analytical solution for tidal propagation in a leaky aquifer extending finite distance under the sea this paper focuses on groundwater dynamics in response to the tidal fluctuation in a coastal aquifer system an analytical solution is derived to describe groundwater level fluctuation in a leaky aquifer extending finite distance under the sea based on this solution the joint effects of various parameters such as the dynamic effect of water table fluctuation and the leakages of the inland and offshore on the behavior of the groundwater level fluctuations in the inland part of the leaky confined aquifer can be thoroughly analyzed when the roof length is increased the dynamic effect of the water table fluctuation on the dimensionless groundwater amplitude intrusion distance and fixed phase shift in the unconfined aquifer become more important and the water table fluctuation approaches constant values when the roof length is greater than a threshold value however given the same values of dimensionless leakage and roof length the dimensionless groundwater amplitude intrusion distance and fixed phase shift in the leaky aquifer with considering the dynamic effects are always larger than those of neglecting such effects

semi numerical simulation of groundwater flow induced by periodic forcing with a case study at an island aquifer summary this paper presents an efficient method to analyze the aquifer response to periodic forcing a complex transformation is used to change the time dependent linear aquifer model into an equivalent time independent problem of a complex function referred to as the complex amplitude hereafter the magnitude of the complex amplitude represents the amplitude of the sinusoidal fluctuation of the groundwater level and the argument of the complex amplitude the phase shift of the fluctuation different iterative methods can be used to obtain the numerical solution a semi analytical numerical solution of the groundwater level which is numerical for the spatial variables and analytical for the time variable can be obtained by implementing a simple inverse complex transform to the complex amplitude the advantages of the time independent method include ruling out of initial conditions direct outputs of the amplitude and phase shift of different sinusoidal components of the water level fluctuations on the contrary the numerical solutions of the time dependent model need further computational efforts for the analyses of the amplitudes and phase shifts of different sinusoidal components and have to consider an initial condition whose impacts on the periodicity have to be eliminated by running sufficiently great number of tidal circles a case study at an unconfined aquifer formed by fill materials near lam chau island hong kong sar pr china shows the applicability of the semi numerical solution to analyze tide induced watertable fluctuations the aquifer parameters were estimated by minimizing a least squares objective function to fit the complex amplitude of semidiurnal and diurnal components of the observed water level fluctuations compared with the parameter estimations of some of the previous analytical solutions the numerical method gives not only better fitting to the water level observations but also more aquifer parameters

tidal fluctuations in a leaky confined aquifer dynamic effects of an overlying phreatic aquifer tidal fluctuations in a leaky confined coastal aquifer are damped significantly due to leakage into an overlying phreatic aquifer jiao and tang 1999 presented an analytical solution to a simple model describing this phenomenon their solution assumes that the tidal fluctuations in the overlying phreatic aquifer are negligible i e a static phreatic aquifer here we examine dynamic effects of the overlying aquifer based on a new approximate analytical solution the numerical results indicate that the dynamic effects can be significant for a relatively large leakage and a high transmissivity of the phreatic aquifer

blake plateau control of miocene sedimentation patterns by large scale shifts of the gulf stream axis the distribution of buried channel networks within cenozoic sequences of the blake plateau and their correlation with global sea level oscillations indicate that the gulf stream axis shifted landward against the florida hatteras slope during sea level high stands and seaward across the central blake plateau during sea level lowstands a sedimentation model incorporating axial shifts of the gulf stream successfully predicts the miocene stratigraphy of the florida hatteras slope and blake plateau as defined by seismic and drill hole data

numerical modeling of tidal effects on groundwater dynamics in a multi layered estuary aquifer system using equivalent tidal loading boundary condition case study in zhanjiang china this paper presents a numerical modeling study of tidal effects on groundwater dynamics of a multi layered estuary aquifer system in this study to generalize the tidal loading effects on the confined aquifer we used equivalent tidal loading boundary condition based on existing analytical solutions for the confined aquifer in a three dimensional numerical model and then good simulated result was obtained using the numerical model the tide induced groundwater level and confined groundwater discharge dynamics were discussed and the following conclusions were obtained the combined action of sea tide and river tide can enhance the tidal response of groundwater level in the aquifer system especially for the confined groundwater the river tidal effect is more significant in the areas away from coastline although the confined aquifer is separated by a layer of clay from the ocean because of the tidal loading effects the confined groundwater discharge also changes periodically displaying a negative correlation with the tide level fluctuations like the unconfined groundwater discharge using linear functions that express the relationship between the mean confined groundwater flux and sea tide level the confined groundwater flux through the tidal loading boundaries can be approximately estimated when the sea tide level is higher than 4 4 m there would be a risk of seawater intrusion into the confined aquifer in the study area this case study is useful for numerical studying of tidal effects on groundwater resource dynamics in a coastal multi layered estuary aquifer system

tide induced head fluctuations in a confined aquifer with sediment covering its outlet at the sea floor 1 a confined coastal aquifer usually extends under the sea for some distance with its submarine outlet covered by a thin layer of sediment with properties dissimilar from the aquifer previous theoretical studies neglected this outlet capping in this paper an analytical solution is derived for a confined aquifer subject to tidal fluctuations with a leaky boundary condition at the outlet capping for the cases that offshore length of the aquifer and or the leakance of the outlet capping are either zero or infinity existing solutions in literature are obtained it is shown that our analytical solution can also be used to describe the tidal wave propagation in a confined aquifer extending infinitely under a tidal river the full solution agreed well with the observations in a piezometer in the jahe river basin in china 200 m inland from the coastline where the tide induced head fluctuations without delay were observed but cannot be explained using previous analytical and numerical solutions ignoring the effect of the outlet capping the sensitivity analysis showed that the combined actions of the tidal loading and outlet capping lead to complex dependency of the head fluctuation on the outlet capping s leakance when the offshore confined aquifer is short the inland head fluctuation increases with the outlet capping s leakance in this case significant negative phase shift occurs if the leakance is small for offshore aquifer length greater than a threshold value the inland head fluctuations are independent of this length and the outlet capping s leakance

tidal groundwater level fluctuations in l shaped leaky coastal aquifer system this paper presents an analytical solution to describe tidal groundwater level fluctuations in a coastal leaky aquifer system bounded by water land boundaries that form a right angle referred to as l shaped coastlines the system consists of an unconfined aquifer a confined aquifer and a leaky layer between them previously published analytical solutions that discuss only single aquifer constitute a special case of the new solution when the permeability of leaky layer approaches zero a simple approximate solution without integral is presented error analysis and hypothetical example show that the approximate solution has adequate accuracy for both groundwater level prediction and parameter estimation for an l shaped leaky aquifer system q 2002 elsevier science b v all rights reserved

the enhancing effect of the elastic storage of the seabed aquitard on the tide induced groundwater head fluctuation in confined submarine aquifer systems summary this paper investigated the tide induced head fluctuation in a coastal confined aquifer with an infinitely extending semipermeable roof the roof is overlain by an unconfined aquifer terminating at the coastline previous analytical studies ignored the effect of the elastic storage of the semipermeable roof our model incorporated this effect the analytical solution indicated that the increase of the elastic storage of the offshore semipermeable roof may significantly enhance the tidal head fluctuation in the confined aquifer at the coastline a threshold leakance exists in the sense that when the leakance is less than the threshold the head fluctuation at the coastline will become insensitive to leakance and the roof can be regarded as impermeable the threshold strongly depends on the roof s elastic storage and decreases as the roof s elastic storage increases indicating that the roof s leakage effect is significantly enhanced by the increase of the roof s elastic storage

an analytical solution for tidal fluctuations in unconfined aquifers with a vertical beach 1 the perturbation technique has been commonly used to develop analytical solutions for simulating the dynamic response of tidal fluctuations in unconfined aquifers however the solutions obtained from the perturbation method might result in poor accuracy for the case of the perturbation parameter being not small enough in this paper we develop a new analytical model for describing the water table fluctuations in unconfined aquifers based on laplace and fourier transforms in the new approach the mean sea level is used as the initial condition and a free surface equation neglecting the second order slope terms as the upper boundary condition numerical results show that the present solution agrees well with the finite different model with the second order surface terms unlike teo et al s 2003 approximation which restricts on the case of shallow aquifers the present model can be applied to most of the tidal aquifers except for the very shallow one in addition a large time solution in terms of sine function is provided and examined graphically with four different tidal periods

early cretaceous shelf edge deltas of the baltimore canyon trough principal sources for sediment gravity deposits of the northern hatteras basin we present evidence that the principal sources for early cretaceous berriasian valanginian gravity flow deposits of the northern hatteras basin were three large shelf edge deltas located along the outer margin of the baltimore canyon trough 100 km southeast of cape charles virginia ocean city maryland and long branch new jersey sedimentary detritus from the central appalachian highlands and the maryland virginia coastal plain was transported across the early cretaceous continental shelf to form the cape charles and ocean city deltas whereas deposits of the long branch delta came chiefly from the adirondack and new england highlands each delta supplied sediment gravity flows to large slope aprons and submarine fan complexes on the early cretaceous continental slope and rise the most conspicuous distributary of sediment on the early cretaceous continental rise extends 500 km basinward from the ocean city delta where its distal deposits were cored at deep sea drilling project site 603

sedimentology and architecture of de geer moraines in the western scottish highlands and implications for grounding line glacier dynamics sedimentary exposures in moraines in a scottish highland valley glen chaorach reveal stacked sequences of bedded and laminated silt sand and gravel interspersed or capped with diamicton units in four examples faults and folds indicate deformation by glaciotectonism and syndepositional loading we propose that these sediments were laid down in an ice dammed lake close to the last ice margin to occupy this glen individual units within cross valley de geer moraine ridges are interpreted by comparison with examples from similar environments elsewhere stratified diamictons containing laminated or bedded lenses are interpreted as subaqueous ice marginal debris flow deposits massive fine grained deposits as hyperconcentrated flow deposits and massive gravel units as high density debris flow deposits using an allostratigraphic approach we argue that glaciotectonically deformed coarsening upward sand and gravel sequences that culminate in deposition of subglacial diamicton represent glacier advances into the ice marginal lake whereas undisturbed cross bedded sand and gravel reflects channel or fan deposits laid down during glacier retreat a flat terrace of bedded sand and gravel at the northern end of glen chaorach is interpreted as subaerial glaciofluvial outwash on the basis of these inferences we propose the following three stage deglacial event chronology for glen chaorach during glacier recession ice separation and intra lobe ponding first led to subaquaeous deposition of sorted and unsorted facies subsequent glacier stabilisation and ice marginal oscillation produced glaciotectonic structures in the ice marginal sediment pile and formed de geer moraines finally drainage of the ice dammed lake allowed a subaerial ice marginal drainage system to become established throughout deglaciation deposition within the lake was characterized by abrupt changes in grain size and in the architecture of individual sediment bodies reflecting changing delivery paths and sediment supply and by dynamic margin oscillations typical of water terminating glaciers

a general solution for groundwater flow in an estuarine s leaky aquifer system after considering aquifer anisotropy this paper presents an analytical model for describing the tidal effects in a two dimensional leaky confined aquifer system in an estuarine delta where ocean and river meet this system has an unconfined aquifer on top and a confined aquifer on the bottom with an aquitard in between the two the unconfined and confined aquifers interact with each other through leakage it was assumed that the aquitard storage was negligible and that the leakage was linearly proportional to the head difference between the unconfined and confined aquifers this model s solution was based on the separation of variables method two existing solutions that deal with the head fluctuation in one dimensional or two dimensional leaky confined aquifers are shown as special cases in the present solution based on this new solution the dynamic effect of the water table s fluctuations can be clearly explored as well as the influence of leakage on the behaviour of fluctuations in groundwater levels in the leaky aquifer system copyright 2015 john wiley sons ltd

sea level periodic change and its impact on submarine groundwater discharge rate in coastal aquifer abstract this study focused on the interaction between groundwater and seawater and the effect of temporal scale of sea level fluctuation on submarine groundwater discharge sgd rate the numerical code feflow diersch 2005 was used to evaluate the influence of periodic changes in sea level on an increase in sgd at various time scales from diurnal to glacial cycles simulation results showed that not only amplitude but also period of the sea level fluctuation controls total amount of sgd the increase ratio of recirculated submarine groundwater discharge rsgd was found to generally be proportional to the amplitude of sea level and the square root of specific storage of an aquifer while inversely proportional to the square root of hydraulic conductivity of an aquifer and period of sea level fluctuation a nondimensional number p that incorporates combined effect of sea level oscillation and aquifer properties is used to represent the magnitude of periodic forcing from sea boundary into an aquifer as a mechanism for enhancement of rsgd simulation results demonstrate that increase ratios of rsgd are well fitted by an exponential curve as a function of p and variations in rsgd occur only when p exceeds a critical number the dimensionless inland recharge rate where recharge rate is divided by hydraulic conductivity was shown to be important in determining the critical number this implies that sea level fluctuation does not lead to enhancement of rsgd until the specified condition is reached and groundwater recharge rate and aquifer hydraulic conductivity are the key factors controlling the increase of rsgd

numerical experiments on interactions between wave motion and variable density coastal aquifers a comprehensive two dimensional cross shore process based numerical model of nearshore hydrodynamics based on the navier stokes equations k e turbulence closure and the volume of fluid method beach morphology and variable density groundwater flow seawat 2000 was developed this model which was applied at the field scale relaxes simplifications in existing models that do not include such detailed mechanistic descriptions numerical experiments were conducted to investigate the effects of varying aquifer beach and wave characteristics e g inland groundwater head sand grain size different wave heights and periods on the coupled system spilling and plunging breakers on dissipative and intermediate beaches were simulated for a given set of boundary conditions the model was run for 1 y without the hydrodynamic sub model to achieve a realistic salt freshwater interface then the hydrodynamic component was run for 15 min and the model results analyzed the main features considered were groundwater circulation saltwater wedge position in exfiltration across the beach face and beach morphology the predictions of the numerical model agree well with existing understanding and experimental measurements for an inland watertable that is lower than the still water level swl such that the groundwater flow is mainly landward on both coarse and fine sand beaches the addition of wave motion moves the saltwater wedge further landward for an inland watertable that is higher than the swl the opposite behavior occurred the numerical experiments showed that more sediment transport takes place on intermediate beaches than on dissipative beaches in addition beach profile variations are greater under plunging breakers while coarse sand beaches are steeper than fine sand beaches for the same wave conditions there is a strong correlation between in exfiltration and beach face deposition erosion for the coarse beaches while in exfiltration has a slight effect on sediment transport for fine beaches the model is capable of simulating the short term evolution of foreshore profile changes and beach watertable and saltwater wedge movement due to interactions between wave motion and coastal groundwater

paleogene neogene depositional history of the middle u s atlantic continental rise mixed turbidite and contourite depositional systems abstract the construction of the middle u s atlantic continental rise from late paleogene to late pliocene time included downslope sediment gravity flows and alongslope contourite deposition some previous studies have suggested that contour following bottom currents were the dominant control on depositional patterns depositional relief from two large contourite drifts largely built in late miocene to late pliocene time continue to dominate present day lower rise morphology detailed seismic sequence mapping presented here suggests that significant turbidite fan systems also were active and are preserved in the stratigraphic section a single thick depositional sequence is mapped for oligocene to middle miocene sediments deposited when inferred fan systems off the norfolk washington canyons were reworked by bottom currents along the lower rise to form the proto hatteras outer ridge detailed mapping of three late miocene to late pliocene depositional sequences indicates that fan channel systems were active at the same time large contourite drifts were being constructed sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the hatteras outer ridge and through entrainment and resuspension was an important source for regional contourite deposition controlled by the deep western boundary undercurrent we suggest that submarine fans and current controlled sediment drifts may develop simultaneously as companion systems types of fan drift interaction include current only modification of fans transitional fan to drift and adjacent or overlapping submarine fan and sediment drift deposition

comment on an analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer by jiu jimmy jiao and zhonghua tang

an analytical solution and case study of groundwater head response to dual tide in an island leaky confined aquifer 1 this paper presents an analytical solution of groundwater head response to dual tide fluctuation in the transect of an island leaky aquifer system comprising a confined aquifer and its overlying semipermeable confining layer both layers terminate at the coastlines on two sides of the island solution analysis indicates that the tidal waves from the two sides of the island transect interfere at the middle of the island and the interference decreases to zero as the horizontal length of the aquifer increases to infinity the leakage of the overlying confining layer enhances the landward attenuation of the tidal head fluctuation and shortens the time lag between the head and tide fluctuations the solution agreed well with the observations in eight piezometers in garden island on the continental shelf of western australia reported by trefry and bekele 2004

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

the early neogene continental rise off the eastern united states abstract single and multichannel seismic data were used to investigate depositional components comprising the late oligocene middle miocene constructional phase of the united states atlantic continental rise in the georges bank blake plateau region the seismic sequence studied was bounded by the eocene oligocene age horizon a u unconformity at the base and a newly defined late middle miocene age horizon g at the top late oligocene middle miocene accretion onlapping the paleocontinental slope and downlapping onto horizon a u basinward was derived primarily from erosion of the uplifted central appalachian mountains this accretion up to 3 2 km thick was characterized by seven depocenters the lower paleocontinental slope upper rise depocenters situated off southern new england and the delmarva peninsula were interpreted as deep sea fan complexes or a broadly overlapping series of sedimentary blankets a whereas the closely associated oceanographer and wilmington depocenters were inferred to be levee complexes immediately north of the developing blake bahama outer ridge two n ne oriented sediment ridges that were probably built by gulf stream flow during an early miocene sea level lowstand were observed on the lower continental rise the proto hatteras and mytilus outer ridges from 0 4 to 0 8 km thick were probably formed from the wilmington and oceanographer levee complexes in early middle miocene time through the interaction of the growing levee complex with the sw moving abyssal current late middle miocene 12 11 5 ma bottom current erosion created the horizon g unconformity and brought to an end this initial stage of continental rise construction nevertheless the basic morphological framework that controlled late neogene and quaternary continental rise evolution was now in place

geostatistical inverse modeling of the transmissivity field of a heterogeneous alluvial aquifer under tidal influence summary the record of the head fluctuations of an alluvial aquifer connected to the estuary of a river submitted to tidal influence can be used to reconstruct the transmissivity field of the aquifer a markov chain monte carlo mcmc sampler is used to invert the transmissivity field at a set of pilot points our approach is first successfully benchmarked on a synthetic case study then we present a real case study in which both a semi confined aquifer and a shallow unconfined aquifer are connected to the estuary of the seine river in the western north of france these aquifers experience strong tidal variations up to 8 4 m this test site is equipped with a series of seventeen wells in which the temporal fluctuations of water heads associated with megatidal forcing were recorded during several years in addition to the use of the pilot points method a zoning parameterization with 45 zones is used to determine the vertical leakage coefficient from the semi confined aquifer into the shallow unconfined aquifer the distribution of the vertical leakage coefficient is determined by inverting the hydraulic head at a set of wells localized in the semi confined aquifer we determine jointly the distribution of the hydraulic transmissivity and the vertical leakage from the semi confined into the unconfined aquifer using 72 pilot points the storage coefficient is assumed to be constant over the whole domain its value is determined from three pumping tests that show a consistent value for this parameter log10s 5 and 0 11 as a variance in our approach there are 119 model parameters 72 transmissivity values at the pilot points 45 values of the vertical leakage coefficient and two parameters sill and range for the isotropic variogram the spatial distribution of the hydraulic transmissivity agrees qualitatively with the soil texture variations observed in the semi confined aquifer and core samples analysis

an analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer an analytical solution is derived to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system the analytical solution developed here is more general than the traditional solution obtained by ferris 1951 which can be regarded as a special case of the solution presented in this paper this solution is based on a conceptual model under the assumption that the groundwater level in the confined aquifer fluctuates in response to sea tide while that of the overlying unconfined aquifer remains constant this conceptual model is supported by numerous field studies by previous researchers which have demonstrated that the tidal response in an unconfined aquifer may be negligible compared to that in a confined aquifer the leakage has a significant impact on the tidal behavior of the confined aquifer hypothetical studies indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the sea tide will be considerably reduced because of the existence of leakage this analytical solution is used to investigate the tidal and piezometer data at the chek lap kok airport hong kong special administrative region people s republic of china

groundwater response to tidal fluctuations in a leaky confined coastal aquifer with a finite length the study on the hydraulic properties of coastal aquifers has significant implications both in hydrological sciences and environmental engineering although many analytical solutions are available most of them are based on the same basic assumption that assumes aquifers extend landward semi infinitely which does not necessarily reflect the reality in this study the general solutions for a leaky confined coastal aquifer have been developed that consider both finitely landward constant head and no flow boundaries the newly developed solutions were then used to examine theoretically the joint effects of leakage and aquifer length on hydraulic head fluctuations within the leaky confined aquifer and the validity of using the simplified solution which assumes the aquifer is semi infinite the results illustrated that the use of the simplified solution may cause significant errors depending on joint effects of leakage and aquifer length a dimensionless characteristic parameter was then proposed as an index for judging the applicability of the simplified solution in addition practical application of the general solution for the constant head inland boundary was used to characterize the hydraulic properties of a leaky confined aquifer using the data collected from a field site at the seine river estuary france and the versatility of the general solution was further justified

analysis of tide and offshore storm induced water table fluctuations for structural characterization of a coastal island aquifer analysis of water table fluctuations can provide important insight into the hydraulic properties and structure of a coastal aquifer system including the connectivity between the aquifer and ocean this study presents an improved approach for characterizing a permeable heterogeneous coastal aquifer system through analysis of the propagation of the tidal signal as well as offshore storm pulse signals through a coastal aquifer offshore storms produce high wave activity but are not necessarily linked to significant onshore precipitation in this study we focused on offshore storm events during which no onshore precipitation occurred extensive groundwater level data collected on a sand barrier island sable island ns canada show nonuniform discontinuous propagation of the tide and offshore storm pulse signals through the aquifer with isolated inland areas showing enhanced response to both oceanic forcing signals propagation analysis suggests that isolated inland water table fluctuations may be caused by localized leakage from a confined aquifer that is connected to the ocean offshore but within the wave setup zone two dimensional groundwater flow simulations were conducted to test the leaky confined unconfined aquifer conceptualization and to identify the effect of key parameters on tidal signal propagation in leaky confined unconfined coastal aquifers this study illustrates that analysis of offshore storm signal propagation in addition to tidal signal propagation provides a valuable and low resource approach for large scale characterization of permeable heterogeneous coastal aquifers such an approach is needed for the effective management of coastal environments where water resources are threatened by human activities and the changing climate

tidal fluctuations in a leaky confined aquifer localised effects of an overlying phreatic aquifer damping of tidal head fluctuations in a leaky confined coastal aquifer is enhanced by leakage into an overlying phreatic aquifer we show that the phreatic aquifer is however resistant to the leakage flow and in particular a deep phreatic aquifer can reduce the leakage effects significantly an analytical solution based on a vertical flow model for the phreatic aquifer is derived for quantifying the role of this upper free water body in tidal propagation in the lower semi confined aquifer