C. Eberhardt

Enhancement of dilution and transverse reactive mixing in porous media: experiments and model-based interpretation.

Transport and natural attenuation of contaminant plumes in groundwater are often controlled by transverse dispersion. The extent of mixing between dissolved reaction partners at the fringe of a plume determines its length and depends strongly on the groundwater flow field. Transient flow conditions as well as the focusing of the flow in high-permeability zones may enhance transverse mixing of dissolved species and, therefore, create favorable conditions for the natural attenuation of contaminant plumes. The aim of the present study is to experimentally test the influence of these processes on solute mixing and to directly compare the results with those under analogous homogeneous and steady-state conditions. We have performed conservative and reactive tracer experiments in a quasi two-dimensional tank filled with glass beads of different sizes. The experiments have been carried out in both homogeneous and heterogeneous porous media under steady-state and transient (i.e. oscillating) flow fields. We used fluorescein as conservative tracer; whereas an alkaline solution (NaOH) was injected into ambient acidic water (HCl) in the reactive experiments. A pH indicator was added to the reacting solutions in order to visualize the emerging plume. We simulated the laboratory experiments with a numerical model and compared the outcomes of the model with the measured concentrations at the outlet of the tank and with the observed tracer plumes. Spatial moments, a newly defined flux-related dilution index, the product mass fluxes and the reaction enhancement factors were calculated to quantify the differences in mixing and reaction extent under various experimental conditions. The results show that flow focusing in heterogeneous porous media significantly enhances transverse mixing and mixing-controlled reactions, whereas temporally changing flow fields appear to be of minor importance.

Time scales of organic contaminant dissolution from complex source zones: coal tar pools vs. blobs

Groundwater contamination due to complex organic mixtures such as coal tar, creosote and fuels is a widespread problem in industrialized regions. Although most compounds in these mixtures are biodegradable, the contaminant sources are very persistent for many decades after the contamination occurred (e.g., more than 100 years ago at gasworks sites). This limited bioavailability is due to slow dissolution processes. This study presents results from a large scale tank experiment (8 m long) on the long-term (354 days) dissolution kinetics of BTEX and PAHs from a 2.5 m long coal tar pool and 0.5 m long (smear) zone containing coal tar blobs distributed in a coarse sand. The results inidicate (1) that Raoults law holds for estimation of the saturation aqueous concentrations of the coal tar constituents, (2) that for the dissolution of smear zones longer than approximately 0.1 m and with more than 3-5% residual saturation, the local equilibrium assumption is valid and (3) that although very small (< 0.1 mm), the transverse vertical dispersivity dominates the pool dissolution processes. Typical time scales for removal of the pollutants from the blob zone and the pool are in the order of a few weeks to more than 10,000 years, respectively.

Evidence of Compound-Dependent Hydrodynamic and Mechanical Transverse Dispersion by Multitracer Laboratory Experiments

Mass transfer, mixing, and therefore reaction rates during transport of solutes in porous media strongly depend on dispersion and diffusion. In particular, transverse mixing is a significant mechanism controlling natural attenuation of contaminant plumes in groundwater. The aim of the present study is to gain a deeper understanding of vertical transverse dispersive mixing of reaction partners in saturated porous media. Multitracer laboratory experiments in a quasi two-dimensional tank filled with glass beads were conducted and transverse dispersion coefficients were determined from high-resolution vertical concentration profiles. We investigated the behavior of conservative tracers (i.e., fluorescein, dissolved oxygen, and bromide), with different aqueous diffusion coefficients, in a range of grain-related Peclet numbers between 1 and 562. The experimental results do not agree with the classical linear parametric model of hydrodynamic dispersion, in which the transverse component is approximated as the sum of pore diffusion and a compound-independent mechanical dispersion term. The outcome of the multitracer experiments clearly indicates a nonlinear relation between the dispersion coefficient and the average linear velocity. More importantly, we show that transverse mechanical dispersion depends on the diffusion coefficient of the compound, at least at the experimental bench-scale. This result has to be considered in reactive-transport models, because the typical assumption that two reactants with different aqueous diffusive properties are characterized by the same dispersive behavior does not hold anymore.

Enhanced biodegradation by hydraulic heterogeneities in petroleum hydrocarbon plumes

In case of dissolved electron donors and acceptors, natural attenuation of organic contaminant plumes in aquifers is governed by hydrodynamic mixing and microbial activity. Main objectives of this work were (i) to determine whether aerobic and anaerobic biodegradation in porous sediments is controlled by transverse dispersion, (ii) to elucidate the effect of sediment heterogeneity on mixing and biodegradation, and (iii) to search for degradation-limiting factors. Comparative experiments were conducted in two-dimensional sediment microcosms. Aerobic toluene and later ethylbenzene degradation by Pseudomonas putida strain F1 was initially followed in a plume developing from oxic to anoxic conditions and later under steady-state mixing-controlled conditions. Competitive anaerobic degradation was then initiated by introduction of the denitrifying strain Aromatoleum aromaticum EbN1. In homogeneous sand, aerobic toluene degradation was clearly controlled by dispersive mixing. Similarly, under denitrifying conditions, microbial activity was located at the plumes fringes. Sediment heterogeneity caused flow focusing and improved the mixing of reactants. Independent from the electron accepting process, net biodegradation was always higher in the heterogeneous setting with a calculated efficiency plus of 23-100% as compared to the homogeneous setup. Flow and reactive transport model simulations were performed in order to interpret and evaluate the experimental results.

CCD camera image analysis for mapping solute concentrations in saturated porous media

AbstractThis paper presents an optical approach, based on the use of a low-cost charge-coupled device (CCD) camera, for the quantitative determination of solute concentrations in saturated porous media. The method is applied to evaluate tracer experiments carried out in a laboratory model tank. The CCD photos deliver RGB values which are transferred into concentrations for the evaluation of vertical concentration profiles over the whole tank area. A specially developed evaluation procedure, including internal referencing for noise reduction, considers the colour of the adjacencies of the evaluated spots and scattering effects. The CCD data evaluation technique is accompanied by conventional sampling and absorption measurements and by numerical flow and transport simulations. This non-invasive technique allows a direct mapping of the concentration distribution without any disturbance of the solute plume. Therefore, it turns out to be an important tool for a detailed investigation of fundamental processes (e.g. transverse dispersion) determining the solute (e.g. contaminant) transport in porous media. FigureCCD camera set-up for solute concentration mapping in saturated porous media

Evaluation of the role of heterogeneities on transverse mixing in bench-scale tank experiments by numerical modeling.

In this work, numerical modeling is used to evaluate and interpret a series of detailed and well-controlled two-dimensional bench-scale conservative tracer tank experiments performed to investigate transverse mixing in porous media. The porous medium used consists of a fine matrix and a more permeable lens vertically aligned with the tracer source and the flow direction. A sensitivity analysis shows that the tracer distribution after passing the lens is only slightly sensitive to variations in transverse dispersivity, but strongly sensitive to the contrast of hydraulic conductivities. A unique parameter set could be calibrated to closely fit the experimental observations. On the basis of calibrated and validated model, synthetic experiments with different contrasts in hydraulic conductivity and more complex setups were performed and the efficiency of mixing evaluated. Flux-related dilution indices derived from these simulations show that the contrasts in hydraulic conductivity between matrix and high-permeable lenses as well as the spatial configuration of tracer plumes and lenses dominate mixing, rather than the actual pore scale dispersivities. These results indicate that local material distributions, the magnitude of permeability contrasts, and their spatial and scale relation to solute plumes are more important for macro-scale transverse dispersion than the micro-scale dispersivities of individual materials. Local material characterization by thorough site investigation hence is of utmost importance for the evaluation of mixing-influenced or -governed problems in groundwater, such as tracer test evaluation or an assessment of contaminant natural attenuation.