Horizontal Hydraulic Conductivity Research Articles

Saturated anisotropic hydraulic conductivity of a compacted lateritic soil

This study focuses on the saturated anisotropic hydraulic conductivity of a compacted lateritic clayey sandy soil. The effects of the molding water content and the confining stress on the anisotropic hydraulic conductivity are investigated. The hydraulic conductivity is measured with a flexible-wall permeameter. Samples are dynamically compacted into the three compaction states of a standard Proctor compaction curve: the dry branch, optimum water content and wet branch. Depending on the molding water content and confining stress, the hydraulic conductivity may increase or decrease. In addition, the results indicate that, when the samples are compacted to the optimum water content, lower hydraulic conductivity is obtained, except at a confining stress equal to 50 kPa. The increase of the confining stress decreases the hydraulic conductivity for each of the evaluated compaction states. In the wet branch, horizontal hydraulic conductivity is about 8 times higher than the vertical value. The anisotropic hydraulic conductivities of the dry and wet branches decrease when the confining stress increases, and the opposite is observed in the optimum water content state.

A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity

A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.

Borehole characterization of hydraulic properties and groundwater flow in a crystalline fractured aquifer of a headwater mountain watershed, Laramie Range, Wyoming

Fractured crystalline aquifers of mountain watersheds may host a significant portion of the world’s freshwater supply. To effectively utilize water resources in these environments, it is important to understand the hydraulic properties, groundwater storage, and flow processes in crystalline aquifers and field-derived insights are critically needed. Based on borehole hydraulic characterization and monitoring data, this study inferred hydraulic properties and groundwater flow of a crystalline fractured aquifer in Laramie Range, Wyoming. At three open holes completed in a fractured granite aquifer, both slug tests and FLUTe liner profiling were performed to obtain estimates of horizontal hydraulic conductivity (Kh). Televiewer (i.e., optical and acoustic) and flowmeter logs were then jointly interpreted to identify the number of flowing fractures and fracture zones. Based on these data, hydraulic apertures were obtained for each borehole. Average groundwater velocity was then computed using Kh, aperture, and water level monitoring data. Finally, based on all available data, including cores, borehole logs, LIDAR topography, and a seismic P-wave velocity model, a three dimensional geological model of the site was built. In this fractured aquifer, (1) borehole Kh varies over ∼4 orders of magnitude (10−8–10−5 m/s). Kh is consistently higher near the top of the bedrock that is interpreted as the weathering front. Using a cutoff Kh of 10−10 m/s, the hydraulically significant zone extends to ∼40–53 m depth. (2) FLUTe-estimated hydraulic apertures of fractures vary over 1 order of magnitude, and at each borehole, the average hydraulic aperture by FLUTe is very close to that obtained from slug tests. Thus, slug test can be used to provide a reliable estimate of the average fracture hydraulic aperture. (3) Estimated average effective fracture porosity is 4.0 × 10−4, therefore this fractured aquifer can host significant quantity of water. (4) Natural groundwater velocity is estimated to range from 0.4 to 81.0 m/day, implying rapid pathways of fracture flow. (5) The average ambient water table position follows the boundary between saprolite and fractured bedrock. Groundwater flow at the site appears topography driven.

Optimization of dewatering schemes for a deep foundation pit near the Yangtze River, China

A deep foundation pit constructed for an underground transportation hub was excavated near the Yangtze River. Among the strata, there are two confined aquifers, between which lies an aquiclude that is partially missing. To guarantee the safety of pit excavation, the piezometric head of the upper confined aquifer, where the pit bottom is located, should be 1 m below the pit bottom, while that of the lower confined aquifer should be dewatered down to a safe water level to avoid uplift problem. The Yangtze River levee is notably close to the pit, and its deformation caused by dewatering should be controlled. A pumping test was performed to obtain the hydraulic conductivity of the upper confined aquifer. The average value of the hydraulic conductivity obtained from analytical calculation is 20.45 m/d, which is larger than the values from numerical simulation (horizontal hydraulic conductivity KH = 16 m/d and vertical hydraulic conductivity KV = 8 m/d). The difference between KH and KV indicates the anisotropy of the aquifer. Two dewatering schemes were designed for the construction and simulated by the numerical models for comparison purposes. The results show that though the first scheme could meet the dewatering requirements, the largest accumulated settlement and differential settlement would be 94.64 mm and 3.3‰, respectively, greatly exceeding the limited values. Meanwhile, the second scheme, in which the bottoms of the waterproof curtains in ramp B and the river side of ramp A are installed at a deeper elevation of −28 m above sea level, and 27 recharge wells are set along the levee, can control the deformation of the levee significantly.

HYDRAULIC CONDUCTIVITY CHARACTERISTICS OF ROAD BASE MATERIALS BLENDED WITH FLY ASH AND BOTTOM ASH

Hydraulic conductivity, is the ability of water to flow through a soil, should be consideredwhen designing roads. Hydraulic conductivity should be employed when designing roads that will providegood drainage as well. Fly ash and bottom ash were utilized as partial substitutes to conventional road basematerials in road base construction. The study aimed to prove that employing fly ash and bottom ash wouldincrease the hydraulic conductivity characteristics of road base while also decreasing the disposal costs of thesaid coal by-products. Series of experiments were conducted to test the horizontal and vertical hydraulicconductivity of pure fly ash, pure bottom ash, pure conventional road base materials, and blends comprisingof the said soil components. It was also established that horizontal and vertical hydraulic conductivity had asignificant difference wherein the flow of water at the horizontal-direction is greater compared to thevertical-direction.

Zone-Integrated Double-Constraint Methodology for Calibration of Hydraulic Conductivities in Grid Cell Clusters of Groundwater Flow Models

This paper deals with the double-constraint methodology for calibration of steady-state groundwater flow models. The methodology is based on updating the hydraulic conductivity of the model domain by comparing the results of two forward groundwater flow models: a model in which known fluxes are specified as boundary conditions and a model in which known heads are specified as boundary conditions. A new zone-integrated double-constraint approach is presented by partitioning the model domain in zones with presumed constant hydraulic conductivity (soft data), and the double-constraint methodology is reformulated accordingly. The feasibility of the method is illustrated by a practical case study involving a numerical steady-state groundwater flow model with about 3 million grid blocks, subdivided into four zones corresponding to the major hydrogeological formations. The results of the zone-integrated double-constraint method for estimating the horizontal and vertical hydraulic conductivities of the zones compare favourably with a classical model calibration based on minimisation of the differences between calculated and measured heads, while the double-constraint method proves to be more robust and computationally less cumbersome.

Contribution to the Ground Water Hydrology of the Quaternary Aquifer in West Ismailia Area, Egypt

Hydrologic conditions of the ground water of the Quaternary aquifer in west Ismailia area, Egypt, were characterized based on new hydrologic data collected in 2017. The Quaternary aquifer consists of alluvial deposits. The grain size distribution results indicate that the major part of the aquifer deposits is formed of medium to coarse grained sand with medium uniformity coefficients ranging between 2.5 and 10. They also reflect the heterogeneity, anisotropy and high productivity of the aquifer. The estimated aquifer parameters range from 29% to 41% for total porosity, from 18.71 m/day to 63.95 m/day for horizontal hydraulic conductivity, from 8.94 m/day to 61.6 m/day for vertical hydraulic conductivity, from 1.01 to 4.27 for anisotropy and from 1870 m2/day to 6549 m2/day for transmissivity. The ground water flows mainly from the north and northeast recharge sources (Ismailia and El Manaief canals) and from the south recharge area (Miocene aquifer), with an average hydraulic gradient of 0.00438. The recharge rate to the aquifer (from the southern area) and the Darcy’s velocity of ground water are estimated to be 447 × 106 m3/year and 0.203 m/day (on average), respectively. A great change in the hydrologic setting of the aquifer is occurred during the period of 1992-2017, where the groundwater flow has changed during that period. The aquifer contains fresh to saline water. An oxidizing (alkaline) environment is reported for the aquifer, as revealed from the relationship between the redox potential (185 - 836 mV) and pH (5.2 - 7.5 standard units). This indicated the capability of ground water to dissolve heavy metals associated to rock-forming minerals. A wide variation in the concentrations of total dissolved solids (320 - 7385 mg/l) and dissolved oxygen (2.13 - 8.4 mg/l) in the Quaternary aquifer is observed, reflecting the local variation of the environmental and geologic conditions and indicating the influence of different recharge sources.

Hydrogeological Characterization of Aquifer in Palla Flood Plain of Delhi Using Integrated Approach

ABSTRACT The Yamuna flood plains spread across the northern part of Indian subcontinent is home to millions of people. The ever-growing population in these plains make it difficult to sustain the demand of freshwater resources. However, the productive aquifers of flood plains could provide solution for these issues. In this context, it is necessary to understand the aquifer characteristics. Thus, the paper attempts to characterize the aquifer in Palla area of the flood plain using integrated approach. Besides, grain size analysis and site-litholog study, the nature of aquifer material was also ascertained from bulk mineralogy of the sediments using X-Ray Diffraction. The aquifer parameters were estimated with help of long duration pumping test data. Moreover, the effect of pumping on salinity variation and hydrochemical facies evolution was also examined. The sand dominant, unconfined aquifer was estimated to have horizontal hydraulic conductivity in the range of 25 m/day and vertical hydraulic conductivity of 6-7 m /day. While the specific yield of the aquifer was estimated in the range of 0.07-0.08 .It is observed that under conducive active flood plain environment, the given sand mineralogy at the site does not allow salinity increase in groundwater even after more than a decade of groundwater pumping. In fact, over years, hydrochemical facies have evolved towards Bicarbonate type. These things put together make the active flood plain aquifer a sustainable groundwater resource.

Modeling of leachate recirculation using combined drainage blanket-horizontal trench systems in bioreactor landfills.

Leachate recirculation in municipal solid waste (MSW) landfills operated as bioreactors offers significant economic and environmental benefits. Combined drainage blanket (DB)-horizontal trench (HT) systems can be an alternative to single conventional recirculation approaches and can have competitive advantages. The key objectives of this study are to investigate combined drainage blanket -horizontal trench systems, to analyze the effects of applying two recirculation systems on the leachate migration in landfills, and to estimate some key design parameters (e.g., the steady-state flow rate, the influence width, and the cumulative leachate volume). It was determined that an effective recirculation model should consist of a moderate horizontal trench injection pressure head and supplementary leachate recirculated through drainage blanket, with an objective of increasing the horizontal unsaturated hydraulic conductivity and thereby allowing more leachate to flow from the horizontal trench system in a horizontal direction. In addition, design charts for engineering application were established using a dimensionless variable formulation.

Spatial variation of saturated hydraulic conductivity of a loess slope in the South Jingyang Plateau, China

Water infiltration in loess soils is an important trigger causing landslides, in which the hydraulic conductivity of the loess plays an important role. By analyzing a soil trench and soil samples from the South Jingyang Plateau, northwest China, a loess slope was investigated. A horizontal adit in a Lishi loess layer was also analyzed. Using variable head permeability tests the saturated horizontal hydraulic conductivity (Kh) and saturated vertical hydraulic conductivity (Kv) of the loess samples were measured. Results showed that the majority of the Kh values are larger than the Kv values, indicating that the loess exhibits permeability anisotropy. Lilliefors test was used to determine the normality of the Kh and Kv results. Test results indicate that Kh and Kv measurements of the loess samples collected in the trench and adit all show log-normal distribution, while a number of the Kh and Kv values also reflect a normal distribution. Geo-statistical analysis shows that the spherical model and the Gaussian model provide the best fit variograms of Kh and Kv values along the trench; analysis along the adit indicated the opposite results. The variogram range values suggest that the spatial correlation of loess Kv is greater in the vertical direction in a horizontal plane, while the spatial correlation of Kh in the adit is greater than that in the vertical direction. Loess structure rather than randomness decides the spatial variation of Kh and Kv measurements (except for Kh variation along the adit which depends on both structural factor and randomness). Kh and Kv values of loess tend to decrease with depth; results which can be described by an exponential decay model.

Hydraulic properties of the Paskapoo Formation in west-central Alberta

In an effort to better understand the hydraulic properties of the Paskapoo Formation, hydraulic conductivity and porosity were evaluated for a region in west-central Alberta. Whereas previous studies have focused mainly on sandstone units in the lower portion of the Paskapoo Formation, in southern and central parts of the province, this study focuses on the middle to upper portions. Hydraulic conductivity values were determined by air permeametry for seven drill cores from the area between Hinton and Fox Creek, Alberta. Thin-section petrology and porosity analyses using photomicrographs were also conducted for three of the seven drill cores. Results confirm previous findings that the Paskapoo Formation has heterogeneous hydraulic properties, with horizontal hydraulic conductivity values ranging from 10−10to 10−5m/s (determined by air permeametry) and porosity values ranging from 0.02% to 15.3%. The first measurements for the upper sandstone units are provided (1.1 × 10−9– 2.6 × 10−5m/s and 0.08%–15.3%) and numerous measurements of the middle siltstone–mudstone unit (1.1 × 10−10– 4.9 × 10−8m/s and 0.02%–1.8%) for the northwestern portion of the Paskapoo Formation. Qualitative petrologic analysis suggests that the degree of cementation, rather than grain size, is the dominant control on the hydraulic properties of this portion of the formation. This study determined primary hydraulic properties for both the highly conductive units often considered as aquifers and the low-conductivity units considered as aquitards or confining layers. When combined with previous findings, this study helps expand the understanding of the Paskapoo Formation and provides critical data for assessing groundwater resources.

Estimation of saturated hydraulic conductivity from ring infiltrometer test taking into account the surface moisture stain extension

Abstract A large single-ring infiltrometer test was performed in order to characterize the saturated hydraulic conductivity below an infiltration basin in the well field of Lyon (France). Two kinds of data are recorded during the experiment: the volume of water infiltrated over time and the extension of the moisture stain around the ring. Then numerical analysis was performed to determine the saturated hydraulic conductivity of the soil by calibration. Considering an isotropic hydraulic conductivity, the saturated hydraulic conductivity of the alluvial deposits is estimated at 3.8 10−6 m s−1. However, with this assumption, we are not able to represent accurately the extension of the moisture stain around the ring. When anisotropy of hydraulic conductivity is introduced, experimental data and simulation results are in good agreement, both for the volume of water infiltrated over time and the extension of the moisture stain. The vertical saturated hydraulic conductivity in the anisotropic configuration is 4.75 times smaller than in the isotropic configuration (8.0 10−7 m s−1), and the horizontal saturated hydraulic conductivity is 125 times higher than the vertical saturated hydraulic conductivity (1.0 10−4 m s−1).

Effects of temperature and thermally-induced microstructure change on hydraulic conductivity of Boom Clay

Boom Clay is one of the potential host rocks for deep geological disposal of high-level radioactive nuclear waste in Belgium. In order to investigate the mechanism of hydraulic conductivity variation under complex thermo-mechanical coupling conditions and to better understand the thermo-hydro-mechanical (THM) coupling behaviour of Boom Clay, a series of permeability tests using temperature-controlled triaxial cell has been carried out on the Boom Clay samples taken from Belgian underground research laboratory (URL) HADES. Due to its sedimentary nature, Boom Clay presents across-anisotropy with respect to its sub-horizontal bedding plane. Direct measurements of the vertical (Kv) and horizontal (Kh) hydraulic conductivities show that the hydraulic conductivity at 80 °C is about 2.4 times larger than that at room temperature (23 °C), and the hydraulic conductivity variation with temperature is basically reversible during heating–cooling cycle. The anisotropic property of Boom Clay is studied by scanning electron microscope (SEM) tests, which highlight the transversely isotropic characteristics of intact Boom Clay. It is shown that the sub-horizontal bedding feature accounts for the horizontal permeability higher than the vertical one. The measured increment in hydraulic conductivity with temperature is lower than the calculated one when merely considering the changes in water kinematic viscosity and density with temperature. The nuclear magnetic resonance (NMR) tests have also been carried out to investigate the impact of microstructure variation on the THM properties of clay. The results show that heating under unconstrained boundary condition will produce larger size of pores and weaken the microstructure. The discrepancy between the hydraulic conductivity experimentally measured and predicted (considering water viscosity and density changes with temperature) can be attributed to the microstructural weakening effect on the thermal volume change behaviour of Boom Clay. Based on the experimental results, a hydraulic conductivity evolution model is proposed and then implemented in ABAQUS. Three-dimensional (3D) numerical simulation of the admissible thermal loading for argillaceous storage (ATLAS) III in situ heating test has been conducted subsequently, and the numerical results are in good agreement with field measurements.

Numerical Simulation of Soil Stress State Variations due to Mini-Pile Penetration in Clay

During installation of prefabricated piles in saturated clayey soils, excess pore water pressure (EPWP) is generated around the pile shaft and tip. The developed excess pore pressure is extremely important for evaluation of changes of stress state in clay and subsequent pile bearing capacity. Therefore, the main objective of this paper is to present a numerical finite-element model (FEM) to simulate a mini-pile installation and subsequent pore water pressure dissipation over time. The mini-pile is inserted from the ground surface into the soil to the desired depth. The numerical model is validated using the results of a physical model in which a piezocone is penetrated into a consolidation chamber containing saturated clay. The effects of Over-Consolidation Ratio (OCR), coefficient of lateral soil pressure (K 0), penetration rate (PR), and soil hydraulic conductivity (K i ) on soil stress state are investigated. The verification results have shown that the numerical model has successfully simulated the penetration of mini-pile, generation, and subsequently dissipation of the excess pore water pressure (EPWP). The parametric study revealed that increasing the OCR and K 0 has resulted in increase of the effective radial stresses on the pile skin compared to its initial value, after dissipation of EPWP. The results indicate that the variations in radial stresses at the end of dissipation are independent from the penetration rate. The dissipation of EPWP is more correlated with horizontal hydraulic conductivity of the soil, because the dissipation rate is accelerated in the radial direction.

Field scale hydraulic conductivity and compressibility of organic clays

This paper uses the results of an extensive subsurface investigation performed along the Inner Harbor Navigational Canal in New Orleans, Louisiana to identify the scale effect of soil hydraulic conductivity (K) and compressibility (mv) for geotechnical and geoenvironmental analyses. The magnitude and variability of soil hydraulic conductivity and compressibility parameters were obtained from laboratory 1-D consolidation and permeameter tests, CPTu dissipation tests, field piezometer slug tests, and multiple well pump tests. The geometric means of horizontal hydraulic conductivity (Kh) measured by the laboratory permeameter, slug, and field pump tests are 2.5×10−7, 1.6×10−6, and 1.3×10−5cm/s, respectively. This represents a scale effect of about 50 times when increasing the sample volume tested from the laboratory to field scale. The uncertainty in vertical hydraulic conductivity (Kv), Kh, and mv values was determined using a coefficient of variation, which ranges from 0.34 to 0.73. While Kh is scale dependent, a comparison of mv evaluated from field pump and 1-D consolidation tests indicates that sample volume does not significantly impact measured values of mv, which signifies that laboratory consolidation tests can be used to predict field scale compressibility.

Quantitative Analysis of Transient Intertidal Submarine Groundwater Discharge in Coastal Aquifer of Western Japan

The work assessed effect of diurnal tidal fluctuation on transient groundwater dynamics with special focus on trace metal mobilization using tide-aquifer interaction technique in the Saijo plain, Japan. Fluctuation of trace metal concentration in groundwater during intertidal phase obtained through geochemical analysis is validated with numerical simulation using two different codes (PHREEQC and SEEP/W) to observe saturation index for different minerals in aquifers and dynamics of submarine groundwater discharge (SGD) respectively. Result for saturation index shows that most of the samples are strongly undersaturated with respect to FeS, goethite, siderite and scorodite, unlike pyrite where it approaches towards saturation. Also during lower low tide situation, water samples relatively getting more undersaturated with pyrite suggesting it as a source for dissolved iron. For numerical simulation, problem domain consists of 368 elements and 3 layers, is an anisotropic unconfined aquifer with horizontal hydraulic conductivities (K X ) ranging from 0.00001 to 0.01 m/P. Coastal side of the domain considered as variable head boundary keeping different diurnal tidal amplitude into account. Simulation result shows that during low tide situation, value of SGD is maximum i.e. 2.1644e−005 and 3.3704e−005 m3/s for lower boundary towards sea at 2 and 4 m below mean sea level respectively. It suggests that tidal height affects the amount and position of SGD to play a positive role for trace metal mobilization through oxidation–reduction process. Strong relation between diurnal fluctuations of the simulated results (SGD) versus observed result oxidation–reduction potential firmly supports and validates the results from chemical analysis.

Analysis of transient groundwater flow through a stochastic modelling approach

This study demonstrates the application of a stochastic procedure to enhance confidence in the predictions of numerical groundwater flow models. The objective was to assess the hydrogeological conditions and groundwater potential of a semi-confined aquifer system in Legon, Accra. The study applied the parameter randomization procedure in the Groundwater Modelling System (GMS) to evaluate model uncertainties arising from uncertainties in the data of model input parameters. A total of 2500 equally likely solutions of the numerical model were simulated through the Latin hypercube stochastic option. Regarding the size of the terrain and the number of parameters used for calibrating the model, the 2500 model realizations were adjudged to be largely sufficient to represent the possible solutions that could arise from the various permutations of the parameter data. High variances would suggest significant differences in the realizations, indicating high levels of non-uniqueness in the solutions. The 2500 solutions were compared using standard deviations computed from the differences in the estimated horizontal hydraulic conductivity, specific yield and specific storage estimates. The transient model calibrated for this domain suggests very close similarities in the model-computed aquifer parameters for all the 2500 solutions. This suggests that any of the solutions could be used to represent the conditions prevailing in the domain. Risk analyses were then performed on abstractions through all the 2500 solutions. The study shows that the domain is sustained largely by lateral subsurface inflows from adjoining areas amounting to 399,278 m3/day, with direct vertical infiltration and percolation of precipitation accounting for an average of 2450 m3/day. The analysis suggests that at this rate of input, the domain can sustain groundwater abstraction to meet all water needs in the area. In addition, the domain has demonstrated resilience against annual increment in abstraction rates by 5 % over a 20-year period at the current recharge conditions. However, increasing abstraction rates by up to 200 % of the current abstraction rates over the 20-year period (2015–2035) but with a gradual decrease in groundwater recharge by up to 10 % over the period will result in drastic lowering of the hydraulic head especially in the south-central portions of the domain. This situation might induce reversal of the current flow and potentially lead to contamination and/or aquifer dewatering in the domain.

Tunnel Engineering Seepage Parameters Intelligent Back Analysis Based on FEM and Differential Evolution Algorithm

To determine the seepage parameters of surrounding rock during excavation of tunnels, a program of groundwater seepage (SEEP) was developed. Then finite element models are established in SEEP with the consideration of seepage equations for continuous medium, and the correctness was verified by the comparison of FLAC3D. Moreover, seepage back analysis program (SBAP) was developed in C++ language by combining differential evolution algorithm (DE) and SEEP, and the measured values of water pressure and flow were used as input data, SBAP was applied to predict the horizontal and vertical hydraulic conductivities of the different stratum. The hydraulic conductivities predicted were inputted to SEEP, the computed values of water pressure and flow calculated by SEEP are in good agreement with the measured values. SBAP shows the faster convergence speed and precise feedback results. This program provides a means for engineers and researchers to determine seepage parameters in the seepage analysis of similar tunnel projects.

Stochastic groundwater modeling of a sedimentary aquifer: evaluation of the impacts of abstraction scenarios under conditions of reduced recharge

A transient finite difference groundwater flow model has been calibrated for the Nasia sub-catchment of the White Volta Basin. This model has been validated through a stochastic parameter randomization process and used to evaluate the impacts of groundwater abstraction scenarios on resource sustainability in the basin. A total of 1500 equally likely model realizations of the same terrain based on 1500 equally likely combinations of the data of the key aquifer input parameters were calibrated and used for the scenario analysis. This was done to evaluate model non-uniqueness arising from uncertainties in the key aquifer parameters especially hydraulic conductivity and recharge by comparing the realizations and statistically determining the degree to which they differ from each other. Parameter standard deviations, computed from the calibrated data of the key parameters of hydraulic conductivity and recharge, were used as a yardstick for evaluating model non-uniqueness. All model realizations suggest horizontal hydraulic conductivity estimates in the range of 0.03–78.4 m/day, although over 70 % of the area has values in the range of 0.03–14 m/day. Low standard deviations of the horizontal hydraulic conductivity estimates from the 1500 solutions suggest that this range adequately reflects the properties of the material in the terrain. Lateral groundwater inflows and outflows appear to constitute significant components of the groundwater budgets in the terrain, although estimated direct vertical recharge from precipitation amounts to about 7 % of annual precipitation. High potential for groundwater development has been suggested in the simulations, corroborating earlier estimates of groundwater recharge. Simulation of groundwater abstraction scenarios suggests that the domain can sustain abstraction rates of up to 200 % of the current estimated abstraction rates of 12,960 m3/day under the current recharge rates. Decreasing groundwater recharge by 10 % over a 20-year period will not significantly alter the results of this abstraction scenario. However, increasing abstraction rates by 300 % over the period with decreasing recharge by 10 % will lead to drastic drawdowns in the hydraulic head over the entire terrain by up to 6 m and could cause reversals of flow in most parts of the terrain.

Interpreting the cross-sectional flow field in a river bank based on a genetic-algorithm two-dimensional heat-transport method (GA-VS2DH)

Interactions between surface waters and groundwater are of great significance for evaluating water resources and protecting ecosystem health. Heat as a tracer method is widely used in determination of the interactive exchange with high precision, low cost and great convenience. The flow in a river-bank cross-section occurs in vertical and lateral directions. In order to depict the flow path and its spatial distribution in bank areas, a genetic algorithm (GA) two-dimensional (2-D) heat-transport nested-loop method for variably saturated sediments, GA-VS2DH, was developed based on Microsoft Visual Basic 6.0. VS2DH was applied to model a 2-D bank-water flow field and GA was used to calibrate the model automatically by minimizing the difference between observed and simulated temperatures in bank areas. A hypothetical model was developed to assess the reliability of GA-VS2DH in inverse modeling in a river-bank system. Some benchmark tests were conducted to recognize the capability of GA-VS2DH. The results indicated that the simulated seepage velocity and parameters associated with GA-VS2DH were acceptable and reliable. Then GA-VS2DH was applied to two field sites in China with different sedimentary materials, to verify the reliability of the method. GA-VS2DH could be applied in interpreting the cross-sectional 2-D water flow field. The estimates of horizontal hydraulic conductivity at the Dawen River and Qinhuai River sites are 1.317 and 0.015 m/day, which correspond to sand and clay sediment in the two sites, respectively.