integrated hydrogeological and geophysical study of depression focused groundwater recharge in the canadian prairies 1 in the northern prairie region of north america numerous seasonal wetlands and ephemeral ponds form as snowmelt water is trapped in small topographical depressions a detailed hydrogeological investigation is combined with electrical resistivity imaging eri to evaluate the roles of the wetlands and ponds on depression focused groundwater recharge at the st denis national wildlife area in saskatchewan canada the analysis of groundwater samples indicated two distinct geochemical zones a zone of salt leaching under wetland 109 and zones of salt accumulation under the adjacent uplands two intermediate zones were identified as partially leached or mixed leached unleached and premodern or mixed saline nonsaline water the resistivity images of the areas around 109 were classified using the correlation between eri derived electrical conductivity ec and the groundwater ec the eri data clearly showed that depression focused recharge occurs under all depressions regardless of size leaching is observed to the depth of a regional intertill aquifer indicating that depression focused recharge contributes to the regional groundwater system the eri data also revealed the complex pattern of salt distribution that could not have been recognized by hydrogeological observations alone the complex distribution of salts appears to be caused by interaction between wetlands and variations in topography index terms 1829 hydrology groundwater hydrology 1045 geochemistry low temperature geochemistry keywords depression focused recharge electrical resistivity imaging groundwater recharge prairie wetland

spatial variability of hydraulic conductivity and bulk density along a blanket peatland hillslope this article presents the results of a field investigation of saturated hydraulic conductivity ksat and bulk density bd in an atlantic blanket bog in the southwest of ireland starting at a peatland stream and moving along an uphill transect toward the peatland interior bd and ksat were examined at regular intervals saturated horizontal hydraulic conductivity khsat and vertical kvsat was estimated at two depths 10 20 and 30 40 cm below the peat surface whereas bd was estimated for the full profile we consider two separate zones one a riparian zone extending 10 m from the stream and a second zone in the bog interior we found that the ksat was higher 10 5 m s 1 in the bog interior than that in the riparian zone 10 6 m s 1 whereas the converse applied to bulk density with lowest density 0 055 g cm 3 at the interior and highest 0 11 g cm 3 at the riparian zone in general we found khsat to be approximately twice the kvsat these results support the idea that the lower ksat at the margins control the hydrology of blanket peatlands it is therefore important that the spatial variation of these two key properties be accommodated in hydrological models if the correct rainfall runoff characteristics are to be correctly modelled stream flow analysis over 3 years at the peatland catchment outlet showed that the stream runoff was composed of 8 base flow and 92 flood flow suggesting that this blanket peatland is a source rather than a sink for floodwaters copyright 2011 john wiley sons ltd

origin of shallow saline groundwater on the island of laeso denmark abstract shallow saline groundwater has a patchy occurrence in superficial postglacial marine sediments beneath coastal meadows and salt marshes on the southern shores of the island of laeso denmark the postglacial sequence consists of 1 2 m of marine sands and gravels resting on a slightly undulating platform of practically impermeable interglacial marine clay which makes up a confining layer or aquiclude the saline groundwater primarily occurs beneath the vegetated fringe and associated minor unvegetated sand pans of coastal meadows and salt marshes the salinity varies from 2 to approximately 17 and analyses of major ions indicate a prevailing marine composition the saline groundwater is supersaturated with respect to carbonates in the entire salinity range and slightly supersaturated with respect to gypsum with salinities above 10 however aragonite is the only precipitate recorded so far stable isotope analyses 2 h and 18 o plot fairly well along the local groundwater seawater mixing line indicating mixing of seawater with fresh meteoric groundwater in the proportion 0 50 in contrast the stable isotopic compositions show no correlation to the wide range of salinities recorded it is proposed that the thin cover of superficial deposits resting on a clay platform results in a relatively closed hydrochemical system in which increasing concentration of dissolved solids in the interstitial water is caused by transpiration which is known to abstract water from groundwater without leading to isotopic fractionation periodical precipitation dissolution of salt in topsoils of sand pans and from salt secreting halophytes may also be contributory causes of increasing salinity in the groundwater the spatial distribution of the saline groundwater most likely depends on the microtopography of the slightly undulating clay platform the dense saline shallow groundwater emerging from the vegetated fringe and sand pans and flowing toward the depressions in the clay platform percolates downward displacing lower salinity seawater and groundwater

late pleistocene holocene eolian deposits of the southern buenos aires province argentina a preliminary model abstract the aim is to reconstruct the sedimentological history of the late pleistocene holocene eolian deposits of southern buenos aires province hence a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle the deposits include sandy loess fine sand sheets and dunefields the southern buenos aires province represents the depositional area whereas the source and transport environments are located in the northern patagonian andes and northern extra andean patagonia according to the mineralogical characteristics of the material explosive volcanism is considered the predominant process of particle formation particularly as generator of pyroclastic deposits which were reworked by different erosive processes glacial fluvial eolian during the late glacial maximum and the holocene alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units were deposited along the floodplains of the colorado and negro rivers including those distal segments located in the emerged offshore platform the fluvial regime depended upon the regime of the glaciers hence it reflected the major glacial fluctuations which took place in the andes floodplains were deflated by southwest winds transporting the material towards the southern buenos aires province sandy loess and loessial sands and to la pampa province dunefields

a multiscale study of the permeability of a thick clayey till we describe and evaluate two previously undocumented methods of bulk permeability k determination and compare results obtained with results of conventional smaller scale tests on the same thick clayey till deposit analysis of the downward propagation of seasonal water table fluctuations yielded bulk k of approximately 10 10 m s 1 in close agreement with results of laboratory consolidation and permeameter tests slug tests and distribution of vertical hydraulic gradient with depth in the deposit this agreement suggests that for such materials high gradient tests conducted at small scales of distance and time can provide reasonable estimates of bulk k the results also imply groundwater residence times in the till of thousands of years the flow pattern observed near a large excavation in the till was consistent with the initial recoveries of piezometers installed in much smaller boreholes assuming perturbation of hydraulic head in the formation due to borehole excavation time scales of these perturbations which prevent interpretation of measured hydraulic head using conventional well hydraulic methods varied from months to tens of years

ionic relations a tool for studying hydrogeochemical processes in pampean shallow lakes buenos aires argentina abstract main geochemical processes that determine the chemical composition of shallow lakes in the lower salado river basin were studied the region is characterized by the total lack of slope and the succession of humid and dry climates where the lakes are shallow alkaline and eutrophic ecosystems with circulation pattern corresponding to polymictic lakes gibbs classical diagrams confirmed that weathering of mineral phases and evaporation crystallization were the principal processes involved nevertheless deviations to the basic pattern were observed the study of ionic relations in meq l 1 determined that a cationic exchange process takes place in the pampean loessic sediments influencing the chemical characteristics of both groundwater s and surface waters

hydrogeology of two saskatchewan tills i fractures bulk permeability and spatial variability of downward flow groundwater flow patterns were compared and contrasted in two deposits of clayey till of different thicknesses both of which overlie regional aquifers in a flat upland recharge area analysis of 67 field and laboratory hydraulic conductivity determinations indicated that conductive fractures in the unoxidized zone of the thinner till dalmeny site give rise to an isotropic bulk permeability of approximately 5 10 9 ms 1 which is two orders of magnitude greater than the bulk permeability of unoxidized till in the thicker deposit warman site vertical pore water velocities estimated using hydraulic and isotopic evidence vary greatly at dalmeny and appear to be of the order of 8 m in 10 000 yr at warman the association of vertical groundwater flux depth to water table and depth of oxidized zone at a given location is referred to as the flow regime there spatially detailed water level monitoring and test drilling indicated substantial variation of the flow regime at dalmeny on a horizontal scale of tens of meters no such variation exists at warman the differentiation of the flow regime at dalmeny is due to higher till permeability which permits a spatially variable response to microtopographically focussed infiltration of surface water recognition of the causes and symptoms of flow regime variability beneath flat till landscapes has implications for environmental assessment procedures agricultural practices wetland management and waste disposal

evaluating the hydraulic and transport properties of peat soil using pore network modeling and x ray micro computed tomography abstract micro scale properties of peat pore space and their influence on hydraulic and transport properties of peat soils have been given little attention so far characterizing the variation of these properties in a peat profile can increase our knowledge on the processes controlling contaminant transport through peatlands as opposed to the common macro scale or bulk representation of groundwater flow and transport processes a pore network model pnm simulates flow and transport processes within individual pores here a pore network modeling code capable of simulating advective and diffusive transport processes through a 3d unstructured pore network was developed its predictive performance was evaluated by comparing its results to empirical values and to the results of computational fluid dynamics cfd simulations this is the first time that peat pore networks have been extracted from x ray micro computed tomography ct images of peat deposits and peat pore characteristics evaluated in a 3d approach water flow and solute transport were modeled in the unstructured pore networks mapped directly from ct images the modeling results were processed to determine the bulk properties of peat deposits results portray the commonly observed decrease in hydraulic conductivity with depth which was attributed to the reduction of pore radius and increase in pore tortuosity the increase in pore tortuosity with depth was associated with more decomposed peat soil and decreasing pore coordination number with depth which extended the flow path of fluid particles results also revealed that hydraulic conductivity is isotropic locally but becomes anisotropic after upscaling to core scale this suggests the anisotropy of peat hydraulic conductivity observed in core scale and field scale is due to the strong heterogeneity in the vertical dimension that is imposed by the layered structure of peat soils transport simulations revealed that for a given solute the effective diffusion coefficient decreases with depth due to the corresponding increase of diffusional tortuosity longitudinal dispersivity of peat also was computed by analyzing advective dominant transport simulations that showed peat dispersivity is similar to the empirical values reported in the same peat soil it is not sensitive to soil depth and does not vary much along the soil profile

hydrological management of a cutover peatland mechanisms for preserving the hydrological balance of thorne moors national nature reserve nnr while allowing peat extraction to continue in the surrounding peatland are examined three strategies exist for modifying seepage losses from the nnr i re direction of pump drainage waters from the peat extraction areas ii provision of a hydrological buffer zone around the nnr and iii mechanical compaction of the boundaries of the buffer zone strategy ii increased width of the buffer zone should be used together with strategy iii compaction for the maximum reduction of seepage losses from the nnr if strategy i water level manipulation is also adopted it may be possible to reduce the width of the hydrological buffer zone around the nnr

flow path oscillations in transient ground water simulations of large peatland systems abstract transient numerical simulations of the glacial lake agassiz peatland near the red lakes in northern minnesota were constructed to evaluate observed reversals in vertical ground water flow seasonal weather changes were introduced to a ground water flow model by varying evapotranspiration and recharge over time vertical hydraulic reversals driven by changes in recharge and evapotranspiration were produced in the simulated peat layer these simulations indicate that the high specific storage associated with the peat is an important control on hydraulic reversals seasonally driven vertical flow is on the order of centimeters in the deep peat suggesting that seasonal vertical advective fluxes are not significant and that ground water flow into the deep peat likely occurs on decadal or longer time scales particles tracked within the ground water flow model oscillate over time suggesting that seasonal flow reversals will enhance vertical mixing in the peat column the amplitude of flow path oscillations increased with increasing peat storativity with amplitudes of about 5 cm occurring when peat specific storativity was set to about 0 05 m 1

the baseflow and storm flow hydrology of a precambrian shield headwater peatland a hydrological investigation was conducted in a small headwater peatland located in the experimental lakes area north western ontario canada to determine the subsurface and surface flow paths within the peatland and between the peatland and an adjacent forested upland during baseflow and storm flow conditions distinct zones of groundwater recharge and discharge were observed within the peatland these zones are similar to those found in much larger flow systems even though the peatland was only influenced by local groundwater flow groundwater emerging in seeps and flowing beneath the peatland sustained the surface wetness of the peatland and maintained a constant baseflow the response of the peatland stream to summer rain events was controlled by peatland water table position when the basin was dry and antecedent moisture storage on the uplands when the basin was wet the magnitude and timing of peak runoa during wet conditions were controlled by the degree of hydrological connectivity between the surrounding upland terrain and the peatland 1998 john wiley sons ltd hydrol process vol 12 57 72 1998

pollen distribution in marine surface sediments from chilean patagonia abstract southern south america is the only mid to high latitude continental landmass in the southern hemisphere and therefore represents one of the key areas to recover terrestrial records for the study of past and future climate variability at these latitudes during cruise md 159 of r v marion dufresne cores were taken off the chilean coast from 40 to 55 s in the open ocean and the fjords pollen spectra of 32 core top samples illustrate the relationships between pollen distribution in the sampled marine sediments and their transport from the vegetation onshore pollen concentrations show two distinct gradients 1 a north to south gradient controlled by latitudinal vegetation changes and by westerly winds and 2 from the fjords to the open ocean a gradient clearly controlled by wind fluvial inputs and pollen diagenesis linked to the distance from the coast and water depth the distribution of pollen percentages in these marine surface sediment samples reflects the local vegetation from the nearby continental area consequently pollen analyses from older sediments in these marine cores will be reliable tools to reconstruct past vegetational changes

hydrological model of peat mound form with vertically varying hydraulic conductivity the form of raised peat mires can be considered to reflect the form of the saturated water mound ingram 1982 calculates this form from hydraulic considerations but assumes a uniform hydraulic conductivity throughout the profile here the form of the mire is calculated for non uniform depth dependent hydraulic conductivity of the peat deposit although requiring more computational effort the alternative model may offer a more appropriate description of the structure of the peat mire

tidal and tidally averaged circulation characteristics of suisun bay california availability of extensive field data permitted realistic calibration and validation of a hydrodynamic model of tidal circulation and salt transport for suisun bay california suisun bay is a partially mixed embayment of northern san francisco bay located just seaward of the sacramento san joaquin delta the model employs a variant of an alternating direction implicit finite difference method to solve the hydrodynamic equations and an eulerian lagrangian method to solve the salt transport equation an upwind formulation of the advective acceleration terms of the momentum equations was employed to avoid oscillations in the tidally averaged velocity field produced by central spatial differencing of these terms simulation results of tidal circulation and salt transport demonstrate that tides and the complex bathymetry determine the patterns of tidal velocities and that net changes in the salinity distribution over a few tidal cycles are small despite large changes during each tidal cycle computations of tidally averaged circulation suggest that baroclinic and wind effects are important influences on tidally averaged circulation during low freshwater inflow conditions exclusion of baroclinic effects would lead to overestimation of freshwater inflow by several hundred m3 s for a fixed set of model boundary conditions likewise exclusion of wind would cause an underestimation of flux rates between shoals and channels by 70 100

the archaeological record in pampean loess deposits abstract archaeological sites in the pampean loess or loess like deposits of southern buenos aires province are discussed the focus in on site formation processes and the main characteristics acquired by the sites pedogenesis and faunal turbation are considered the most important post depositional processes which can generate transformations the environmental conditions did not favor the preservation of organic remains other than bones and shells the poor preservation of archaeological features i e floors and pits is interpreted as a consequence of post depositional processes whereas the reduced amount of charcoal and hearths could result from environmental conditions

geomorphology morphometry spatial distribution and ages of mafic monogenetic volcanoes of the peinado and incahuasi fields southernmost central volcanic zone of the andes abstract the back arc southern puna region south of 24 s in the central volcanic zone of the andes contains hundreds of miocene to quaternary mafic monogenetic volcanoes roughly distributed in several volcanic fields analysis of the geomorphology morphometry spatial distribution and age of 36 mafic monogenetic centers in the peinado and incahuasi fields at the southwestern margin of the puna plateau gives insights on the links that mafic monogenetic volcanism can have with main and secondary structures and the effects of degradation on scoria cone shape degree of preservation morphometry and six new unspiked k ar ages indicate two distinct age groups old centers 2 ma and young centers 1 ma the older mafic centers consist of degraded scoria cones and lava flows with an estimated cumulative bulk volume of 2 2 km3 the younger mafic centers display a variety of well preserved landforms scoria cones lava flows maars tuff rings domes and have an estimated cumulative volume of 6 2 km3 their distribution in a main nne ssw and secondary ene wsw trends suggests a control by the regional nne ssw striking peinado and san francisco fault systems in a dextral strike slip configuration scoria cone and lava flow morphometries are similar in both fields but a marked difference indicating hydrovolcanic activity is the presence of tuff rings maars and tephra fallout deposits only in the peinado field morphometric analysis of dated pristine and degraded scoria cones of the southern puna region shows that parameters related to crater depth and inner crater steepness decrease consistently with age suggesting that crater infilling is the degradation related process that is most sensitive to age at scoria cones in the southern puna high values of crater depth crater width crater depth cone height and mean inner crater slope are indicative of a young 1 ma age as young cones can have initially low values

groundwater surface water interactions in headwater forested wetlands of the canadian shield abstract groundwater and surface water interaction in two conifer swamps located in headwater catchments with contrasting till depth typical of the southern canadian shield were studied from june 1990 to august 1992 both swamps had little influence on the regulation or attenuation of seasonal runoff response in the catchment the two valley bottom swamps were connected to local aquifers but the upland wetland connection was continuous in the catchment with deeper till and ephemeral in the catchment with thin till rock ridges groundwater movement through the wetlands was restricted mainly to the surface peat layer in both wetlands because a large portion of inputs from shallow soil layers and stream inflows enter near the peat surface however differences in upland wetland connections resulted in contrasting hydrologic regimes in the two swamps during seasons with larger inputs both swamps were hydrologically connected to uplands and had a similar hydrology characterized by a high water table rapid storm response and predominance of saturated overland flow in summer upland inputs were absent in the catchment with thin till rock ridges resulting in cessation of baseflow and a lower water table that varied in response to variations in rainfall continuous upland inputs throughout the summer in the catchment with deeper tills 1 3 m sustained baseflow and kept the water table near the peat surface this study demonstrates the control of morphology and shallow subsurface geology on the hydrology of valley bottom swamps influenced by local aquifers

controls on depression focused recharge during spring snowmelt on the oak ridges moraine southern ontario canada abstract the oak ridges moraine orm is a key hydrogeologic feature in southern ontario previous work has emphasized the value of groundwater recharge r estimates and information on the timing and location of r to incorporating groundwater protection into land use planning on the orm and has suggested that ponding and depression focused recharge dfr occurs in closed topographic depressions on the orm during winter and spring however the influence of land cover topography and soil type on the occurrence and extent of dfr is unclear sixteen closed topographic depressions on the crest and flanks of the orm were chosen for study observations of ponding and measurements of pond extent during the snowmelt period were related to depression land cover percent coverage of open vs forested cover depression geometry i e contributing area average slope and relief ratio and soil texture sand silt and clay ponding occurred at 10 of 16 depressions all of which were in predominantly open agricultural cover ponded and non ponded depressions differed significantly in terms of percentage of open area and average slope ponded depressions were gently sloped with high proportions 85 of open coverage while non ponded depressions were steep and largely forested extent of ponding was not related to any of the depression characteristics however pond volume showed a significant positive non linear relationship with average slope for a subset of sites with similar land cover and soil type results suggest that ponding and dfr during winter and spring on the orm is restricted to open agricultural areas and that this phenomenon may be more pronounced in steeper depressions the relationship between land cover and dfr presented here could have implications for the quantity and quality of water recharging the orm s aquifers and further work is recommended to better define this relationship

geomorphology and spatial distribution of monogenetic volcanoes in the southern puna plateau nw argentina abstract monogenetic volcanoes are the most common volcanic landforms on earth several works have shown that the study of geomorphology and spatial distribution of such small volcanoes reveals important aspects related to the volcanic field from eruption dynamics to tectonic processes the altiplano puna plateau 15 to 27 s central andes comprises the largest non collisional plateau on earth characterized by intensive deformation uplift and volcanism this region is marked by the presence of hundreds of mafic monogenetic volcanoes despite its discrete occurrence the monogenetic volcanoes show a close relationship with large scale lithospheric foundering events in the region in this contribution we applied a geographic information system gis approach in order to build the first complete catalog of monogenetic landforms present in the southern puna plateau 24 30 to 27 00 s nw argentina exploring their evolution and relationship with geotectonic events combining spatial analysis with a geomorphological study of each volcano it was possible to identify the presence of 285 monogenetic volcanoes in the region which could be grouped in seven clusters southern puna shows a predominance of cinder cones and lava domes suggesting a small influence of water in the eruption dynamics despite its high clustering degree southern puna monogenetic vents present a low density 8 3 10 3 vents km2 when compared to other monogenetic fields around the world in addition puna plateau cones show a close relationship with monogenetic cones associated with an extensional environment considering their back arc context based on relative and available absolute ages it was possible to map the volcanism evolution through time the results indicate that the first monogenetic volcanoes appeared in the central region of the plateau sierra de calalaste mainly associated with large scale nne ssw reactivated faults the activity then expanded antofalla and antofagasta regions and finally reached the borders of the plateau pasto ventura and arizaro regions which show volcanoes associated with nw se normal faults

silica dynamics in a pampean lake lake chascomus argentina abstract silica dynamics in a pampean lake lake chascomus chaco pampa plain argentina is described pampean lakes are shallow alkaline ecosystems with circulation pattern corresponding to polymictic lakes due to the nearly continuous vertical mixing that promotes a high concentration of suspended particulate matter and low transparency a silica budget was calculated in lake chascomus in 1999 2000 using a one box model that constitutes the first one for a shallow pampean lake in general terms the dissolved silica in lake chascomus contributed by streams and groundwater the latter responsible for 70 of the incoming dissolved silica was able to support the diatom production development however in the algal bloom period spring and summer the winter input contributed by the streams and groundwater was insufficient and therefore an extra source of dissolved silica was necessary dissolution of silicate mineral phases or silica desorption from metallic oxides might provide the dissolved silica needed for the seasonal increase in diatom abundance

on lagrangian residual currents with applications in south san francisco bay california the lagrangian residual circulation has often been introduced as the sum of the eulerian residual circulation and the stokes drift unfortunately this definition of the lagrangian residual circulation is conceptually incorrect because both the eulerian residual circulation and the stokes drift are eulerian variables in this paper a classification of various residual variables are reviewed and properly defined the lagrangian residual circulation is then studied by means of a two stage formulation of a computer model the tidal circulation is first computed in a conventional eulerian way and then the lagrangian residual circulation is determined by a method patterned after the method of markers and cells to demonstrate properties of the lagrangian residual circulation application of this approach in south san francisco bay california is considered with the aid of the model results properties of the eulerian and lagrangian residual circulation are examined it can be concluded that estimation of the lagrangian residual circulation from eulerian data may lead to unacceptable error particularly in a tidal estuary where the tidal excursion is of the same order of magnitude as the length scale of the basin a direction calculation of the lagrangian residual circulation must be made and has been shown to be feasible

groundwater interaction with a kettle hole lake relation of observations to digital simulations abstract observations made on a small fully penetrating kettle hole lake are used to test predictions made using digital stimulation about the exchange of water between lakes and groundwater systems on the upgradient side of lower nashotah lake groundwater inseepage decreases exponentially away from the shore water exchange on the downgradient side is controlled by the presence and strength of a groundwater mound but that mound and its effects are ephemeral when the mound increases in strength it generates a local groundwater flow cell beneath it which contains water of chemical composition distinct from deeper regional groundwater the growing mound also causes local inseepage of groundwater to the lake in an expanding zone along the downgradient shore all of these phenomena occur as they have been predicted in published studies where digital simulation of relatively simple groundwater lake systems has been carried out the field observations thus confirm the validity of the simulation studies and their usefulness as a tool for understanding water exchanges in unmonitored systems

fracture effects in the shallow groundwater zone in weathered sarnia area clay the sarnia area of southwestern ontario is underlain by thick deposits of clay rich glacial till from ground surface to between 4 and 6 m depth the clay till is fractured oxidized and penetrated by root holes the water table fluctuates seasonally between the ground surface and the bottom of the weathered zone water level response tests in conventional piezometers installed in augered holes in the weathered zone typically provide very low values of hydraulic conductivity 10 7 10 9 cm s which are similar to values obtained from the deeper unweathered and unfractured till the augering process creates a smeared clay zone along the borehole walls that significantly reduces measured hydraulic conductivity values in this study the weathered zone hydraulic conductivity was measured using unconventional piezometers a large diameter well and a tracer experiment between two test pits the smeared zone was removed or reduced around these installations and the measured weathered zone hydraulic conductivi

hydraulic reversals and episodic methane emissions during drought cycles in mires reversals in hydraulic head and extensive methane losses were identified during a drought cycle from changes in hydraulic head and dissolved methane profiles in peat glacial lake agassiz peatlands northern minnesota united states during the summer 1990 drought regional groundwater discharged from the mineral soil underlying the peat to raised bogs concentrations of dissolved ch sub 4 in pore water showed that much of the peat column was supersaturated with respect to a reference standard of 1 atm partial pressure ch sub 4 by the summer of 1991 the severity of the drought lessened and the upper peat column was resaturated with precipitation derived water groundwater flow was then controlled by local precipitation driven recharge flow systems and the regional groundwater that was discharged affected only the peat immediately above the mineral soil peat interface in contrast during the summer of 1991 concentrations of dissolved ch sub 4 in pore water were all undersaturated with respect to 1 atm partial pressure ch sub 4 23 refs 5 figs

abundance and distribution of fluoride concentrations in groundwater la ballenera catchment southeast of buenos aires province argentina high concentrations of fluoride have negative effects on human health and it is one of the most widespread contaminants in groundwater having being found in high concentrations in different countries over the entire world fluoride concentrations exceeding the drinking water levels are very frequent in the region known as pampa plain in argentina associated with loess like sediments bearing volcanic shards and calcium carbonate concretions in this study the behavior of fluoride in the catchment of la ballenera creek is performed to contribute to the understanding of the processes controlling its distribution at catchment scale the catchment is around 100 km2 mostly in a flat landscape outflowing into the atlantic ocean the concentration of major ions silica and fluoride was measured on 34 groundwater and 9 surface water samples obtained in october 2013 all the samples both streamwater and groundwater were sodium bicarbonate type with an average ph of 8 and 7 3 respectively the fluoride concentrations in groundwater ranged between 1 1 and 2 5 mg l with an average value of 1 9 mg l fluoride concentrations in streamwater were close to groundwater concentrations indicating the predominance of baseflow on surface water composition the variations in the contents of fluoride are due to two main factors one geological geomorphological and the other geochemical the first one is related to the dominance of loess like sediments formed by minerals bearing fluoride and the second one is related to the control resulting on calcite precipitation adsorbing fluoride ions

predominance of arid climates indicated by mammals in the pampas of argentina during the late pleistocene and holocene abstract south american climates during the pleistocene and holocene are poorly known especially in the lowlands east of the andean ranges however there is relatively good biostratigraphic and climatic information for the late lujanian and platan local ages latest pleistocene to holocene of the pampas region most of the climatic data from the pampean area are based on land mammal assemblages pollen records and geology contrasting with north american faunal associations there are many non analogue assemblages in the holocene of the pampean area the climate was arid and relatively cold during most of the early lujanian however several relatively wet and warm events are suggested by the presence of subtropical mammals and evidence of tchernozoid paleosoils the late lujanian is well represented by the fossiliferous sediments of the la chumbiada and guerrero members of the lujan formation ca 30 000 and 21 000 13 000 yr b p respectively cropping out in stream valleys in the buenos aires province mammal assemblages suggest a change from a temperate to a cool climate from the la chumbiada member to the guerrero member the latter representing the last glacial maximum the lower portion of the eolian la postrera formation has been dated at 11 000 8000 yr b p this section was deposited on basin divides under dry and cool conditions the mammal association was similar to that of the gueffero member and includes the last megaherbivore mammals in south america e g megatherium americanum mammals occurring in platan beds usually indicate dry but not cool conditions warm and probably wetter conditions were detected in continental beds deposited synchronously with a mid holocene marginal marine ingression ca 7000 6000 yr b p a holocene non analogue assemblage of subtropical mammals and dry cold indicators occurred at latitude 38 30 s at ca 2000 yr b p subtropical mammals that require wetter conditions have been migrating southwards in the pampean region since 1500 1000 yr b p eolian sediments in the la postrera formation dated ca 440 yr b p yielded an arid to semiarid mammal assemblage which could relate to the little ice age period remarkably most of the present day manual biocenoses and climatic conditions in the pampas seem to have been recently established paleogeographic maps for the last glacial maximum and the mid holocene are also provided

imaging a magma plumbing system from mash zone to magma reservoir the puna plateau of the central andes is a well suited location to investigate the processes associated with the tectono magmatic development of a cordilleran system these processes include long lived subduction including shallow and steep phases substantial crustal thickening the emplacement of large volumes of igneous rocks and probably delamination to elucidate the processes associated with the development of a cordilleran system we pair common conversion point derived receiver functions with rayleigh wave dispersion data from ambient noise tomography the resulting high resolution shear wave velocity model of the southern puna plateau reveals the details of a lithospheric scale magma plumbing system slow velocities near the crust mantle transition are interpreted as a mash zone a partially molten zone where mantle derived melts interact with the lithosphere and undergo density differentiation with 4 9 4 9 melt after differentiation less dense and presumably more felsic melts propagate to shallower depths within the crust 20 km below surface and comprise vertically 10 km and laterally 75 km extensive slow velocity bodies that span the frontal arc and plateau interior these large slow velocity bodies represent a partially molten mid crust up to 22 where magma can further evolve to higher silica concentrations the periodic influx of melt from the underlying mash zone into these mid crustal bodies may serve as a trigger to the eruption of the voluminous ignimbrites observed in the southern puna plateau many of the active tectonic processes operating along the southern puna plateau are thought to be analogous to the processes that formed the north american cordillera thus these results could provide insight into some of the processes associated with the development of a cordilleran margin

sedimentological and mineralogical records from drill core skd1 in the jianghan basin central china and their implications for late cretaceous early eocene climate change abstract the jianghan basin central china contains a thick continuous sequence of early cenozoic evaporite bearing deposits that provide a wealth of information about early cenozoic climate change and evaporites accumulation in east asia however the paleoclimatic history of the basin and its effect on the evaporites are largely unknown we obtained a deep drill core skd1 2346 5 m long from the late cretaceous to early eocene evaporite bearing deposits in the jianghan basin detailed analyses of the lithofacies mineralogy and geochemistry of the core indicate that salinization and deepening of the lake occurred corresponding to the yuyang formation to the lower xingouzui formation followed by desalting and shallowing corresponding to the upper xingouzui formation reconstruction of the spatiotemporal evolution of the evaporites in the jiangling depression suggests a transition from semi arid conditions in the late cretaceous to extremely arid conditions in the initial stage of the eocene the climate then became semi humid during the deposition of the upper xingouzui formation indicating a monsoon or monsoon like climate in east asia during the early eocene the southward migration of the depocenter of the jiangling depression coupled with a warm and extremely arid climate resulted in the precipitation of evaporites during the accumulation of the upper shashi formation and the lower xingouzui formation these results indicate great potential for identifying potash resources within the subsag in the southern jiangling depression

hydrogeochemistry and isotope techniques to determine water interactions in groundwater dependent shallow lakes wet pampa plain argentina this paper gives an account of the implementation of hydrochemical and isotopic techniques to identify and explain the processes that govern solute exchange in two groundwater dependent shallow lakes in the southeastern pampa plain of argentina water samples lakes streams spring water and groundwater for hydrochemical and stable isotopic determination were collected and the main physical chemical parameters were measured the combination of stable isotope data with hydrogeochemical techniques was used for the identification of sources and preferential recharge areas to these aquatic ecosystems which allowed the explanation of the lake water origin the hydrochemical processes which explain los padres lake water chemistry are evaporation from groundwater co2 input calcite dissolution na release by ca2 and mg2 exchange and sulfate reduction the model that best aligns with la brava lake hydrochemical constraints includes mixing co2 and calcite dissolution cationic exchange with na release and mg2 adsorption and to a lesser extent ca na exchange this model suggests that the fractured aquifer contribution to this water body is greater than 50 an isotopic specific fingerprint for each lake was identified finding a higher evaporation rate for la brava lake compared to los padres lake isotopic data demonstrate the importance of these shallow lakes as recharge areas to the regional aquifer becoming areas of high groundwater vulnerability the tandilia range system considered in many hydrogeological studies as the impermeable bedrock of the pampean aquifer acts as a fissured aquifer in this area contributing to low salinity waters and with a fingerprint similar to groundwater isotopic composition

structure of peat soils and implications for water storage flow and solute transport a review update for geochemists abstract global peatlands are a valuable but vulnerable resource they represent a significant carbon and energy reservoir and play major roles in water and biogeochemical cycles peat soils are highly complex porous media with distinct characteristic physical and hydraulic properties pore sizes in undecomposed peat can exceed 5 mm but significant shrinkage occurs during dewatering compression and decomposition reducing pore sizes the structure of peat soil consists of pores that are open and connected dead ended or isolated the resulting dual porosity nature of peat soils affects water flow and solute migration which influence reactive transport processes and biogeochemical functions advective movement of aqueous and colloidal species is restricted to the hydrologically active or mobile fraction of the total porosity i e the open and connected pores peat may attenuate solute migration through molecular diffusion into the closed and dead end pores and for reactive species also through sorption and degradation reactions slow diffusion limited solute exchanges between the mobile and immobile regions may give rise to pore scale chemical gradients and heterogeneous distributions of microbial habitats and activity in peat soils while new information on the diversity and functionalities of peat microbial communities is rapidly accumulating the significance of the geochemical and geomicrobial study on peat stands to benefit from a basic understanding of the physical structure of peat soils in this paper we review the current knowledge of key physical and hydraulic properties related to the structure of globally available peat soils and briefly discuss their implications for water storage flow and the migration of solutes this paper is intended to narrow the gap between the ecohydrological and biogeochemical research communities working on peat soils

modelling two dimensional steady state groundwater flow and flow sensitivity to boundary conditions in blanket peat complexes this study used a two dimensional steady state finite element groundwater flow model to simulate groundwater flow in two newfoundland blanket peat complexes and to examine flow system sensitivity to changes in water table recharge and aquifer properties the modelling results were examined within the context of peat forming processes in the two complexes modelled flow compared favourably with observed flow the sensitivity analyses suggested that more highly decomposed bog peat along bog margins probably has had a positive impact on net peat accumulation within bog interiors peat with lower hydraulic conductivity along bog margins effectively impedes lateral drainage localizes water table drawdown to extreme bog margins and elevates water tables along bog interiors peat formation and elevated water tables in adjacent poor fens laggs currently rely on placic and ortstein horizons impeding vertical drainage and water flow inputs from adjacent bogs modest reductions in atmospheric recharge were found to govern bog flow system geometries in a way that would adversely affect paludification processes in adjacent fens laggs copyright 2004 john wiley sons ltd

anisotropy and depth related heterogeneity of hydraulic conductivity in a bog peat ii modelling the effects on groundwater flow anisotropy and heterogeneity of hydraulic conductivity k are seldom considered in models of mire hydrology we investigated the effect of anisotropy and heterogeneity on groundwater flow in bog peat using a steady state groundwater model in five model simulations four sets of k data were used the first set comprised measured k values from an anisotropic and heterogeneous bog peat these data were aggregated to produce the following simplified data sets an isotropic and heterogeneous distribution of k an isotropic and homogeneous distribution and an anisotropic and homogeneous distribution we demonstrate that where anisotropy and heterogeneity exist groundwater flow in bog peat is complex fine scale variations in k have the potential to influence patterns and rates of groundwater flow however for our data at least it is heterogeneity and not anisotropy that has the greater influence on producing complex patterns of groundwater flow we also demonstrate that patterns and rates of groundwater flow are simplified and reduced when measured k values are aggregated to create a more uniform distribution of k for example when measured k values are aggregated to produce isotropy and homogeneity the rate of modelled seepage is reduced by 28 we also show that when measured k values are used the presence of a drainage ditch can increase seepage through a modelled cross section our work has implications for the accurate interpretation of hydraulic head data obtained from peat soils and also the understanding of the effect of drainage ditches on patterns and rates of groundwater flow

miocene pliocene shortening extension and mafic magmatism support small scale lithospheric foundering in the central andes nw argentina

application of three dimensional lagrangian residual transport three dimensional lagrangian residual circulation computed from an intratidal hydrodynamic model is used to drive an intertidal water quality transport model the sum of the eulerian residual and stokes drift is used as a first order approximation for the lagrangian residual current the stokes drift approximates residual currents induced by the nonlinear interactions of the tidal currents computed residual velocities show close agreement with a two dimensional vertical longitudinal analytical result the methodology is applied to chesapeake bay for the entire year of 1985 salinity computed with the intertidal transport model i e with lagrangian residuals shows good agreement with observed salinity data and that computed with the intratidal hydrodynamic model the basic effect of the stokes drift is to transport water mass e g salinity up estuary the procedures provide a practical means of interfacing intratidal hydrodynamic model information to water quality models for making long term in

paleosalinity evolution of the paleogene perennial qaidam lake on the tibetan plateau climatic vs tectonic control as the largest cenozoic terrestrial intermountain basin in the tibetan plateau the qaidam basin is an ideal basin to understand the coupling controls of tectonics and climate on plateau lake evolution more than 10 000 data of chloride content from the paleogene sediments of the endorheic qaidam basin have been collected to monitor changes in lake paleosalinity results show that there were two prominent salinity increase events in the paleogene paleolake one in the late eocene 40 ma and the other in the transition from the eocene to oligocene eot 35 5 ma the first salinity increase event was evidenced by the expansion of saline water the connection of two previous separated saline centers and the deposition of thick bedded halite in a local sag which correlated with the late eocene s frequently fluctuated climate the other salinity increase event at the eot was characterized by the expansion of mesosaline water range which corresponded well to the drier condition of the oligocene icehouse climate the synchronous migration of the saline centers and depocenters demonstrates that the qaidam basin was a tectonically active saline basin where the activity of main faults controlled the occurrence and disappearance of supersaline center under the overall brackish background the deposition of over 200 m thick bedded halite during the late eocene in a local sag was the result of coupled effects by active tectonics and fluctuated climate

fracture permeability and groundwater flow in clayey till near saskatoon saskatchewan field and laboratory studies of a clayey unweathered glacial till have shown that its bulk permeability exceeds its matrix permeability by two orders of magnitude such findings are common for weathered till but are rare for unweathered till and have important implications for groundwater recharge and contaminant transport the till in question is 6 m thick and appears to be unweathered and unfractured it is overlain by 12 m of weathered and highly fractured till results of consolidation tests on the unweathered till indicate a mean vertical hydraulic conductivity of 3 5 10 11 m s 1 and a mean specific storage of 1 3 10 4 m 1 results of slug tests yield a horizontal hydraulic conductivity of about 5 10 9 m s 1 drawdowns in the till in response to pumping from the aquifer below indicate a vertical hydraulic diffusivity of 4 5 10 5m2 s 1 the slug test results combined with specific storage results from consolidation tests indicate a similar value for horizontal hydraulic diffusivity of 4 10

lagrangian trajectories residual currents and rectification process in the northern gulf of california abstract considering a semi implicit approximation of the coriolis terms a numerical solution of the vertically integrated equations of motion is proposed to test the two dimensional numerical model several experiments for the calculation of euler stokes and lagrange residual currents in the gulf of california were carried out to estimate the lagrangian residual current trajectories of particles were also simulated the applied tidal constituents were m2 s2 k2 n2 k1 p1 and o1 at spring tides strong tidal velocities occur in the northern half of the gulf in this region of complex geometry depths change from a few meter in the northern shelf zone to more than 3000 m in the southern part in the archipelago region the presence of islands alters amplitude and direction of tidal currents producing a rectification process which is reflected in a clockwise circulation around tiburon island in the lagrangian residual current the rectification process is explained by the superposition of the euler and stokes residual currents residual current patterns show several cyclonic and anticyclonic gyres in the northern gulf of california numerical experiments for individual and combinations of several tidal constituents revealed a large variability of lagrangian trajectories

hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from la pampa argentina groundwaters from quaternary loess aquifers in northern la pampa province of central argentina have significant quality problems due to high concentrations of potentially harmful elements such as as f no3 n b mo se and u and high salinity the extent of the problems is not well defined but is believed to cover large parts of the argentine chaco pampean plain over an area of perhaps 106 km2 groundwaters from la pampa have a very large range of chemical compositions and spatial variability is considerable over distances of a few km dissolved as spans over 4 orders of magnitude 4 5300 g l 1 and concentrations of f have a range of 0 03 29 mg l 1 b of 0 5 14 mg l l v of 0 02 5 4 mg l 1 no3 n of 0 2 140 mg l 1 mo of 2 7 990 g l 1 and u of 6 2 250 g l 1 of the groundwaters investigated 95 exceed 10 g as l 1 the who guideline value and 73 exceed 50 g as l 1 the argentine national standard in addition 83 exceed the who guideline value for f 1 5 mg l 1 99 for b 0 5 mg l 1 47 for no3 n 11 3 mg l 1 39 for mo 70 g l 1 32 for se 10 g l 1 and 100 for u 2 g l 1 total dissolved solids range between 730 and 11400 mg l 1 the high values resulting mainly from evaporation under ambient semi arid climatic conditions the groundwaters are universally oxidising with high dissolved o2 concentrations groundwater phs are neutral to alkaline 7 0 8 7 arsenic is present in solution predominantly as as v groundwater as correlates positively with ph alkalinity hco3 f and v weaker correlations are also observed with b mo u and be desorption of these elements from metal oxides especially fe and mn oxides under the high ph conditions is considered an important control on their mobilisation mutual competition between these elements for sorption sites on oxide minerals may also have enhanced their mobility weathering of primary silicate minerals and accessory minerals such as apatite in the loess and incorporated volcanic ash may also have contributed a proportion of the dissolved as and other trace elements concentrations of as and other anions and oxyanions appear to be particularly high in groundwaters close to low lying depressions which act as localised groundwater discharge zones concentrations up to 7500 g l 1 were found in saturated zone porewaters extracted from a cored borehole adjacent to one such depression concentrations are also relatively high where groundwater is abstracted from close to the water table presumably because this zone is a location of more active weathering reactions the development of groundwaters with high ph and alkalinity results from silicate and carbonate reactions facilitated by the arid climatic conditions these factors together with the young age of the loess sediments and slow groundwater flow have enabled the accumulation of the high concentrations of as and other elements in solution without significant opportunity for flushing of the aquifer to enable their removal

paleoenvironments and paleoclimates in tierra del fuego and southernmost patagonia south america abstract a continuous paleoclimatic history for the past 14 000 years is presented based on palynological records from south of latitude 50 s in south america prior to 12 500 yr b p dry empetrum heathlands not mesic tundra dominated throughout the high southern latitudes indicating high winds annual precipitation of less than 300 mm and freezing temperatures year round after 12 500 yr b p steppe replaced the heathlands suggesting decrease in wind intensity increase in effective moisture and increase in temperatures when forests expanded at 9000 yr b p along the rainward side and at 8000 yr b p in the rainshadow side of the andes precipitation reached holocene levels judging from the greater openness of the early holocene forests including those in the rainward part of the region precipitation levels probably ranged between 500 and 800 mm comparable to those from today s forest steppe transition after a pronounced mid holocene dry event the late holocene forests appeared more closed than the early holocene forests suggesting greater effective moisture probably related to cooler temperatures the high latitude paleoclimatic history differs from that of lower latitudes as far as the actual amplitude of change and the climatic signal is concerned low moisture levels prior to 12 500 yr b p between latitudes 45 and 55 c suggest that the westerly stormtracks responsible for precipitation patterns in southern south america may have been located year round closer to the equator than today the precipitation increase at 12 500 yr b p that extended only as far south as 50 s indicates that the stormtracks had shifted poleward but did not reach tierra del fuego by 9000 yr b p the stormtracks had shifted to the high southern latitudes but the large seasonal latitudinal shift comparable to the modern situation equatorwards in winter and polewards in summer did not develop until after 4500 yr b p

high chloride concentrations in the soil and groundwater under an oak hedge in the west of france an indicator of evapotranspiration and water movement chloride is a major anion in soil water and its concentration rises essentially as a function of evapotranspiration compared to herbaceous vegetation high transpiration rates are measured for isolated trees shelterbelts or hedgerows this article deals with the influence of a tree hedge on the soil and groundwater cl concentrations and the possibility of using cl as an indicator of transpiration and water movements near the tree rows cl concentrations were measured over 1 year at different depths in the unsaturated zone and in the groundwater along a transect intersecting a bottomland oak hedge we observed a strong spatial heterogeneity of cl concentrations with very high values up to 2 g l 1 in the unsaturated zone and 1 2 g l 1 in the upper part of the groundwater this contrasts with the low and homogeneous concentrations 60 70 mg l 1 in the deeper part of the groundwater cl accumulation in the unsaturated zone at the end of the vegetation season allows us to identify the active root zone extension of trees in winter upslope of the tree row downwards leaching partly renews the soil solution in the root zone while the slow water movement under the trees or farther downslope results in cl accumulation and leads to a salinization of the soil and groundwater this salinization is of the same order as experimental conditions produce negative effects on oak seedlings the measurement of cl concentrations in the unsaturated zone under tree rows at the end of the vegetation season would indicate whether certain topographic pedological or climatic conditions are likely to favour a strong salinization of the soil as observed in the present study copyright 2009 john wiley sons ltd

effect of tropical cyclones on residual circulation and momentum balance in a subtropical estuary and inlet observation and simulation 1 a three dimensional 3 d hydrodynamic model curvilinear grid hydrodynamics in 3 d ch3d and observation data are used to investigate the effects of tropical storms fay and hanna in 2008 on the spatial structure of residual circulation and momentum balance inside the subtropical estuarine system of the northern coastal basin and st augustine inlet during a typical tidal cycle four residual eddies are formed two strong eddies inside of the inlet and two weaker eddies outside of the inlet during tropical storms fay and hanna the alongshore component of the northeasterly winds disrupted the ocean side eddies first and forced the alongshore southeasterly current owing to not so strong local winds residual flow at the mouth of the inlet never became complete inflow however following tropical storm fay s landfall flow inside the inlet became complete outflow owing to relaxing wind and the remote wind effect created by the alongshore component of the southeasterly wind the residual circulation returned to its prestorm condition in 4 days owing to relatively short storm duration model results revealed that the cross sectional residual flow pattern observed at the inlet mouth is highly variable with time and location of the cross section comparison of various terms in the horizontal momentum equations shows that the balance is primarily between the barotropic pressure gradient and nonlinear advection although the wind stress and bottom friction briefly become significant in the balance they are still at least an order of magnitude smaller baroclinic pressure gradient is weak despite the increase in precipitation and river discharge

effect of ground water recharge on configuration of the water table beneath sand dunes and on seepage in lakes in the sandhills of nebraska u s a abstract analysis of water level fluctuations in about 30 observation wells and 5 lakes in the crescent lake national wildlife refuge in the sandhills of nebraska indicates water table configuration beneath sand dunes in this area varies considerably depending on the configuration of the topography of the dunes if the topography of an interlake dunal area is hummocky ground water recharge is focused at topographic lows causing formation of water table mounds these mounds prevent ground water movement from topographically high lakes to adjacent lower lakes if a dune ridge is sharp the opportunity for focused recharge does not exist resulting in water table troughs between lakes lakes aligned in descending altitudes parallel to the principal direction of regional ground water movement generally have seepage from higher lakes toward lower lakes

the nature of a dynamic water table in a system of non tidal freshwater coastal wetlands abstract the miller woods wetland system contains over 150 individual minerotrophic fens the fens are established in progradational beach complex of relict beach ridges and parabolic dunes and are in hydraulic connection with lake michigan large scale flow in the wetland system is horizontal to the northwest the position of a ground water divide which forms the southern hydrologic boundary does not coincide with geographical boundaries of the system and migrates in time and space the position of the divide is not solely a function of topography but is dependent on an interplay among evapotranspirative demands antecedent soil moisture and recent recharge small scale local flow systems develop and dissipate on variable temporal scales along the margins of individual wetlands in response to evapotranspirative demands by wetland vegetation and other climatic influences

hydrological hotspots in blanket peatlands spatial variation in peat permeability around a natural soil pipe 1 measurements were made of the hydraulic conductivity k of peat around a natural soil pipe in a blanket bog this is the first investigation of decimeter scale variability in both vertical k and horizontal k in blanket peats which were found to be higher than indicated by previous research this information suggests that it may be appropriate to reconsider i the spatial sampling strategies employed to investigate subsurface flow in blanket peatlands and ii how field data are used to parameterize flow models critically there was spatial structure in the heterogeneity with a wedge of high k peat directly above the pipe forming a hydrological conduit between near surface peat and the perennially flowing pipe there was also significantly greater horizontal k parallel to the pipe s orientation compared with horizontal k perpendicular to the pipe determinations of the triaxial anisotropy of k undertaken for the first time in peat soils revealed substantial directional variations in k the k around the pipe peat interface was investigated however sample length dependency of k for peat samples precluded the investigation of a hypothesized low k skin around the pipe

a two channel laterally averaged estuarine circulation model laecim a two dimensional two channel laterally averaged estuarine circulation model laecim with a sigma coordinate in the vertical is developed for tides tidal currents and salinity in a two channel estuary with any arbitrary angle the model is forced by oceanic tides composed of 16 major constituents and freshwater discharge the two channels are governed by the same dynamic equations a control volume at the junction between the two channels is designed to conserve mass and volume transport and to guarantee the continuity of momentum in the finite difference schemes based on the physical principle in continuum medium of fluid an explicit leapfrog scheme in time and centered differencing in space are employed together with a time filter to remove the computational mode an euler backward scheme is periodically used to correct retard the phase frequency which is overestimated due to the explicit leapfrog scheme used the model is applied to the entire san francisco bay with its two channels the main north south channel and the golden gate channel which is perpendicular to the former i e with a right angle to the former the simulated time series of tides tidal currents and salinities compare favorably with the observed tidal gauge data and moored current and salinity time series during the summer 1980 the simulated along channel salinity section also compares well to the conductivity temperature depth sectional observations during the same period the model simulated eulerian residual current is consistent with two layer flow theory

the role of recharge and evapotranspiration as hydraulic drivers of ion concentrations in shallow groundwater on everglades tree islands florida usa recently evapotranspiration has been hypothesized to promote the secondary formation of calcium carbonate year round on tree islands in the everglades by influencing groundwater ions concentrations however the role of recharge and evapotranspiration as drivers of shallow groundwater ion accumulation has not been investigated the goal of this study is to develop a hydrologic model that predicts the chloride concentrations of shallow tree island groundwater and to determine the influence of overlying biomass and underlying geologic material on these concentrations groundwater and surface water levels and chloride concentrations were monitored on eight constructed tree islands at the loxahatchee impoundment landscape assessment lila from 2007 to 2010 the tree islands at lila were constructed predominately of peat or of peat and limestone and were planted with saplings of native tree species in 2006 and 2007 the model predicted low shallow groundwater chloride concentrations when inputs of regional groundwater and evapotranspiration to recharge rates were elevated while low evapotranspiration to recharge rates resulted in a substantial increase of the chloride concentrations of the shallow groundwater modeling results indicated that evapotranspiration typically exceeded recharge on the older tree islands and those with a limestone lithology which resulted in greater inputs of regional groundwater a sensitivity analysis indicated the shallow groundwater chloride concentrations were most sensitive to alterations in specific yield during the wet season and hydraulic conductivity in the dry season in conclusion the inputs of rainfall underlying hydrologic properties of tree islands sediments and forest structure may explain the variation in ion concentration seen across everglades tree islands copyright 2012 john wiley sons ltd

hydrochemical and isotopical evidence of ground water salinization processes on the coastal plain of samborombon bay argentina summary salinization in coastal areas of the province of buenos aires argentina is a controlling factor of water quality and limits its exploitation for human uses and local ecological values coastal plains such as the one in the southern part of samborombon bay have a tidal morphology consisting of fine sediment deposits tidal channels and aeolian sand sheets each of them playing a hydrological role in this paper we describe the hydrological processes in the samborombon bay coastal plain using hydrochemical and isotopical data to provide evidence of salinization processes specific ionic relationships for surface and ground water samples from each formation are differ from those of sea water isotopic data also reveal infrequent participation of sea water in the coastal area water samples showing an evaporation trend in both surface and ground water we conclude that salinization has its origin in the occurrence of gypsum and halite as well as other chlorides from the coastal plain deposits rainfall recharge continental water inputs evaporation and long residence times define the hydrochemical and isotopical features of ground water which shows no signs of present sea water contribution sand sheets constitute local aquifers with lower salinity content continental surface waters as well as rainfall drainage via tidal channels have a dominant effect on surface water hydrochemistry notwithstanding tidal oscillations exploitation of water resources is thus limited to regionally extended aeolian sand sheet formations and to surface waters specifically those related to contributions of freshwater discharge from inland areas

paleoenvironmental studies of alfar archaeological site mid holocene southeastern pampas of argentina silicophytoliths gastropods and archaeofauna the firsts results of the silicophytolith research carried out for the sedimentary sequence of alfar archaeological site are presented the aims of this study were to identify the vegetation associations and to contribute to the reconstruction of past environments during the hunter gatherers occupation of the site in the mid holocene this siliceous microfossil data is also complemented with other proxy evidence derived from the freshwater gastropods recorded throughout the stratigraphic sequence and the zooarchaeological study of the bone remains recovered from the site the results of this multi proxy approach show the existence of a permanent cover of gramineous communities on a lagoon margin and a considerable pedogenetic activity during human occupation the obtained data indicates that the human populations occupied alfar site under dry conditions but with the warm climate of the mid holocene hypsithermal

wind driven surface transport in stratified closed basins direct versus residual circulations a numerical model has been used to investigate the wind driven circulation in a stratified basin of moderate size lake tahoe california nevada two types of circulation are identified direct circulations in which the current directions remain relatively constant and the mean circulation formed over several days resembles the instantaneous circulation and residual circulations in which the currents fluctuate continuously and the mean circulation is characterized by small net displacements of parcels after large oscillations previous studies of stratified closed basins have emphasized residual circulations caused by cyclonically propagating internal basin modes resulting in a single cyclonic mean gyre during light to moderate winds observations at lake tahoe have shown currents which are more constant in direction with a double gyre pattern of surface circulation dominated by an anticyclonic northern gyre model experiments of lake tahoe demonstrate that the curl of the wind stress must be included to obtain a direct double gyre circulation similar to the observations horizontally uniform winds cause a residual circulation similar to that reported at other lakes use of the model to calculate the vorticity budget clarifies the role of the wind stress curl in creating the direct double gyre

late quaternary vegetational and climatic changes in the pampa grassland of argentina abstract the vegetation and climate of the pampa grassland argentina during the late quaternary are reconstructed from pollen recovered from dated stratigraphic sections from arroyo walls and from archaelogical excavations prior to 10 500 yr b p herbaceous psammophytic steppe existed in the central part of the pampa grassland while xerophytic woodland associated with psammophytic and halophytic steppe occurred in the southwestern part of the pampa these types of vegetation and the continental conditions that prevailed in the area of the present day coast 38 s indicate subhumid dry climate and annual precipitation 100 mm lower than present a subsequent change toward a vegetation characteristic of ponds swamps and foodplains or toward environments with locally more effective moisture occurred ca 10 500 yr b p suggesting annual precipitation close to modern levels or a higher availability of water in the central part of the pampa grassland this type of vegetation existed until 8000 yr b p when it was replaced by grassland communities that lasted until 7000 yr b p in the southwestern part of the pampa grassland this vegetation developed before 7000 yr b p and persisted until ca 5000 yr b p sea level higher than the present ca 6200 yr b p is consistent with sharp modification of the vegetation and development of local halophytic communities dominant at 38 s a return to subhumid dry conditions occurred after 5000 yr b p the late holocene vegetation is characterized by pollen assemblages similar to the psammophytic and halophytic communities of the southern pampa grassland associated with communities with more edaphic conditions at the same time at 38 s a sea level regression is suggested by the dominance of fresh water pollen assemblages and micropaleontological remains the trend toward humid temperate conditions ca 1000 yr b p suggested by vertebrate remains characteristic of temperate and humid conditions as well as a very short but dry episode during the 18th century suggested by the geology are not clearly evidenced in the pollen sequences vegetational and climatic changes are explained by the latitudinal shifts and changes in intensity of the southern atmospheric circulation and changes in sea level

loess of southern south america abstract loess and loessoid loess like reworked loess sediments extend across the chaco pampean plains and the nw mountain environments of argentina and in neighbouring countries paraguay brazil uruguay bolivia loessoid sediments are much more abundant than primary loess the beginning of the loessoid sedimentation cycle was related to a phase of late miocene orogeny in the andes the plio pleistocene record is mostly composed of loessoid sediments modified by pedogenesis which produced welded palaeosols the late pleistocene holocene loess record reveals a heterogeneous composition across the region coarse textures and an andean derived volcanic composition prevail in the southern pampas finer textures and material coming from the andes and the other two sources sierras pampeanas and the parana basin characterize the loess deposits of the northern pampas and the eastern chaco a southern patagonian source is also suggested for the mountain valley loess of tucuman although a western andean provenance has been proposed it is believed that the material in the western chaco was derived from the bolivian andes loess deposition was related to a multistage transport mechanism involving fluvial and aeolian processes inferred westerly and southwesterly wind directions as dominant carriers of the aeolian deposits are in agreement with westerly palaeowind simulations using climate models however the role played by westerly tropospheric winds and northerly winds remains to be established

structures and hydrologic function of soil landscapes with kettle holes using an integrated hydropedological approach the hummocky post glacial soil landscapes with kettle holes as internal drainage systems are characterized by ponds that trap lateral fluxes in topographic depressions a quantitative description is mostly limited by the unknown complexity of hydraulically relevant soil and sediment structures this paper is focussing on a structure based approach to identify relevant field scale flow and transport processes illustrative examples demonstrate extreme variations in water table fluctuation for adjoining kettle holes explanations require a pedohydrologic concept of the arable soil landscape identification of structures is based on geophysical methods and soil hydraulic measurements electrical resistivity imaging yields 0 5 m scale spatial structures that correspond with soil texture distributions electromagnetic induction provides larger scale field maps that reflect major soil and sediment features results of both methods correspond within the limits of the different spatial resolutions with geophysical exploration methods colluvial areas with textural differences between upper and deeper soil layers coarse textured sediment lenses and stony colluvial regions around kettle holes are identified as potentially relevant flow structures the colluvial fringe around the pond seems to be a sensitive area with important lateral exchange fluxes tensiometer measurements perpendicular to this boundary indicate hydraulic gradients directed from the pond towards the partially saturated soil the localized infiltration of trapped water in kettle holes can control large fractions of ground water recharge and may have implications for the fate of agricultural chemicals in post glacial landscapes while surface and subsurface hydraulic structures may be inferred using minimal invasive techniques better understanding of processes and properties governing lateral exchange fluxes between pond and surrounding soil are required

on tide induced lagrangian residual current and residual transport 2 residual transport with application in south san francisco bay california the transports of solutes and other tracers are fundamental to estuarine processes the apparent transport mechanisms are convection by tidal current and current induced shear effect dispersion for processes which take place in a time period of the order of a tidal cycle however as emphasis is shifted toward the effects of intertidal processes the net transport is mainly determined by tide induced residual circulation and by residual circulation due to other processes the commonly used intertidal conservation equation takes the form of a convection dispersion equation in which the convective velocity is the eulerian residual current and the dispersion terms are often referred to as the phase effect dispersion or sometimes as the tidal dispersion the presence of these dispersion terms is merely the result of a fickian type hypothesis since the actual processes are not fickian thus a fickian hypothesis obscures the physical significance of this equation recent research results on residual circulation have suggested that long term transport phenomena are closely related to the lagrangian residual current or the lagrangian residual transport in this paper a new formulation of an intertidal conservation equation is presented and examined in detail in a weakly nonlinear tidal estuary the resultant intertidal transport equation also takes the form of a convection dispersion equation without the ad hoc introduction of phase effect dispersion in a form of dispersion tensor the convective velocity in the resultant equation is the first order lagrangian residual current the sum of the eulerian residual current and the stokes drift the remaining dispersion terms are important only in higher order solutions they are due to shear effect dispersion and turbulent mixing there exists a dispersion boundary layer adjacent to shoreline boundaries an order of magnitude estimate of the properties in the dispersion boundary layer is given the present treatment of intertidal transport processes is illustrated by an analytical solution for an amphidromic system and by a numerical application in south san francisco bay california the present formulation reveals that the mechanism for long term transport of solutes is mainly convection due to the lagrangian residual current in the interior of a tidal estuary this result also points out the weakness in the tidal dispersion formulation and explains the large variability of the observed values for tidal dispersion coefficients further research on properties of the dispersion boundary layer is needed

hydrochemistry of pampasic ponds in the lower stream bed of salado river drainage basin argentina chemical analyses of major ions were performed in some of the pampasic ponds in the lower stream bed of salado river drainage basin results indicated that total dissolved solids had a mean value of 1341 mg l where sodium was the most abundant of the cations 80 2 and chloride 44 2 and bicarbonate 37 4 were the predominant anions saturation index showed oversaturation with respect to calcite and dolomite since groundwater is the main source of surface water high sodium concentration in the ponds may be explained by a cation exchange process in the loessic sediments of the basin where calcium is replaced by sodium

groundwater and soil chemical changes under phreatophytic tree plantations 1 the onset of groundwater consumption by plants can initiate a pathway of chemical inputs from aquifers to ecosystems typically absent in groundwater recharge areas we explored this biogeochemical transfer and its influence on soils in phreatophytic eucalypt plantations and native grasslands of the pampas argentina groundwater and soil chemical observations at three grassland plantation pairs were complemented with more detailed analyses along a 400 m long grassland plantation transect although tree plantations showed a widespread and homogeneous salinization of groundwater and soils at all study sites chemical contrasts between the plantation edge and core were evident along the study transect nonsalty slightly acidic bicarbonate dominated waters in the grassland changed sharply within the plantation with dissolved chloride sulfate calcium and magnesium peaking at the plantation core 200 m away from the grassland and dissolved sodium carbonate bicarbonate and ph peaking toward the edge 0 50 m away from the grassland and declining toward the core in agreement with these differences soil alkalinization was the strongest at the plantation edge but absent in the core groundwater flow simulations using flownet suggested trajectories of increasing length and depth and older groundwater ages confirmed by tritium analyses toward the plantation core explaining the hydrochemical contrasts within the plantation flow simulations and chloride mass balances suggested discharges of 250 500 mm yr 1 to the plantations in our sites phreatophytic discharge controlled solute transfers from groundwater through 1 altered flow within the aquifer affecting solute transport to the rooting zone and 2 water uptake plus solute exclusion concentrating solutes in the rooting zone while the first mechanism may be restricted to the core of large phreatophytic areas the second is likely to occur more generally in phreatophytic ecosystems

predicting the vertical structure of tidal current and salinity in san francisco bay california a two dimensional laterally averaged numerical estuarine model is developed to study the vertical variations of tidal hydrodynamic properties in the central north part of san francisco bay california tidal stage data current meter measurements and conductivity temperature and depth profiling data in san francisco bay are used for comparison with model predictions an extensive review of the literature is conducted to assess the success and failure of previous similar investigations and to establish a strategy for development of the present model a plane transformation is used in the vertical dimension to alleviate problems associated with fixed grid model applications in the bay where the tidal range can be as much as 20 25 of the total water depth model predictions of tidal stage and velocity compare favorably with the available field data and prototype salinity stratification is qualitatively reproduced conclusions from this study as well as future model applications and research needs are discussed

holocene sea level fluctuation in the southern hemisphere abstract if rising sea levels dominate in the northern hemisphere nh falling or fluctuating sea levels predominate in the southern hemisphere sh endogenic processes tectonics isostasy or geoidal changes could explain local or regional mean sea level msl fluctuations but not an hemispherical one evidence from south america africa antarctica australia and the pacific and indian oceans suggest that the holocene transgression rose above the present msl in higher latitudes before the tropics by plotting latitude against the age of msl arrival at present coasts good correlation is observed oceanic salinity mixing has been already proposed to explain this mid holocene sea level fluctuation climate could be the only factor responsible for this hemisphere wide behavior of msl it has been suggested previously that the climate of the sh precedes that of the nh by 3000 years the climatic optimum or maximum warmth occurred predominantly about 6000 bp in the nh but about 10 9000 bp in the sh short term climatic effects on the sea level monsoons southern oscillation el nino phenomena should have significant occurrences during the past in the windiest oceanic hemisphere this latitudinal trend in former msl should be considered when using shorelines as reference points for measuring vertical crustal movements

paleoenvironmental reconstruction by pollen analysis from loess sequences of the southeast of buenos aires argentina abstract a model of the dynamics of pollen dispersion and deposition in relation to loess deposition is proposed it is based on pollen analysis of three discontinuous loess sequences of late pleistocene holocene age that are located at the archaeological locality of cerro la china buenos aires province argentina 37 57 s 58 37 w deposition and accumulation of loess and pollen were interpreted in terms of their provenance and transport using the tauber 1967 and pye 1987 models for pollen and dust dispersal respectively three pollen zones can be distinguished based on changes in pollen assemblages the final late pleistocene early holocene interval is represented by pollen zone 3 loess deposits have been accumulated in an environment with a locally high effective moisture loess deposition would have occurred synsedimentarily with extra regional local and regional pollen taxa the relationship between pollen dispersal and possible extra regional pollen sources indicate predominantly w sw winds this zone is truncated by an erosive episode hence the successional process of the community could not be reconstructed for the early holocene the middle holocene is represented by pollen zone 4 deposition of loess began in an environment with predominantly herbaceous communities pollen concentration increases towards the top of the zone and may represent a buried soil this would imply a change towards more stable and humid conditions than those at the beginning of the deposition extra regional pollen concentration suggests a comparatively high local and regional pollen productivity rather than a change in wind direction a slight erosive episode constitutes the upper boundary of zone 4 and is related to new drier conditions zone 5 occurs during historical times and is represented by graminous steppe communities and over representation of local taxa

paleoenvironments in the paso otero locality during late pleistocene holocene pampean region argentina an interdisciplinary approach the recently excavated paso otero 4 site is located within the paso otero archaeological locality necochea district pampean region the site is characterized by the presence of a great variety of archaeological materials and a rich and complex stratigraphic record which allows use of an interdisciplinary approach the objective of this paper is to present the results of the geoarchaeological palynological phytolith and diatom analyses in order to reconstruct the paleoenvironmental conditions for the final late pleistocene through holocene these lines of research allow inference of paleoenvironmental and palaeoclimatic conditions on both local and regional scales at the site the rio salado member unit 2 where the archaeological deposit is buried underwent sudden and fluctuating changes in water availability conditions although the setting was basically characterized by calm depositional processes within and along the margins of interconnected lagoons paleoclimatic reconstructions indicate the existence of abrupt climatic changes throughout the sequence early holocene human occupations took place under regionally temperate warm climates while the end of the middle holocene witnessed cold and arid or cold temperate climates in general terms paleoenvironmental conditions inferred for paso otero 4 are in accordance with regional reconstructions proposed for the pampas

the interaction of lakes with variably saturated porous media numerical simulation of variably saturated porous media indicates that groundwater recharge is variable in time and space depending on the thickness of the unsaturated zone through which infiltrating water must move the resulting complex transient groundwater flow systems have significant impact on contiguous surface water in very permeable media small local closed groundwater flow systems can develop and dissipate within a few weeks to several months after major recharge these have a direct effect on contiguous surface water by alternately causing seepage to and seepage from the surface water the transient nature of these flow systems indicates that reversals of the direction of groundwater flow may be common in less permeable media the same complex flow systems may occur but the time for development and dissipation is much greater for example it is conceivable that small local flow systems may exist for many months or years as a result of major recharge therefore directions of flow in such systems are more stable and the effect on contiguous surface water also is more stable the findings of this study indicate that wells and groundwater quality sampling sites need to be carefully located to define accurately water table configuration groundwater recharge direction of seepage through the beds of surface water bodies and complex geochemical processes related to changing directions of groundwater flow

soil water flow dynamics in a managed cutover peat field quebec field and laboratory investigations in this paper concerned with soil water dynamics in a managed cutover peat field the microscale hydrological processes and parameters governing water flow and storage through variably saturated peat are investigated an open water ditch reservoir enhanced wetting of adjacent cutover peat maintaining the water table depth above 43 cm during the summer surface soil moisture above 45 and water tension in the surface layer above 245 mbar desaturation of pores was noted in the 22 and 210 cm depths but at 230 and 250 cm a decrease in moisture content of several percent was associated with compression of the peat as the water table dropped air entry occurred only at pressures below 215 mbar seasonal subsidence resulted in cumulative vertical displacement in excess of 10 cm during the study period typical settlements in the peat ranged between 11 and 23 of the lowering of the water table considerable hysteresis was observed and vertical displacement was 5 times greater in response to water loss compared to rewetting the specific storage s s in the 180 cm thick deposit averaged 9 4 3 10 24 cm 21 during drying periods but averaged only 2 6 3 10 24 cm 21 on rewetting s s was more important than specific yield sy in the overall aquifer storativity transient hydraulic properties resulted from the shifting soil structure the increase in peat bulk density caused by drying increased the water retention capacity and decreased hydraulic conductivity mean saturated hydraulic conductivity was 15 cm d 21 and decreased 2 orders of magnitude as the degree of saturation dropped from 1 to 0 4 the horizontal vertical anisotropy ratio was 4 the changing surface elevation in response to seasonal subsidence had a profound influence on the nature of the storage changes and hydraulic parameters of the peat soil

carbon 14 dating of groundwater in confined aquifers implications of aquitard diffusion the influence of diffusive losses into fine grained aquitards on the 14c age interpretation of groundwater in long thin confined aquifers is examined an analytical model is developed by coupling two one dimensional steady state differential equations one representing advective dispersive transport in the aquifer and the other representing diffusion into the aquitards the results indicate that aquitard diffusion is a mechanism that can cause a significant reduction in 14c concentrations in a confined aquifer groundwater velocities derived from 14c measurements that are unadjusted for diffusion would therefore be underestimated the error that ensues is highly sensitive to the aquifer thickness and the magnitude of the diffusion coefficient in contrast the steady state distribution of 14c is shown to be relatively insensitive to the magnitude of the longitudinal dispersivity value of the aquifer dispersivities representative of the upper range of values that have been reported in the literature produced a negligible difference in steady state 14c concentration compared to results obtained using small dispersivities an equation is presented which permits the adjustment of 14c data to account for diffusion providing that the aquifer aquitard system is not stratigraphically complex and heterogeneous the analysis can be extended to include age dating isotopes that have half lives different from that of 14c the results of this study also apply to age dating in fractured porous media

harmonic analysis of tides and tidal currents in south san francisco bay california abstract water level observations from tide stations and current observations from current meter moorings in south san francisco bay south bay california have been harmonically analysed at each tide station 13 harmonic constituents have been computed by a least squares regression without inference tides in south bay are typically mixed there is a phase lag of approximately 1 h and an amplification of 1 5 from north to south for a mean semi diurnal tide because most of the current meter records are between 14 and 29 days only the five most important harmonics have been solved for east west and north south velocity components the eccentricity of tidal current ellipse is generally very small which indicates that the tidal current in south bay is strongly bidirectional the analyses further show that the principal direction and the magnitude of tidal current are well correlated with the basin bathymetry patterns of eulerian residual circulation deduced from the current meter data show an anticlockwise gyre to the west and a clockwise gyre to the east of the main channel in the summer months due to the prevailing westerly wind opposite trends have been observed during winter when the wind was variable

microdepression focused recharge in a coastal wetland la plata argentina depression focused recharge dfr has been well documented for the clay till plains of western canada kettle lakes of southeastern wisconsin the sandhills region of nebraska and elsewhere in this study the importance of dfr in a fine grained estuarine coastal plain environment with extremely low topographic relief is shown the southwestern coastal plain of the rio de la plata near the city of la plata argentina extends 8 9 km inland from the river and ranges in elevation from 0 to 3 m above sea level the holocene postpampeano sediments predominantly composed of silty clay in the study area mantle the coastal plain and exert a major control on the hydrodynamics of groundwater of the region the average hydraulic conductivity of the unit ranges from 1 0 10 9 m s 1 when estimated on a small scale single well and oedometer tests to approximately 1 0 10 8 m s 1 when estimated on a larger scale tritium profiles and seepage meters a study of the groundwater flow system has established that recharge is occurring in the marshes of the central coastal plain which are 0 5 1 0 m lower than the surrounding land a broad 50 cm high groundwater mound a significant feature in the context of a regional hydraulic gradient of 0 0005 was documented below the marshland using nests of monitoring wells recharge rates are further controlled by subtle 10 40 cm elevation variations within the marsh four nests of six monitoring wells each were installed in the marsh and sampled for tritium recharge rates were then estimated through calibration simulations using a 1 d analytical solute transport model in the two nests in the lower wetter areas recharge rates were estimated at 4 6 cm year 1 whereas estimated recharge rates in the higher drier areas ranged from 0 3 cm year 1 these differences are consistent with 1 near neutral vertical hydraulic gradients in higher areas vs persistent downward gradients in lower areas 2 chloride concentrations almost an order of magnitude lower in groundwater underlying lower areas than in that of higher areas due to flushing of older more saline groundwater in the lower areas and 3 isotopic evidence for evapotranspiration only in the higher areas the present study suggests that ponding of surface water in local depressions can lead to significant recharge from coastal wetlands this finding has implications for the study of water balance soil salinity and nutrient budgets in these environments 1997 elsevier science b v all rights reserved

groundwater s dynamic role in regulating acidity and chemistry in a precipitation dominated lake abstract the role of groundwater in regulating the hydrologic and chemical regime ofan acid susceptible lake situated in a sandy silicate aquifer in northern wisconsin was determined by direct groundwater measurements using a network of 68 piezometers groundwater inputs were compared with precipitation and dryfall components of the hydrologic and chemical budgets for the lake groundwater contributed only 10 of the water to crystal lake in 1982 but was the major source of alkalinity calcium magnesium sodium potassium silicon and chloride precipitation contributed 90 of the water and was the major source of hydrogen and sulfate ions continuing analyses suggest that groundwater consistently supplies the bulk of the inoorganic constituents to the lake on annual basis the flux of groundwater and dissolved solids to the lake is highly seasonal with large pulses of groundwater inflow occuring after spring snowmelt and in the fall for certain nonconservative elements such as silicon the periodic influx of chemically concentrated groundwater can trigger significant changes in the chemical and biological balance in the lake despite the fact that the aquifer contains no carbonate minerals and the area experiences acid precipitation ph 4 6 the alkalinity of the small amount of groundwater entering crystal lake is more than enough to neutralize all of the acidity contributed by precipitation in lake sulfate reduction may generate an equal amount of alkalinity this buffering of ph helps to regulate aluminum concentrations at low levels thus minimizing potential aluminum toxicity in the lake most lakes in the region probably receive even larger amounts of alkalinity rich groundwater than crystal lake because most lie further downgradient in the regional flow system this may help to explain why most lakes in this acid susceptible area are no more acidic today than they were 50 years ago