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Contaminant transport and fate is fundamentally different in fractured rock than in unconsolidated (sand and gravel) aquifers. Significantly more uncertainty exists as to the direction and rate of contaminant migration, as well as the processes and factors that control chemical and microbial transformations (USGS 2010). Depending upon the orientation and type of fracture or solution channel, just finding all of the contaminated zones can be a challenge. In addition, contaminated and uncontaminated fracture zones can lie above or below each other and drilling stands the risk of opening a conduit between the two that was not present before.
Rock formations can have both secondary and primary porosity. Secondary porosity, the result of chemical leeching of minerals or the generation of a fracture system, is the primary source of fluid movement in rocks. The network of interconnected fractures (and solution channels in the case of karst) allows fluid movement through rock formations with very low primary porosity. Dissolved contaminants and fluids will generally spread through fracture networks.
Primary porosity is the inherent ability of the rock matrix to accept and transport fluids. Poorly cemented sandstones have relatively high primary porosity and will allow fluids to move through them. Rock with a crystalline matrix, such as granite, will not readily transport fluids, and the matrix acts as a barrier to water and contaminants. The primary porosity of a rock also is important to fate and transport considerations in that the more porous a rock matrix is, the greater the potential for dissolved contaminants to diffuse into it. Shales, for example, whose matrix comprises primarily clay and silt particles, are moderately impermeable to water flow through the matrix but still have sufficient porosity to allow for the diffusion of contaminants into the matrix (Vitolins et al. 2004). This diffusion is an important consideration in the construction of a conceptual site model and selection of a site remedy because the contaminants in the rock matrix could become a contaminant source zone if the concentration in the fractures falls below the concentration of contaminants in the rock matrix (Doe 2010a).
In some cases contaminants can be introduced to the fractured rock system as a non-aqueous phase liquid. These liquids are hydrophobic and can be lighter or heavier than water. In this situation, multi-phase flow will occur. Doe (2010a) describes the flow as follows:
In multiphase flow, density-based gravitational forces come into play along with capillary forces that act along the interfaces between the fluids (or gasses) and the solid surfaces of the pores or fractures. The significance of these forces — gravity, viscosity, and capillarity — vary with the pore size or fracture aperture, where capillarity dominates in smaller pores or fractures and gravity dominates in larger ones. In a multiphase flow system, capillary pressures can immobilize wetting phase fluid in the smallest aperture fractures. In larger fractures, flow occurs according to Darcy's law but with different permeabilities for each phase that depend on their saturations. In the largest fractures, gravity dominates flow, producing a variety of non-Darcian flow processes that may be very rapid and are still poorly understood.
Doe (2010b) has an extensive discussion of flow in fractured rock in its Appendix A.
Volcanic Systems (Extrusive Igneous)
Aquifers in basaltic and other volcanic rocks are widespread in Washington, Oregon, Idaho, and Hawaii, and extend over smaller areas in California, Nevada, and Wyoming. Volcanic rocks have a wide range of chemical, mineralogic, structural, and hydraulic properties. The variability of these properties is due largely to rock type and the way the rock was ejected and deposited.Pyroclastic rocks, such as tuff and ash deposits, might be emplaced by flowage of a turbulent mixture of gas and pyroclastic material, or might form as windblown deposits of fine-grained ash. Where they are unaltered, pyroclastic deposits have porosity and permeability characteristics like those of poorly sorted sediments; where the rock fragments are very hot as they settle, however, the pyroclastic material might become welded and almost impermeable. Silicic lavas, such as rhyolite or dacite, tend to be extruded as thick, dense flows and have low permeability except where they are fractured. Basaltic lavas tend to be fluid and form thin flows that have a considerable amount of primary pore space at the tops and bottoms of the flows. Numerous basalt flows commonly overlap and the flows commonly are separated by soil zones or alluvial material that form permeable zones. Basalts are the most productive aquifers of all volcanic rock types (Miller 1999).
Intrusive Igneous and Metamorphic Systems
Spaces between the individual mineral crystals of the crystalline rocks are few, microscopically small, and generally unconnected. Consequently, the intergranular primary porosity of crystalline rocks is so small as to be insignificant. Secondary fracture permeability in crystalline rocks is the result of the cooling of igneous rocks, deformation of igneous and metamorphic rocks, faulting, jointing and weathering. Openings commonly are present along relict bedding planes, cleavage planes, foliation, and other zones of weakness in the rocks; these openings typically are heterogeneous in spacing, orientation, size, and degree of interconnection. Generally, openings in the rocks are most prevalent near land surface and decrease in number and size with depth (Olcott 1995).
Aquifers in sandstone are more widespread than those in all other kinds of consolidated rocks. Although the porosity of well-sorted, unconsolidated sand may be as high as 50 percent, the porosity of most sandstones is considerably less. During the process of conversion of sand into sandstone (lithification), compaction by the weight of overlying material reduces not only the volume of pore space as the sand grains become rearranged and more tightly packed, but also the interconnection between pores (permeability). The deposition of cementing materials such as calcite or silica between the sand grains further decreases porosity and permeability. Sandstones retain some primary porosity unless cementation has filled all the pores, but most of the porosity in these consolidated rocks consists of secondary openings such as joints, fractures, and bedding planes. Groundwater movement in sandstone aquifers primarily is along bedding planes, but the joints and fractures cut across bedding and provide avenues for the vertical movement of water between bedding planes (Miller 1999). Note that poorly cemented sandstones are capable of transmitting fluids through primary porosity.
Shale, Claystone, Mudstone
Shale, claystone, and mudstone are sedimentary rocks made from varying degrees of silts and clays. Shale is different from claystone and mudstone in that it is fissile and readily splits along lamination planes. These rocks have low permeability, and fluid flow of any volume will be through fractures, primarily along bedding planes. Their permeability is such, however, that dissolved contaminants may diffuse into the rock matrix, which can complicate cleanup efforts (Miller 1999).
Aquifers in carbonate rocks are most prominent in the central and southeastern parts of the nation, but also occur in small areas as far west as southeastern California and as far east as northeastern Maine. Most of the carbonate-rock aquifers consist of limestone, but dolomite and marble locally are sources of water. The water-yielding properties of carbonate rocks are highly variable; some yield almost no water and are considered to be confining units, whereas others are among the most productive aquifers (Floridan) known (Miller 1999).
Most carbonate rocks form from calcareous deposits that accumulate in marine environments ranging from tidal flats to reefs to deep ocean basins. The deposits are derived from calcareous algae or the skeletal remains of marine organisms that range from foraminifera to molluscs. Minor amounts of carbonate rocks are deposited in fresh to saline lakes, as spring deposits, geothermal deposits, or dripstone in caves. The original texture and porosity of carbonate deposits are highly variable because of the wide range of environments in which the deposits form. The primary porosity of the deposits can range from 1 to more than 50 percent. Compaction, cementation, and dolomitization are diagenetic processes that act on the carbonate deposits to change their porosity and permeability (Miller 1999).
The principal post-depositional process that acts on carbonate rocks is dissolution. Carbonate rocks are readily dissolved to depths of about 300 feet below land surface where they crop out or are covered by a thin layer of material. Precipitation absorbs some carbon dioxide as it falls through the atmosphere, and even more from organic matter in the soil through which it percolates, thus forming weak carbonic acid. This acidic water partially dissolves carbonate rocks, initially by enlarging pre-existing openings such as pores between grains of limestone or joints and fractures in the rocks. These small solution openings become larger, especially where a vigorous groundwater flow system moves the acidic water through the aquifer. Eventually, the openings join as networks of solution openings, some of which may be tens of feet in diameter and hundreds to thousands of feet in length. The end result of carbonate-rock dissolution is expressed at the land surface as karst topography, characterized by caves, sinkholes, and other types of solution openings, and by few surface streams. Where saturated, carbonate-rock aquifers with well-connected networks of solution openings yield large volumes of water to wells that penetrate the solution cavities, even though the undissolved rock between the large openings may be almost impermeable. Because water enters the carbonate-rock aquifers rapidly through large openings, any contaminants in the water can rapidly enter and spread through the aquifers (Miller 1999).
Technical and Regulatory Challenges Resulting from VOC Matrix Diffusion in a Fractured Shale Bedrock Aquifer
Vitolins, A.R., K.J. Goldstein, D. Navon, G.A. Anderson, S.P. Wood, B. Parker, and J. Cherry.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 13-15, 2004, Portland, Maine. p 115-126, 2004
Analysis of Organic Carbon (foc) in Fractured Bedrock
Rawson, J. and T.R. Eschner.
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, ME, 9 pp, 2007
U.S. Geological Survey website.
This Web page provides a general description of various aquifer types and where they are found in the United States.
Characterization, Modeling, Monitoring, and Remediation of Fractured Rock
National Research Council.
National Academies Press, Washington, DC. ISBN: 978-0-309-37898-7, 244 pp, 2015
This report examines the state of practice and state of art in the characterization of fractured rock and the chemical and biological processes related to subsurface contaminant fate and transport. Many new characterization tools have been developed in the last 20 years along with a greater appreciation for the importance of chemical and biological processes that can occur in the fractured rock environment. The findings of this report are particularly applicable to engineered repositories for buried or stored waste and to fractured rock sites that have been contaminated as a result of past disposal or other practices.
Chlorinated Solvent Source and Plume Behavior in Fractured Sedimentary Rock from Field Studies
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine, 19 pp, 2007
Conceptual Models of Flow and Transport in the Fractured Vadose Zone
National Research Council.
National Academies Press, Washington, DC. ISBN-10: 0-309-07302-2, 392 pp, 2001
The Effect of Matrix Diffusion on the Results of Large-Scale Tracer Experiments in a
Fractured Rock Conference: State of the Science and Measuring Success in Remediation, September 24-26, 2007, Portland, Maine.
Matrix Diffusion-Derived Plume Attenuation in Fractured Bedrock
Lipson, D., B. Kueper, and M. Gefell.
Ground Water 43:30-39(2005)
Rock Fractures and Fluid Flow: Contemporary Understanding and Applications
National Research Council (NRC).
National Academies Press, Washington, DC. ISBN-10: 0-309-10371-1, 382 pp, 1996