comparison of radar and in situ measurements of atmospheric turbulence 1 we compare measurements of refractive index structure constant cn2 and energy dissipation rate e by vhf radar with in situ observations by high resolution thermosondes during a campaign near adelaide australia in august 1998 a total of 17 thermosonde soundings was performed within a distance of 120 km to the radar site the variance between estimates of cn2 from backscattered radar power and in situ thermosonde observations is found to be comparable to the observed variance of the radar estimates due to temporal and spatial variations of the turbulent parameters angle brackets denote average over the radar range resolution of 1 km the high resolution thermosonde observations allow us to test the applicability of the turbulent volume fraction model to the estimation of e from cn2 the data indicate that the turbulent volume fraction should not be modeled as the fraction of unstable layers with gradient richardson number ri 0 25

how paleosols influence groundwater flow and arsenic pollution a model from the bengal basin and its worldwide implication in the bengal basin the land surface exposed during the last lowstand of sea level around 20 ka and now buried by holocene sediment is capped by an effectively impermeable clay paleosol that we term the last glacial maximum paleosol lgmp the paleosol strongly affects groundwater flow and controls the location of arsenic pollution in the shallow aquifers of our study site in southern west bengal and by implication in shallow aquifers across the bengal basin and as polluted deltaic aquifers worldwide the presence of the lgmp defines paleointerfluvial areas it is absent from paleochannel areas a paleosol model of pollution proposed here predicts that groundwater in paleochannels is polluted by arsenic while that beneath paleointerfluvial areas is not paleointerfluvial aquifers are unpolluted because they are protected by the lgmp from downward migration of arsenic and from downward migration of organic matter that drives as pollution via reductive dissolution of as bearing iron oxyhydroxides horizontal groundwater flow carries arsenic from paleochannels toward paleointerfluvial aquifers in which sorption of arsenic minimizes the risk of pollution

evidence for widespread nonlinear strong ground motion in the mw 6 9 loma prieta earthquake we exploit 55 repeating microearthquake sequences on the san andreas fault just south of the rupture zone of the 1989 m w 6 9 loma prieta earthquake to search for time dependent properties of the earth s crust using moving window waveform cross correlation we identify clear and systematic delays as large as 20 msec for the direct s wave and exceeding 50 msec in the early s wave coda following the loma prieta mainshock others have also identified phase delays velocity reductions associated with damaging earthquakes and they have suggested a myriad of possible causal mechanisms here we present new evidence for a mechanism to produce velocity reductions correlated in time and space with an earthquake a strong correlation between the spatial patterns of s delays and the intensity of strong ground motion in the loma prieta earthquake suggests that physical damage the formation or growth of cracks during strong ground motion to the earth s shallow crust is responsible for the observed velocity reductions the strong spatial variability in s delays over short distances and the strong correlation of the magnitude of delays with surface geology indicate that the phase delays accumulate primarily near the receiver the effect is stronger at stations on young soft rocks than at stations on old hard rock disproportionately larger s coda delays than p coda delays suggest that the cracks formed by the strong shaking are fluid filled in the 10 years after loma prieta the initial increase in travel times reduces logarithmically with respect to time often back to the premainshock levels we attribute this behavior to the same slow dynamic healing observed in laboratory studies of the recovery of materials from transient nonlinear strain manuscript received 14 january 2004

integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the mekong delta abstract arsenic is contaminating the groundwater of holocene aquifers throughout south and southeast asia to examine the biogeochemical and hydrological processes influencing dissolved concentrations and transport of as within soils sediments in the mekong river delta a 50 km2 field site was established near phnom penh cambodia where aqueous as concentrations are dangerously high and where groundwater retrieval for irrigation is minimal dissolved as concentrations vary spatially ranging up to 1300 g l in aquifer groundwater and up to 600 g l in surficial clay pore water groundwaters with high as concentrations are reducing with negligible dissolved o2 and high concentrations of fe ii nh 4 and dissolved organic c within near surface environments these conditions are most pronounced in sediments underlying permanent wetlands often found within oxbow channels near the mekong river there labile c co deposited with as bearing fe hydr oxides under reducing conditions drives the reductive mobilization inclusive of fe and as reduction of as here conditions are described under which as is mobilized from these sediments and near surface as release is linked to aquifer contamination over long time periods 100s to 1000s of years site biogeochemistry is coupled with extensive hydrologic measurements and accordingly a comprehensive interpretation of spatial as release and transport within a calibrated hydraulic flow field is provided of an as contaminated aquifer that is representative of those found throughout south and southeast asia

groundwater flow dynamics and arsenic source characterization in an aquifer system of west bengal india numerical groundwater flow modeling reverse particle tracking and environmental tracers are used to locate the source of geogenic as affecting an aquifer in west bengal the aquifer is hosted by point bar sands deposited in a meandering fluvial environment wells tapping the aquifer exhibit as concentrations up to 531 g l high as groundwaters are recharged in ponds marking an abandoned river channel the source of as is traced to the underlying fine grained channel fill sediments arsenic release within these sediments is accompanied by a concomitant release of br and doc indicating that these species may be decay products of natural organobromines codeposited along with as mass transfer of as to the dissolved phase and its flushing from source sediments are described using a simplified reactive solute transport model based on this model a characteristic reaction time for mass transfer is estimated at 6 7 years average groundwater residence times in the source are estimated to have declined from 16 6 to 6 6 years with the advent of intensive irrigation pumping the ratio of residence and reaction times a damkohler number has declined correspondingly from 2 49 to 0 99 indicating a shift from transport to reaction rate limited as mobilization greater insight into the as problem in se asia may be achieved by shifting the focus of field investigations from aquifers to potential contamination sources in aquitards

retardation of arsenic transport through a pleistocene aquifer holocene aquifers are the source of much arsenic poisoning in south and southeast asia whereas pleistocene aquifers are mostly safe here the delayed arsenic contamination of a pleistocene aquifer is described and modelled millions of people across southeast asia are exposed to arsenic contaminated drinking water drawn from holocene aquifers layers of sand deposited less than 5 000 years ago by contrast pleistocene aquifers deposited about 12 000 years ago have lower levels of contamination and are increasingly being exploited as safe sources of drinking water this study reports the gradual penetration of arsenic into a low arsenic pleistocene aquifer south of hanoi vietnam changes in groundwater flow and the redox state of the aquifer sands induced by pumping are introducing contamination from the high arsenic holocene aquifer contamination so far is limited owing to the absorption of arsenic onto aquifer sands which delays arsenic movement by decades groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast asia exposes an estimated population of over a hundred million people to toxic levels of arsenic1 holocene aquifers are the source of widespread arsenic poisoning across the region2 3 in contrast pleistocene sands deposited in this region more than 12 000 years ago mostly do not host groundwater with high levels of arsenic pleistocene aquifers are increasingly used as a safe source of drinking water4 and it is therefore important to understand under what conditions low levels of arsenic can be maintained here we reconstruct the initial phase of contamination of a pleistocene aquifer near hanoi vietnam we demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a holocene aquifer into a previously uncontaminated pleistocene aquifer we also find that arsenic adsorbs onto the aquifer sands and that there is a 16 20 fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period our findings suggest that arsenic contamination of pleistocene aquifers in south and southeast asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport

reversible adsorption and flushing of arsenic in a shallow holocene aquifer of bangladesh the spatial heterogeneity of dissolved arsenic as concentrations in shallow groundwater of the bengal basin has been attributed to transport of as and reactive carbon from external sources or to the release of as from within grey sand formations we explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream within the 10 20 m depth range groundwater ages along the transect determined by the 3h 3he method increase from 10 yr in the recharge area to a maximum of 40 yr towards the stream concentrations of groundwater as within the same grey sands increase from 10 to 100 to 500 g l along this transect evidence of reversible adsorption of as between the groundwater and sediment was obtained from a series of push pull experiments traditional batch adsorption experiments and the accidental flooding of a shallow monitoring well assuming reversible adsorption and a distribution coefficient kd of 0 15 1 5 l kg inferred from these observations a simple flushing model shows that the increase in as concentrations with depth and groundwater age at this site and at other sites in the bengal and red river basins can be attributed to the evolution of the aquifer over 100 1000 years as aquifer sands are gradually flushed of their initial as content a wide range of as concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates without external inputs of reactive carbon or as from other sources

nonlinearity liquefaction and velocity variation of soft soil layers in port island kobe during the hyogo ken nanbu earthquake clear nonlinear behavior is analyzed from the acceleration records of the 1995 hyogo ken nanbu earthquake at port island kobe from four triaxial in struments placed at four different depths the surficial effects during strong ground motions were compared with those during weak motions before and after the main shock we used a spectral ratio technique and a nonlinear inversion for velocity structure to analyze the data from the spectral analysis we observed a large variation of the spectral ratios between the surface and different depths during the strong ground motions and during the liquefied state the spectral ratios after the mainshock i e after the liquefied state are different from those before the mainshock the peak frequencies in the spectral ratios after the mainshock are shifted to lower fre quencies with respect to those in the spectral ratios before the mainshock we in verted the s wave velocities using a genetic algorithm technique to determine the velocity structure before during and after the mainshock the s wave velocity struc ture before and after the mainshock was found to be different specifically the s wave velocity of the second layer 5 m to 16 m depth after the mainshock was 20 lower than before our analysis shows that the liquefied state remains at least 3 hr after the mainshock but no more than 24 hr the rigidity of the soil decreased close to zero when liquefaction happened and later increases gradually following a trend that resembles a consolidation curve the strong influence of nonlinearity during the mainshock yielded a big reduction of the horizontal surface ground motions so that the observed horizontal peak acceleration was only about 25 of the peak acceler ation expected from the linear theory however the nonlinear effects in the vertical peak acceleration were not significant

impact of local recharge on arsenic concentrations in shallow aquifers inferred from the electromagnetic conductivity of soils in araihazar bangladesh 1 the high degree of spatial variability of dissolved as levels in shallow aquifers of the bengal basin has been well documented but the underlying mechanisms remain poorly understood we compare here as concentrations measured in groundwater pumped from 4700 wells 22 m 75 ft deep across a 25 km2 area of bangladesh with variations in the nature of surface soils inferred from 18 500 measurements of frequency domain electromagnetic induction a set of 14 hand auger cores recovered from the same area indicate that a combination of grain size and the conductivity of soil water dominate the electromagnetic signal the relationship between pairs of individual em conductivity and dissolved as measurements within a distance of 50 m is significant but highly scattered r2 0 12 n 614 concentrations of as tend to be lower in shallow aquifers underlying sandy soils and higher below finer grained and high conductivity soils variations in em conductivity account for nearly half the variance of the rate of increase of as concentration with depth however when the data are averaged over a distance of 50 m r2 0 50 n 145 the association is interpreted as an indication that groundwater recharge through permeable sandy soils prevents as concentrations from rising in shallow reducing groundwater

groundwater dynamics and arsenic contamination in bangladesh although arsenic contaminated groundwater in bangladesh is a serious health issue little is known about the complex transient patterns of groundwater flow that flush solutes from aquifers and carry solutes into the subsurface hydrologic modeling results for our field site in the munshiganj district indicate that groundwater flow is vigorous flushing the aquifer over time scales of decades to a century and also transporting solute loads into the aquifer with recharge from ponds rivers and rice fields the combined hydrologic and biogeochemical results from our field site imply that the biogeochemistry of the aquifer system may not be in steady state and that the net effect of competing processes could either increase or decrease arsenic concentrations over the next decades modeling results suggest that irrigation has greatly changed the location timing and chemical content of recharge to the aquifer flushing water through the system more quickly and also cycling large fluxes of water through rice fields during the dry season that could mobilize arsenic from oxides in near surface sediments furthermore the hydrologic model reveals that ponds many of which have been excavated over the last 50 years now provide much of the groundwater recharge these ponds receive most of the waste from the villages and thus provide another potential source of organic carbon to the groundwater system

nonlinear ground motion amplification by sediments during the 1994 northridge earthquake it has been known since at least 1898 ref 1 that sediments can amplify earthquake ground motion relative to bedrock for the weak ground motion accompanying small earthquakes the amplification due to sediments is well understood in terms of linear elasticity hooke s law 2 but there has been a long standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes the view of geotechnical engineers based largely on laboratory studies is that hooke s law breaks down at larger strains causing a reduced nonlinear amplification seismologists on the other hand have tended to remain sceptical of this nonlinear effect mainly because the relatively few strong motion observations seemed to be consistent with linear elasticity although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances3 4 the significance of nonlinearity for the type of stiff soil sites found in the greater los angeles region remains unresolved5 here we report that ground motion amplification due to sediments for the main shock of the 1994 northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks these observations imply significant nonlinearity in such amplification and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites

evidence of decoupling between arsenic and phosphate in shallow groundwater of bangladesh and potential implications reductive dissolution of iron oxyhydroxides and reduction of arsenic are often invoked as leading causes of high dissolved as levels in shallow groundwater of bangladesh the second of these assumptions is questioned here by comparing the behavior as and phosphate p a structural analogue for as v which also adsorbs strongly to fe oxyhydroxides but is not subject to reduction the first line of evidence is provided by a detailed groundwater time series spanning two years for three wells in the 6 9 m depth range showing removal of as iii from shallow groundwater during the monsoon without of loss of p the data indicate a loss of 90 of the dissolved as from groundwater in the intermediate well relative to a level of 3 mol l as predicted by conservative mixing between groundwater sampled from the shallower and the deeper well in contrast p concentrations of 30 mol l in the intermediate well closely match the prediction from conservative mixing reduction therefore appears to inhibit the release of as to groundwater at this site relative to p instead of enhancing it a re analysis of existing groundwater as and p data from across the country provides a broader context for this finding and confirms that without reduction elevated concentrations of as would probably be even more widespread in shallow aquifers of bangladesh without providing definite proof x ray absorption spectroscopy of sediment from the time series site and elsewhere suggests that the loss of as from groundwater may be coupled to precipitation of as sulfide further study is needed to assess the implications of these observations for shallow aquifers that have been subjected to increased withdrawals for irrigation in recent decades

nonlinear dynamics granular media and dynamic earthquake triggering the landers earthquake of 1992 triggered a series of earthquakes in other parts of california focusing attention on the mechanism of remote triggering laboratory experiments on granular material under pressure suggest that triggering is a result of seismic waves impinging on a fault and inducing elastic nonlinear behaviour of the fault core accompanied by instantaneous weakening and failure the resulting softening to weakening model fits in well with field observations of real earthquakes the 1992 magnitude 7 3 landers earthquake triggered an exceptional number of additional earthquakes within california and as far north as yellowstone and montana1 2 3 since this observation other large earthquakes have been shown to induce dynamic triggering at remote distances for example after the 1999 magnitude 7 1 hector mine1 and the 2002 magnitude 7 9 denali4 earthquakes and in the near field as aftershocks5 the physical origin of dynamic triggering however remains one of the least understood aspects of earthquake nucleation1 2 3 4 5 the dynamic strain amplitudes from a large earthquake are exceedingly small once the waves have propagated more than several fault radii for example a strain wave amplitude of 10 6 and wavelength 1 m corresponds to a displacement amplitude of about 10 7 m here we show that the dynamic elastic nonlinear behaviour of fault gouge perturbed by a seismic wave may trigger earthquakes even with such small strains we base our hypothesis on recent laboratory dynamic experiments conducted in granular media a fault gouge surrogate6 7 from these we infer that if the fault is weak8 9 10 seismic waves cause the fault core modulus to decrease abruptly and weaken further if the fault is already near failure this process could therefore induce fault slip

first observations of turbulence parameters in the troposphere over the bay of bengal and the arabian sea using radiosonde 1 this paper discusses the turbulence characteristics of the free atmosphere over the bay of bengal and the arabian sea during the premonsoon season of 2006 the turbulence scaling concepts developed over the past decades for application to oceanic mixing are used for retrieving turbulence properties in the free atmosphere from high resolution soundings as described by clayson and kantha 2008 the variability in the vertical turbulence structure over a large spatial region is studied using this novel method for the first time investigations revealed the presence of a highly turbulent band at upper troposphere below the cold point tropopause cpt with significant enhancements in thorpe length and eddy diffusion coefficient this increased turbulence is studied in conjunction with the observations of the tropical tropopause layer ttl it is seen that the lower and upper boundaries of this turbulence band nearly coincide with the height where the potential temperature lapse rate begins to show a decrease lrhmin and cold point tropopause respectively the ttl is the transition regime from convectively dominated troposphere to radiatively dominated stratosphere and is characterized by a decrease in the static stability the enhanced zonal and meridional wind shear observed in this region shows that the stability in this region is further weakened because of the breaking of kelvin helmholtz waves which results in the enhancement of turbulence the study reveals the potential of high resolution soundings to be used in the investigations of turbulence in the free atmosphere

nonlinear and linear site response and basin effects in seattle for the m 6 8 nisqually washington earthquake we used recordings of the m 6 8 nisqually earthquake and its m l 3 4 aftershock to study site response and basin effects for 35 locations in seattle washington we determined site amplification from fourier spectral ratios of the recorded horizontal ground motions referenced to a soft rock site soft soil sites generally national earthquake hazard reduction program nehrp class e on artificial fill and young alluvium have the largest 1 hz amplifications factors of 3 7 for both the mainshock and aftershock these amplifications are correlated with areas of higher damage from the mainshock to major buildings and liquefaction there are several indications of nonlinear response at the soft soil sites for the mainshock ground motions despite relatively modest peak accelerations in the s waves of 15 22 g first the mainshock spectral ratios do not show amplification at 2 8 hz as do the aftershock spectral ratios spectral peaks at frequencies below 2 hz generally occur at lower frequencies for the mainshock spectral ratios than for the aftershock ratios at one soft soil site there is a clear shift of the resonant frequency to a lower frequency for the mainshock compared with the aftershock the frequency of this resonance increases in the coda of the mainshock record indicating that the site response during the weaker motions of the coda is more linear than that of the initial s wave three of the soft soil sites display cusped one sided mainshock accelerograms after the s wave these soft soil sites also show amplification at 10 20 hz in the s wave relative to the rock site that is not observed for the aftershock the cusped waveforms and 10 20 hz amplification are symptomatic of nonlinear response at the soft soil sites these sites had nearby liquefaction the largest amplifications for 0 5 hz occur at soft soil sites on the southern portion of the seattle basin stiff soil sites nehrp classes d and c on pleistocence age glacial deposits display similar spectral amplification for the mainshock and aftershock indicating approximately linear response the stiff soil sites generally have moderate amplification factors of 1 1 2 4 at 0 5 and 1 hz amplifications at 1 and 5 hz for all sites generally increase with decreasing shear wave velocity measured in the top 30 m v s 30 however larger amplifications at 0 5 and 1 hz for sites with similar v s 30 values are observed for sites in the seattle basin illustrating the amplification from the deeper 30 m sediments and the contribution from basin surface waves record sections for the mainshock and aftershock show that basin surface waves produce the peak velocities for many of the sites in the seattle basin and often dominate the amplitude at 1 hz and lower frequencies

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

s wave velocity structure of sediments in anchorage alaska estimated with array measurements of microtremors the array measurements of microtremors were carried out at nine sites to estimate the subsurface s wave velocity structures of the sedimentary deposits beneath the metropolitan area of anchorage alaska the data were recorded by ten three component accelerometers arranged in a triangular manner for three different array sizes the phase velocities c f were estimated at each site from the vertical component of the recorded microtremor data by using the frequency wavenumber technique the c f data from different arrays were combined after checking their consistency for a series of overlapped frequency bands from different arrays and were inverted using a stochastic least squares inversion technique to estimate the 1d structure underneath each site the inversion results show that the engineering basement 750 m sec lies at a relatively shallower depth 40 m in the eastern part of the basin along the foothills of the chugach mountains cm and at deeper depths toward the southcentral 100 m and western 150 m parts of the basin in accord with the general dip of the basin below the engineering basement a well developed low velocity zone lvz with values in the range of 900 1040 m sec is found to be present in the eastern as well as along the knik arm side in the western part of the basin at a depth of 200 m and 900 m respectively moreover the central part of the basin is associated with a weakly developed lvz below the engineering basement depth in the rest of the basin the value increases gradually with depth the spatial variations of in the basin at different depths from 20 to 500 m are represented by using 2d interpolation of the structures obtained from the inversion of c f data the depth to the crystalline basement of the basin however could not be ascertained and it seems to be much deeper than the maximum depth 2000 m resolved by the data gathered in this study to validate the results of inversion the spectral ratio between the horizontal and vertical components h v of the recorded microtremor data at each site has been compared with the computed h v of rayleigh waves based on the respective structure the results showed good agreement in the frequency range of about 0 4 6 0 hz in this frequency range the h v peaks are due to the overall effect of the velocity contrasts between layers representing the subsurface structure

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

the impact of aquifer flushing on groundwater arsenic across a 35 km transect perpendicular to the upper brahmaputra river in assam india well testing in the floodplain of the brahmaputra river in golaghat and jorhat districts of assam india shows that groundwater arsenic as concentrations increase with distance from the river to establish the origin of this pattern an additional 900 wells 60 m deep were tested for as and 9 sites were drilled along a 35 km transect perpendicular to the river the field data show no relation between groundwater as concentrations ranging from 1 to 660 g l along the transect and a as concentrations of 1 5 mg kg in cuttings of aquifer sand recovered while drilling or b the degree of reduction of iron oxides in these cuttings the drilling indicates however a marked increase in the thickness of a clay layer capping the aquifer starting from 1 5 m near the river to over 60 m at the most distant site towards the base of the naga foothills organic radiocarbon ages of 18 46 kyr obtained from all but one of 13 clay samples indicate pre holocene deposition of the underlying sands across the entire transect radiocarbon ages of dissolved inorganic carbon of 0 2 4 7 and 17 8 kyr were measured in groundwater from 3 monitoring wells installed to 30 60 m depth at distances of 10 20 and 40 km from the river respectively a conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and as a result preserved higher as levels in groundwater

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

hysteretic and dilatant behavior of cohesionless soils and their effects on nonlinear site response field data observations and modeling in this study we present evidence that nonlinearity can be directly observed in acceleration time histories such as those recorded at the wildlife refuge and kushiro port downhole arrays from the 1987 superstition hills california and the 1993 kushiro oki japan earthquakes respectively these accelerograms and others compiled in this study present a characteristic waveform composed of intermittent high frequency peaks riding on a low frequency carrier in addition soil amplification of the surface records is strongly observed compared to their downhole counterpart this is contrary to the expected amplification reduction produced by the nonlinear soil behavior laboratory studies show that the physical mechanism that produces such phenomena is the dilatant nature of cohesionless soils which introduces the partial recovery of the shear strength under cyclic loads this recovery translates into the ability to produce large deformations followed by large and spiky shear stresses the spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure these results are significant in strong motion seismology because these spikes produce large if not the largest acceleration they are site related not source related using the in situ observations from the kushiro port downhole array we have modeled the 1993 kushiro oki earthquake the synthetic accelerograms show the development of intermittent behavior high frequency peaks as observed in the recorded acceleration time histories shear modulus degradation due to pore pressure produces large strains in the soil with large amplification in the low frequency band of the ground motion we also modeled data from the 1987 superstition hills earthquake recorded at the wildlife refuge station the results show the importance of better soil characterization when pore pressure may develop and the effects of dilatancy in the understanding of nonlinear site response

spatial gradient analysis for linear seismic arrays i incorporate the spatial gradient of the wave field recorded from one dimensional arrays into a processing method that yields the horizontal wave slowness and the change of geometrical spreading with distance in general the model for seismic wave propagation is enough to be appropriate for body and surface waves propagating from nearby seismic sources but can be simplified into a plane wave model although computation of the spatial gradient requires that array elements be closer than 10 of the horizontal wavelength seismic array apertures in the usual sense may extend over many horizontal wavelengths and illuminate changes within the wave field array images of horizontal slowness and the relative geometrical spreading changes of seismic waves are derived using filter theory and used to interpret observed array wave fields errors in computing finite difference spatial gradients from array nodes are explicitly considered to avoid spatial aliasing in the estimates i apply the method to interpret waves in strong ground motion and small scale refraction data sets use of the wave spatial gradient accentuates spatial differences in the wave field that can be theoretically exploited in fine scale tomographic studies of structure and is complementary to frequency wavenumber or beam forming array processing techniques

simultaneous study of the source path and site effects on strong ground motion during the 1989 loma prieta earthquake a preliminary result on pervasive nonlinear site effects we considered the effects of source propagation path and local site conditions on strong ground motion from the loma prieta earthquake in the following steps first the local site effect was removed from the strong motion records applying the weak motion amplification factors estimated from nearby stations of usgs regional network using the coda method then we confirmed the approximate validity of the 1 r law of geometrical spreading factor for the crustal model applicable to the region and we determined the attenuation parameter q f and source spectrum s f empirically from the site effect corrected spectral amplitude both q f and s f are consistent with the results obtained earlier for major california earthquakes we then synthesized the time history of ground acceleration by applying the stochastic simulation technique the agreement between the observed and predicted records was good for duration and spectral content we found however a strong systematic discrepancy with regard to peak acceleration namely for distances less than 50 km the predicted peak acceleration consistently overestimated the observed for sediment sites and underestimated it for the franciscan formation sites since the above systematic discrepancy applies to all the available stations within about 50 km from the hypocenter it is unlikely that the effect is due to the radiation pattern near source structure or topography it appears that the strong difference in the amplification factor between the franciscan and the sediment sites observed for weak motion disappears at acceleration levels higher than about 0 1 to 0 3 g if this is due to the nonlinear response of the sediment sites it is in the range expected from geotechnical engineering studies

controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the red river flood plain vietnam abstract geological and hydrogeological processes controlling recharge and the mobilization of as were investigated in a shallow holocene aquifer on the red river flood plain near hanoi vietnam the geology was investigated using surface resistivity methods geophysical borehole logging drilling of boreholes and installation of more than 200 piezometers recharge processes and surface groundwater interaction were studied using i time series of hydraulic head distribution in surface water and aquifers ii the stable isotope composition of waters and iii numerical groundwater modeling the red river and two of its distributaries run through the field site and control the groundwater flow pattern for most of the year there is a regional groundwater flow towards the red river during the monsoon the red river water stage rises up to 6 m and stalls the regional groundwater flow the two distributaries recharge the aquifer from perched water tables in the dry season whilst in the flooding period surface water enters the aquifer through highly permeable bank sediments the result is a dynamic groundwater flow pattern with rapid fluctuations in the groundwater table a transient numerical model of the groundwater flow yields an average recharge rate of 60 100 mm a through the confining clay and a total recharge of approximately 200 mm a was estimated from 3h 3he dating of the shallow groundwater thus in the model area recharge of surface water from the river distributaries and recharge through a confining clay is of the same magnitude being on average around 100 mm a the thickness of the confining clay varies between 2 and 10 m and affects the recharge rate and the transport of electron acceptors o2 no 3 and so 4 2 into the aquifer where the clay layer is thin an up to 2 m thick oxic zone develops in the shallow aquifer in the oxic zone the as concentration is less than 1 g l but increases in the reduced zone below to 550 g l in the holocene aquifer as is mobilized at a rate of around 14 g l a an as mass balance for the field site shows that around 1100 kg of as is annually leached from the holocene sand and discharged into the red river corresponding to 0 01 of the total pool of as now present in the holocene sand

nonlinear soil amplification its corroboration in taiwan nonlinear ground response at two strong motion arrays in taiwan is studied using the spectral ratio technique at the smart1 array we calculate the frequency dependent soil amplification functions as a ratio of the spectra at alluvium to rock sites and study their dependence on the excitation level hor izontal components of shear waves are considered we compare 1 the average spectral ratios on weak and strong motions 2 the ratios for the mainshocks and aftershocks and 3 the ratios for the strong shear waves and their coda at the smart1 array weak motions have a peak horizontal acceleration pha less than 30 gal strong motions are in the range of 100 to 267 gal comparison of the average weak and strong motion spectral ratios shows a significant deamplification of strong motion between 2 and 9 hz exceeding the error margin estimated by the standard deviations the maximum deamplifi cation occurs at approximately 6 5 hz where the average weak motion ampli fication is 2 9 versus 0 40 in the strong motion a similar pattern is exhibited by the ratios calculated for the mainshocks and the aftershocks as well as for the shear waves and their coda the spectral ratio calculated from a single re alization of coda is identical to the average ratio obtained from many small earthquakes at the smart2 we analyze spectral ratios between the stations on pleistocene terrace deposits and recent alluvium which characterize the rel ative response at these two types of sediments weak motion is pha less than 13 gal while strong motion extends from 100 to 295 gal strong motion spec tral ratios between terrace and alluvial sites are consistently reduced in the fre quency range from 1 to 10 hz compared with the weak motion this effect is insensitive to the variation in distance between stations from 7 9 to 11 4 km as well as the azimuthal change of up to 80 in the station pair strike we attribute the observed discrepancies between weak and strong motion ampli fications to the differential nonlinear response occurring at terrace and alluvial sites our results document a significant nonlinear ground response at both ar rays

turbulence at the tropopause due to breaking kelvin waves observed by the equatorial atmosphere radar 1 the equatorial atmosphere radar ear installed in sumatra island indonesia observed significant enhancement of turbulence in the tropopause region 15 17 km intermittently for 5 days in november 2001 the turbulence intensity was estimated with the spectral width of the radar echo power spectrum and the turbulence during the period was a factor of up to 5 larger in kinetic energy than that in other periods further analyses confirm that the enhanced turbulence was convectively generated in the breaking phase of an equatorial kelvin wave between july and december 2001 we observed at least three more prominent cases of the turbulence generation by breaking kelvin waves in the tropopause region

seismic velocity reductions caused by the 2003 tokachi oki earthquake 1 we use four repeating earthquake sequences located near hokkaido to identify velocity changes caused by the mw 8 0 2003 tokachi oki earthquake using a moving window cross correlation technique we identify delays in the arrival time of seismic waves that accumulate linearly with time into the seismogram this behavior is indicative of multiple scattering within a medium where the seismic velocity has been reduced for all of our earthquake receiver geometries we find evidence of significant velocity reductions close to the receiver the correlation of the size of the velocity reductions with both strong shaking and site characteristics suggests that these velocity reductions are caused by damage to near surface materials created by nonlinear strong ground motion for earthquake receiver geometries where the seismic waves cross the tokachi oki rupture zone we identify particularly large reductions in velocity as a result of the earthquake for these geometries we believe that the rupture zone of the tokachi oki earthquake or the shallow crust above it represents a second region where seismic velocities are reduced as a result of the main shock

faster better shear wave velocity to 100 meters depth from refraction microtremor arrays current techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use at most construction sites they require large sources to be effective in noisy urban settings or specialized independent recorders laid out in an extensive array this work shows that microtremor noise recordings made on 200 m long lines of seismic refraction equipment can estimate shear velocity with 20 accuracy often to 100 m depths the combination of commonly available equipment simple recording with no source a wavefield transformation data processing technique and an interactive rayleigh wave dispersion modeling tool exploits the most effective aspects of the microtremor spectral analysis of surface wave sasw and multichannel analysis of surface wave masw techniques the slowness frequency wavefield transformation is particularly effective in allowing accurate picking of rayleigh wave phase velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities higher mode rayleigh waves body waves air waves and incoherent noise two locations illustrate the application of this technique in detail coincident with a large accelerometer microtremor array in reno nevada and atop a borehole logged for shear velocity in newhall california refraction equipment could duplicate microtremor results above 3 hz but could not estimate velocities deeper than 100 m refraction microtremor cannot duplicate the detail in the velocity profile yielded by a suspension logger but can match the average velocity of 10 to 20 m depth intervals and suggest structure below the 100 m logged depth of the hole eight additional examples from southern california and new zealand demonstrate that the refraction microtremor technique quickly produces good results from a wide range of hard and soft sites

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

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

solid phases and desorption processes of arsenic within bangladesh sediments abstract arsenic is a contaminant in the groundwater of bangladesh and west bengal india where an estimated 57 million people may be drinking water with unsafe arsenic levels the source of arsenic appears to be natural solid phase arsenic in the sediments and various theories have been put forth regarding the modes of arsenic release ranging from the oxidative or reductive degradation of arsenic bearing solids to competitive ligand displacement by phosphate currently however reductive dissolution of fe iii hydr oxides and concomitant arsenic release is the most widely accepted explanation of high arsenic groundwater concentrations using micro x ray fluorescence elemental mapping and micro x ray absorption near edge structure spectroscopy we detect arsenic bearing sulfides in the aquifer sediments from our field site in munshiganj bangladesh reduction of fe and as in surface soil layers is apparent but fe iii hydr oxides are not detected in the holocene aquifer materials rapid abiotic desorption of arsenic from sediments is observed in batch experiments and positive controls with ferrihydrite dismiss the role of ferric hydr oxides in arsenic retention within the aquifer sediments based on our laboratory results we do not see evidence for ferric hydr oxide reductive dissolution at well depths in the holocene aquifer in contrast our data suggest that arsenic is only released via redox cycling in surface soils sediments and thus must then be transported to well depth through the sandy aquifer

organic carbon mobilization in a bangladesh aquifer explained by seasonal monsoon driven storativity changes abstract currently the most widely accepted hypothesis to explain high as concentrations in bangladesh groundwaters is that dissolved organic c doc reduces solid fe hydr oxides and mobilizes sorbed arsenate the nature of the doc and its release mechanism are still controversial based on weekly to biweekly sampling over the course of one monsoon cycle at six monitoring wells of different depths it is proposed that storativity changes drive natural doc release from clay peat layers to the adjacent aquifers with a decrease in hydraulic heads during the dry season total mineralization and doc concentrations increased with the onset of the rainy season and an increase in hydraulic heads release of clay peat derived components stopped and vertical water displacement due to groundwater recharge from rainwater occurred causing aquifer flushing and a decrease in total mineralization and doc concentrations total as and doc concentrations correlated over depth however at the depth of maximum concentrations the as peak was observed during the rainy season at present the reason for this inverse seasonal trend between as and doc is unclear higher mineralization or doc concentrations could lead to increased as sorption or the increased arsenite release is a time lag abiotic or microbial response to the doc peak the vulnerability of the pleistocene aquifer towards increased as concentrations was found to be much higher than previously assumed though sorption capacities were determined to be higher than in the holocene aquifer probably due to intact fe hydr oxides long term continuous as input from overlying clay and peat layers by the proposed seasonal storativity changes has led to increased aqueous as concentrations of 85 g l considerably higher than drinking water standards until now aquifer and especially aquitard and aquiclude hydraulics have not been considered sufficiently when attempting to explain as mobilization in bangladesh

groundwater flow in an arsenic contaminated aquifer mekong delta cambodia abstract to advance understanding of hydrological influences on as concentrations within groundwaters of southeast asia the flow system of an as rich aquifer on the mekong delta in cambodia where flow patterns have not been disturbed by irrigation well pumping was examined monitoring of water levels in a network of installed wells extending over a 50 km2 area indicates that groundwater flow is dominated by seasonally variable gradients developed between the river and the inland wetland basins while the gradient inverts annually net groundwater flow is from the wetlands to the river hydraulic parameters of the aquifer k 10 4 ms 1 and overlying clay aquitard k 10 8 ms 1 were determined using grain size permeameter and slug test analyses when coupled with observed gradients they indicate a net groundwater flow velocity of 0 04 0 4 ma 1 downward through the clay and 1 13 ma 1 horizontally within the sand aquifer producing aquifer residence times on the order 100 1000 a the results of numerical modeling support this conceptual model of the flow system and when integrated with observed spatial trends in dissolved as concentrations reveal that the shallow sediments upper 2 10 m of fine grained material are an important source of as to the underlying aquifer

small scale wind fluctuations in the tropical tropopause layer from aircraft measurements occurrence nature and impact on vertical mixing abstractthe contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer ttl is poorly constrained partly due to a lack of direct observations here the authors use high resolution 20 hz airborne measurements to study the occurrence and properties of small scale 100 m wind fluctuations in the ttl 14 19 km over the tropical pacific the fluctuations are highly intermittent and appear localized within shallow 100 m patches furthermore active turbulent events are more frequent at low altitude near deep convection and within layers of low gradient richardson number a case study emphasizes the link between the turbulent events and the occurrence of inertio gravity waves having small horizontal or vertical scale in order to evaluate the impact of the observed fluctuations on tracer mixing their characteristics are examined during active events they are in broad agreement with inertial range turbulence theory the motions are close to 3d isotropic and the sp

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

induced dynamic nonlinear ground response at garner valley california we present results from a prototype experiment in which we actively induce observe and quantify in situ nonlinear sediment response in the near surface this experiment was part of a suite of experiments conducted during august 2004 in garner valley california using a large mobile shaker truck from the network for earthquake engineering simulation nees facility we deployed a dense accel erometer array within meters of the mobile shaker truck to replicate a controlled laboratory style soil dynamics experiment in order to observe wave amplitude dependent sediment properties ground motion exceeding 1g acceleration was pro duced near the shaker truck thewave field was dominated by rayleigh surface waves and ground motions were strong enough to produce observable nonlinear changes in wave velocity we found that as the force load of the shaker increased the rayleigh wave phase velocity decreased by as much as 30 at the highest frequencies used up to 30 hz phase velocity dispersion curves were inverted for s wave velocity as a function of depth using a simple isotropic elastic model to estimate the depth depen dence of changes to the velocity structure the greatest change in velocity occurred nearest the surface within the upper 4 m these estimated s wave velocity values were used with estimates of surface strain to compare with laboratory based shear modulus reduction measurements from the same site our results suggest that it may be possible to characterize nonlinear soil properties in situ using a noninvasive field technique

groundwater arsenic concentrations in vietnam controlled by sediment age arsenic contamination of groundwater threatens the health of millions of people in southeast asia measurements in an arsenic contaminated aquifer in vietnam point to sediment age as a key determinant of groundwater arsenic concentrations

arsenic migration to deep groundwater in bangladesh influenced by adsorption and water demand elevated concentrations of arsenic are common in shallow groundwater in many parts of south and southeast asia field experiments and model simulations suggest that adsorption of arsenic to sediments could help to limit contamination of deep groundwater

tropospheric turbulence over the tropical open ocean role of gravity waves a large set of soundings obtained in the indian ocean during 3 field campaigns is used to provide statistical characteristics of tropospheric turbulence and its link with gravity wave gw activity the thorpe method is used to diagnose turbulent regions of a few hundred meters depth above the mixed layer turbulence frequency varies from 10 in the lower troposphere up to 30 around 12km heights gw are captured by their signature in horizontal wind normalized temperature and balloon vertical ascent rate these parameters emphasize different parts of the wave spectrum from longer to shorter vertical wavelengths respectively composites are constructed in order to reveal the vertical structure of the waves and their link with turbulence the relatively longer wavelength gw described by their signature in temperature gwt are more active in the lower troposphere where they are associated with clear variations in moisture turbulence is then associated with minimum static stability and vertical shear stressing the importance of the former and the possibility of convective instability conversely the short waves described by their signature in balloon ascent rate gww are detected primarily in the upper troposphere and their turbulence is associated with a vertical shear maximum suggesting the importance of dynamic instability furthermore gww appear to be linked with local convection whereas gwt are more active in suppressed and dry phases in particular of the madden julian oscillation these waves maybe associated with remote sources such as organized convection or local fronts such as those associated with dry air intrusions

first observations of elevated ducts associated with intermittent turbulence in the stable boundary layer over bosten lake china nocturnal radiative cooling is a main driver for atmospheric duct formation within this atmospheric process the impacts of intermittent turbulence on ducting have seldom been studied in this paper we reported two confusing ducting events observed in the early morning in august 2014 over bosten lake china when a stable boundary layer sbl still survived by using tethered high resolution gps radiosondes elevated ducts with strong humidity inversions were observed during the balloon ascents but were absent during observations made upon the balloon descent several minutes later this phenomenon was initially hypothesized to be attributable to turbulence motions in the sbl and the connection between the turbulence event and the radar duct was examined by the statistical thorpe method turbulence patches were detected from the ascent profiles but not from the descent profiles the possible reasons for the duct formation and elimination were discussed in detail the turbulent transport of moisture in the sbl and the advection due to airflows coming from the lake are the most probable reasons for duct formation in one case the downward transport of moisture by turbulence mixing within a kelvin helmholtz billow at the top of the low level jet resulted in duct elimination in another case the passage of density currents originating from the lake may have caused the elimination of the duct few studies have attempted to associate intermittent turbulence with radar ducts thus this work represents a pioneering study into the connection between turbulent events and atmospheric ducts in a sbl

rate limited mass transfer or macrodispersion which dominates plume evolution at the macrodispersion experiment made site we present a model of solute transport that explains the large scale behavior of the solute tracer test plumes at the macrodispersion experiment made site as the result of advection and rate limited mass transfer between mobile and small scale immobile domains this model does not consider the process of dispersion and yet provides an alternative explanation of the evolution of the observed concentration profiles compared to the macrodispersion model the mass transfer model better represents the change in mobile dissolved mass with time the peak of the concentration profile and the profile asymmetry specifically unlike the macrodispersion model the mass transfer model explains the facts that the observed mass of the plume was greater than the injected mass in early snap shots of the plume and less than the injected mass at late times we suggest that the injected mass advects through the mobile domain and diffuses into and out of the immobile domain the immobile domain consists of a combination of low permeability zones on the scale of centimeters to decimeters the darcy scale immobile domain and intragranular porosity dead end pores and surface sorption the pore scale immobile domain we suggest that the mobile domain was sampled preferentially when water was extracted therefore at early times relatively clean water in the immobile domain was not sampled and incorrectly assumed to contain high solute concentrations similarly the mass at late times was underestimated because solute trapped in the pore scale immobile domain was not extracted during sampling and therefore ignored the combination of advection and slow mass transfer is consistent with the fact that the peak of the plume migrated only 5 m by the termination of the experiment as well as the different behavior of bromide and tritium tracers

resonance and elastic nonlinear phenomena in rock in a great variety of laboratory experiments over large intervals in stress strain and frequency rocks display pronounced nonlinear elastic behavior here we describe nonlinear response in rock from resonance experiments two important features of nonlinear resonant behavior are a shift in resonant frequency away from the linear resonant frequency as the amplitude of the disturbance is increased and the harmonics in the time signal that accompany this shift we have conducted young s mode resonance experiments using bars of a variety of rock types limestone sandstone marble chalk and of varying diameters and lengths typically samples with resonant frequencies of approximately 0 5 1 5 khz display resonant frequency shifts of 10 or more over strain intervals of 10 7 to 10 6 and under a variety of saturation conditions and ambient pressure conditions correspondingly rich harmonic spectra measured from the time signal progressively develop with increasing drive level in our experiments to date the resonant peak is observed to always shift downward if indeed the peak shifts indicating a net softening of the modulus with drive level this observation is in agreement with our pulse mode and static test observations and those of other researchers resonant peak shift is not always observed even at large drive levels however harmonics are always observed even in the absence of peak shift when detected strain levels exceed 10 7 or so this is an unexpected result important implications for the classical perturbation model approach to resonance results from this work observations imply that stress strain hysteresis and discrete memory may play an important role in dynamic measurements and should be included in modeling this work also illustrates that measurement of linear modulus and q must be undertaken with great caution when using resonance

seasonal variability of vertical eddy diffusivity in the middle atmosphere 1 three year observations by the middle and upper atmosphere radar the vertical eddy diffusivity k due to atmospheric turbulence with spatial scales of 100 102 m has been computed from the echo power spectral width observed by the middle and upper atmosphere radar for almost every month from january 1986 to december 1988 the method of analysis follows lilly et al 1974 sato and woodman 1982 and hocking 1983a 1985 1988 and the contamination due to beam broadening vertical shear and transience has been removed although observations for horizontal wind speeds larger than approximately 40 m s such as occur near the tropopause jet stream in winter have been omitted because of excessive beam broadening sufficient numbers of observations have been accumulated to produce a reasonable climatology for the upper troposphere and lower stratosphere 6 20 km altitude and for the mesosphere 60 82 km altitude the monthly median of k shows a local maximum near the tropopause jet stream altitude it becomes larger in the mesosphere increasing gradually with height maxima of k are observed in winter near the tropopause and in summer in the mesosphere and the seasonal variability of k reaches approximately an order of magnitude a semiannual variability is apparent in the mesosphere with minima in the equinoctial seasons

temporal change in site response caused by earthquake strong motion as revealed from coda spectral ratio measurement 1 the spectral ratios of coda waves of local earthquakes have been often used as measures of relative amplification factors of different sites applying this method to coda waves registered by seismometers installed on the surface and at the bottom of a borehole we succeeded in stably measuring the temporal change in site response associated with the occurrence of a large earthquake strong motion a remarkable drop of coda spectral ratio and a shift of the peak frequency were observed during strong shake at two sites by the 2000 western tottori earthquake and at a site by the 2003 tokachi oki earthquake in japan the reduction of the peak frequency reached 30 70 at all the sites after that the peak frequency logarithmically recovered to the value before the strong motions for a few years at two sites whereas the other one quickly recovered in a few tens of minutes

slow dynamics in the nonlinear elastic response of berea sandstone a typical resonance curve measured accel eration versus drive frequency made on a thin bar of rock shows peak bending with a softening nonlinear modulus as drive levels are increased the shapes of these nonlinear resonance curves were found in earlier work to depend somewhat on sweep rate these slow dynamics are now examined and quantified we have measured slow dynamics in a 0 3 m long 50 mm diam eter bar of berea sandstone under ambient conditions peak strain levels during the experiments ranged from 10 to 10 at driving frequencies near 4 khz the fun damental longitudinal resonance frequency of the bar slow dynamics begin to manifest themselves at strain amplitudes above 10 a at ambient conditions and at the onset of nonlinear peak bending strains above this value condition the rock altering its response for min utes to hours after the drive has been turned off ter the pressure on the rock was changed the work presented here is the first to specifically document the slowly varying time response to periodic excitation i e slow dynamics of a berea sandstone sample moreover we demonstrate that the slow dynamics produce promi nent effects even at room conditions slow dynamics manifest themselves in a sandstone sample during high intensity longitudinal elastic reso nance we first report the effects of high strain on a sample by comparing conditioned resonance curves e g after the sample is driven at high intensity with a set of fully recovered curves e g after the sample is left untouched for a long period we illustrate that the first resonance curve made on a recovered sample differs remarkably from subsequent curves we present several results that illustrate aspects of the sample s memory and concludiwith a discussion of measurements to de termine how the rock recovers and to understand what

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

in situ measurements of nonlinear and nonequilibrium dynamics in shallow unconsolidated sediments abstract we present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow unconsolidated sediments an array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g we determined high strain rayleigh wave dispersion across the array in the band from 5 to 60 hz rayleigh wave phase velocities at frequencies above 10 hz are wave amplitude dependent and a function of the driving force amplitude as driving force was increased phase velocity decreased consistent with nonlinear dynamics we demonstrate the existence of a temporary nonequilibrium state occurring in the near surface soils during and after the induced nonlinear behavior nonlinear conditioning is demonstrated by measuring changes in rayleigh wave phase velocity for input signals with the same applied driving force amplitude a logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle measurements from weak motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible we demonstrate that an active source field based approach has the potential to expand our knowledge of how sediments respond to strong ground motions provide additional insight into the poorly understood slow dynamic recovery process and possibly even lead to a new site specific and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits

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

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

role of sulfur redox cycling on arsenic mobilization in aquifers of datong basin northern china sulfur redox cycling potentially exerts important influences on arsenic as fate in shallow groundwater systems hydrochemical and sediment geochemical analysis combined with thermodynamic modeling study were conducted at datong basin to elaborate the effects of sulfur redox cycling on as speciation and mobilization under a strongly reducing environment dissolved as and sulfide concentration in 32 groundwater samples with depths of 19 40 m below the land surface varied from 8 to 2700 g l and from 5 to 490 g l respectively while dissolved fe ii was relatively low ranging from 20 to 280 g l the apparent co increase in dissolved sulfide and as concentration especially for samples with as content larger than 500 g l indicates that sulfidogenesis may significantly contribute to the mobilization of as via sulfide induced reduction of both as bearing fe iii oxide minerals and as v thermodynamic calculations indicate that groundwater as might be also thiolated in the presence of high level sulfide particularly to a large extent for as v speciation instead of sequestration by as sulfide precipitates results of sequential extraction and scanning electron microscopy array on sediments indicate the presence of fe ii sulfide mineral phases and an appreciable amount of co existent as in the sediments suggesting the precipitation of fe ii sulfides can restrict the build up of dissolved fe ii and sequester as from groundwater but not strongly enough thereby lowering down as to a moderate level of about 500 g l thus redox processes involving as s and fe species under sulfidic conditions as observed in datong basin not only facilitate the enrichment of as iii species and as v potentially existing as thiolated species but also the depletion of fe ii concentration in groundwater due to fe ii sulfide formation

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

kelvin helmholtz instability around the tropical tropopause observed with the equatorial atmosphere radar 1 in november 2001 the equatorial atmosphere radar 0 20 s 100 32 e observed a continuous strong eastward wind shear 10 50 m s 1 km 1 westward wind 2 27 m s 1 and the radar echo layer tilted downward to the west in the region 0 1 km above the tropopause during the same period the richardson number calculated with hourly averaged horizontal wind and radiosonde temperature data was almost continuously 0 5 and sometimes 0 25 which seems to indicate that the kelvin helmholtz instability khi frequently occurs in that region the existence of the tilted radar echo layer can be explained by khi billows a spurious updraft caused by the khi induced tilted echo layer and by the strong westward wind was also observed in the region