Macro- and microscale gaseous diffusion in a Stagnic Luvisol as affected by compaction and reduced tillage

Intensification of mechanical agriculture has increased the risk for soil compaction and deformation. Simultaneously, reduced tillage practices have become popular due to energy saving and environmental concerns, as they may strengthen and improve the functioning of structured soil pore system. Soil aeration is affected by both compaction and reduced tillage through changes in soil structure and in the distribution of easily decomposable organic matter. We investigated whether a single wheeling by a 35 000 kg sugar-beet harvester in a Stagnic Luvisol derived from loess near Göttingen, Germany, influenced the gas transport properties (air permeability, gaseous macro- and microdiffusivities, oxygen diffusion rate) in the topsoil and subsoil samples, and whether the effects were different between long-term reduced tillage and mouldboard ploughing. Poor structure in the topsoil resulted in slow macro- and microscale gas transport at moisture contents near field capacity. The macrodiffusivities in the topsoil under conventional tillage were slower compared with those under conservation treatment, and soil compaction reduced the diffusivities by about half at the soil depths studied. This shows that even one pass with heavy machinery near field capacity impairs soil structure deep into the profile, and supports the view that reduced tillage improves soil structure and aeration compared with ploughing, especially in the topsoil

Theoretical and Computational Modeling of Contaminant Removal in Porous Water Filters

Contaminant transport in porous media is a well-researched problem across many scientific and engineering disciplines, including soil sciences, groundwater hydrology, chemical engineering, and environmental engineering. In this thesis, we attempt to tackle this multiscale transport problem using the upscaling approach, which leads to the development of macroscale models while considering a porous medium as an averaged continuum system. First, we describe a volume averaging-based method for estimating flow permeability in porous media. This numerical method overcomes several challenges faced during the application of traditional permeability estimation techniques, and is able to accurately provide the complete permeability tensor of a porous sample in a single simulation. Several anisotropic unit cells are created in two- and three-dimensions based on three different parameters: (1) unit-cell size, (2) particle shape, and (3) aspect ratio of the particles inside the unit cells. The results from the volume averaging-based method show good agreement on comparison with the conventional Stokes-Darcy flow technique for the two- and three-dimensional models. We also find that the proposed method provides much faster results than the Stokes-Darcy flow technique for 3D unit-cell geometries. Next, the cartridges used in commercial water filters are mostly created by packing particles or beads that can be assumed to be of mono-modal size distribution and thus create single-scale porous media. In this thesis, we employ the volume averaging method to upscale the phenomenon of solute transport (which include both diffusion and advection) accompanied with adsorption in such homogeneous porous media. Our novel contribution in this research is the development of a micro-macro coupling between the microscopic and macroscopic length scales, which forms the basis of our macroscale models to reflect the macroscopic behaviour of the system. Two versions of the macroscale models are proposed: (a) complete Volume Averaged Model (VAMc) and (b) simplified Volume Averaged Model (VAMs), which involve two effective transfer coefficients, namely, the total dispersion tensor and the adsorption-induced vector. Further, in order to investigate one of the critical design parameters of a porous water filter, the \u27hydraulic detention time\u27 of the polluted water in the filter, we carry out an extensive numerical investigation of the proposed macroscale models. For this, first we nondimensionalize the pore-scale and macroscale models, which leads to surfacing of two important dimensionless numbers, namely, the Damkohler number and the Peclet number. Next, we develop a 2-D geometry of porous media made up of a chain of 100 identical unit cells for testing the above-mentioned models. The numerical simulations corresponding to the dimensionless pore-scale model, which are referred to as the Direct Numerical Simulation (DNS), and the dimensionless macroscale models, which are referred to as the Volume Averaged Model (VAM), are conducted on the chain-of-unit-cells geometry. The intrinsic average concentration predictions from the macroscale models display excellent results on comparison with the pore-scale (or DNS) outcomes. We also assess the importance of large fluid-solid interfacial area inherent in porous adsorbents by varying the porosity and number of particles inside the artificially-prepared porous-media models. The total dispersion tensor coefficient is validated and found to be in excellent agreement with the literature. Our findings reveal that an increase in the interfacial area of the models leads to higher effective transfer coefficient values. Last, we perform adsorption experiments in an effort to evaluate the effectiveness of the proposed macroscale models. For this, three trials of column-flow experiment are conducted using an adsorbent made up of functionalized zeolite material to remove phosphorus from synthetically prepared influent. Micro-CT scans of zeolite material are used to develop a unit-cell representative of the pore space inside the actual adsorbent medium. The numerical simulations on the unit-cell provide realistic effective transfer coefficient values; however, a large difference between the concentration predictions from theory and experimental results is noted. The lack of adherence to the time-scale constraints is assessed to be the primary reason behind this discrepancy. We offer different recommendations in order to improve the experiments and accurately gauge the effectiveness of the macroscale models. Overall, these models have the potential to improve the state-of-the-art technologies for modeling contaminant transport in porous water filters by providing useful recommendations based on numerical simulations, and may be used as a tool for the optimization of the design of porous water filters

Quantifying in situ β-glucosidase and phosphatase activity in groundwater

Enzymes play an important role in the environment, they breakdown natural-occurring and anthropogenic molecules so that they can be transported into cells and utilized. Enzyme assays are routinely used in soil science and oceanography to measure the activities of specific processes and to serve as general indicators of microbial activity. Conventional enzyme assays are conducted as batch incubation of sediment and water samples. During these assays the concentration of product is measured and enzyme activity is then determined as the rate of product formation. Few studies have measured enzyme activities of groundwater. This work investigates the use of β-glucosidase and phosphatase assays for quantifying in situ enzyme activities in groundwater. Improvements to conventional enzyme assays using p-nitrophenyl substituted compounds were made by developing a high performance liquid chromatography method to improve quantitation limits of the product and to quantify concentrations of both the substrate and the product. An in situ single-well push pull test was then conducted to measure β-glucosidase activity in situ and to estimate the Michaelis constant (K[subscript m]) and the maximum reaction velocity (V[subscript max]) in petroleum-contaminated groundwater at a field site near Newberg, Oregon. An important feature of the single-well push pull test is the nonlinear drop in pore water velocity that the test solution experiences as it moves out from the injection point. The nonlinear drop in pore water velocity is of particular interest because enzyme-mediated reactions are very fast and changes in the hydraulic properties during the test may give rise to mass-transport limitations. Fast reactions lead to the simultaneous depletion of substrate and accumulation of product at the site of the reaction so substrate and product concentrations near the enzyme can be different then the concentrations in bulk solution. And the rates obtained from a single-well push pull tests may be a combination of the rates at which substrate diffuses to the microorganism and at which the reaction occurs. Laboratory experiments with sediment-packed columns were conducted with a range of pore water velocities typically achieved in the subsurface during as push-pull test as a means for examining the potential effects of inhibition and diffusion on phosphatase enzyme kinetics. In this set of column experiments rates of phosphatase-mediated reactions were investigated instead of β-glucosidase, which is an inducible enzyme. Numerical investigations were then conducted to examine the importance of diffusion limitations for describing the influence of transport processes on the observed rates of reaction. The theoretical investigation was conducted by formally upscaling the proposed sub-pore-scale processes to develop a macroscale (or Darcy scale) description of the transport of the substrate. These results indicate that mass-transfer limitations due to the diffusion of the substrate to the enzyme cause an increase in the apparent K[subscript m] but have no effect on V[subscript max]. In this study an analytical method was developed to measure rates of enzyme-mediated reaction in situ so that the measured rates reflected actual rates of microorganism in their natural environment. More carefully controlled laboratory experiments demonstrated that rates of enzyme-mediated reactions measured at low substrate concentrations depended on the flow properties of the test solution

Dynamics of partial anaerobiosis denitrification, and water in soil : experiments and simulation

Dynamic interactions between biological respiration and denitrification, and physical transport processes that modify the abiotic soil environment in which bacteria live, were studied through the development of a new type of experimental respirometer system and an explanatory simulation model.The respirometer system enables one to measure simultaneously the distribution of water, oxygen, nitrate, ammonium, and pH as a function of space and time in an unsaturated, artificially made, homogeneous, cylindrical soil aggregate. The coherent data sets that were obtained by this experimental system served to test the explanatory simulation model.The simulation model comprises four submodels: 1) biological respiration and denitrification, 2) water transport including a description to account for hysteresis, 3) solute transport, and 4) gas transport including a new description to simulate the integral soil atmosphere. Besides evaluation of the integral model with the results of the respirometer system, three of the submodels were also separately tested, either by means of experiments (submodel 1 and 2) or by analytical solutions (a special case of submodel 4).It was found that the new respirometer system yields valuable data to test the simulation model, and that the simulation model gives a fair description of the measured data. However, it appears that only the combined study of the results of experiments and simulations will deepen the understanding of the complicated interactions that occur in this soil biological ecosystem.It was the objective of this study to describe the respirometer system, the explanatory simulation model, and the tests that were done to evaluate the integral model and the separate submodels