PORE CONNECTIVITY, EPISODIC FLOW, AND UNSATURATED DIFFUSION IN FRACTURED TUFF

Impact of fluid–rock chemical interactions on tracer transport in fractured rocks

It is well known that matrix diffusion (mass transfer between fractures and the rock matrix through molecular diffusion) can significantly retard solute transport processes in fractured rock, and therefore is important for analyzing a variety of problems, including geological disposal of nuclear waste. Matrix-diffusion-coefficient values measured from small rock samples in the laboratory are generally used for modeling field-scale solute transport in fractured rock. However, by compiling results from a number of field tracer tests corresponding to different geological settings, this study demonstrates that the effective matrix diffusion coefficient at field scale is generally larger than that at lab scale and tends to increase with testing scale. Preliminary interpretations of this observation are also investigated. We found that this interesting scale dependence may be related to the complexity of flow-path geometry in fractured rock.

Transport of solutes in fractured media, based on a channel network model : B. Gylling, L. Moreno & I. Neretnieks, in: Groundwater quality: remediation and protection. proc. conference, Prague, 1995, ed K. Kovar & J. Krasny, (IAHS; Publication, 225), 1995, pp 107–113

Influence of colloid and adsorption parameters on contaminant transport in fractured rocks - A triple continuum model

Analytical solutions for reactive transport of N-member radionuclide chains in a single fracture

Precipitation-dissolution reactions are important for a number of applications such as isotopic tracer transport in the subsurface. Analytical solutions have been developed for tracer transport in both single-fracture and multiple-fracture systems associated with these reactions under transient and steady-state transport conditions. These solutions also take into account advective transport in fractures and molecular diffusion in the rock matrix. For studying distributions of disturbed tracer concentration (the difference between actual concentration and its equilibrium value), effects of precipitation-dissolution reactions are mathematically equivalent to a “decay” process with a decay constant proportional to the corresponding bulk reaction rate. This important feature significantly simplifies the derivation procedure by taking advantage of the existence of analytical solutions for tracer transport associated with radioactive decay in fractured rock. It is also useful for interpreting tracer breakthrough curves, because the impact of a decay process is relatively easy to analyze. Several illustrative examples are presented, which show that the results are sensitive to fracture spacing, matrix diffusion coefficient (fracture surface area), and bulk reaction rate (or “decay” constant), indicating that the relevant flow and transport parameters may be estimated by analyzing tracer signals.

An interpretation of potential scale dependence of the effective matrix diffusion coefficient

This paper presents a methodology for upscaling matrix‐material transport parameters in fractured‐flow dominated systems with multimodal reactive mineral facies. The upscaling method provides a theoretical and practical link between controlled experimental results at the laboratory/bench scale and multikilometer field scales at which contaminant remediation and risk assessment are actually conducted. As sorption reactions in matrix are in part determined by mineral properties, a new conceptual model is developed to reflect the hierarchical structure of reactive mineral facies at the microform, mesoform, and macroform scales. The conceptual model of hierarchical reactive matrix mineral facies is integrated with a dual‐porosity model for simulating diffusion of solutes out of fractures and sorption onto the matrix minerals. By assuming that sorption reactions primarily occur in the rock matrix, we develop a multimodal spatial random function for characterizing both the tortuosity (physical heterogeneity) and sorption coefficient (chemical heterogeneity) at different scales in the rock matrix. The effective tortuosity at the field scale is derived by volume averaging of mass transfer coefficient for a conservative species. Subsequently, using a sorbing species (e.g., uranium), we derive the equations for upscaling the sorption coefficients in a saturated, fractured‐rock system for field‐scale simulations. The effective field‐scale tortuosity and sorption coefficient are related to their mean, variance, integral scale, and domain size along a pathway through a three‐dimensional flow field. The upscaled values increase with the integral scale and are larger than their geometric mean. Simulations conducted with upscaled sorption coefficients and tortuousities are compared very well with high‐resolution Monte Carlo simulations capturing the parameter spatial variations. Results of this study can be extended to explore scale dependence of other important transport parameters for fractured‐rock solute transport.

One of soil microbiology's most intriguing puzzles is how so many different bacterial species can coexist in small volumes of soil when competition theory predicts that less competitive species should decline and eventually disappear. We provide evidence supporting the theory that low pore connectivity caused by low water potential (and therefore low water content) increases the diversity of a complex bacterial community in soil. We altered the pore connectivity of a soil by decreasing water potential and increasing the content of silt- and clay-sized particles. Two textures were created, without altering the chemical properties or mineral composition of the soil, by adding silt- and clay-sized particles of quartz to a quartz-based sandy soil at rates of 0% (sand) or 10% (silt+clay). Both textures were incubated at several water potentials, and the effect on the active bacterial communities was measured using terminal restriction fragment length polymorphism (TRFLP) of bacterial 16S rRNA. Bacterial richness and diversity increased as water potential decreased and soil became drier (P < 0.012), but they were not affected by texture (P > 0.553). Bacterial diversity increased at water potentials of <or=2.5 kPa in sand and <or=4.0 kPa in silt+clay, equivalent to <or=56% water-filled pore space (WFPS) in both textures. The bacterial community structure in soil was affected by both water potential and texture (P < 0.001) and was correlated with WFPS (sum of squared correlations [delta(2)] = 0.88, P < 0.001). These findings suggest that low pore connectivity is commonly experienced by soil bacteria under field conditions and that the theory of pore connectivity may provide a fundamental principle to explain the high diversity of bacteria in soil.

ABSTRACTAn in situ experiment to study migration in a single fracture in granitic rock has been performed. Both sorbing and nonsorbing tracers have been used. Results from a tracer run with a nonsorbing tracer (Uranine) are presented. The breakthrough curves have been explored with a model which takes into account hydrodynamic dispersion as well as diffusion into the rock matrix. Diffusivities and porosities of the rock matrix measured in the laboratory are used to interpret the field data. A short description of the experimental design is given. This investigation is a part of the STRIPA PROJECT phase I.

We develop a theoretical model for power law tailing behavior of transport in fractured rock based on the relative dominance of the decay rate of the advective travel time distribution, modeled using a Pareto distribution (with tail decaying as ∼ time−(1+α)), versus matrix diffusion, modeled using a Lévy distribution. The theory predicts that when the advective travel time distribution decays sufficiently slowly (α&lt;1), the late‐time decay rate of the breakthrough curve is −(1+α/2) rather than the classical −3/2. However, if α&gt;1, the −3/2 decay rate is recovered. For weak matrix diffusion or short advective first breakthrough times, we identify an early‐time regime where the breakthrough curve follows the Pareto distribution, before transitioning to the late‐time decay rate. The theoretical predictions are validated against particle tracking simulations in the three‐dimensional discrete fracture network simulator dfnWorks, where matrix diffusion is incorporated using a time domain random walk.

Contaminant transport in fractured rocks with significant matrix permeability, using natural fracture geometries

Pore connectivity characterization of shale using integrated wood’s metal impregnation, microscopy, tomography, tracer mapping and porosimetry

Microbiological characterizations of contaminant biodegradation in fractured sedimentary rock have primarily focused on the biomass suspended in groundwater samples and disregarded the biomass attached to fractures and in matrix pores. In fractured sedimentary rock, diffusion causes nearly all contaminant mass to reside in porous, low-permeability matrix. Microorganisms capable of contaminant degradation can grow in the matrix pores if the pores and pore throats are sufficiently large. In this study, the presence of dechlorinating microorganisms in rock matrices was investigated at a site where a fractured, flat-lying, sandstone-dolostone sequence has been contaminated with a mixture of chlorinated and aromatic hydrocarbons for over 40 years. The profile of organic contaminants as well as the distribution and characterization of the microbial community spatial variability was obtained through depth-discrete, high-frequency sampling along a 98-m continuous rock core. Dechlorinating microorganisms, such as Dehalococcoides and Dehalobacter, were detected in the rock matrices away from fracture surfaces, indicating that biodegradation within the rock matrix blocks should be considered as an important component of the system when evaluating the potential for natural attenuation or remediation at similar sedimentary rock sites.

Radioactive nuclides moving with the fluid flow in fractured rocks is one of the most important safety issues of nuclear waste repositories. The nuclides are retarded by sorption and diffusion in and out of the rock matrix as well. Since 1970s, a large number of analytical, semi-analytical and numerical schemes were proposed to simulate transport process in single fracture, regular fracture network, and complicated random fracture network. These results provided important insights to understanding transport phenomenon in fractured crystalline rocks like granite. Berkowitz (2002), Neuman (2005) systematically reviewed the developments of modeling flow and transport in fractured geological medium in the past.