedded sand cores of different grain sizes the significance of our results is discussed with respect to particulate organic matter flux and mineralization in shelf sands and we suggest the concept of a decomposition layer in contrast to muddy sea beds with low permeabilities where transport of solutes is mainly driven by diffusion water can flow through marine sands providing a fast carrier for the exchange of substances between the water column and the upper sediment layers surface gravity waves cause pressure oscillations that increase fluid exchange at the sediment water interface and dispersion of solutes within the

metal fractionation in oxic sediments of an estuary affected by acid mine drainage south western spain abstract fractionation of fe cu zn as cd and pb was studied in 17 surface sediment samples of the ria of huelva in all the cases total concentration of these elements was high especially for fe 11 2 zn 1519 ppm followed by cu 1413 ppm pb 716 ppm as 442 ppm and cd 3 26 ppm the enrichment factors of these metals follow this order cd cu zn pb as fe with respect to the regional background the highest concentrations associated with exchangeable or labile fractions are shown by zn 85 of total concentration followed by cd 76 12 fe 58 41 cu 40 80 pb 37 44 and as 33 88 the main sources of metal input are the tinto and odiel rivers which discharge acidic water ph

spatial periodicity in bed form scale solute and thermal transport models of the hyporheic zone spatially periodic solute boundaries force symmetry across a model domain by ensuring that concentrations and concentration gradients are identical at the same location on opposite boundaries they have been used in multiple publications on a hyporheic zone model of a single ripple or dune style bed form including variable density flow and reactive transport variants we evaluate simulations of multibed form models without imposing spatially periodic transport to demonstrate that nonphysical solute distributions arise from the periodic solute transport assumption that is the flow field within the single bed form model leads to a transport scenario that violates the forced symmetry of periodic solute boundary conditions culminating in a physically unrealistic solute distribution our results show that lack of symmetry between boundaries is a function of the vertical concentration gradient and two dimensionless parameters characterizing the hyporheic and underflow flow regimes and the solute exchange between them we assess the error associated with the spatially periodic assumption based on an analysis of solute fluxes across the lateral bed form model boundaries while the focus is on steady state concentration distributions the implications for transient solute transport models are also discussed we conclude that periodic solute transport boundary conditions should be applied only to bed form models that have minimal vertical dispersive and diffusive solute transfer this includes gaining systems and tracers such as temperature for which a temporally periodic flux reversal occurs across the top boundary

tide driven deep pore water flow in intertidal sand flats sulfidic seeps with methane ebullition were observed at the low water line of intertidal sand flats at a number of locations in the wadden sea bioturbating fauna was absent in the seep areas but abundant in the more central areas of the tidal flat at one site the vertical methane and sulfate distribution in pore water was determined along transects from the low water line toward the interior of the sand flat the resulting two dimensional distributions showed a plume of methane rich and sulfate depleted pore water reaching from a depth below 1 2 m beneath the sand surface up to the sediment surface at the low water line the 13c of methane released at the seeps was 68 6 indicating a biological origin the 14c signature of methane was clearly elevated by anthropogenic radiocarbon which shows that the methane was formed less than 50 yr ago the observations indicate an internal circulation where water enters the sand flats in the central area and exits at the low water line pore water flow patterns in the sand flat during the tidal cycle were calculated from the surface topography and from the pressure distribution at the flat surface across the tidal cycle the calculated flow patterns explain the measured methane and sulfate distributions and predict a residence time of the seepage water of about 30 yr intertidal sand flats act as one way valves passing water from the central surface through the interior of the flat to an outflow zone at and below the low water line with a velocity of millimeters to centimeters per day the flow causes permeable tidal flats to emit methane to the surface water and atmosphere in substantial amounts 2008 by the american society of limnology and oceanography inc

arsenic from gold mining in marine and stream sediments in baja california sur mexico the environmental impacts associated with arsenic as from historical mining areas may persist for many years we evaluated the influence of as in smelter and tailing wastes from a historic gold mine near san antonio in the south east of the baja california mexico on fluvial and marine sediments in the san juan de los planes basin and la ventana bay respectively arsenic levels in the smelter and in the surrounding tailing wastes ranged from 82 000 to 208 000 mg kg 1 and from 4220 to 92 000 mg kg 1 respectively the as concentrations in fluvial sediments were much lower as these derived from wastes that had been scattered and had been subject to weathering for some time as concentrations in surface sediments in the san antonio stream close to the smelter and further downstream ranged from 477 to 1100 mg kg 1 and from 64 to 613 mg kg 1 respectively as concentrations were between 0 05 and 9 mg kg 1 in surface shelf sediments from the coast adjacent to la ventana bay the concentration was highest in sediment collected close to the outlet of the main stream la bocana of the los planes basin the as concentrations in all the solid waste samples from near san antonio exceeded the effect range medium erm of the sediment quality guidelines 70 mg kg 1 overall 71 and 43 of the stream samples exceeded the effect range low erl 8 2 mg kg 1 and the erm respectively indicating potential risks for biota and humans

magnitudes of submarine groundwater discharge from marine and terrestrial sources indian river lagoon florida 1 magnitudes of terrestrial fresh and marine saline sources of submarine groundwater discharge sgd are estimated for a transect across indian river lagoon florida two independent techniques seepage meters and pore water cl concentrations show terrestrial sgd decreases linearly to around 22 m offshore and these techniques together with a model based on the width of the outflow face indicate a cumulative discharge of between 0 02 and 0 9 m3 d per meter of shoreline seepage meters and models of the deficiencies in 222rn activity in shallow sediments indicate marine sgd discharges of roughly 117 m3 d per meter of shoreline across the entire 1800 m wide transect two surface streams nearest the transect have an average discharge of about 28 m3 d per meter of shoreline marine sgd is thus 4 times greater then surface water discharge and more than 2 orders of magnitude greater than terrestrial sgd the magnitude of the terrestrial sgd is limited by the amount of regional precipitation evaporation recharge and groundwater usage while marine sgd is limited only by processes circulating marine water into and out of the sediments the large magnitude of marine sgd means that it could be important for estuarine cycling of reactive components such as nutrients and metals with only slight modification from estuarine water compositions the small magnitude of terrestrial sgd means that large differences from estuarine water composition would be required to affect chemical cycling

dayside ionosphere of mars empirical model based on data from the marsis instrument 1 we present results of a systematic study of electron densities in the dayside martian ionosphere measured by the mars advanced radar for subsurface and ionosphere sounding marsis instrument on board the mars express spacecraft there are two distinct regions controlled by different physical mechanisms the first region is located at altitudes up to about 5 neutral scale heights above the altitude of peak electron density electron densities in this region are well described by the basic chapman theory the observed small deviations can be most probably explained by the neutral scale height and electron temperature increasing with altitude rather than being constant the second region is located at altitudes higher than about 10 neutral scale heights above the altitude of peak electron density it is controlled primarily by diffusion and the observed electron densities decrease exponentially with increasing altitude the corresponding diffusion scale height increases with increasing solar zenith angle which can be probably explained by nearly horizontal magnetic fields in the ionosphere induced by interaction with the solar wind the obtained dependencies can be used as a simple empirical model of the dayside martian ionosphere

saltwater freshwater mixing dynamics in a sandy beach aquifer over tidal spring neap and seasonal cycles the biogeochemical reactivity of sandy beach aquifers is closely linked to physical flow and solute transport processes thus a clearer understanding of the hydrodynamics in the intertidal zone is needed to accurately estimate chemical fluxes to the marine environment a field and numerical modeling study was conducted over a 1 year timeframe to investigate the combined effects of tidal stage spring neap variability in tidal amplitude and seasonal inland water table oscillations on intertidal salinity and flow dynamics within a tide dominated microtidal sandy beach aquifer measured and simulated salinities revealed an intertidal saline circulation cell with a structure and cross sectional mixing zone area that varied over tidal spring neap and seasonal time scales the size of the circulation cell and area of the mixing zone were shown for the first time to be most affected by seasonal water table oscillations followed by tidal amplitude and tidal stage the intertidal circulation cell expanded horizontally and vertically as the inland water table declined displacing the fresh discharge zone and lower interface seaward over monthly spring neap cycles the center of the circulation cell shifted from beneath the backshore and upper beachface to the base of the beach salinity variations in the intertidal zone over semidiurnal tidal cycles were minimal the dynamics of the circulation cell were similar in simulations with and without a berm the highly transient nature of intertidal salinity over multiple time scales may have important implications for the types and rates of chemical transformations that occur in groundwater prior to discharge to the ocean

mars global surveyor radio science electron density profiles neutral atmosphere implications the mars global surveyor mgs radio sci ence rs experiment permits retrieval of electron density prof iles versus height 90 200 km from occultation mea surements an initial set of electron profiles is examined spanning high northern latitudes early morning solar local times and high solar zenith angles 78 to 81 near aphelion sampling for these 32 profiles is well distributed over longi tude the height of the photochemically driven ionospheric peak is observed to respond to the background neutral den sity structure with a mean height during this season at this location of 134 4 km strong wave 3 oscillations about this mean are clearly observed as a function of longitude and correspond to neutral density variations measured by the mgs accelerometer acc experiment the wave 3 tidal pattern implicated by both the rs and acc datasets is consistent with a semi diurnal wave frequency clearly the height of the martian dayside ionospheric peak is a sensitive indicator of the state of the underlying mars atmosphere this ionospheric peak height can be used as a proxy of the longitude specific non migrating tidal variations present in the mars lower thermosphere

arsenic content and groundwater geochemistry of the san antonio el triunfo carrizal and los planes aquifers in southernmost baja california mexico the san antonio el triunfo mining district located at a mountainous region 45 km south east of la paz baja california has been worked since the late 1700s mine waste material produced during 200 years of mineral extraction area poses a risk of local groundwater pollution and eventually regional pollution to the carrizal west basin and the los planes east basin aquifers there are different types of deposits in the mining area these are dominated by epithermal veins in which arsenopyrite is an important component carrillo and drever 1998a concluded that even though the amount of mine waste is relatively small in comparison to the large scale area significant as in groundwater derived from the mine waste piles is found locally in the groundwater this paper shows the results of geochemical analyses of groundwater samples from the san antonio el triunfo area and the carrizal and los planes aquifers during several years of monitoring 1993 1997 the highest values of total dissolved solids tds and as are in the mineralized area where the mining operations occurred 1500 ppm tds and 0 41 ppm as the lowest concentrations of tds and as are in general away from the mineralized area 500 ppm tds and 0 01 ppm as sulfate and bicarbonate alkalinity are in general high near the mineralized area and low away from it the arsenic concentrations vary seasonally especially after the heavy summer thunderstorms geochemical modeling minteqa2 and netpath and analysis of the regional geochemical evolution of the groundwater from the mining area towards the aquifer of los planes shows that the most likely hydrochemical processes include dilution precipitation of calcite and adsorption of as onto surfaces of iron oxyhydroxides ferrihydrite these processes act as natural controls to the extent and amount of as pollution in the carrizal and los planes aquifers

nutrient biogeochemistry in a gulf of mexico subterranean estuary and groundwater derived fluxes to the coastal ocean we hypothesize that biogeochemical processes in a subterranean estuary in the northeastern gulf of mexico significantly alter nutrient concentrations that are discharged into the ocean via submarine groundwater discharge sgd this subterranean estuary is a site of steep gradients over small spatial scales so selecting the appropriate groundwater endmember for flux calculations is difficult two endmember mixing models typically applied to investigate biogeochemical processes in surface estuaries are not useful in this case because of aquifer heterogeneity and complex solid fluid interactions groundwater residence times redox potential source strength and aquifer matrix appear to be the most important drivers for dissolved organic carbon doc and nutrient concentrations in this pristine coastal plain aquifer the distribution of dissolved species was consistent with a sequence of reactions in a narrow 2 m surface layer in which nitrate is initially exhausted likely because of denitrification organic nitrogen is remineralized releasing ammonium and some doc remains on the basis of a 222rn derived non steady state box model total terrestrial marine sgd rates were estimated to oscillate around 11 cm d21 the sgd is composed almost entirely of recirculated seawater accounting for 95 of total sgd fluxes conservative estimates of groundwater derived nitrogen inputs into the coastal ocean were 8 2 6 1 2 mmol m22 d21 consisting of ammonium 58 organic nitrogen 28 and nitrate 14 we suggest that these mainly represent recycled rather than new nutrient additions when extrapolated to the florida gulf coast the nitrogen and doc fluxes were comparable to the main regional rivers supporting the proposition that sgd plays a major role in coastal biogeochemistry the biogeochemical implications of submarine groundwater discharge sgd into coastal environments are becoming increasingly recognized globally groundwater discharge has been estimated to be only a few percent of the total freshwater flux to the oceans burnett et al 2006 in spite of that small fraction the input of nutrients to coastal environments via groundwater pathways could be globally more important because nutrient concentrations in groundwater are often much higher than those in river water in addition high dissolved n p ratios in groundwater relative to surface waters might drive the coastal ocean toward p limitation within the coming decades perhaps changing the present n limited coastal primary production slomp and van cappellen 2004

solid phases controlling the mobility of potentially toxic elements and the generation of acid drainage in abandoned mine gold wastes from san antonio el triunfo mining district baja california sur mexico in many countries of latin america mexico included as and other toxic elements in water and soils may have a natural geogenic origin however the anthropogenic activities specially in mining areas trigger the metal loid s mobilization mainly by acid mine drainage aggravating the environmental pollution problem in san antonio el triunfo mining district sa et md baja california sur state mexico 200 years of intermittent mining activities have left around 800 000 tons of mine waste scattered across 400 km2 of the sa et area some environmental studies in the area assume mining wastes as the main source of contamination of groundwater not considering the geothermal source moreover recent studies have shown that the pollution in the region has occurred as consequence of seasonal erosion caused by winds and irregular episodes of heavy rainfall nevertheless the actual potential to release as and other potentially toxic elements as well as the hazardousness derived from this mine waste and the geochemical mineralogical mechanism that influence such mobilization has not been assessed yet through the detailed physicochemical and mineralogical characterization x ray diffraction and scanning electron microscopy this novel study shows that jarosite and arsenolite are the key mineral phases controlling the potential release and arsenic mobilization and therefore the hazardousness of the mining wastes what is more from ph and acid base accounting results it has been found that 35 of the analyzed samples maintain basic to near neutral conditions due to the neutralizing potential of aluminosilicates feldspar muscovite etc while in the remaining 65 of analyzed samples the mineral composition is not enough to neutralize the generated acidity hence those samples present the potential of generating acid mine drainage and consequently they are considered hazardous

geologic effects on groundwater salinity and discharge into an estuary summary submarine groundwater discharge sgd can be an important pathway for transport of nutrients and contaminants to estuaries a better understanding of the geologic and hydrologic controls on these fluxes is critical for their estimation and management we examined geologic features porewater salinity and sgd rates and patterns at an estuarine study site seismic data showed the existence of paleovalleys infilled with estuarine mud and peat that extend hundreds of meters offshore a low salinity groundwater plume beneath this low permeability fill was mapped with continuous resistivity profiling extensive direct sgd measurements with seepage meters n 551 showed fresh groundwater discharge patterns that correlated well with shallow porewater salinity and the hydrogeophysical framework small scale variability in fresh and saline discharge indicates influence of meter scale geologic heterogeneity while site scale discharge patterns are evidence of the influence of the paleovalley feature beneath the paleovalley fill fresh groundwater flows offshore and mixes with saltwater before discharging along paleovalley flanks on the adjacent drowned interfluve where low permeability fill is absent fresh groundwater discharge is focused at the shoreline shallow saltwater exchange was greatest across sandy sediments and where fresh sgd was low the geologic control of groundwater flowpaths and discharge salinity demonstrated in this work are likely to affect geochemical reactions and the chemical loads delivered by sgd to coastal surface waters because similar processes are likely to exist in other estuaries where drowned paleovalleys commonly cross modern shorelines the existence and implications of complex hydrogeology are important considerations for studies of groundwater fluxes and related management decisions

estimation of the total electron content of the martian ionosphere using radar sounder surface echoes 1 the martian ionosphere s local total electron content tec and the neutral atmosphere scale height can be derived from radar echoes reflected from the surface of the planet we report the global distribution of the tec by analyzing more than 750 000 echoes of the mars advanced radar for subsurface and ionospheric sounding marsis this is the first direct measurement of the tec of the martian ionosphere the technique used in this paper is a novel transmission mode sounding of the ionosphere of mars in contrast to the active ionospheric sounding experiment ais on marsis which generally operates in the reflection mode this technique yields a global map of the tec for the martian ionosphere the radar transmits a wideband chirp signal that travels through the ionosphere before and after being reflected from the surface the received waves are attenuated delayed and dispersed depending on the electron density in the column directly below the spacecraft in the process of correcting the radar signal we are able to estimate the tec and its global distribution with an unprecedented resolution of about 0 1 deg in latitude 5 km footprint the mapping of the relative geographical variations in the estimated nightside tec data reveals an intricate web of high electron density regions that correspond to regions where crustal magnetic field lines are connected to the solar wind our data demonstrates that these regions are generally but not exclusively associated with areas that have magnetic field lines perpendicular to the surface of mars as a result the global tec map provides a high resolution view of where the martian crustal magnetic field is connected to the solar wind we also provide an estimate of the neutral atmospheric scale height near the ionospheric peak and observe temporal fluctuations in peak electron density related to solar activity

simultaneous ground satellite observations of structured pc 1 pulsations structured pc 1 pulsations are investigated using simultaneous multipoint ground satellite observations recorded on september 10 1986 during viking orbit 1103 the multipoint pc 1 observations were acquired using the viking magnetic field experiment at 13 550 km altitude from l 5 1 to 5 5 and three finnish ground stations at rovaniemi ivalo and kilpisjarvi these stations are all located within 30 min magnetic local time and 1 in latitude of the ionospheric field line footprint of the pc 1 source field line structured pc 1 pulsations between 0 5 and 1 0 hz were observed both on the ground and in space at similar frequencies with similar frequency dispersion and with a similar repetition period the wave source region based on viking langmuir probe observations was just inside the plasmapause the wave transit time between viking and the ground was 12 2 s where viking leads the ground this implies that the waves propagate downward from viking to the ground consistent with previous downward poynting flux estimates

quantification of point source groundwater discharges to the ocean from the shoreline of the big island hawaii aerial thermal infrared imaging has revealed that submarine groundwater discharge sgd along the western coast of the big island of hawaii is often focused as point source discharges that create buoyant groundwater plumes that mix into the coastal ocean we quantified the sgd fluxes associated with several plumes using natural geochemical tracers offshore transects of 222rn and 224ra show elevated activities and corresponding low salinities in the nearshore waters within the plumes indicating that these naturally occurring radionuclides can be useful tracers of groundwater inputs in this area using a series of simultaneous mass balance equations for water salt and radon we determined the groundwater fluxes of six plumes near kona hawaii the average sgd fluxes ranged from 1100 m3 d21 to 12 000 m3 d21 of total fresh saline sgd the fresh meteoric groundwater equivalents for the same flows modeled by adjusting the groundwater end member to reflect freshwater rather than brackish groundwater composition ranged from 630 m3 d21 to 8600 m3 d21 these fluxes are in general agreement with earlier results obtained by hydrological water budgets and physical oceanographic analyses of fresh sgd rates in this region

transient layers in the topside ionosphere of mars 1 radar soundings from the marsis instrument on board the mars express spacecraft have shown that distinct layers can occur in the topside ionosphere of mars well above the main photo ionization layer these layers appear as cusps or sometimes steps in plots of the time delay as a function of frequency usually only one topside layer is observed typically at altitudes from 180 to 240 km however occasionally an additional layer occurs at even higher altitudes the layers are transient features and are present about 60 of the time near the subsolar point decreasing with increasing solar zenith angle to less than 5 at the terminator and the nightside the transient nature of the layers suggests that they are produced by a dynamical process most likely involving an interaction with the solar wind in the upper levels of the ionosphere