Trend Analysis and Spatial Prediction of Groundwater Levels Using Time Series Forecasting and a Novel Spatio-Temporal Method

Water Resources Management - Overexploitation of groundwater in the Malayer Plain has resulted in a continuous decline of groundwater levels over recent years with associated risks to water...     

Influence of Spatially Variable Side Friction on Single Drilled Shaft Resistance and LRFD Resistance Factors

Load and resistance factor design (LRFD) is a method that aims at meeting specified target reliabilities (probabilities of failure) of engineered systems. The present work focuses on ultimate side friction resistance for axial loads on single cylindrical drilled shaft foundations in the presence of spatially variable rock/soil strength. Core sample data are assumed to provide reliable information about local strength in terms of mean, coefficient of variation and spatial correlation structure (variogram) at a site. The geostatistical principle of support up-scaling is applied to quantify the reduction in variability between local strength and the average ultimate shaft side friction resistance without having to recur to lengthy stochastic finite difference/element simulations. Site and shaft specific LRFD resistance factors (Φ values) are given based on the assumption of lognormal load and resistance distributions and existing formulas recommended by the Federal Highway Administration. Results are efficiently represented in dimensionless charts for a wide range of target reliabilities, shaft dimensions, and geostatistical parameters including nested variograms of different types with geometric and/or zonal anisotropies. Field data of local rock strength is used to demonstrate the method and to evaluate the sensitivity of obtained resistance factors to potentially uncertain variogram parameters.     

Constructal design of permeable reactive barriers: groundwater-hydraulics criteria

Unidirectional, steady-state, Darcian flow in a confined homogeneous aquifer is partially intercepted by a permeable reactive barrier (PRB), the shape of which is optimized with the following hydraulic criteria: seepage flow rate through a PRB (equivalent to the width and frontal area of the intercepted part of the plume in 2-D and 3-D cases, correspondingly) and travel time of a marked particle through the PRB interior along streamlines. The wetted perimeter, cross-sectional area and volume of the reactive material are selected as isoperimetric constraints. The PRB contour is modeled as either a constant head line (if the reactive material is much more permeable than the aquifer) or as a refraction boundary (if the reactive material has an arbitrary permeability), on which the hydraulic head and normal flux components in the barrier and aquifer are continuous. In the former case, the complex potential domain of the flow is a tetragon and a broad class of PRBs can be studied. In the latter case, analytical solutions are available for ellipses and ellipsoids (only these classes of shapes are considered in optimization). In the 2-D case and constant head PRB, a novel shape-control technique through the kernels of singular integrals is implemented: the Zhukovskii function is introduced; a Dirichlet boundary-value problem is solved for this function by setting the orientation (with respect to the incident flow direction) of the Darcian velocity vector on the PRB contour as a control function. Unlike similar controls for impermeable airfoils in aerodynamic design, the kernel has two discontinuities, which reflect the flow topology near a hinge (stagnation) point and the PRB tip. The integral is evaluated for V-shaped and curve-shaped PRBs and parametric expressions for the contours are obtained resulting (for the latter case) in a “pointy banana” shape. In the class of a V-shaped PRB, it is proved that a straight-line barrier minimizes the perimeter if the plume width is fixed. In 2- and 3-D refracting PRBs, the Pilatovskii (ellipse) and Poisson (ellipsoid) solutions for the flow field inside and outside the PRB are used for obtaining explicit formulae for the magnitude of the velocity, which is uniform inside the PRB. Simple expressions for the longest travel time within the PRB and the discharge intercepted by it are obtained. The ellipse/ellipsoid axes ratio/ratios are used as control variables in optimization. Extrema are obtained and analyzed for different PRB-aquifer conductivity ratios and for varying angles between the incident velocity vector and the ellipse/ellipsoid axes.     

Field-scale evaluation of the passive flux meter for simultaneous measurement of groundwater and contaminant fluxes

A new method, passive flux meter (PFM), has been developed and field-tested for simultaneously measuring contaminant and groundwater fluxes in the saturated zone at hazardous waste sites. The PFM approach uses a sorptive permeable medium placed in either a borehole or monitoring well to intercept contaminated groundwater and release "resident" tracers. The sorbent pack is placed in a groundwater flow field for a specified exposure time and then recovered for extraction and analysis. By quantifying the mass fraction of resident tracers lost and the mass of contaminant sorbed, groundwater and contaminant fluxes are calculated. Here, we assessed the performance of PFMs at the Canadian Forces Base Borden field site in Ontario, Canada. Two field tests were conducted under imposed groundwater flow fields: (1) radial flow to a well and (2) linear flow in a test channel confined by sheet pile walls on three sides. Both tests demonstrate that the local fluxes measured by PFM and averaged overthe screen interval were within 15% of imposed groundwaterflow and within 30% of measured contaminant mass flux. Patterns in depth variations in groundwater and contaminant fluxes, determined by the PFM approach, allow for site characterization at a higher spatial resolution. These results support the PMF method as a potential innovative alternative for measuring groundwater and contaminant fluxes in screened wells.     

Initial test results for a passive surface water fluxmeter to measure cumulative water and solute mass fluxes

The theoretical concept and initial test results of a Passive Surface Water Fluxmeter (PSFM) to directly and simultaneously measure cumulative water and solute mass fluxes in surface water flow systems are presented. The PSFM consists of a symmetric hydrofoil that is vertically installed in a stream and one or more sorbent columns that are connected to the nonuniform flow field around the hydrofoil. Depending on the ambient flow velocity, a flow occurs through each column, which elutes portions of initially present "resident" tracers in the column, while, at the same time, solutes in the water (e.g., contaminants or nutrients) are retained in the sorbent column. Quantification of the resident tracer mass remaining and the mass of solutes sorbed in the column enables determination of the local cumulative or time-averaged water and solute mass fluxes. Laboratory flume experiments show good agreement with independent measurements (R(2) > or = 0.96) for instantaneous water fluxes (tested range: 0.3-0.7 m/s), cumulative water fluxes (50-600 L/cm(2)), and cumulative nitrate fluxes (0.4-5.1 g/cm(2)). Future work is required to validate the PSFM performance under a larger range of flow velocities, transient flow, and transport conditions and for different hydrofoil shapes.     

Magnitude and directional measures of water and Cr(VI) fluxes by passive flux meter

A new configuration of the passive fluxmeter (PFM) is presented that provides for simultaneous measurements of both the magnitude and the direction of ambient groundwater specific discharge qo and Cr(VI) mass flux J(Cr). The PFM is configured as a cylindrical unit with an interior divided into a center section and three outer sectors, each packed with a granular anion exchange resin having high sorption capacity for the Cr(VI) oxyanions CrO4(2-) and HCrO4-. The sorbent in the center section is preloaded with benzoate as the "resident" tracer. Laboratory experiments were conducted in which PFMs were placed in porous packed bed columns, through which was passed a measured volume of synthetic groundwater containing Cr(VI). During the deployment period, some of the resident tracer is depleted while the Cr(VI) is sorbed. The resin was then removed from the four sectors separately and extracted to determine the "captured" mass of Cr(VI) and the residual mass of the resident tracer in each. Cumulative specific discharge, q0t, values were assessed using the residual mass of benzoate retained in the center section. The direction of this discharge theta was ascertained from the mass distribution of benzoate intercepted and retained in the outer three sections of the PFM. Cumulative chromium fluxes, J(Cr)t, were quantified using the total Cr(VI) mass intercepted and retained on the PFM. Experiments produced an average measurement error for direction theta of 3 degrees +/- 14 degrees, while the average measurement errors for q0 and J(Cr) were, respectively, -8% +/- 15% and -12% +/- 23%. Results demonstrate the potential utility of the new PFM configuration for characterizing groundwater and contaminant fluxes.