Transport Of Conservative Contaminants Research Articles

Modelling transport in fractured media using the fracture continuum approach

Two of the commonly used approaches in modelling flow and transport in fractured geological media are the discrete fracture network (DFN) approach and the stochastic continuum (SC) approach. The former approach is computationally demanding and requires large input parameters, whereas the latter approach is computationally efficient and requires fewer parameters but at the expense of not preserving fracture network details and properties. The fracture continuum (FC) approach combines the advantages of both the DFN and SC approaches. The main objective of this research is to develop a mapping technique utilizing the FC approach to preserve transport characteristics between DFN and the mapped continuum. A two-dimensional particle tracking model is developed to simulate conservative contaminant transport through DFN. The fracture network is randomly generated in a stochastic Monte Carlo framework, and the flow problem is solved using mass conservation at the fracture junctions. The transport problem is then solved via the developed particle tracking model on the DFN. The obtained transport solution is used as a reference solution. The fracture network is mapped onto a finite difference grid at four different grid cell sizes: 1 m × 1 m, 2 m × 2 m, 5 m × 5 m, and 10 m × 10 m, and the flow problem is solved via MODFLOW. The transport problem is solved on the grid using a particle tracking method. Comparisons between the transport characteristics for both approaches (DFN and FC) are performed, and the percentage error in each case is quantified. It is found that a new correction factor is needed to preserve conservative transport characteristics on the grid. The developed correction factor improves the ability of the FC technique to preserve transport on the grid for the case of conservative contaminant transport.

Unsaturated zone travel time to groundwater on a vulnerable site

A bromide (Br) tracing experiment was conducted to ascertain unsaturated zone travel time to groundwater on a site with a karstified limestone aquifer overlain by a thin free-draining overburden. Br tracer was applied to areas surrounding two boreholes; soil solution and groundwater Br concentrations were monitored. Bromide was first detected after eight and 34 days in the soil solution and groundwater. The quick break-through of the applied Br in the soil solution and groundwater indicates the presence of preferential flow in the soil at this site. The time to maximum groundwater Br concentration supports a dominant matrix flow path through the overburden and then preferential flow through the unsaturated limestone bedrock. The results indicated that the transport of conservative contaminants, such as nitrate, can be expected to occur in a single recharge season. The occurrence of preferential flow raises concerns over rapid transport of non-conservative contaminants such as faecal coliforms and this merits further investigation.

Using Unscented Kalman Filter in Subsurface Contaminant Transport Models

Using a traditional numerical approach in subsurface contaminant transport models generates unavoidable deviation due to unknown or uncertain sources, inaccurate transport and hydraulic parameters, and numeric scheme errors. As a result, stochastic data assimilation or filtering techniques have been employed in the subsurface simulation processes to improve the accuracy of model results. The Kalman Filter (KF) has been widely used for estimation and tracking of linear systems. In the subsurface transport model, even if the system dynamics are linear, it can become a nonlinear one because of the presence of the unknown parameters. The Unscented Kalman Filter (UKF) is one of the data assimilation filters that offer a potential solution to the problem of model development with noisy and incomplete data when the system is nonlinear. The objective of this study was to apply the UKF in subsurface contaminant transport models and to find the contaminant plume. The performance was then evaluated in comparison with the KF and numerical model. A two dimensional transport model with advection and dispersion was used as the deterministic model of a conservative contaminant transport in the subsurface. Random Gaussian noises were added to the numerical method result to simulate the true solution and the observation data. Then the UKF and KF filtering techniques were applied for the data assimilation. An Error Standard Deviation (ESD) of pollutant concentrations was used to examine the effectiveness. The UKF can reduce 6~75% and 2~52% of prediction errors when compared with the numerical and KF results, respectively.

Extended Kalman Filtering to Improve the Accuracy of a Subsurface Contaminant Transport Model

Modeling the behavior of contaminants in a subsurface flow is important in predicting the fate of the pollutants, in risk assessment, and as a preliminary step of the mitigation process. A two-dimensional transport model with advection and dispersion is used as the deterministic model of a conservative contaminant transport in the subsurface. With the system model alone, it is very difficult to predict the true dynamic state of the pollutant. Therefore, observation data are needed to guide the deterministic system model to assimilate the true state of the contaminant. Extended Kalman Filter (EKF), which is essentially a first order approximation to an infinite dimensional problem, is used in this study to predict the contaminant plume transport. A traditional root mean square error (RMSE) of pollutant concentrations is used to examine the effectiveness of the EKF in contaminant transport modeling. The result shows that EKF can reduce 74 to 91% of prediction errors compared to the numerical method while wo...

Mathematical modeling and practical verification of groundwater and contaminant transport in a chosen natural aquifer

The paper addresses the 2D mathematical equation of conservative contaminant transport in an aquifer for chosen contaminants. The contaminants (chlorides and sulfates) are subject to instantaneous reversible part of sorption process. The term of instantaneous reversible sorption in the presented equation has been described by the non-linear Freundlich adsorption isotherm, widely applied in practice in relation to static processes (for local equilibrium). The numerical solution (using the finite difference method) has been based on the previously calculated values of longitudinal and transverse dispersion coefficients and the non-linear adsorption parameters for the chosen contaminants. Based on this model, the values of chloride and sulfate concentration isolines have been calculated and compared with the measured maximal concentrations in the chosen natural aquifer (installed piezometers). Additionally, the values of chloride concentrations have been calculated taking into account the influence of radioactive decay term, using the numerical value of the firstorder decay rate constant for an adopted theoretical radionuclide.

Three Dimensional Hydrodynamic Model with Multiquadtree Meshes

This study presents a three dimensional model for the transport of conservative contaminants, which can be used for bodies of water which are affected by winds and/or tides. The model solves the equation of mass transport, based on results obtained using a hydrodynamic model for shallow waters that works in a finite volume scheme and a type of hierarchical grid, called multiquadtree, which is adaptable to the bathymetry. To solve the vertical coordinates, the coordinate z is transformed into a sigma (s) coordinate, thus allowing the same number of layers in the vertical, regardless of depth. This hydrodynamic model is validated using two cases: a long wave propagated in a channel of variable width and bottom and wind action in a rectangular basin. Finally, the results obtained are presented for a hypothetical single port outfall in the bay of Campeche, Mexico. The model developed here is both quick and easy to use and is efficient when compared with models presented by other authors since it uses adaptable grids which allow detailed solutions to be obtained for areas of interest such as coastlines and the area around an outfall.

Numerical modeling of tidal influence on density‐dependent contaminant transport

Transport of contaminants in coastal aquifers subject to tidal fluctuation is an important topic in hydrogeology as a consequence of the significant development of human activities near the shoreline. Despite this, relatively little work has been done to investigate the joint effect of variable water density flow and tidal saltwater head fluctuation. In particular, numerical modeling results have rarely been validated with experimental data. The primary aim of this work was to develop and validate a modeling strategy to incorporate the effect of tides in a variable density simulation using a widely adopted simulation package. The numerical model reproduced data from a laboratory experiment designed to investigate the impact of tides on conservative contaminant transport in a coastal aquifer, where the polluted water is denser than the ambient groundwater. Both the plume shape and size were reproduced when tidal fluctuations were incorporated in the model. Model simulations were then used to examine in detail how the flow patterns are modified during one tidal cycle. According to the simulations, the contaminant was mainly discharged from the beach surface. The model was also used to investigate the contact between the contaminant plume and the saltwater intrusion in the freshwater aquifer. We observed that the two dense water phases never completely mix during the simulation. Instead, a slice was continuously removed from the front of the polluted plume because of the discharge of freshwater. The observed mechanism is cyclical with a frequency compatible with the tide.

Analytical models of contaminant transport in coastal aquifers

The Henry formulation, which couples subsurface flow and salt transport via a variable-density flow formulation, can be used to evaluate the extent of sea water intrusion into coastal aquifers. The coupling gives rise to nontrivial flow patterns that are very different from those observed in inland aquifers. We investigate the influence of these flow patterns on the transport of conservative contaminants in a coastal aquifer. The flow is characterized by two dimensionless parameters: the Péclet number, which compares the relative effects of advective and dispersive transport mechanisms, and a coupling parameter, which describes the importance of the salt water boundary on the flow. We focus our attention on two regimes – low and intermediate Péclet number flows. Two transport scenarios are solved analytically by means of a perturbation analysis. The first, a natural attenuation scenario, describes the flushing of a contaminant from a coastal aquifer by clean fresh water, while the second, a contaminant spill scenario, considers an isolated point source.

Unsaturated zone travel time to groundwater on a vulnerable site

A bromide (Br) tracing experiment was conducted to ascertain unsaturated zone travel time to groundwater on a site with a karstified limestone aquifer overlain by a thin free-draining overburden. Br tracer was applied to areas surrounding two boreholes; soil solution and groundwater Br concentrations were monitored. Bromide was first detected after eight and 34 days in the soil solution and groundwater. The quick break-through of the applied Br in the soil solution and groundwater indicates the presence of preferential flow in the soil at this site. The time to maximum groundwater Br concentration supports a dominant matrix flow path through the overburden and then preferential flow through the unsaturated limestone bedrock. The results indicated that the transport of conservative contaminants, such as nitrate, can be expected to occur in a single recharge season. The occurrence of preferential flow raises concerns over rapid transport of non-conservative contaminants such as faecal coliforms and this merits further investigation.

Dispersion Measurements in Highly Heterogeneous Laboratory Scale Porous Media

Laboratory experiments and numerical simulations investigate conservative contaminant transport in a heterogeneous porous medium. The laboratory experiments were performed in cylindrical columns 1m long and 3.5cm inside diameter filled with spherical glass beads. Concentration breakthrough curves are measured at a scale much finer than the size of the heterogeneity. Numerical simulations are based on a random walk in a known constant velocity field. The heterogeneity is a distinct, discontinuous change in the local permeability field. Fluid flow is miscible, flowing in a saturated porous medium. Previous work has shown this to be a very poorly understood phenomenon. The measurements reported here help to better understand how dispersion evolves through and past a heterogeneity.

Vertical transport processes in unconfined aquifers

We derive simple two-dimensional mathematical models describing the unsteady transport of conservative contaminants through an unconfined aquifer with a gently sloping aquiclude subject to advection, recharge, and vertical dispersion. The inclusion of vertical transport terms permits the proper nonreactive analysis of closed and open chemical systems, with the latter allowing dispersion of volatile constituents across the water table. These systems exhibit conservative and pseudoreactive behavior respectively when the pollution is analyzed on a depth-integrated basis, as is common in present one-dimensional models of groundwater contamination. Vertical and longitudinal chloride and total inorganic carbon observations at the well-documented Babylon, Long Island sanitary landfill plume are used to calibrate and test the analyses with a modest level of accuracy, using the vertical dispersivity as a calibration factor in this testing process. The parameter is important in the determination of reaeration rates across the water table and nutrient mixing from below in the related problem of biological transformations near the free surface.