natural uranium and strontium isotope tracers of water sources and surface water groundwater interactions in arid wetlands pahranagat valley nevada usa summary near surface physical and chemical process can strongly affect dissolved ion concentrations and stable isotope compositions of water in wetland settings especially under arid climate conditions in contrast heavy radiogenic isotopes of strontium 87sr 86sr and uranium 234u 238u remain largely unaffected and can be used to help identify unique signatures from different sources and quantify end member mixing that would otherwise be difficult to determine the utility of combined sr and u isotopes are demonstrated in this study of wetland habitats on the pahranagat national wildlife refuge which depend on supply from large volume springs north of the refuge and from small volume springs and seeps within the refuge water budgets from these sources have not been quantified previously evaporation transpiration seasonally variable surface flow and water management practices complicate the use of conventional methods for determining source contributions and mixing relations in contrast 87sr 86sr and 234u 238u remain unfractionated under these conditions and compositions at a given site remain constant differences in sr and u isotopic signatures between individual sites can be related by simple two or three component mixing models results indicate that surface flow constituting the refuge s irrigation source consists of a 65 25 10 mixture of water from two distinct regionally sourced carbonate aquifer springs and groundwater from locally sourced volcanic aquifers within the refuge contributions from the irrigation source and local groundwater are readily determined and depend on proximity to those sources as well as water management practices

hydrological changes in the mediterranean sea over the last 30 000 years sea surface temperatures were reconstructed over the last 30 000 years from alkenone paleothermometry sst alk and planktonic foraminifera assemblages using the modern analog technique mat sst foram along two cores of the mediterranean sea md84 632 levantine basin and md04 2797 siculo tunisian strait oxygen isotope of planktonic foraminifera g bulloides for core md04 2797 and g ruber for core md84 632 were also determined sst alk in the levantine basin indicate colder values at the last glacial maximum lgm 14 c than earlier established from mat and a cooling amplitude of 6 7 c comparable to the central mediterranean sea climatic events such as the younger dryas yd and heinrich events 1 and 2 h1 and h2 were times of significant cooling in the two cores in the eastern basin values of local seawater oxygen isotope w indicate relatively saltier waters during the lgm and deglaciation than today with increasing w values higher salinity in the eastern basin and decreasing ones lower salinity in the central mediterranean sea during cold stadials the observed alterations of surface water properties t and w in the central and eastern mediterranean at the lgm are consistent with model experiments showing slightly lower evaporation in the mediterranean than today except for the eastern basin

deciphering interaction of regional aquifers in southern tunisia using hydrochemistry and isotopic tools groundwater is the most important source of water supply in southern tunisia previous hydrogeologic and isotopic studies carried out in this region revealed the existence of two major aquifer systems the complex terminal ct and the continental intercalaire ci turonian carbonates constitute one of the major aquifer levels of the ct multilayered aquifer it extends over most of southern tunisia and its hydrodynamic regime is largely influenced by tectonics lithology and recharge conditions forty eight groundwater samples from the ci and turonian aquifers were collected between january and april 2004 for chemical and isotopic analyses hydrochemistry and isotopic tools were combined to get an insight into the processes controlling chemical composition of groundwater and wide scale interaction of these two aquifer systems analysis of the dissolved constituents revealed that several processes control the observed chemical composition i incongruent dissolution of carbonate minerals ii dissolution of evaporitic minerals and iii cation exchange dissolution alone cannot account for the observed high supersaturation states of groundwater with respect to calcite and dolomite the observed supersaturation is most probably linked to geogenic co2 entering water bearing horizons of the ct and ci aquifers via deep tectonic faults and discontinuities and subsequent degassing in the exploitation wells presence of geogenic co2 in the investigated region was confirmed by c isotope data of the dic reservoir the radiocarbon content of the turonian samples varied between 9 5 and 43 pmc for ci samples generally lower values were recorded between 3 8 and 22 5 pmc stable isotope composition of turonian groundwater samples varied from 8 3 to 5 3 for d 18 o and from 60 to 25 for d 2 h the corresponding ranges of d values for the continental intercalaire samples were from 8 9 to 6 9 for d 18 o and from 68 2 to 45 7 for d 2 h stable isotope composition of groundwater representing ct and ci aquifers

uranium colloidal transport and origin of the 234u 238u fractionation in surface waters new insights from mount cameroon in order to evaluate the spatial variability of the 234u 238u fractionation and its origin in surface waters 234u 238u activity ratios a r and trace element concentrations were measured in 34 streams and springs draining mount cameroon volcano in addition tangential ultrafiltration techniques were applied to 10 of the samples to assess the role of colloids on the transport of the uranium isotopes u concentration variations in the different ultrafiltered fractions indicate that inorganic colloids can significantly influence the transport of this chemical element even in diluted and organic poor waters such influence which may be related to the chemical and physical nature of colloids does induce fractionations between u and sr along with filter size 234u 238u a r variations among the different ultrafiltered fractions of a sample remain small or even negligible only two samples exhibit a significant isotopic difference between colloid associated and solute u colloids of these two samples seem to correspond to a phase which is distinct from the dissolved and particulate loads of the waters at the scale of the volcano the 234u 238u a r in surface waters is large ranging from 1 038 to 1 397 small 234u 238u fractionations between 1 038 and about 1 10 are attributed to surface and meteoric weathering of the bedrock by contrast the highest 234u 238u fractionations up to 1 397 are associated with groundwater contributions thus variations in the 234u 238u a r could correspond to the water source and not to the surface weathering processes consequently 234u 238u disequilibrium may represent a powerful tool for detecting and quantifying the influence of groundwater inputs on the chemical fluxes carried by surface waters

a general power equation for predicting bed load transport rates in gravel bed rivers 1 a variety of formulae has been developed to predict bed load transport in gravel bed rivers ranging from simple regressions to complex multiparameter formulations the ability to test these formulae across numerous field sites has until recently been hampered by a paucity of bed load transport data for gravel bed rivers we use 2104 bed load transport observations in 24 gravel bed rivers in idaho to assess the performance of eight different formulations of four bed load transport equations results show substantial differences in performance but no consistent relationship between formula performance and degree of calibration or complexity however formulae containing a transport threshold typically exhibit poor performance furthermore we find that the transport data are best described by a simple power function of discharge from this we propose a new bed load transport equation and identify channel and watershed characteristics that control the exponent and coefficient of the proposed power function we find that the exponent is principally a factor of supply related channel armoring transport capacity in excess of sediment supply whereas the coefficient is related to drainage area a surrogate for absolute sediment supply we evaluate the accuracy of the proposed power function at 17 independent test sites

a new explanation for extreme 234u238u disequilibria in a dolomitic aquifer high 234u238u activity ratios are found in the shallow groundwater of the phreatic transvaal dolomite aquifer the aquifer is uranium poor while the waters are oxygen rich and young tritium and 14c are used to show that the disequilibrium increases the longer the water resides in contact with the dolomite host the 228ra and 222rn isotopes were used as supporting evidence that ion exchange between the 238u series nuclides in the water and the carbonate wall rock is necessary in assisting in the fractionation process the wolkberg cave speleothems preserve a record of the uranium isotopic fractionations that evolved as water flowed through the aquifer extremely variable and elevated 234u238u ratios of 2 12 are characteristic individual caverns may exhibit very large 234u238u variability in their drip water and in derivative carbonate precipitates speleothems deeper chambers where water has spent a longer time in contact with the aquifer tend to exhibit a greater degree of uranium isotopic fractionation in their speleothems the recorded fractionation process has been an ongoing one from at least the middle late pleistocene to the present the speleothem study supports the contention that ion exchange processes that produce sufficient enrichment of uranium on carbonate can provide ideal conditions for generating uranium isotopic fractionation in the water of carbonate aquifers in the absence of redox changes and within a uranium poor environment

timing and dynamics of the last deglaciation from european and north african δ13c stalagmite profiles comparison with chinese and south hemisphere stalagmites abstract the last deglaciation and its climatic events such as the bolling allerod ba and the younger dryas yd have been clearly recorded in the 13 c profiles of three stalagmites from caves from southern france to northern tunisia the three 13 c records dated by thermal ionization mass spectrometric uranium thorium method tims show great synchroneity and similarity in shape with the chinese cave 18 o records and with the marine tropical records leading to the hypothesis of an in phase between 15 5 and 16 ka 0 5 ka postglacial warming in the northern hemisphere up to at least 45 n the ba transition appears more gradual in the speleothem records than in the greenland records and the allerod seems warmer than the bolling showing here close similarities with other marine and continental archives a north south gradient is observed in the ba trend it cools in greenland and warms in our speleothem records several climatic events are clearly recognizable a cooler period at about 14 ka older dryas od the intra allerod cold period at about 13 3 ka the yd cooling onset between 12 7 and 12 9 0 3 ka similar to the ba the yd displays a gradual climate amelioration just after its onset at 12 75 0 25 ka up to the preboreal and is punctuated by a short climatic event at 12 15 ka even though the southern hemisphere stalagmite records seem to indicate that the postglacial warming started about 3 ka 1 8 ka earlier in new zealand 41 s and about 1 to 2 ka earlier in south africa 24 1 s large age uncertainties essentially due to slow growth rates make the comparison still perilous the overall 13 c speleothem record seems to follow a baseline temperature increase controlled by the increase in insolation and punctuated by cold events possibly due to the n america freshwater lake discharges

hydrogeochemistry of sodium bicarbonate type bedrock groundwater in the pocheon spa area south korea water rock interaction and hydrologic mixing the pocheon spa land area south korea occurs in a topographically steep fault bounded basin and is characterized by a hydraulic upwelling flow zone of thermal water up to 44 c in its central part hydrogeochemical and environmental isotope data for groundwater in the study area suggested the occurrence of two distinct water types a ca hco3 type and a na hco3 type the former water type is characterized by relatively high concentrations of ca so4 and no3 which show significant temporal variation indicating a strong influence by surface processes in contrast the na hco3 type waters have high and temporally constant temperature ph tds na cl hco3 and f indicating the attainment of a chemical steady state with respect to the host rocks granite and gneiss oxygen hydrogen and tritium isotope data also indicate the differences in hydrologic conditions between the two groups the relatively lower 18o d and tritium values for na hco3 type waters suggest that they recharged at higher elevations and have comparatively long mean residence times considering the geologic and hydrogeologic conditions of the study area na hco3 type waters possibly have evolved from ca hco3 type waters mass balance modeling revealed that the chemistry of na hco3 type water was regulated by dissolution of silicates and carbonates and concurrent ion exchange particularly low ca concentrations in na hco3 water was mainly caused by cation exchange multivariate mixing and mass balance modeling m3 modeling was performed to evaluate the hydrologic mixing and mass transfer between discrete water masses occurring in the shallow peripheral part of the central spa land area where hydraulic upwelling occurs based on q mode factor analysis and mixing modeling using phreeqc an ideal mixing among three major water masses surface water shallow groundwater of ca hco3 type deep groundwater of na hco3 type was proposed m3 modeling suggests that all the groundwaters in the spa area can be described as mixtures of these end members after mixing the net mole transfer by geochemical reaction was less than that without mixing therefore it is likely that in the hydraulic mixing zone geochemical reactions are of minor importance and therefore that mixing regulates the groundwater geochemistry

234u 238u activity ratios in freshwaters as tracers of hydrological processes the strengbach watershed vosges france abstract 234u 238u and 87sr 86sr isotopic ratios as well as major and trace u ba sr rb element concentrations were analyzed in dissolved loads of the strengbach stream vosges france in order to constrain the sources of u isotopes 234u 238u to river waters the variations of the 234u 238u activity ratios along the stream indicate a clear dependence between the 234u 238u activity ratios of the surface waters and the different types of rocks forming the watershed with near equilibrium values for the waters draining granites and high 234u 238u disequilibria 1 4 for those flowing over carbonates the high 234u 238u disequilibria are related to a supply of 234u enriched groundwaters located within the carbonate rocks the 234u 238u activity ratios of the waters at the outlet of the catchment collecting the sources of the stream on the granitic lithology decrease from 1 02 to 0 96 when the discharge of the stream increases such a tendency requires mixing between a water body enriched in 234u which weathered the granitic bed rock at secular equilibrium and a water with a 234u 238u activity ratio below unity representing a mobilization of u from material that has already been weathered comparison of the geochemical characteristics of waters collected during the year and those collected during a flood event reveals the involvement of two different weathered end members depending on the hydrological conditions during the year the dissolved u transported by the river originates from bed rock and deep horizons of the weathering profile whereas a significant part of u during the flood event is supplied by superficial horizons of soils probably complexed by organic colloids these results outline the potential of 234u 238u activity ratios to investigate hydrological processes and emphasize that their use as tracers of rock weathering should be made cautiously

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

humidity changes in southern tunisia during the late pleistocene inferred from u th dating of mollusc shells abstract calcareous deposits mainly consisting of mollusc shell accumulations which have been dated by the u th disequilibrium method mark the shorelines of paleolake highstands in the great chotts area of southern tunisia the 5 sites studied consist of discontinuous accumulations of fossils of marine like organisms e g cerastoderma glaucum melania tuberculata melanopsis praemorsa cerithium rupestre u th isochron plots and age frequency histograms for 39 shell samples are reported here limited variations for u content and 234u 238u activity ratios ar of shells support the hypothesis of closure of the geochemical system with respect to this element it is remarkable that 234u 238u ar of shells collected in chott fejej or chott jerid are clustered around different values reflecting probably different groundwater recharge from the continental intercalaire ci or complexe terminal ct aquifers furthermore waters collected near wadi el akarit show 234u 238u ar values comparable to those observed for shells 14c determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka bp the u th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30 95 100 130 150 and 180 200 ka for the episode centred around 30 ka the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event moreover the comparison of 13c and 18o with those of older humid pleistocene phases when very large palaeolakes have been recorded suggests that these young carbonate shells are not related to a true highstand lake it is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place it should be noted that this period was less arid than the present

relationship between senonian and deep aquifers in southern tunisia abstract steady increase in the salinity of water resources and the decline in the groundwater level are the most widespread processes that degrade water quality and endanger future water exploitation in southern tunisia hydrological geochemical and environmental isotope data were evaluated to characterize the recharge conditions of the senonian aquifer and the flow pattern of groundwater mixing the results show that groundwaters can be classified into two major water facies ca so4 to mixed and na cl the main hydrogeochemical processes that typically control the groundwater mineralization are dissolution of evaporites halite gypsum and anhydrite evaporation cation exchange and mixing process related to the up flow leakage of deep groundwaters the stable isotope signatures reveal that the main part of groundwater is significantly affected by evaporation process wide range 4 5 to 8 4 and spatial heterogeneity of 18o in groundwater indicate that the aquifer is characterized by a slow process of natural groundwater in its lateral extent the 14c activity varies between 43 and 0 33 pmc the 13c contents of the total dissolved inorganic c tdic range between 10 8 and 3 6 the calculated 13c contents of the co2 in equilibrium with the tdic varying between 15 1 to 11 4 indicate two origins of c in solution the carbonate matrix 13c 2 and the soil co2 13c from 26 and 22 for the cultivated areas mean residence times have been determined after correction of the initial activities for dead c from the rock matrix the mean residence times confirm a modern recharge of the groundwater from dahar upland and indicate the presence of palaeowaters in the northern and southern parts of the basin detailed investigations on identifying the potential recharge zones flow paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization

a review of single sample based models and other approaches for radiocarbon dating of dissolved inorganic carbon in groundwater abstract numerous methods have been proposed to estimate the pre nuclear detonation 14c content of dissolved inorganic carbon dic recharged to groundwater that has been corrected adjusted for geochemical processes in the absence of radioactive decay 14c0 a quantity that is essential for estimation of radiocarbon age of dic in groundwater the models approaches most commonly used are grouped as follows 1 single sample based models 2 a statistical approach based on the observed curved relationship between 14c and 13c data for the aquifer and 3 the geochemical mass balance approach that constructs adjustment models accounting for all the geochemical reactions known to occur along a groundwater flow path this review discusses first the geochemical processes behind each of the single sample based models followed by discussions of the statistical approach and the geochemical mass balance approach finally the applications advantages and limitations of the three groups of models approaches are discussed the single sample based models constitute the prevailing use of 14c data in hydrogeology and hydrological studies this is in part because the models are applied to an individual water sample to estimate the 14c age therefore the measurement data are easily available these models have been shown to provide realistic radiocarbon ages in many studies however they usually are limited to simple carbonate aquifers and selection of model may have significant effects on 14c0 often resulting in a wide range of estimates of 14c ages of the single sample based models four are recommended for the estimation of 14c0 of dic in groundwater pearson s model ingerson and pearson 1964 pearson and white 1967 han plummer s model han and plummer 2013 the iaea model gonfiantini 1972 salem et al 1980 and oeschger s model geyh 2000 these four models include all processes considered in single sample based models and can be used in different ranges of 13c values in contrast to the single sample based models the extended gonfiantini zuppi model gonfiantini and zuppi 2003 han et al 2014 is a statistical approach this approach can be used to estimate 14c ages when a curved relationship between the 14c and 13c values of the dic data is observed in addition to estimation of groundwater ages the relationship between 14c and 13c data can be used to interpret hydrogeological characteristics of the aquifer e g estimating apparent rates of geochemical reactions and revealing the complexity of the geochemical environment and identify samples that are not affected by the same set of reactions processes as the rest of the dataset the investigated water samples may have a wide range of ages and for waters with very low values of 14c the model based on statistics may give more reliable age estimates than those obtained from single sample based models in the extended gonfiantini zuppi model a representative system wide value of the initial 14c content is derived from the 14c and 13c data of dic and can differ from that used in single sample based models therefore the extended gonfiantini zuppi model usually avoids the effect of modern water components which might retain bomb pulse signatures the geochemical mass balance approach constructs an adjustment model that accounts for all the geochemical reactions known to occur along an aquifer flow path plummer et al 1983 wigley et al 1978 plummer et al 1994 plummer and glynn 2013 and includes in addition to dic dissolved organic carbon doc and methane ch4 if sufficient chemical mineralogical and isotopic data are available the geochemical mass balance method can yield the most accurate estimates of the adjusted radiocarbon age the main limitation of this approach is that complete information is necessary on chemical mineralogical and isotopic data and these data are often limited failure to recognize the limitations and underlying assumptions on which the various models and approaches are based can result in a wide range of estimates of 14c0 and limit the usefulness of radiocarbon as a dating tool for groundwater in each of the three generalized approaches single sample based models statistical approach and geochemical mass balance approach successful application depends on scrutiny of the isotopic 14c and 13c and chemical data to conceptualize the reactions and processes that affect the 14c content of dic in aquifers the recently developed graphical analysis method is shown to aid in determining which approach is most appropriate for the isotopic and chemical data from a groundwater system

the hydro geochemical characterization of ground waters in tunisian chott s region tunisian chott s region is one of the most productive artesian basins in tunisia it is located in the southwestern part of the country and its groundwater resources are developed for water supply and irrigation the chemical composition of the water is strongly influenced by the interaction with the basinal sediments and by hydrologic characteristics such as the flow pattern and time of residence the system is composed of an upper unconfined plio quaternary aquifer with a varying thickness of 20 200 m an intermediate confined unconfined complex terminal aquifer about 100 m in thickness and a deeper continental intercalaire aquifer about 150 m in thickness separated by thick clay and marl layers the dissolution of evaporites and carbonates explains part of the contained na ca2 mg2 k so42 and cl but other processes such carbonate precipitation also contributes to the water composition the stable isotope composition of waters establishes that the deep groundwater depleted as compared to present corresponding local rainfall is ancient water recharged probably during the late pleistocene and the early holocene periods the relatively recent water in the plio quaternary aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater

u isotope ratios as tracers of groundwater inputs into surface waters example of the upper rhine hydrosystem major elements trace elements and sr and u isotopes were analyzed in the dissolved load of water samples collected in the main reservoirs of the rhine graben i e the rhine river its main tributaries and the shallow and deep ground waters in several different aquifers our results confirm that the variations of major elements trace elements especially sr and rb and sr isotope ratios of the water samples are mainly correlated to the lithological variations of the watershed the atmospheric inputs can be considered as a minor cause for the chemical variability of the surface waters in the rhine hydrosystem except for a few chemical elements in the mountain streams human activity in particular the pollution by the exploitation of the alsace potash mines mainly affects the na and cl concentrations of the rhine and the south alluvial groundwaters but remains negligible for the other water samples the specific study of the lauter stream located in the lower triassic sandstones buntsandstein sequences demonstrates the importance of groundwater influences onto the chemical composition of river waters up to 50 of the total dissolved load tds of this stream is supplied by deep waters this contribution significantly modifies the chemical composition of the stream especially the 238u 234u radioactive disequilibrium which in turn can be used as a specific tracer of deep water inputs into surface waters and could be useful to distinguish in the dissolved load of rivers surface weathering fluxes from deeper ones

origin and mobility of humic colloids in the gorleben aquifer system the origin and mobility of humic colloids in the gorleben aquifer system have been examined for this purpose the distribution of humic colloids and relevant hydrological and geochemical parameters were examined an investigation area was selected where sediments have been disturbed by salt dome uplift and glacial events it is shown that on a local scale considerable groundwater movement and intermixing takes place from the surface down to the salt dome consequently no effective separation of groundwater layers occurs two different humic colloid sources are identified influx from the humus horizon with recharge water and continuous in situ generation via mineralization of sedimentary organic carbon soc the in situ generation leads to groundwaters with humic colloid concentrations approaching 0 4 g l compared to concentrations of less than 0 005 g l in recharge waters young groundwaters no 14c decay detected between approximately 50 and 200 m depth exhibit these highly elevated humic colloid concentrations at greater depth salt brines are found with low humic colloid concentrations this can be attributed to precipitation of humic acid and or hampering of the in situ generation process due to the high salt content there is no indication of retention or decomposition of humic colloids the fate of in situ generated humic colloids cannot be precisely evaluated due to analytical limitations and insufficient understanding of groundwater movement

radioelements radiogenic helium and age relationships for groundwaters from the granites at stripa sweden abstract the solution of radioelements and radiogenic 4he by groundwaters in fractured rocks is dependent upon the radioelement distribution in the rock matrix and the extent of the rock water interface the 234 u 238 u activity ratio and the dissolved u rn and he contents of such groundwaters respond to changes in the flow regime with time although 234 u 238 u activity ratios change with groundwater residence time as a consequence of 234th recoil induced solution of 234u the activity ratio is strongly influenced by the u distribution within fractures by the extent of the rock water interface and by the amount of 238u in solution a model for the quantitative evaluation of these effects is presented groundwaters from depths up to 880 m in the stripa granite have variable dissolved uranium contents and 234 u 238 u activity ratios the uranium geochemistry is primarily determined by variations in flow path rather than by groundwater age dissolved radiogenic 4he in the groundwaters increases with their depth of origin and is dependent upon the u content of the granite and upon its fracture porosity it increases with groundwater residence time but movement of 4he by diffusion and transport processes make the actual groundwater age indeterminate

uranium isotopes as tracers of groundwater evolution in the complexe terminal aquifer of southern tunisia abstract the complexe terminal ct multi layer aquifer is formed by neogene paleogene sand deposits upper senonian campanian maastrichtian limestones and turonian carbonates the chemical composition and isotopes of carbon and uranium were investigated in groundwater sampled from the main hydrogeological units of the ct aquifer in southern tunisia we paid special attention to the variability of uranium contents and isotopes ratio 234u 238u to provide a better understanding of the evolution of the groundwater system uranium concentrations range from 1 5 to 19 5 ppb typical of oxic or mildly reducing conditions in groundwaters the lowest concentrations are found southeast of the study area where active recharge is supposed to take place when looking at the isotope composition it appears that all the samples including those from carbonate levels are in radioactive disequilibrium with significant 234u excess a clear cut distinction is observed between turonian and senonian carbonate aquifers on the one hand with 234u 238u activity ratios between 1 1 and 1 8 and the sandy aquifer on the other hand showing higher ratios from 1 8 to 3 2 the distribution of uranium in this complex aquifer system seems to be in agreement with the lithological variability and are ultimately a function of a number of physical and chemical factors including the uranium content of the hosting geological formation water rock interaction and mixing between waters having different isotopic signatures significant relationships also appear when comparing the uranium distribution with the major ions composition it is noticeable that uranium is better correlated with sulfate calcium and magnesium than with other major ions as chloride or bicarbonate the 14c activities and 13c values of dic cover a wide range of values from 1 1 pmc to 30 2 pmc and from 3 6 to 10 7 respectively 14c model ages estimated by the fontes and garnier model are all younger than 22 ka and indicate that the recharge of ct groundwater occurred mainly during the end of the last glacial and throughout the holocene

uranium isotopic disequilibrium for groundwater classification first results on complexe terminal and continental intercalaire aquifers in southern tunisia a fractionation of uranium u series into parent daughter pairs 234u and 238u always occurs in natural waters and the disequilibria between these is commonly used as a tracer of groundwater flow we report here an interpretation of the u content and u isotope disequilibria in groundwater sampled from the deep complexe terminal and continental intercalaire aquifers of southern tunisia variations in both the contents of these isotopes 0 006 2 4 ppb and 234u 238u activity ratios ars 1 7 15 4 were observed the data could be plotted in two distinct fields of reciprocal u concentration versus 234u 238u ar according to groundwater flow and regional bedrock differences an initial assessment aimed at verifying whether the results of this investigation support those of previous hydrogeological and isotope studies thereby suggesting that the disequilibrium between u isotopes in groundwater may represent a useful tool for hydrogeological investigations of deep and fossil groundwater in addition the disequilibrium can be used for quantifying the recharge or mixing rates between different formations with the aim of delineating the preferential outflow pattern or determining residence times of waters

uranium series radionuclides in fluids and solids milk river aquifer alberta canada abstract samples from 21 wells completed in the milk river aquifer were sampled and analysed for u isotopes in general u concentrations decrease with distance from the recharge area groundwater samples can be delineated into a small oxic group u content 101t 5 mmol 1 and a more predominant anoxic group u content of 10 7 mmol 1 all groundwater samples were undersaturated with respect to the common u minerals such as uraninite coffinite and rutherfordine the dominant u complexes were uranyl carbonates the234u 238u activity ratios increased from 2 to 11 with distance downflow of the outcrop the highest activity ratio of 11 8 was measured at a well defined redox front whereupon a steady decrease downdip to 4 was observed this decrease was interpreted to be due to the decay of234u excess with age of the groundwater and due to u loss by sorption precipitation processes from solution to solid surfaces two u isotope dating models were used to estimate the age of the groundwater in the milk river aquifer model 1 is a simple model of downdip decay of234u excess in solution under steady state conditions model 2 is a phenomenological model for the evolution of u isotope groundwater composition downdip based on a transport dispersion equation incorporating radioactive decay and assuming that sorption processes are of first order kinetics both models were applied to the u isotope data collected along the same flow path in the aquifer they yielded flowrates of 0 1 0 2 m a and 0 2 0 6 m a respectively the latter range is in good agreement with flowrates of 0 3 m a obtained from hydraulic considerations the lower value derived from model 1 is interpreted in terms of u migration velocity rather than as groundwater flowrate model 2 was also applied to an additional flowpath the calculated flowrate range of 0 1 0 4 m a is in excellent agreement with the hydraulic model flowrate of 0 2 m a the main conclusion of this work is that u isotopes can be used for dating very old groundwaters provided the local hydrogeology and u geochemistry are understood well

groundwater evolution in the continental intercalaire aquifer of southern algeria and tunisia trace element and isotopic indicators abstract the geochemical processes taking place along an 800 km flow line in the non carbonate continental intercalaire aquifer ci aquifer in north africa are described using chemical major and trace element and isotopic indicators the aquifer is hydraulically continuous from the atlas mountains in algeria to the chotts of tunisia and the geochemical evidence corroborates this the highest discharge temperature is 73 c but silica geothermometry indicates a maximum temperature of 94 c at depth chloride concentrations increase from 200 to 800 mg l 1 and the br cl ratios confirm the dissolution of non marine evaporites or interstitial waters as the main source of salinity fluoride concentrations are low and are likely to be derived from rainfall recording oscillations in source radiocarbon ages except near outcrop are at or near detection limits and the 18o and 2h values indicate a cooler recharge regime with rainfall having lower primary evaporation than today this is shown by the fact that mean isotope ratios of ci waters are around 3 lighter than the present day weighted mean value for rain major ion ratios and most trace elements indicate that despite the complex structure and stratigraphy uniform evolution with continuous water rock interaction takes place along the flow lines which are only disturbed near the tunisian chotts by groundwater converging from additional flow lines the ageing of the water can also be followed by the smooth increase in several indicator elements such as li k and mn which are least affected by solubility controls similarly the influence of marine facies in the tunisian sector may be recognised by the changing mg ca and higher br cl as well as trace element indicators the groundwaters are oxidising up to 300 km from outcrop dissolved o2 has persisted for at least 20 ka and within this zone the concentrations of several elements forming oxy anions such as u and cr increase and no3 remains conservative beyond 300 km from outcrop the groundwaters are reducing and contain high fe concentrations the basin contains huge reserves of fresh brackish waters but these need careful development due to the limiting high salinity and scaling tendency resulting from the high temperatures and mineral super saturation caused during abstraction as well as high concentrations of some harmful elements such as cr in the oxidising section

the u contents and 234u 238u activity ratios of dissolved uranium in groundwaters from some triassic sandstones in england abstract the isotopic compositions of dissolved uranium in groundwaters of age up to 36 000 a from the east midlands triassic sandstone and in saline fluids from deep geothermal boreholes in the wessex basin have been investigated groundwaters from the east midlands have u contents which lie on a normal distribution and average 0 75 g kg 1 with a standard deviation of 0 4 g kg 1 the 234 u 238 u activity ratios for the dissolved u in these groundwaters have minimum values between 3 and 4 some groundwaters have further enhancement of the 234 u 238 u activity ratio up to 7 but this is not related to groundwater age it is shown that uranium solution is primarily controlled by carbonate solution involving the cement phase of the sandstone and that the 234 u 238 u activity ratios are the results of preferential chemical etch of 234 u relative to 238 u an eh ph stability field diagram calculated for the average groundwater hco 3 contents in the aquifer shows that the uranyl carbonate complexes uo 2 co 3 2 2 and uo 2 co 3 3 4 stabilise uranium solution in the east midlands groundwaters in the wessex basin the redox conditions indicate that uranium deposition should occur and this is reflected in the very low u content of the geothermal fluids g kg 1 the residence time of these thermal waters probably exceeds 15 ma and a steady state 234 u 238 u activity ratio due to alpha recoil induced solution of 234 u should therefore have been attained the activity ratio 2 0 is very much less than the theoretical steady state ratio 10 20 for recoil processes in the interstitial fluids of the sandstone it is concluded that u in the interstitial surfaces must have been sealed against alpha recoil processes by a phase of low u content if the thermal water has evolved from groundwater similar to that in the east midlands triassic sandstone decay of excess 234 u must have occurred the time required for this decay is at least 0 25 ma if no alpha recoil solution of 234 u occurs and is 1 25 ma if an alpha recoil steady state has been attained the general implications of the study on the use of 234 u 238 u activity ratio changes for the estimation of groundwater residence times are discussed

trench normal variation in observed seafloor displacements associated with the 2011 tohoku oki earthquake 1 seafloor geodetic measurements using the gps acoustic technique just above the rupture area of the 2011 tohoku oki earthquake revealed strong trench normal variation in the horizontal crustal displacement associated with the earthquake one set of observations at a site only 50 km away from the trench yielded the most trench ward data ever reported with an accuracy better than 1 m it showed a coseismic displacement of up to 31 m whereas another set of observations 100 km away from the trench showed a 15 m displacement these horizontal data as well as vertical data from pressure gauges and tsunami observations strongly indicate that the seismic slip reached the trench they are also likely to contribute quantitatively to further investigations that reveal the coseismic behavior of the frontal wedge of the overriding plate in the source region along the japan trench

revision of fontes garnier s model for the initial 14c content of dissolved inorganic carbon used in groundwater dating abstract the widely applied model for groundwater dating using 14c proposed by fontes and garnier fg or 2 using different fractionation factors if the exchange process is dominated by soil co2 gas as opposed to solid carbonate mineral typically calcite an analysis of the fg however 14c0 will be slightly underestimated finally in carbonate systems undergoing complex geochemical reactions such as oxidation of organic carbon radiocarbon ages are best estimated by inverse geochemical modeling techniques

geochemistry and residence time estimation of groundwater from miocene pliocene and upper cretaceous aquifers of southern tunisia abstract miocene pliocene and upper cretaceous formations in tunisia contain one of the most productive artesian aquifers in the country they consist of mio pliocene sands and senonian turonian carbonates intensive pumping in kebili and nefzaoua regions over the past decades resulted in gradual lowering of water table at the rate of approximately 0 85 m y 1 forty two groundwater samples have been collected from different sectors of the studied aquifers for chemical and isotopic analyses the observed large spatial variability of chemical composition of groundwater in the study area is most probably linked to two processes i dissolution of dolomite and gypsum combined with calcite precipitation dedolomitisation and ii partial evaporation of water the first process plays an important role in the study area due to abundance of evaporites partial evaporation occurs in the upper part of the unsaturated zone during infiltration especially for groundwater sampled in the kebili and djerid regions apart from these processes there are others which influence the salinity of the aquifers in the mio pliocene aquifer which behaves as an open system to gases and which receives inputs of co2 gas derived from intensive tectonic activity in the area the interaction of carbon dioxide with carbonate matrix of the aquifer produces an increase in the alkalinity of water in the senonian and turonian aquifers the process of dedolomitisation evolves in a closed system with respect to co2 gas ca2 na cation exchange and halite dissolution processes are also important stable isotope composition of water 18o 2h indicates that the recharge occurs from the dahar upland the 14c activity varies between 89 5 1 5 and 3 7 2 1 pmc the 13c content in the total dissolved inorganic carbon tdic range between 13 9 and 3 6 the calculated concentrations of 13c in the co2 gas in equilibrium with the tdic vary between 22 and 11 indicating two sources of carbon in the solution carbonate matrix 13c 2 and soil co2 13c from 25 to 21 for the cultivated areas mean residence times of water have been determined after correction of the initial 14c activities for 14c dilution processes including carbonate dissolution calcite precipitation and cation exchange the dilution processes were quantified on the basis of geochemical and 13c mass balance equations the calculated mean residence times of water confirm modern recharge from dahar upland and the mountains surrounding depressions and indicate the presence of paleowaters in the east and south west region and in the discharge zone

quantifying the modern recharge of the fossil sahara aquifers 1 the north western sahara aquifer system nwsas one of the world s largest groundwater systems shows an overall piezometric decline associated with increasing withdrawals estimating the recharge rate in such a semiarid system is challenging but crucial for sustainable water development in this paper the recharge of the nwsas is estimated using a regional water budget based on grace terrestrial water storage monthly records soil moisture from the gldas a land data system that assimilates hydrological information and groundwater pumping rates a cumulated natural recharge rate of 1 40 0 90 km3 yr 1is estimated for the two main aquifers our results suggest a renewal rate of about 40 which partly contradicts the premise that recharge in this area should be very low or even null aquifer depletion inferred from our analysis is consistent with observed piezometric head decline in the two main aquifers in the region annual recharge variations were also estimated and vary between 0 and 4 40 km3 yr 1for the period 2003 2010 these values correspond to a recharge between 0 and 6 75 mm yr 1 on the 650 000 km2of outcropping areas of the aquifers which is consistent with the expected weak and sporadic recharge in this semiarid environment these variations are also in line with annual rainfall variation with a lag time of about 1 year

preferential dissolution of 234u and radiogenic pb from α recoil damaged lattice sites in zircon implications for thermal histories and pb isotopic fractionation in the near surface environment abstract zircons from two archean samples a 2732 ma trondhjemite and a 2728 ma dacite both from the western superior province were washed in hydrofluoric acid hf at moderate temperatures zircon residues and hf wash solutions were analyzed for u pb isotope systematics and 234u 238u ratios in an attempt to study the effect of hf washing on selective dissolution of radiogenic isotopes in areas damaged by recoil on a conventional concordia diagram the datum from one analyzed wash solution plots below concordia near the array of discordant data from magnetic unabraded zircon fractions which record a lower concordia intersection of about 230 ma data points from hf wash residues of two different zircon fractions from the trondhjemite are also discordant and collinear with near concordant data from abraded unleached fractions which record the true age of crystallization however this array has a negative lower concordia intercept of 749 ma the residue and wash from hf treatment of a single zircon fraction from the dacite are collinear with the crystallization age of the sample as defined by near concordant data from unleached fractions this line has a lower concordia intersection of 1388 ma also negative but distinct from the trondhjemite residue line the discordance of strongly washed residues is proportional to the calculated recoil dosage of the grains leach residues show negative 234u 238u anomalies while the leaches themselves show 234u excess on the order of 10 these results can be explained by the bulk dissolution of altered and metamict crystal domains during the hf wash accompanied by leaching of 234u and radiogenic pb from recoil damaged lattice sites in alteration free zircon residues studies were also carried out on zircons washed in nitric acid these show small ca 0 5 negative 234u anomalies this suggests that small amounts of radiogenic pb can be leached from damaged lattice sites by nitric acid the negative age lower concordia intercepts of pb loss lines defined by hf leach residues can be explained if early radiation damage has been annealed so that only more recently formed radiogenic pb atoms are in damaged crystal lattice sites and are susceptible to leaching since this pb fraction is relatively young the residue pb is biased toward anomalously old values a model is proposed for early burial and heating of the rocks above the annealing temperature for recoil damage followed by uplift and cooling several hundred million years after crystallization this model predicts a cooling time of 2500 20 ma for the trondhjemite and 2295 35 ma for the rhyolite comparison with metamorphic and cooling histories from ar ar studies suggest that the long term annealing temperature for low levels of recoil damage in zircon is in the range 250 300 c approximately the same as for fission tracks these results demonstrate that hf and possibly to a small extent hno3 can preferentially leach radioactive daughter elements from damaged zircon lattice sites in addition to dissolving metamict domains the threshold recoil dosage for dissolution of damaged zircon appears to be slightly in excess of 1 5 1015 decay events mg the isotopic composition of the leached pb is sensitive to the annealing history of the zircon thus in addition to supplying the magmatic crystallization age of the sample the hf leaching of zircon might be useful for constraining its later low temperature thermal history leaching from recoil damaged sites also occurs in the natural environment and this may lead to significant enhancement of 206pb 204pb in groundwater

processes controlling 234u and 238u isotope fractionation and helium in the groundwater of the st lawrence lowlands quebec the potential role of natural rock fracturing abstract the goal of this study is to explain the origin of 234 u 238 u fractionation in groundwater from sedimentary aquifers of the st lawrence lowlands quebec canada and its relationship with 3 he 4 he ratios to gain insight regarding the evolution of groundwater in the region 234 u 238 u activity ratios or 234 u 238 u act were measured in 23 groundwater samples from shallow quaternary unconsolidated sediments and from the deeper fractured regional aquifer of the becancour river watershed the lowest 234 u 238 u act 1 14 0 01 was measured in ca hco 3 type freshwater from the quaternary shallower aquifer where bulk dissolution of the carbonate allows u to migrate into water with little 234 u 238 u isotopic fractionation the 234 u 238 u act increases to 6 07 0 14 in na hco 3 cl type groundwater preferential migration of 234 u into water by recoil is the underlying process responsible for this isotopic fractionation an inverse relationship between 234 u 238 u act and 3 he 4 he ratios has been observed this relationship reflects the mixing of newly recharged water with 234 u 238 u act close to the secular equilibrium and containing atmospheric tritiogenic helium and mildly mineralized older water 14 c ages of 6 6 kyrs with 234 u 238 u act of 6 07 and large amounts of radiogenic 4 he in excess of the steady state amount produced in situ the simultaneous fractionation of 234 u 238 u act and the addition of excess 4 he could be locally controlled by stress induced rock fracturing this process increases the surface area of the aquifer matrix exposed to pore water from which produced 4 he and 234 u can be released by recoil and diffusion this process would also facilitate the release of radiogenic helium at rates greater than those supported by steady state u th production in the rock consequently sources internal to the aquifers could cause the radiogenic 4 he excesses measured in groundwater

palaeoclimate and the formation of sapropel s1 inferences from late quaternary lacustrine and marine sequences in the central mediterranean region abstract synchronous responses to climatic changes during the late pleistocene holocene transition are inferred from marine and lacustrine stratigraphic records in the central mediterranean region new stratigraphic data are presented from well dated sequences in the meso adriatic depression mad two crater lakes in the lazio region and in the tyrrhenian sea the sequences all span the last termination and the holocene but we focus here on the evidence in each record for the time period during which sapropel s1 formed in the mediterranean ca 9 0 to 6 8 cal kyr b p the new records provide evidence of palaeoenvironmental changes on land and sea that can be reconstructed at a high temporal resolution and which throw some light on the processes which led to the formation of s1 the collective evidence indicates that i organic rich sediments occurred in both the marine and the crater lake sites during the time of formation of the s1 sapropel ii there is evidence of increased stratification and anoxia in the sea water column during the period of s1 formation iii the s1 period in the study area is divisible into two sub phases s1a and s1b which reflect short term variations in oceanographic conditions stratification and anoxia iv changes in stratification in the marine column were contemporaneous with regional climate variations that are inferred from the terrestrial records we conclude that the key factor that initiated the formation of s1 was increased discharge of freshwater into the mediterranean following a change post 9 0 cal kyr b p to a warmer and wetter climate furthermore the period of s1 formation was interrupted by a short lived episode ca 500 years of comparatively cooler and drier conditions during the early middle holocene transition emht

quantifying paleorecharge in the continental intercalaire ci aquifer by a monte carlo inversion approach of 36cl cl data abstract past variations of the recharge is estimated in the atlas mountains the main recharge area of the continental intercalaire ci aquifer one of the major saharan aquifers over the last 775 kyr in this mediterranean climatic context continental archives generally record single humid events grouped in hypothetical time interval and do not offer continuous chronicles of precipitation we propose to use spatially distributed 36 cl cl data as a temporal constraint to infer the past recharge in the atlas area based on a simplified but robust climatic scenario assuming a piston model we apply a markov chain monte carlo mcmc inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods the interglacial recharge values vary from a few mm yr 1 to more than 60 mm yr 1 glacial recharge is less than 1 mm yr 1 these values are then analyzed in terms of intensity and allow questioning some initial hypothesis especially the generally accepted value of the initial 36 cl cl value around 133 10 15 at at 1 our analysis suggests a higher value around 175 10 15 at at 1 this approach allows us to bring out paleoclimatic information retained in these continental archives especially computed recharges provide reliable evidence that marine isotope stage mis 5 was more humid than the holocene period in agreement with marine archives that document wet condition in the north sahara during that period

updated geochemical and isotopic data from the continental intercalaire aquifer in the great occidental erg sub basin south western algeria abstract due to its size and to its strategic importance as a huge transboundary aquifer the continental intercalaire c i has been widely studied making use of the most classical groundwater research approaches as well as the latest techniques investigations focused more on the shared parts and neglected the great occidental erg sub basin occurring exclusively in algeria the present research deals with recent hydrochemical and isotope results obtained for that sub basin the latter has seen major developments and an increase in water demand during the last two decades the occidental sub basin occurs in an arid region limited by the atlas mountains north tademait plateau south saoura valley west and the m zab uplift east rainfall is relatively high only in the mountainous recharge area saharan atlas rainfall decreases southward with decreasing altitude from 200 mm to 20 mm yearly in the discharge area the sub basin is characterized by a dense run off network wadis flowing southward from the southern atlas foothills the c i reservoir is contained in the continental formations of the lower cretaceous the aquifer is mostly unconfined unlike the eastern part its depth increases from north to south it has a shape of a depression that disappears further south where it becomes confined tds increases southward along the recognized flowpath sample points align parallel to the halite dissolution line on a cl na plot confirming the origin of those elements major elements increase with the increase of tds moreover a good correlation between calcium and sulfates suggests gypsum and anhydrite leaching as water flows underground isotopically speaking groundwater are enriched and exhibit large variations in stable isotopes 18 o and 2 h they are distributed along an evaporation line corresponding to a mixing of two end members the erg aquifer that feeds the c i in the north and unaffected c i groundwater in the south in the west saoura valley alluvial aquifer samples align in the continuation of the evaporated c i line mirroring the intermingling of two aquifers during wadi saoura flooding radiocarbon data showed that appreciable amount of 14 c in the c i is traceable only up to few distances from the aquifer exposure zones up north atlas escarpments the confined part of the reservoir shows depleted stable isotopes data and low 14 c content reflecting the absence of modern water at the discharge zone

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago

mediterranean pluvial periods and sapropel formation over the last 200 000 years abstract three mediterranean deep sea cores have been studied to reconstruct the hydrological conditions at the time of deposition of interglacial and glacial sapropels during the time interval 200 60 kyr bp the isotopic and sea surface temperature records of planktonic foraminifera were used to estimate the oxygen isotopic composition of surface water w in the tyrrhenian sea and levantine basin our results show a strong w salinity decrease in both basins associated with each sapropel these w decreases reflect a drastic change in the local freshwater budget and were comparable to that observed during the last sapropel s 1 centred at ca 8 kyr bp a strong precipitation increase transformed the whole mediterranean sea into a non concentration basin the low surface salinity was responsible for the water column stratification while nutrients brought in by the runoff enhanced productivity it resulted in a significant reduction in the rate of deep water oxygenation and sapropel formation in the eastern basin in addition the mediterranean sea 18 o record is very similar to that of the bay of bengal as the latter is very sensitive to the summer monsoon rain this similarity suggests that both glacial and interglacial sapropels result from enhanced monsoon like precipitation due to an increase in summer insolation driven by precession variations

234u 238u evidence for local recharge and patterns of ground water flow in the vicinity of yucca mountain nevada usa uranium concentrations and 234u 238u ratios in saturated zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of yucca mountain a potential high level radioactive waste disposal site the u data were obtained by thermal ionization mass spectrometry on more than 280 samples from the death valley regional flow system large variations in both u concentrations commonly 0 6 10 g l 1 and 234u 238u activity ratios commonly 1 5 6 are present on both local and regional scales however ground water with 234u 238u activity ratios from 7 up to 8 06 is restricted largely to samples from yucca mountain data from ground water in the tertiary volcanic and quaternary alluvial aquifers at and adjacent to yucca mountain plot in 3 distinct fields of reciprocal u concentration versus 234u 238u activity ratio correlated to different geographic areas ground water to the west of yucca mountain has large u concentrations and moderate 234u 238u whereas ground water to the east in the fortymile flow system has similar 234u 238u but distinctly smaller u concentrations ground water beneath the central part of yucca mountain has intermediate u concentrations but distinctive 234u 238u activity ratios of about 7 8 perched water from the lower part of the unsaturated zone at yucca mountain has similarly large values of 234u 238u these u data imply that the tertiary volcanic aquifer beneath the central part of yucca mountain is isolated from north south regional flow the similarity of 234u 238u in both saturated and unsaturated zone ground water at yucca mountain further indicates that saturated zone ground water beneath yucca mountain is dominated by local recharge rather than regional flow the distinctive 234u 238u signatures also provide a natural tracer of downgradient flow elevated 234u 238u in ground water from two water supply wells east of yucca mountain are interpreted as the result of induced flow from 40 a of ground water withdrawal elevated 234u 238u in a borehole south of yucca mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic tertiary volcanic aquifer where it becomes diluted by regional flow in the fortymile system