4.6 Velocity measurements in fractured media

The hydrologic cycle tells us that energy from the sun drives water (primarily) from the oceans into the atmosphere. However, the Earth is not a static planet and eventually conditions prevail that cause the water to rain out of the sky and return to the surface — sometimes the land surface. From there, water seeks the lowest elevation point it can find under the influence of gravity — ultimately returning to the oceans. So relentless is this quest that almost no place that offers space for water molecules is not invaded by them as they make their journey. In the earlier sections of this book, the spaces at issue are those between the grains of sediment or regolith that lie between the sky and the rocky surface of the planet. But the Earth’s crust is also a dynamic thing, and the rock that composes it is frequently unable to withstand the tectonic, volcanic, isostatic, or erosional factors that bend, fold, uplift, heat, cool, subside, rotate or weather it. The result is that the shallow rock layers of the planet begin to break up, developing fractures, joints, partings, or solution openings that inevitably fill with water. When the fracturing is sufficiently pronounced, the rock strata can behave as aquifers.

Not surprisingly, the occurrence of fractured rock aquifers is very common. They occur in crystalline, igneous or metamorphic shield rock where the only openings for water are the fractures (Figure 26a), and in sedimentary rocks composed of grains that are consolidated with mineral cements. These rocks exhibit both primary porosity and permeability inherent to the rock matrix, as well as secondary porosity and permeability associated with later fracturing, including partings along contacts between beds (Figure 26b). In cases where sedimentary rock is composed of thick layers of soluble material, such as limestone or dolostone, fractures can become enlarged through dissolution, vastly increasing the capacity to carry water (Figure 26c). This phenomenon underlies the formation of karst topography, a geomorphic descriptor that applies to 25% of the Earth’s land surface and which is discussed in greater depth later in this section.

The application of conventional Darcy’s Law approaches to characterize fractured rock aquifers is commonly used but subject to misleading outcomes. In the simplest cases, the density of fractures is very high i.e., fracture spacings are hundreds or thousands of times smaller than the spatial scale of the investigation and the aquifer behaves much like an equivalent porous medium (EPM), so the use of Darcy’s Law is well founded (van der Kamp, 1992). The EPM assumption is also appropriate in cases where the rock matrix has high permeability and does not depend on the fractures to conduct flow. Permeable matrix rock is susceptible to invasion by pollutants due to slow advective flow or diffusion between the fractures and the matrix. This aspect of the fractured media problem is qualitatively similar to Figure 21 and discussed in more detail elsewhere in the Groundwater Project books. Here, the focus is on flow in the fractures.

In many cases involving the investigation of groundwater pollution, the scale of the site is not sufficiently large, compared to the fracture spacing, to support the use of velocity measurement methods that depend on the EPM assumption (Figure 27). In these cases, a form of Darcy’s Law (derived from the ‘Cubic Law’ that relates flow in a fracture to the cube of its aperture) might still be applied for single fractures or fracture sets within a defined section of a borehole. To apply such methods, detailed hydraulic testing of the fracture interval is required, in part to estimate the fracture hydraulic conductivity, K_f, hydraulic aperture, 2b, and fracture porosity, n_f. It is worth noting that the application of Darcy’s Law in granular media, porosity is in the range of 0.2 to 0.5 while in fractured rock the range extends to much lower values, typically 10^–5 to 10^–3 (Morris and Johnson, 1967). Using Equation 3 of Figure 27 leads to an estimated seepage velocity orders of magnitude higher in fractured media than granular media, with important implications for the assessment of risk. This responsibility should not rest entirely on the shoulders of Darcy’s Law since the parameters used in the calculation come with substantial uncertainties. Independent measurements of velocity are highly desirable to validate the Darcy predictions.

Building on the issues raised above, fractured aquifers pose special challenges for hydrogeologists because unlike granular aquifers, the openings that conduct water behave as discrete pathways rather than a continuous medium. A given borehole — which can be very expensive to drill — may or may not intersect a productive fracture or fracture set, while a second borehole a meter away yields abundant water. Furthermore, not all fractures are created equal; some may have apertures smaller than the width of a human hair while others may be hundreds of microns wide, leading to vast differences in water productivity. In the case of karst aquifers, open channels large enough for a person to enter (i.e., caves) may be present and control the speed and direction of water flow. All of this can add up to counterintuitive water level data when Darcy’s Law-based surveys are undertaken with a conventional porous medium mindset. For example, the discrete distribution of fractures can cause the local flow directions to vary widely from regional trends (Figure 27). Water is restricted to flow through the available openings whether or not they align with predictions based on Darcy’s Law (Figure 28a).

The importance of this insight is illustrated with the following hypothetical scenario: a receptor (e.g., a water supply well) is apparently off the path of contaminant transport according to conventional determinations of hydraulic gradient and appears safe from pollution emanating from a buried tank. However, the receptor draws its water from the same fracture carrying polluted groundwater from the leaking tank. Without knowledge of the flow direction in the fracture, it is difficult to assess the risk to the receptor.

A special case that falls into the category of rock-aquifers is karst, as mentioned earlier in this section. In addition to the issues raised above, karst landscapes are characterized by large solution voids in the underlying rock that can conduct subsurface water in what are essentially channels. This can lead to uncharacteristically high (for groundwater) linear velocities that reach magnitudes approaching kilometers per day (Figure 29). Therefore, tools used for porous media or fractured rock, which typically conduct water at lower velocities, may not be suitable for measuring the high flow rates in some karst settings. A favored method for determining groundwater velocity in karst settings is dye tracing (Aley, 2002). These tests are conducted by introducing fluorescent dyes (most commonly) into sinkholes or other recharge locations and monitoring downstream springs for the appearance of the dyes. The success of the method depends in large part on the low detection limits (parts per trillion range) possible with the fluorescent dyes. The method is most commonly used to identify overall directions of flow and times of first arrival of the dyes at the springs. Details of the pathways taken between the sources and springs are not generally discoverable by this method. Also, average linear velocities, based on the travel time of the tracer center of mass, are not be possible to determine in many cases; the range of tracer mass balances (mass detected at springs/mass released) is <1% to nearly 100%, with a median value of about 5% (Tom Aley, personal communication). The most successful tests for achieving tracer mass balances are those involving flow through a single ‘pipe’, or similarly simple pathway. The poorer mass recoveries are thought to result from a combination of dilution in the subsurface channels, where turbulent mixing is possible, and distributary drainage that occurs in many karst systems. Loss of tracer mass to the rock matrix is also possible where the primary porosity of the matrix is notable, as in many clastic sedimentary rocks. Also, in some cases, tracer loss to biotic or abiotic transformations can occur, though the dyes are usually selected to minimize this possibility over the time period of a test (hours to weeks).

Neglecting karst settings for a moment, fractures in rock tend to be micron-scale openings and are therefore incapable of conducting large water flow on an individual basis (note that fracture sets may cumulatively conduct volumes of groundwater that rival porous media aquifers). However, what they lack in volume they can make up for in speed. The small aperture of a fracture behaves like the small space between the thumb and the hose in Figure 4, so transport rates in fractures can be surprisingly high — many meters per day. As a result, the time between a pollutant release and a detection at a receptor can be grievously small, leaving relatively little time to take preventative or reactive measures.

Another challenge encountered in fractured media is the inadvertent creation of new flow paths by boreholes that intersect hitherto isolated fractures (Figure 28b). These kinds of unintended connections can spread pollutants from contaminated zones to clean zones, as well as obscure the ambient flow directions and magnitudes in the aquifer(s) (Sterling et al., 2005). Moreover, even without interconnecting boreholes, fractures that are hydraulically inactive or dry under normal (average) conditions can become hydraulically active at times when recharge rates are high and water levels in the ground rise — for example, after rainstorms. This can result is unexpected changes in directions and rates of pollutant migration compared to those observed under ‘normal’ conditions.

The challenges are formidable, but methods are available — and new methods are being developed or adapted — to take on those challenges. As mentioned above, a favored and well-established method to investigate transport in fractured systems or karst is the introduction of tracer dyes near suspected source areas and the monitoring of their breakthroughs at selected points downstream, commonly at natural springs, but also in wells, and discharge zones in streambeds or lakes. Dye tracing is most effectively used to identify preferred flow paths, which is ideally suited for assessing pathways in fractured rock aquifers. The method circumvents the assumptions implicit in Darcy’s Law calculations and addresses three primary questions (Aley, 2002): 1) where is the groundwater going? 2) how long does it take to get there? 3) what happens to solutes (substances dissolved in groundwater) along the way? Unfortunately, as mentioned previously, the known mass of tracer released to the subsurface is rarely recovered at the discharge locations, so large fractions of the tracer have unknown fates. In general, the larger the scale of the tracer test, the lower the fraction of tracer mass recovered. Nevertheless, much information can be gleaned from these tests. Question 1 is answered on the basis of which monitored points detect tracer, and question 2 is addressed on the basis of the time of first detection at each of these points. The answers to these questions will tend to be most influenced by the highly conductive pathways in the aquifer, which may be advantageous in many circumstances but will not identify mechanisms by which, or locations where, the tracer (and thus the pollutant) mass collects. Answering the third question can help in this respect. One strategy for addressing question 3 is the inclusion of multiple tracers in a test, each with unique transport characteristics, including partitioning into organic liquids such as petroleum products, solvents (collectively known as non-aqueous phase liquids, or NAPL), and comparing the timing and mass recoveries of the various tracers at the sampling points (Geyer et al., 2007).

The most recent developments for direct measurement of groundwater flux or velocity in fractured media tend to be borehole methods. Advances in borehole geophysical methods are relevant to measuring groundwater velocity in fractured rocks advances but are outside the scope of this discussion. These borehole tools, many of which have been introduced in the earlier sections of this book, are particularly well suited for fractured rock characterization when they can isolate individual fractures, or closely spaced fracture sets, for testing. The IWPVP is an example of a tool that can be deployed to focus on specific, small scale features like these. Also, borehole dilution methods can focus on individual fractures, if used in conjunction with packers. The PFM was redesigned to become the FRPFM by infusing fluorescent tracers into an elastic, inflatable fabric that is held in place in a borehole between packers. The device is emplaced at depths corresponding to fracture locations and the flow from the fractures leaves a visible record, under ultra-violet light, where the tracers are leached from the fabric.

FLUTe™ liners offer some interesting opportunities for fractured rock aquifer characterization (Figure 30). The FLUTe liner is a sleeve that is installed in a well effectively sealing the borehole and preventing cross-depth flow (Keller et al., 2013). Pressure profiles gathered as the sleeve is installed can provide insight into the depths and transmissivities of fractures. If temperature sensing equipment is emplaced outside the liner prior to installation, depths where temperature variations occur indicate water flow and fracture locations. Either thermistors or optical cable (distributed temperature sensing, DTS) can, in principle, be used to gather such temperature profiles.