Multi-functional heat pulse probe measurements of coupled vadose zone flow and transport

Simultaneous measurement of coupled water, heat, and solute transport in unsaturated porous media is made possible with the multi-functional heat pulse probe (MFHPP). The probe combines a heat pulse technique for estimating soil heat properties, water flux, and water content with a Wenner array measurement of bulk soil electrical conductivity (EC_bulk). To evaluate the MFHPP, we conducted controlled steady-state flow experiments in a sand column for a wide range of water saturations, flow velocities, and solute concentrations. Flow and transport processes were monitored continuously using the MFHPP. Experimental data were analyzed by inverse modeling of simultaneous water, heat, and solute transport using an adapted HYDRUS-2D model. Various optimization scenarios yielded simultaneous estimation of thermal, solute, and hydraulic parameters and variables, including thermal conductivity, volumetric water content, water flux, and thermal and solute dispersivities. We conclude that the MFHPP holds great promise as an excellent instrument for the continuous monitoring and characterization of the vadose zone.     

Effect of mineral reactions on the hydraulic properties of unsaturated soils: Model development and application

The selective radius shift model was used to relate changes in mineral volume due to precipitation/dissolution reactions to changes in hydraulic properties affecting flow in porous media. The model accounts for (i) precipitation/dissolution taking place only in the water-filled part of the pore space and further that (ii) the amount of mineral precipitation/dissolution within a pore depends on the local pore volume. The pore bundle concept was used to connect pore-scale changes to macroscopic soil hydraulic properties. Precipitation/dissolution induces changes in the pore radii of water-filled pores and, consequently, in the effective porosity. In a time step of the numerical model, mineral reactions lead to a discontinuous pore-size distribution because only the water-filled pores are affected. The pore-size distribution is converted back to a soil moisture characteristic function to which a new water retention curve is fitted under physically plausible constraints. The model equations were derived for the commonly used van Genuchten/Mualem hydraulic properties. Together with a mixed-form solution of Richards’ equation for aqueous phase flow, the model was implemented into the geochemical modelling framework PHREEQC, thereby making available PHREEQC’s comprehensive geochemical reactions. Example applications include kinetic halite dissolution and calcite precipitation as a consequence of cation exchange. These applications showed marked changes in the soil’s hydraulic properties due to mineral precipitation/dissolution and the dependency of these changes on water contents. The simulations also revealed the strong influence of the degree of saturation on the development of the saturated hydraulic conductivity through its quadratic dependency on the van Genuchten parameter α. Furthermore, it was shown that the unsaturated hydraulic conductivity at fixed reduced water content can even increase during precipitation due to changes in the pore-size distribution.     

A model to couple overland flow and infiltration into macroporous vadose zone

Most vegetated land surfaces contain macropores that may have a significant effect on the rate of infiltration of water under ponded conditions on the ground surface. Owing to the small-scale variations of the land topography (microtopography), only portions of the land area may get ponded during the process of overland flow. As the macropores transmit water at much higher rates than the primary soil matrix, higher macropore activation in ponded areas produces larger effective infiltration rates into the soil. Therefore, overland flow and infiltration into the macroporous vadose zone are interrelated. Representing the microtopographic variation of the land surface by a simple sine wave function, a method was developed to relate the ponding area to the average ponding depth which was determined by overland flow. A numerical model coupling overland flow and infiltration into the macroporous vadose zone was developed. Overland flow was simulated using the St. Venant equations with the inertia terms neglected. A single macropore model was used to simulate the infiltration into the macroporous vadose zone. The interaction between overland flow and the infiltration into the macroporous vadose zone was analyzed for a hypothetical watershed. The sensitivity analysis revealed that the interaction of macropore flow and overland flow is significant. For the conditions tested, the macropore flow and the overland flow were found to be more sensitive to the macroporosity and less sensitive to the microtopographic surface variation.     

Analysis of radionuclide migration through a 200-m Vadose zone following a 16-year infiltration event

Atomic weapons testing at the Nevada Test Site has introduced many tracers for quantifying subsurface hydrologic transport processes in arid climates. In 1975, groundwater adjacent to the Cambric test, conducted beneath Frenchman Flat 10 years earlier, was pumped steadily for 16 years to elicit information on the migration of residual radioactivity through the saturated zone. Radionuclides in the pumping well effluent, including tritium, ¹⁴C, ³⁶Cl, and ⁸⁵Kr, were extensively monitored prior to its discharge in an unlined ditch, where approximately a third of the flow infiltrated over a distance of 1 km. Radionuclide infiltration through a 220-m thick vadose zone created a second, and rather unique long-term field experiment. Effluent data have been utilized in conjunction with geologic data, new radionuclide measurements, isotopic age-dating estimates, and vadose zone flow and transport models to better understand the movement of radionuclides between the ditch, the water table and a nearby groundwater monitoring well. Detection of tritium in the monitoring well occurred approximately 16 years after its initial discharge into the ditch. Modeling and tritium age dating have suggested 3–5 years of this 16-year transit time occurred solely in the vadose zone. They also suggest considerable recirculation of the pumping well discharge back into the original pumping well. Notably, there have been no observations of ¹⁴C or ⁸⁵Kr in the monitoring well, suggesting their preferential retention or volatilization during transit to the water table.     

Analysis of solute transport with a hyperbolic scale-dependent dispersion model

An empirical hyperbolic scale-dependent dispersion model, which predicts a linear growth of dispersivity close to the origin and the attainment of an asymptotic dispersivity at large distances, is presented for deterministic modelling of field-scale solute transport and the analysis of solute transport experiments. A simple relationship is derived between local dispersivity, which is used in numerical simulations of solute transport, and effective dispersivity, which is estimated from the analysis of tracer breakthrough curves. The scale-dependent dispersion model is used to interpret a field tracer experiment by nonlinear least-squares inversion of a numerical solution for unsaturated transport. Simultaneous inversion of concentration-time data from several sampling locations indicates a linear growth of the dispersion process over the scale of the experiment. These findings are consistent with the results of an earlier analysis based on the use of a constant dispersion coefficient model at each of the sampling depths.     

Time-lapse borehole radar monitoring of an infiltration experiment in the vadose zone

Ground-penetrating radar (GPR) is an effective tool for imaging the spatial distribution of water content. An artificial groundwater recharge test was conducted in Nagaoka City in Japan, and time-lapse crosshole GPR data were collected to monitor the infiltration process in the vadose zone. Since electromagnetic wave velocities in the vadose zone are largely controlled by variations in water content, an increase in traveltime is interpreted as an increase in saturation. In the test zone, the infiltrated water penetrated downward with an average velocity of about 2.7 m/h. A finite-difference time-domain method using two-dimensional cylindrical coordinates is applied to simulate radargrams associated with the advancing wetting front and to quantify the effects of critical refraction. Standard zero-offset profiling for which all first-arrivals are assumed to be direct waves results in an underestimation of water content in the transition zone above the wetting front. As a result, correct velocity analysis requires identification of first-arriving critically refracted waves from the traveltime profile to accurately determine a water content profile.     

Analysis of groundwater flow and travel times for a landfill site in an arid region with a thick vadose zone

Groundwater flows and travel times were analyzed under a landfill site characterized by a thick unsaturated zone, in an arid environment, using two-dimensional models. Transport in the unsaturated zone is important in arid environments with thick vadose zones and usually determines the travel time of groundwater from the landfill site to the regulatory compliance surface. The combined use of a parallel flow (Dupuit-Forchheimer) areal model and vertical cross-sectional model is explored at a site typical in the utility industry. Flow paths and travel times to the compliance zone are obtained for use in landfill site design or environmental impact assessments. Sensitivity analysis shows that boundary conditions are important factors influencing the entire analysis and thus need to be confirmed with field measurements.     

NUMERICAL SIMULATION OF SEDIMENT TRANSPORT IN DYNAMIC FLOW CONDITIONS

1 INTRODUCTION Rivers as a source of life can at the same time impose devastating conditions on the environment. It is , therefore, imperative to analyse and predict river behaviour for different given conditions and engineering activities. Therefore, the use of simulation tools in this field has become a necessity. Many computational tools for simulation of sediment transport in rivers are now available that can be used for prediction and design under different flow conditions. However, …     

Multidimensional analytical models for isotope ratios in groundwater pollutant plumes of organic contaminants undergoing different biodegradation kinetics

This work presents analytical models which are able to predict contours of concentrations and isotope ratios of organic pollutants in homogeneous aquifers. Four analytical solutions of the advective–dispersive transport equation for reactive transport from the literature differing in assumptions regarding biodegradation kinetics were used. Stable isotope ratios are computed after modelling the individual reactive transport of isotopic species in the aquifer, which respond differently to fractionation by biodegradation or sorption. The main finding of this study is that the isotope ratios in the plumes are very sensitive to the assumptions underlying the biodegradation kinetics in the models. When biodegradation occurs throughout the core of the plume as first-order reaction, the transversal gradients in isotope ratios are smooth. When biodegradation occurs in a bi-molecular reaction with an electron acceptor (modelled by double-Monod kinetics), steep transversal isotope gradients are predicted. When the reaction rates approach instantaneous reaction along the plume fringes, isotope shifts in the core of the plume disappear. A model incorporating plume and fringe degradation produces the most plausible predictions of isotope ratios in this study. It is shown furthermore that isotope fractionation by sorption causes an even different pattern of isotope ratios, with positive shifts restricted to near the forerunning front of an expanding plume. The models developed in this work can serve for the validation of numerical models and may be incorporated in natural attenuation support systems such as e.g. BIOSCREEN.     

Stochastic modelling of groundwater flow and solute transport in aquifers

This paper presents an introductory overview of recently developed stochastic theories for tackling spatial variability problems in predicting groundwater flow and solute transport. Advantages and limitations of the theories are discussed. Lastly, strategies based on the stochastic approaches to predict solute transport in aquifers are recommended.     

Numerical modelling of channel migration with application to laboratory rivers

The paper presents the development of a morphological model and its application to experimental model rivers.The model takes into account the key processes of channel migration,including bed deformation,bank failure and wetting and drying.Secondary flows in bends play an important role in lateral sediment transport,which further affects channel migration.A new formula has been derived to predict the near-bed secondary flow speed,in which the magnitude of the speed is linked to the lateral water level gradient.Since only non-cohesive sediment is considered in the current study,the bank failure is modelled based on the concept of submerged angle of repose.The wetting and drying process is modelled using an existing method.Comparisons between the numerical model predictions and experimental observations for various discharges have been made.It is found that the model predicted channel planform and cross-sectional shapes agree generally well with the laboratory observations.A scenario analysis is also carried out to investigate the impact of secondary flow on the channel migration process.It shows that if the effect of secondary flow is ignored,the channel size in the lateral direction will be seriously underestimated.