Saturated-unsaturated Groundwater Flow Research Articles

Finite-infinite element analysis for flow simulation in a phreatic aquifer

Groundwater and surface water interaction is a common process of the hydrologic cycle but still challenging to be addressed in a general context. A special case of groundwater-surface water interaction is the seepage from a river/stream or canal into an adjacent phreatic aquifer, where the groundwater flow in the vicinity of the stream is crucial while the aquifer is supposed to extend to infinity, especially for the mathematical analysis. Herein, numerical solutions of 1-D Boussinesq equation and 2-D Richards equation are presented by embedding infinite elements in the finite element analysis to discretise only the key subsurface flow region close to the stream. By using the infinite elements, which extend to infinity to one direction according to appropriate shape functions, the total number of elements required to discretise the simulation area is substantially reduced. As a result, computationally efficient solutions are obtained, which is particularly important for the two-dimensional saturated-unsaturated groundwater flow described by the Richards equation, as well as sufficiently accurate, both in terms of groundwater recharge and water table in the vicinity of the stream. The finite-infinite element approach could be useful to derive accurate and fast numerical solutions under the investigation of several scenarios.

Hydrological controls on methylmercury distribution and flux in a tidal marsh.

The San Francisco Estuary, California, contains mercury (Hg) contamination originating from historical regional gold and Hg mining operations. We measured hydrological and geochemical variables in a tidal marsh of the Palo Alto Baylands Nature Preserve to determine the sources, location, and magnitude of hydrological fluxes of methylmercury (MeHg), a bioavailable Hg species of ecological and health concern. Based on measured concentrations and detailed finite-element simulation of coupled surface water and saturated-unsaturated groundwater flow, we found pore water MeHg was concentrated in unsaturated pockets that persisted over tidal cycles. These pockets, occurring over 16% of the marsh plain area, corresponded to the marsh root zone. Groundwater discharge (e.g., exfiltration) to the tidal channel represented a significant source of MeHg during low tide. We found that nonchannelized flow accounted for up to 20% of the MeHg flux to the estuary. The estimated net flux of filter-passing (0.45 μm) MeHg toward estuary was 10 ± 5 ng m(-2) day(-1) during a single 12-h tidal cycle, suggesting an annual MeHg load of 1.17 ± 0.58 kg when the estimated flux was applied to present tidal marshes and planned marsh restorations throughout the San Francisco Estuary.

Combination of Genetic Algorithm and Inverse Solution Technique for Estimating the Hydraulic Properties of Unsaturated Soft Rock

Unsaturated hydraulic property (relation between the water retention curve and hydraulic conductivity function) of soft rock must be properly estimated for analyzing the saturated-unsaturated groundwater flow such as the flow around a tunnel, rock surface and so forth. In this study a practical technique for estimating the unsaturated hydraulic property of soft rock was developed on the basis of the Campbell model and Genetic Algorithm (GA). The major objectives of the present study are the following two; 1) investigation whether or not the Campbell model can be applied to soft rock for approximating the unsaturated hydraulic properties and 2) application of GA technique for estimating two parameters included in the Campbell model. These parameters were estimated through the inverse analysis of the transient change of evaporation rates from a surface of disk shaped specimen of Tertiary sedimentary rock which are sampled from the Rokkasho area, Aomori prefecture. Two transient evaporation experiments under different atmospheric conditions were proposed for the estimation. The Campbell's parameters of 13 soft rock specimens (5 sandstone and 8 tuff breccia specimens) were estimated through those types of experiments. As the results it was clearly found that the transient change of evaporation rates can be well fitted by the Campbell model except for the region of low saturation and this fact implies that the Campbell model can be applied to the soft rock. And also it was found that, the GA is a promising technique for parameters estimation in the inverse solution method. It can be concluded that the techniques are practically for estimating the unsaturated hydraulic properties of soft rock.

An Inverse Method to Estimate Soil Hydraulic Properties in Saturated-unsaturated Groundwater Flow Model

An Inverse Method to Estimate Soil Hydraulic Properties in Saturated-unsaturated Groundwater Flow Model

Saturated-Unsaturated 3D Groundwater Model. I: Development

A new saturated-unsaturated three-dimensional (3D) groundwater flow model (SU3D) has been developed that calculates the pressure distribution over the entire groundwater flow domain in response to rainfall and evapotranspiration. Recent advances in solving the saturated-unsaturated groundwater flow equation are incorporated into SU3D, which solves the nonlinear, 3D, modified mixed form of the Richards equation continuously throughout the groundwater flow domain. The block-centered, finite-difference method with a variably sized grid is employed to solve the governing partial differential equation. The non-linear terms of the governing equation are linearized using a modified Picard iteration scheme, and the preconditioned conjugate gradient method is used to solve the linearized system of equations. SU3D can simulate the effects of pumping from an aquifer, and it has an option to calculate potential evapotranspiration (PET) from meteorological data. The PET is partitioned into potential evaporation and po...

Improved three-dimensional finite-element techniques for field simulation of variably saturated flow and transport

Accurate three-dimensional simulations of saturated-unsaturated groundwater flow and contaminant transport in highly heterogeneous subsurface media require extreme agility in the numerical techniques of solution. The primary limitations of simulating large-scale complex field problems are the available computing power, and the considerable data management effort required to calibrate, validate and perform sensitivity analyses to these systems. An efficient finite-element model is presented which overcomes many of the challenges encountered in solving complex problems. The Galerkin and upstream weighted residual procedures are employed for solving the flow and transport equations. Options for using chord-slope Newton-Raphson linearization and upstream weighting of the relative permeabilities are necessary for highly nonlinear flow problems. Computational savings are otherwise achieved using the less robust Picard scheme. Underrelaxation formulas further alleviate convergence difficulties in the solution of the nonlinear equations. Hexahedral orthogonal curvilinear elements are used to accurately and efficiently discretize domains with irregular formation geometry, and transition elements are employed to grade the finite-element mesh for finer discretization only in regions of high gradients. Element matrices are generated using influence coefficient formulas, thus avoiding costly numerical integration. The 7-, 11- and 27-point lattice connectivity structures are provided as options that increase the accuracy of solution, or enhance computational speed and storage requirements. Galerkin and upstream weighting schemes alleviate stability problems in solving the transport equations without excessive smearing of concentrations. Preconditioned conjugate gradient (PCG) and ORTHOMIN techniques are used to solve the large symmetric and asymmetric matrix equations arising from the assembly of the elemental flow and transport equations, respectively, and repeated matrix assembly and decomposition are eliminated from the calculations, if the situation permits. Assembly of the entire model is discussed, with reference to previously published works for details of schemes already tested and implemented. Only further developments of the schemes are detailed herein. Application of the model is demonstrated by selected simulation examples involving assessment of moisture movement and contaminant migration from a shallow waste disposal design above a multilayer unconfined aquifer system.

Comparison of the single-phase and two-phase numerical model formulation for saturated-unsaturated groundwater flow

Two numerical methods are developed for transient saturated-unsaturated two-dimensional ground-water flow. One formulation uses the usual single-phase formulation with a passive air phase at constant pressure. The second model uses a full two-phase air-water formulation. The numerical results for both formulations are compared on some test problems. In some cases, the single-phase formulation and the two-phase formulations give greatly different results. This can be explained in terms of the fractional flow curve for the air-water system. The single-phase technique generally requires less computational work than the two-phase formulation, although there are some situations where the numerical performance of the single-phase formulation is very poor.

水みちを持つ斜面内の非定常地下水流れの性質 ３次元有限要素法による斜面モデル内の飽和・不飽和浸透流の解析

Most of hydrogeologists are facing many problems closely related to groundwater flow in slope, such as runoff in mountaneous area, slope failure, land slide and so on. It has been pointed by many authors that soil pipes developing in slope play an important role on the flow. From the reason, it can be said that the study on effect of soil pipe on the groundwater flow is indispensable to solving the problems.From the view point, transient behavior of saturated-unsaturated groundwater flow in idealized slope having a soil pipe in it was studied by the use of finite element method of three dimensional form. In addition to the numerical analysis, two types of experiment were performed to prove the analytical results.Obtained results are as follows;(1) Finite element method of three dimensional form is very useful to study the effect of soil pipe on the transient behavior of groundwater flow in slope.(2) Flow velocity becomes high only in soil pipe. Although the total amount of effluent quantity from seepage face become larger, velocity distribution on the face is almost uniform under the condition that the pipe does not reach the face.(3) Pore pressure become considerably high at terminal point of the pipe in slope.