Numerical Modelling of Reactive Mass Transport in Fractured Sedimentary Rock
Numerical modelling of mass transport within fractured sedimentary rock presents several conceptual and
computational challenges. Widely varying spatial and temporal scales, heterogeneities and complex
geometries can pose severe constraints on the types of hydrogeological systems that can be realistically
simulated. Accounting for reactive processes in fractured systems adds further challenges since multiple
components must be transported simultaneously while accounting for intra-aqueous and/or water-rock
reactions. Steep concentration gradients, which control the degree of component mixing, must also be
accurately resolved. To circumvent these challenges, simplifications are often introduced, including neglecting
advection and/or dispersion in the matrix, reducing dimensionality, or by assuming that the fractured rock can
be treated as an equivalent porous medium. This presentation will address some of these issues by
comparing reactive and non-reactive contaminant transport behaviour under various scenarios of fractured
sedimentary rock characterized by a porous and permeable matrix. The simulations show that a discrete
fracture network (DFN) approach is critical for reproducing high concentration gradients and mixing along
fracture interfaces. Transport processes within the matrix and system dimensionality are also shown to play
critical roles. The results have implications for predicting the behaviour of dissolved contaminants in fractured
sedimentary rock and in assessing the efficiency of remediation measures.
Solute Transport in Fractured Porous Media: Methods for Determination of Spatially- Resolved Porosity and Diffusion Coefficient Measurements
Determination of the porosity and diffusion properties of porous sedimentary rocks is an important prerequisite for quantification of diffusive mass transfer between fractures and the matrix in fractured porous rock flow systems. Porosity may be measured in a variety of ways, but few methods are available to provide quantitative, spatially-resolved measurements in heterogeneous porous media. Similarly, most of the common techniques for measuring diffusion coefficients are not able to provide spatially-resolved information on the diffusion properties of heterogeneous porous media. We have developed and tested X-ray radiography, γ-ray transmission, and magnetic resonance imaging (MRI) techniques for quantifying the porosity and tracer-concentration distributions in rock samples. These are non-destructive techniques and are therefore capable of acquiring spatially and temporally resolved concentration distributions for a single sample. Diffusion coefficients for a specific tracer in the porous medium can then be obtained by fitting the observed concentration distributions with a quantitative diffusion model. Examples of spatially-resolved distributions of porosity are provided in 1D, 2D and 3D for common rock types at scales from μm to cm. Spatially-resolved tracer concentration distributions are demonstrated, and diffusion coefficients are derived by fitting a diffusion model to the tracer concentration data. In addition, we test the applicability of an established Archie's-Law relationship between porosity and the effective diffusion coefficient at a scale of several tens of μm. These methods have the potential to provide a means for quantifying the feedback between geochemical reactions and the physical properties of the porous media, such as porosity and diffusion coefficient.
Conceptual Models for the Fracture Network in Contaminated Shale Based on Different Lines of Evidence
In investigations of groundwater flow in fractured sedimentary rock, there is typically a large discrepancy between the number of fractures identified by different methods in boreholes. The methods directed at fracture geometry such as inspection of continuous core and borehole imaging (acoustic, optical, electrical televiewing, borehole camera, etc.) commonly identify numerous fractures. In contrast, the methods that infer fractures from measurements in the open borehole water column (borehole flow meters, temperature, electrical conductance, full borehole dilution) show far fewer fractures. These two different categories of data support two very different conceptual models for the fracture networks in which groundwater flow occurs. A study was conducted at a contaminated industrial site in an area of approximately 150m by 100m located on a fractured Ordovician shale in New York State where a fracture network conceptual model was initially developed based primarily on borehole flow metering and related cross-borehole hydraulic tests. In this conceptual model based on eight boreholes having a maximum depth of 50 m, the total number of flow zones identified was 14 over 140 m of open hole and ranged from none to five per hole. PCE DNAPL released decades ago has caused substantial VOC contamination (PCE, TCE, cis-DCE, and VC) and this contamination was delineated by means of a large number of contaminant analyses on samples from continuous rock core at an average spacing of 0.3 m. Although groundwater flow occurs almost entirely in the fractures, almost all the contaminant mass resides in the rock matrix (porosity 2-5%) because of long term diffusion-driven mass transfer from fractures to the matrix. The rock core contaminant profiles indicate that advective transport has occurred over decades through numerous fractures in each borehole. Therefore, many of the fractures identified from corelogs and televiewing must have active groundwater flow. This supports a different fracture network conceptual model in which groundwater flow occurs in a very large number of interconnected fractures. This conceptual model is supported further by the results of three large scale injections of permanganate solution conducted to destroy contaminants by oxidation. The spread of permanganate away from the injection locations was determined using depth-discrete multilevel monitoring systems (MLS)(Westbay, FLUTe, CMT) indicating transport in numerous fractures. The existence of numerous well connected fractures throughout the 50 m thick zone of investigation is consistent with hydraulic head profiles measured in MLSs. Although both conceptual models are based on properly executed measurements in the boreholes, the conceptual model relevant to contaminant migration and remediation is the one having a large number of well interconnected fractures. Although there are a large number of interconnected fractures there is likely a much more limited number of fracture zones allowing rapid large scale hydraulic responses between holes.
Characteristics of Fracture Networks and Hydrogeologic Units: Implications Provided by Detailed Hydraulic Head Profiles
Plume characterization in fractured rock is particularly challenging because of the inherent complexity that is difficult to characterize using conventional data. This field study demonstrates how exceptionally detailed head profiles provide definition of the hydrogeologic framework for a sedimentary rock aquifer system impacted by an extensive mixed organic contaminant plume. The site is located in south central Wisconsin and overlies nearly flat lying fractured Paleozoic sandstones and dolostones. Many of these bedrock geologic units were deposited in a marine setting and as a result are laterally extensive across the cratonic interior of North America. Several regional groundwater flow models exist for south central Wisconsin. In the hydrogeologic framework for these models, the various bedrock units are lumped into a lower aquifer, an aquitard, and an upper aquifer. While this framework may be sufficient for regional groundwater flow models, historical data from the field site indicate it is not an appropriate framework for contaminant transport at the site scale. As a result, field studies were designed to collect detailed data from continuous core, geophysical and hydrophysical logging of the corehole, and detailed multilevel systems (MLSs) to define the hydrogeologic framework for the site. A preliminary study by Meyer et al. (2008) involved the installation of one very detailed MLS, 36 monitoring zones over 120.7 m, in the MP-6 corehole upgradient of the plume. The MP-6 hydraulic head profile is characterized by segments with minimal vertical hydraulic gradient separated by distinct inflections. Meyer et al. (2008) concluded that the sharp inflections observed in the MP-6 head profile delineated the positions of low vertical hydraulic conductivity interfaces which are not distinguishable based on stratigraphy, geophysics, or other conventional indirect indicators alone. The inflections in the head profile were interpreted as the contacts between hydrogeologic units (HGUs) and were used to delineate 11 HGUs at the MP-6 location. The sections of the head profile with minimal vertical gradient indicate an interconnected fracture network and a dominance of horizontal flow within each HGU. In the current study, seven additional detailed multilevel systems were installed across the site to investigate the lateral continuity of the hydraulic head inflections observed at MP-6. The head profiles measured from all eight MLSs have similar simple geometries: sections of minimal hydraulic gradient separated by sharp vertical inflections. The elevations of the hydraulic head inflections at each of the eight coreholes are strongly correlated despite separation distances of up to 3 km. The inflections observed in the detailed head profiles allow for the delineation of up to 13 bedrock HGUs at the site in contrast to the three bedrock HGUs commonly used in regional groundwater flow models. These 13 bedrock HGUs will provide the framework for site scale numerical modeling of groundwater flow and contaminant transport. The results of this study demonstrate that pre-existing regional stratigraphic frameworks are generally not an appropriate hydrogeologic framework, particularly in dual porosity/permeability systems where contaminant transport and fate is a concern. In addition, the simple geometry of the head profiles suggests an ordered and interconnected fracture network within each HGU and a poor vertical hydraulic connection between the fracture networks of adjacent HGUs.
Field Evidence of Trichloroethene Degradation in Fractured Sandstone
Fractured sedimentary rock, such as sandstone, shale, limestone and dolostone, forms the bedrock in most of the populated parts of North America and Europe. Trichloroethene (TCE) is the most commonly found organic solvent contaminant in groundwater and this type of rock; however, documented TCE degradation studies in these bedrock environments are rare. A field investigation of TCE degradation in fractured sandstone was conducted at an industrial site in southern California where TCE as dense non-aqueous phase liquid entered the fractures at many locations decades ago. Long-term sampling (5 to 20 years) of monitoring wells shows common occurrences of dissolved-phase TCE along with the primary degradation product cis 1,2- dichloroethene and minor amounts of degradation products trans 1,2-dichloroethene, 1,1-dichloroethene, and vinyl chloride. TCE degradation products were also found in many rock core samples from intact rock situated between fractures. Evidence for both microbial reductive dechlorination and abiotic dechlorination was obtained from analysis of monitoring wells sampled for compound specific carbon isotope ratios, inorganic ion hydrochemistry, and dissolved gases (O2, CH4, H2). Abiotic degradation was indicated by the presence of acetylene, which is consistent with the literature based on laboratory studies indicating propensity for abiotic degradation of chlorinated ethenes in the presence of iron-bearing minerals such as pyrite and biotite, both are common in the sandstone at this field site. Active groundwater flow and contaminant transport occur only in the interconnected fracture network but nearly all groundwater contaminant mass and reactive mineral surfaces occur in the rock matrix blocks between the fractures where the pore water is nearly immobile. The effective diffusion coefficients for TCE and the degradation products and diffusion distances in the rock matrix blocks accommodate active mass transfer of all contaminants between the fractures and the rock matrix. This study indicates the importance of assessing in conjunction with biotic degradation, the influence of abiotic degradation and the rock matrix in studies of natural attenuation of TCE in sedimentary bedrock.
Evaluation of a Chlorinated Compounds Plume in a Fractured Sandstone Aquifer in Mid- West, US
A study was carried out in the sedimentary fractured rock site located in Mid West, US, which was impacted by a DNAPL spill estimated to occur in the 1950's and 1960's. The majority of the DNAPL has accumulated in the upper portion of the Lone Rock Formation (referred to as Layer 5) and a VOC plume of more than 3km long has formed. The DNAPL is mainly composed of 1,1,1-TCA, PCE, TCE and BTEX, while large amounts of degradation products such as cis-DCE and 1,1-DCA have been found in the plume. Detailed geochemical and carbon isotope analysis in September 2007 showed that complete degradation of PCE and TCE to cis-DCE in Layer 5 had been achieved from the source to the middle of the plume and the dechlorination reaction stalled at cis-DCE, which is in agreement with the redox condition in this part of the plume. On the other hand, degradation of 1,1,1-TCA to 1,1-DCA was incomplete. The fringes of the plume are characterized by the presence of PCE and TCE in agreement with aerobic conditions in this part of the plume. A historical data review from 1992 to 2006 revealed two phases of degradation in Layer 5. The first phase corresponded with the period before 2001, when there was no significant degradation, while the second phase corresponded with the period after 2001, when significant degradation occurred. The occurrence of the second phase was related to a large scale DNAPL pumping in the source zone during 1999 to 2002, which caused a great increase of contaminant concentrations in the plume including large amounts of ketones and BTEX serving as electron donors and substrates for microbial dechlorination. Thus, subsequent degradation of chlorinated compounds occurred extensively in the plume. The contaminant concentration and the shape of the plume has been modified since 2003 by a hydraulic barrier system. This case study shows that the long term degradation pattern and contaminant distribution at the site has been controlled by plume management practices including DNAPL pumping in the source area and the creation of a hydraulic barrier system in the middle of the plume.
Fingerprinting TCE Sources and Natural Attenuation Capacity of a Sedimentary Bedrock Aquifer using Compound Specific Isotope Analysis - A Case Study in the City of Guelph, Ontario.
Chlorinated volatile organic compounds were detected in an industrial facility (site) in the upper part of the underlying dolomite bedrock aquifer. The primary chlorinated solvent detected at the site is trichloroethene (TCE), and associated daughter products, such as cis-1,2- dichloroethene (cis-1,2-DCE) and vinyl chloride (VC), are also present. Several conventional monitoring wells installed at the site were recently sampled for VOCs analysis, inorganic sensitive REDOX parameters and Compound Specific Isotope Analyses (CSIA) of 13C and 37Cl in order to evaluate the natural attenuation capacity of the local upper bedrock aquifer. A municipal well was also included in this study. The municipal well is located approximately 500 meters southwest from the investigated site and consists of anuncased bedrock well completed to 102 meters below ground surface (mbgs). Chlorinated volatile organic compounds were detected in this well during the evaluation of municipal groundwater resources performed in the early 1990s by the City of Guelph. Neither the City nor the Ministry of the Environment of the Province of Ontario (MOE) have been able to satisfactorily determine the origin of these compounds. Groundwater for VOCs analysis and CSIA were therefore collected in the municipal well in order to evaluate the relationship between the TCE detected in the municipal well and the TCE plume detected in the nearby industrial facility. The inorganic data indicated that the local upper aquifer is under sulfate-reducing conditions in some areas where typical VOC's daughter products and highly enriched δ13C and δ 37Cl values of TCE and cis-1,2-DCE were observed. These findings indicated that microbial reductive dechlorination of TCE is occurring in some areas of the local aquifer. The δ 13C values of TCE obtained in groundwater from the conventional wells were approximately 10 ‰ more depleted than δ 13C values obtained for the deep municipal production well. These data indicate that there is no relationship between the TCE plume at the site and the TCE present in the water of the municipal well, suggesting that other potential regional TCE sources are likely to be present in the area.