Stream Aquifer Interface Research Articles

Semi-Analytical Modeling of Transient Stream Drawdown and Depletion in Response to Aquifer Pumping.

Analytical and semi-analytical models for stream depletion with transient stream stage drawdown induced by groundwater pumping are developed to address a deficiency in existing models, namely, the use of a fixed stream stage condition at the stream-aquifer interface. Field data are presented to demonstrate that stream stage drawdown does indeed occur in response to groundwater pumping near aquifer-connected streams. A model that predicts stream depletion with transient stream drawdown is developed based on stream channel mass conservation and finite stream channel storage. The resulting models are shown to reduce to existing fixed-stage models in the limit as stream channel storage becomes infinitely large, and to the confined aquifer flow with a no-flow boundary at the streambed in the limit as stream storage becomes vanishingly small. The model is applied to field measurements of aquifer and stream drawdown, giving estimates of aquifer hydraulic parameters, streambed conductance, and a measure of stream channel storage. The results of the modeling and data analysis presented herein have implications for sustainable groundwater management.

Thermal reactivity at the stream–aquifer interface

The evolution of water temperature at the stream–aquifer interface and the associated heat fluxes have a major influence on the ecological state. Here, a synthetic case is developed to characterize the behavior of the stream–aquifer interface submitted to meteorological and hydrological forcings as well as to various hydraulic and thermal properties, representative of a wide range of lithofacies from clay to gravel/sand. The thermal regime of the stream–aquifer system is driven by two pseudo-periodic cycles: the annual cycle and the diurnal cycle. A thermo-hydrogeological model coupled with a parameter sampling script is used. The results highlight the drivers that have the greatest influence on heat fluxes: (1) the meteorological conditions through the seasonal thermal gradients established between the stream and the aquifer during winter and summer periods, and (2) the hydraulic conductivity at the stream–aquifer interface. Depending on the hydraulic conductivity values at the stream–aquifer interface, two thermal regimes exist: for high hydraulic conductivities, advective fluxes clearly prevail, while for lower hydraulic conductivities, conductive fluxes predominate in most cases. This depth is reduced in upwelling cases and modulated by the thermal regime. The predominance of either the streambed- or aquifer-property effects depends on the equivalent hydraulic conductivity. In downwelling conditions under a given meteorological setup, stream–aquifer disconnection leads to increased advective fluxes compared with its connected counterpart. Results from this study provide better insight into heat fluxes at the stream–aquifer interface, which will ultimately result in a better stream heat-balance model.

Solute dynamics across the stream‐to‐riparian continuum under different flood waves

AbstractTo enhance the understanding of solute dynamics within the stream‐to‐riparian continuum during flood event‐driven water fluctuation (i.e., flood wave), a variable saturated groundwater flow and solute transport model were developed and calibrated against in situ measurements of the Inbuk stream, Korea, where seasonal flooding prevails. The solute dynamics were further investigated for flood waves (varying by amplitude [A], duration [T], roundness [r], and skewness [tp]) that were parameterised by real‐time stream stage fluctuations. We found that the solute transferred faster and farther in the riparian zone, especially within the phreatic zone, above which in the variable saturated zone the concentration required a significantly longer time, particularly at higher altitudes, to return to the initial state. By comparison, solute transferred shallowly in the streambed where the solute plume exhibited an exponential growth trend from the centre to the bank. The dynamic changes of solute flux and mass along the stream–aquifer interface and stream concentration were linked to the shape of flood wave. As the flood wave became higher (A↗), wider (T↗), rounder (r↘), and less skewed (tp↗), the maximum solute storage in aquifer increased. Maximum stream concentration (Cstrˍmax) not only presented a positive linear relationship with A or tp but also showed a negative logarithmic trend with increasing T or r. The sensitivity of Cstr_max to A was approximately two times that of tp, and between these values, the r was slightly more sensitive than T. Cstrˍmax linearly increased as hydraulic conductivity increased and logarithmically increased as longitudinal dispersivity increased. The former relationship was more sensitive than the latter.

Spectral analysis of continuous redox data reveals geochemical dynamics near the stream–aquifer interface

AbstractChanges in streamflow and water table elevation influence oxidation–reduction (redox) conditions near river–aquifer interfaces, with potentially important consequences for solute fluxes and biogeochemical reaction rates. Although continuous measurements of groundwater chemistry can be arduous, in situ sensors reveal chemistry dynamics across a wide range of timescales. We monitored redox potential in an aquifer adjacent to a tidal river and used spectral and wavelet analyses to link redox responses to hydrologic perturbations within the bed and banks. Storms perturb redox potential within both the bed and banks over timescales of days to weeks. Tides drive semidiurnal oscillations in redox potential within the streambed that are absent in the banks. Wavelet analysis shows that tidal redox oscillations in the bed are greatest during late summer (wavelet magnitude of 5.62 mV) when river stage fluctuations are on the order of 70 cm and microbial activity is relatively high. Tidal redox oscillations diminish during the winter (wavelet magnitude of 2.73 mV) when river stage fluctuations are smaller (on the order of 50 cm) and microbial activity is presumably low. Although traditional geochemical observations are often limited to summer baseflow conditions, in situ redox sensing provides continuous, high‐resolution chemical characterization of the subsurface, revealing transport and reaction processes across spatial and temporal scales in aquifers.

Long-Term Quantification of Stream-Aquifer Exchange in a Variably-Saturated Heterogeneous Environment

A variably-saturated finite element model HYDRUS-2D was used to simulate the spatiotemporal dynamics of stream-aquifer exchange for a perennial stream flowing through an undulating catchment and underlain by heterogeneous geology. The model was first calibrated and validated using piezometric heads measured near the stream. The model was then used a) to quantify the long-term dynamics of exchange at stream-aquifer interface and the water balance in the domain, and b) to evaluate the impact of anisotropy of geological materials, thickness (w) and hydraulic conductivity (K s ) of the low permeability layer at the streambed, and water table fluctuations on the extent of exchange. Simulated pressure heads in the domain revealed that seasonal groundwater fluctuations were more pronounced near the stream. Daily discharge to the stream varied from 0.05 to 0.3 mm/day, annual discharge ranged from 59 to 74 mm, and the overall water balance showed a discharge (−54 mm) from the domain during 2000–2012. A five-fold increase in K s of the low permeability layer enhanced discharge to the stream by 14% (10 mm/year) whereas an increase in the thickness of the layer by 1 m had a low impact (2.4 mm/year). A 2-m drawdown of the water table transformed a connected and gaining system into a losing, disconnected system. These results suggest that depletion of groundwater due to climate change or excessive pumping could have a pronounced impact on the availability of water resources and sustainability of the existing water-dependent ecosystem.

Monitoring an artificial tracer test within streambed sediments with time lapse underwater 3D ERT

The stream-aquifer interface is considered a hotspot for environmental and ecological issues. Due to their complexity, the exchange mechanisms occurring between groundwater and surface water at this interface are not yet fully understood. Many studies have focused on the characterization of the two-dimensional distribution of an artificial tracer (generally injected into the stream) within and outside the streambed, but there is insufficient information about the 3D spatial distribution of the tracer fluxes and their temporal variations. We monitored the transport of an artificial solute tracer transport with 3D electrical resistivity tomography (ERT) in order to improve the 3D spatial resolution in the imaging of the first tens of centimeters of streambed sediments and propose an innovative approach of the three-dimensional and temporal observation of the water fluxes. The hydro-geophysical field measurements were made on a small stream located within the Orgeval watershed (Seine et Marne, France). Using a resistivimeter connected to 180 electrodes, 3D electrical resistivity tomograms were made on a riverbed section, as a brine tracer was injected directly into the hyporheic zone. Before the tracer monitoring, the static 3D resistivity tomograms were consistent with the lithological heterogeneities identified at the site. However, this study defines some prerequisites to high-resolution 3D underwater resistivity measurements: for instance, a precise knowledge of an eventual weak electrode contact and a spatial resolution identical in every spatial direction. First results show a rapid development and persistence of a conductive plume around the injection point which disappears progressively after the injection. Within the sediments top layer, preferential flowpaths were highlighted due to the highly heterogeneous medium and hydraulic conductivity. The riverbed topography showed some pool-riffle sequences which conduct the formation of local entering and exiting zones. It seems clear that riverbed heterogeneities drive some local exchanges between surface water and pore water, despite the gaining condition of the stream. Moreover, inversion and data processing appear very sensitive to the boundary condition variations, such as the thickness and the resistivity of the water layer. This makes a quantitative interpretation of tracer fluxes within the hyporheic zone difficult. We demonstrate that for this type of study, knowledge of these conditions and precise monitoring of their fluctuations in time are required.

Characterizing the Impact of River Barrage Construction on Stream-Aquifer Interactions, Korea

This study investigated changes in stream–aquifer interactions during the period shortly after the construction of the Changnyeong-Haman River barrage (CHRB) on the Nakdong River in South Korea. The hydraulic diffusivity (α) and river resistance (R) values at the semipervious stream–aquifer interface were estimated by using a one-dimensional (1-D) analytical solution with Fourier transform (FT). Prior to the application of the 1-D analytical solution, the noise effects on the groundwater levels were removed by using fast Fourier transform and low-pass filtering techniques. Sinusoidal variation of the river stages was applied to the 1-D analytical solution. For the study period, the R values showed a decreasing trend, while the α values showed an increasing trend, and results showed that the average of the median values of flood duration times (td) and flood amplitudes were reduced to 78% and 59%, respectively. Moreover, the ratio of flood peak time to td demonstrated a decreasing tendency after the construction of the CHRB. Hence, it is concluded that the dredging and increase of river-water storage due to CHRB construction enhanced stream–aquifer interactions during the period shortly after the construction of the CHRB.

Continental hydrosystem modelling: the concept of nested stream–aquifer interfaces

Abstract. Coupled hydrological-hydrogeological models, emphasising the importance of the stream–aquifer interface, are more and more used in hydrological sciences for pluri-disciplinary studies aiming at investigating environmental issues. Based on an extensive literature review, stream–aquifer interfaces are described at five different scales: local [10 cm–~10 m], intermediate [~10 m–~1 km], watershed [10 km2–~1000 km2], regional [10 000 km2–~1 M km2] and continental scales [>10 M km2]. This led us to develop the concept of nested stream–aquifer interfaces, which extends the well-known vision of nested groundwater pathways towards the surface, where the mixing of low frequency processes and high frequency processes coupled with the complexity of geomorphological features and heterogeneities creates hydrological spiralling. This conceptual framework allows the identification of a hierarchical order of the multi-scale control factors of stream–aquifer hydrological exchanges, from the larger scale to the finer scale. The hyporheic corridor, which couples the river to its 3-D hyporheic zone, is then identified as the key component for scaling hydrological processes occurring at the interface. The identification of the hyporheic corridor as the support of the hydrological processes scaling is an important step for the development of regional studies, which is one of the main concerns for water practitioners and resources managers. In a second part, the modelling of the stream–aquifer interface at various scales is investigated with the help of the conductance model. Although the usage of the temperature as a tracer of the flow is a robust method for the assessment of stream–aquifer exchanges at the local scale, there is a crucial need to develop innovative methodologies for assessing stream–aquifer exchanges at the regional scale. After formulating the conductance model at the regional and intermediate scales, we address this challenging issue with the development of an iterative modelling methodology, which ensures the consistency of stream–aquifer exchanges between the intermediate and regional scales. Finally, practical recommendations are provided for the study of the interface using the innovative methodology MIM (Measurements–Interpolation–Modelling), which is graphically developed, scaling in space the three pools of methods needed to fully understand stream–aquifer interfaces at various scales. In the MIM space, stream–aquifer interfaces that can be studied by a given approach are localised. The efficiency of the method is demonstrated with two examples. The first one proposes an upscaling framework, structured around river reaches of ~10–100 m, from the local to the watershed scale. The second example highlights the usefulness of space borne data to improve the assessment of stream–aquifer exchanges at the regional and continental scales. We conclude that further developments in modelling and field measurements have to be undertaken at the regional scale to enable a proper modelling of stream–aquifer exchanges from the local to the continental scale.

Nitrogen Changes between Rural and Peri-Urban Stream Subsurface Waters (Yzeron Stream, France)

Urbanization subjects stream and groundwater to increased loads of organic nitrogen, nitrate, and ammonium. Therefore, studying nitrogen species at the urban stream-aquifer interface is important for water resource management. We report here results on water 18O/16O ratios and on nitrogen species in stream subsurface waters upstream and downstream of several combined sewer overflows (CSOs) in a rural area and peri-urban area, respectively. Water 18O/16O ratios were measured to trace the mixing of subsurface waters with stream channel water. Organic nitrogen (ON) and carbon (OC) slightly increased between rural and peri-urban environments in the cold season, but not in the warm season. The highest nitrate levels were observed in rural subsurface waters in the cold season. The lowest nitrate levels were found in peri-urban subsurface waters in the warm season; they corresponded to slow exchange of subsurface waters with channel water.

A general framework of stream–aquifer interaction caused by variable stream stages

Stream–aquifer interface is a critical zone where surface water and groundwater exchange mass, energy, and chemicals. Previous analytical studies of stream–aquifer interaction consider stream stage fluctuations that are only functions of time. This study improves those solutions by simultaneously considering temporally and spatially variable stream stages, low-permeability streambeds, and pumping wells nearby the streams. Semi-analytical solutions of drawdown and stream–aquifer flow rate in the Laplace–Fourier domain are provided and used to obtain solutions in the real time–space domain after the inverse Laplace–Fourier transform, where the stream–aquifer flow rate refers to the rate of discharge between a stream and an adjacent aquifer. The stream–aquifer flow rate caused by the temporally variable stream stage is found to decline with time and becomes negligible when time is sufficiently long. A closed-form analytical solution for the stream–aquifer flow rate caused by a diffusive-type flood wave is obtained, and it can be used to predict the response of an aquifer upon a passing flood wave. For a diffusive-type flood wave that dies out in about 24 h, we find that the stream–aquifer flow rate drops nearly exponentially with time and dies out at about 13.4 h for a sandy aquifer and an alluvial streambed. Sensitivity analysis shows that the stream–aquifer flow rate increases with the diffusion coefficient of the flood wave ( D ) which controls the width and amplitude of the wave, and decreases with the average flood wave velocity ( v ). Low-permeability streambeds reduce stream–aquifer flow rate and also delay the response to the stream stage fluctuations. For a time-dependent cosine stream stage function with a period of 30 days, the maximal stream–aquifer flow rate with a 0.5 m thick low-permeability streambed is 24% less than that obtained without the streambed.

New methodology to investigate potential contaminant mass fluxes at the stream–aquifer interface by combining integral pumping tests and streambed temperatures

The spatial pattern and magnitude of mass fluxes at the stream–aquifer interface have important implications for the fate and transport of contaminants in river basins. Integral pumping tests were performed to quantify average concentrations of chlorinated benzenes in an unconfined aquifer partially penetrated by a stream. Four pumping wells were operated simultaneously for a time period of 5 days and sampled for contaminant concentrations. Streambed temperatures were mapped at multiple depths along a 60 m long stream reach to identify the spatial patterns of groundwater discharge and to quantify water fluxes at the stream–aquifer interface. The combined interpretation of the results showed average potential contaminant mass fluxes from the aquifer to the stream of 272 μg m −2 d −1 MCB and 71 μg m −2 d −1 DCB, respectively. This methodology combines a large-scale assessment of aquifer contamination with a high-resolution survey of groundwater discharge zones to estimate contaminant mass fluxes between aquifer and stream.

Response of sloping unconfined aquifer to stage changes in adjacent stream: II. Applications

Summary We convolve the system response functions of the linearised 1D equation of Boussinesq (extended for slope, Dupuit approximation) derived in the companion paper of Akylas and Koussis [Akylas, E., Koussis, A.D., (submitted for publication). Response of sloping unconfined aquifer to stage changes in adjacent stream: I. Theoretical analysis and derivation of system response functions, J. Hydrol] and develop solutions for the interaction of a fully penetrating stream with a sloping unconfined aquifer, taking also into account a low-conductivity streambed layer. These solutions give the aquifer stage and flow rate, the flow exchange rate at the stream–aquifer interface and the exchanged water volumes (bank storage/release). The solutions are analytical, when the flow-inducing stream stage variations are common functions; otherwise, the convolution integral is evaluated numerically. Responses are compared for aquifers on positive, negative and zero base slopes to a wave-like change in the stage of the stream. Analytical and numerical convolution-derived solutions are used to model the interaction of Cedar River and its adjacent unconfined aquifer near Cedar Rapids, Iowa. In the model, the aquifer base is taken to be horizontal or inclined and the streambed clogged by a low-conductivity sediment layer; the model is verified against data not used in the calibration and is found to perform very well.

Response of sloping unconfined aquifer to stage changes in adjacent stream. I. Theoretical analysis and derivation of system response functions

We study the interaction of a stream with a sloping unconfined aquifer that the stream is assumed to fully penetrate. The analysis applies to flow in a vertical section, considers the existence of a low-conductivity streambed layer and the flow in the aquifer to be induced by variations of the stream stage. Invoking the Dupuit assumption yields the 1-D Boussinesq equation, extended for a sloping base. The Boussinesq equation is linearised, the derived flow model is critiqued and an objective procedure for determining the linearisation level is developed. We solve the linear governing equation by the method of Laplace transform, with analytical inversion; the horizontal-aquifer case is treated in the zero-slope limit. The system response function is derived for the general case (sloping aquifer, sediment bed layer) and for several specific cases, and solutions are verified against known analytical results. Responses are contrasted for aquifers on positive, negative and zero slopes to step changes in the stage of streams with and without a sediment bed layer. The solutions give the aquifer stage and flow rate, the flow exchange rate at the stream-aquifer interface and the exchanged water volumes (bank storage/release).

Erratum/

GroundwaterVolume 45, Issue 2 p. 125-125 Free Access Erratum/ This article corrects the following: A Simple Constant-Head Injection Test for Streambed Hydraulic Conductivity Estimation M. Bayani Cardenas, Vitaly A. Zlotnik, Volume 41Issue 6Groundwater pages: 867-871 First Published online: December 13, 2005 First published: 29 January 2007 https://doi.org/10.1111/j.1745-6584.2006.00283.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat In the Technical Note “A simple constant-head injection test for streambed hydraulic conductivity estimation,” M.B. Cardenas and V.A. Zlotnik (2003) presented a constant-head injection test (CHIT) method for mapping hydraulic conductivity of unconsolidated streambed sediments. There is a typographical error (namely, a misplaced parenthesis) in their equation (11) that could lead to erroneous values for the shape factor P . Equation (11) should read: (11) where A and B are coefficients originally defined by Bouwer and Rice (1976), b is the aquifer saturated thickness below the stream, l is the depth to the top of the screen from the stream-aquifer interface, L is the screen length and rw is the piezometer radius. Note that b in the natural logarithm of the second term in the right-hand side of the equation is also divided by rw. Not doing so not only results in incorrect values for P but may also result in negative arguments for the terms in the brackets leading to complex values after taking the natural logarithm. In general, this error does not change conclusions in the original note or the applicability of the CHIT method. We thank Charles F. Pitz of the Washington State Department of Ecology for bringing this error to our attention. References Bouwer, H., and R.C. Rice. 1976. A slug test for determining hydraulic conductivity of unconfined aquifers with completely of partially penetrating wells. Water Resources Research 12, no. 3: 423– 428. Wiley Online LibraryWeb of Science®Google Scholar Cardenas, M.B., and V.A. Zlotnik. 2003. A simple constant-head injection test for streambed hydraulic conductivity estimation. Ground Water 41, no. 6: 867– 871. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Volume45, Issue2March–April 2007Pages 125-125 ReferencesRelatedInformation

Comment on “Using numerical modelling to evaluate the capillary fringe groundwater ridging hypothesis of streamflow generation” by H. L. Cloke, et al. [J. Hydrol. 316 (2006) 141–162

Cloke et al. (2006) discuss an interesting topic of runoff generation, a central theme in hydrology. While their numerical experiment reproduced the physical experiment of Abdul and Gillham (1984) with good agreement, I feel that their proposed generalization of the model results to more realistic natural settings of riparian zones is not perfectly well supported. This is so because the stream channel initially is disconnected from the groundwater in Abdul and Gillham (1984) experiment which is replicated by Cloke et al. (2006) (Figs. 2, 4, and 6) in their above numerical study. In my opinion the initially disconnected stream setting ought to be replaced by a more common (at least in humid climates) perennially gaining stream example where groundwater is in continuous contact with the channel through the streamaquifer interface permitting immediate water fluxes across it when responding to precipitation. This more realistic boundary condition at the stream-aquifer interface will allow rapid groundwater inflow to the stream and should be

Stream depletion predictions using pumping test data from a heterogeneous stream–aquifer system (a case study from the Great Plains, USA)

This uniquely designed study investigates a fundamental issue—the feasibility of predicting stream depletion rates using linear uniform two-dimensional models. Required input for these models includes the hydraulic parameter estimates of the aquifer and the stream–aquifer interface, which may be obtainable through pumping test data analysis. This study utilizes pumping test data collected near the naturally meandering Prairie Creek, Platte River watershed, Nebraska, USA. Drawdown data were obtained in eight piezometer clusters, located on both sides of the stream, each containing three piezometers screened at different aquifer depths. Parameter estimates and, thus, stream depletion predictions varied over a wide range. Large parameter variance and a low degree of goodness of fit between the calculated and measured data encountered during the analysis suggest deficiencies of the uniform aquifer models in describing significant physical processes. This was also shown by additional field experiments that indicate lateral and vertical aquifer heterogeneity. Hydrogeological and sedimentological considerations of the meandering stream architecture (point bar versus cut bank) and the application of a linear piecewise-homogeneous model yielded a higher degree of goodness of fit and higher confidence in stream depletion predictions. Aquifer heterogeneity appears to be the major reason for uncertainty in stream depletion predictions, though other possible sources of uncertainty should be considered. These include the model linearity, the Dupuit assumption, the simplified representation of the stream–aquifer interface, the approximation of the stream as a straight line or a strip, and the impact of regional groundwater flow.

Explicit Estimation of Aquifer Diffusivity from Linear Stream Stage

Different approaches are available for estimation of aquifer hydraulic diffusivity from a linear stream-stage variation and corresponding groundwater heads. These approaches require interpolation from tabulated values or computation of hydraulic gradient at the stream aquifer interface. Certain methods use approximation or interpolation of tabulated values for an infinite series. These methods are prone to errors in the estimated aquifer hydraulic diffusivity. An alternative approach is to use a closed-form solution of the problem and develop an explicit expression for the aquifer hydraulic diffusivity, which is free from errors of using infinite series, interpolation, and computation of hydraulic gradient. Such an alternative method is developed for a linearly varying stream stage. The new method can yield the estimate of hydraulic diffusivity even from a single observation. The proposed method would be applicable to practical hydraulic engineering problems keeping in view that most of a rising part of a stream stage hydrograph can be approximated by a linear rise and certain rivers may show a linearly varying stream stage. Use of the new method is demonstrated on published and field data, which shows that the estimates obtained using the new method are comparable to that obtained using an optimization approach.

Analysis of pumping-induced stream–aquifer interactions for gaining streams

This paper presents analytical solutions that can be used to evaluate stream infiltration and baseflow reduction induced by groundwater pumping in nearby aquifers. Critical time, infiltration reach, and travel times can also be calculated to determine the hydraulic connectivity between the well and the stream. The critical time indicates the earliest time of reversal of hydraulic gradient occurring along the stream–aquifer interface, the infiltration reach is the stream segment where stream water recharges the aquifer, and the shortest travel time for the stream water particle to get into a pumping well is along the meridian line. The transient features of the two stream depletion components, baseflow reduction and stream infiltration, are evaluated separately. The rate of baseflow reduction can be greater than the rate of stream infiltration for a stronger gaining stream. However, for a given distance between the stream and well, a higher pumping rate or a weaker gaining stream results in higher rate of stream infiltration, although the total depletion rate is the same for different pumping rates or varied hydraulic gradient of the baseflow. When a steady-state condition is assumed for a transient flow, the rate and volume of stream infiltration can be overestimated; this overestimation can be very significant in the early stage of pumping.

Explicit Aquifer Diffusivity Estimation Using Linearly Varying Stream Stage

For a stream fully penetrating an unconfined aquifer, different approaches are available to estimate aquifer diffusivity. However, these approaches involve an interpolation of tabulated values or type-curve matching. Certain solutions also require curve fitting in order to compute hydraulic gradient at the stream-aquifer interface. Thus, in these approaches it is possible to incur errors. An alternative is to develop explicit equations of aquifer diffusivity that may not require any sort of curve fitting to extract hydraulic gradient or interpolation of tabular values. For a linearly varying stream stage, it has been possible to develop such an alternative approach. The present study develops an explicit relationship for the estimation of aquifer diffusivity using the available series or tabular solutions in the literature. An error analysis indicates that the proposed equation is capable of estimating the point-aquifer diffusivity within a maximum error of 3%.

Estimation of Aquifer Diffusivity in Stream-Aquifer Systems

By specifying an extra boundary condition in terms of hydraulic gradient at the stream-aquifer interface, and using the Green’s function of the linearized Boussinesq equation, analytical solutions have been developed to determine aquifer diffusivity explicitly. Four different types of Dirichlet boundary condition describing stream gage hydrographs have been employed. They are: (1)Linearly rising water level in the stream; (2)exponentially rising water level in the stream; (3)water level represented by a sinusoid; and (4)hydrographs approximated by cubic splines. The last one is quite general and is capable of analyzing different shapes of smooth stream hydrographs expected to occur in nature. Hydraulic gradient at the interface was computed using splines. The solutions were validated by analyzing the stream-aquifer data observed in a sand tank model. They yielded consistent values of the aquifer diffusivity at different times, and therefore are useful in determining the parameter of the aquifers contiguous to streams.