1 Introduction

The word ‘velocity’ is a familiar one in the public lexicon. It brings to mind a baseball flying past a swinging bat, or a car hurtling down the highway. For most people, velocity is synonymous with ‘speed’. However, for those in the sciences, the word velocity contains two important components: speed, as alluded to above, and the direction of movement. For those who study groundwater, both of these quantities are conveniently available through the application of Darcy’s Law (discussed in depth in the following section), which relates flow rate to measurable physical characteristics of aquifers and dates back to 1856. However, though Darcy’s Law is widely used to estimate groundwater velocity, it is only one of several methods currently available (Figure 1).

As contaminants in groundwater have gained attention, and been found to depend — sometimes profoundly — on the details of aquifer structure for their fate and transport behaviors, opportunities to augment Darcy’s Law with alternative methods, such as those in Figure 1, have gained attention and value. The efforts to develop these technologies have varied both in approach and level of success. The technologies, discussed in later sections, that have shown promise are summarized in Table 1 and a graphical representation of the areas of strength, by classification from Figure 1, is given in Figure 2.

Table 1 – Summary of groundwater velocity measurement techniques discussed in the text. Within each of the method categories, the range of measurements can extend from a few centimeters per day to tens of meters per day, though this full range of performance is both tool specific and site specific, depending on conditions encountered. Cells colors are matched to Figure 2. [View a full-width version of  Table 1 on a separate web page.]

Figure 2 is provided only as a general indication of the methods’ areas of strength. For example, the greatest strengths of the single borehole methods arise from their ability to identify relatively small-scale geologic features (centimeters to meters in size) that affect contaminant transport in important ways, such as preferred flow channels. Such features can be continuous over large scales, making single borehole methods relevant over any scale of practical value to hydrogeological studies. However, the larger the scale the more measurement points are required to ensure an accurate characterization. This could become cost prohibitive in many cases, so single borehole methods are likely to be most used in investigations at relatively small spatial scales.

Of the single borehole methods, the dedicated instrument methods (probes not expected to be reclaimed from the borehole and reused elsewhere) are expected to be the most reliable because they are subject to fewer sources of bias, such as filter packs and well screens. Offsetting this advantage, is their dependence on good contact between the instrument and the aquifer sediments and this restricts their use to non-cohesive aquifers (with generally high components of sand or fine gravel) and carefully executed methods of emplacement. As geologic complexity increases, an aquifer may be more reliably accessed with a well and the in-well methods may be preferred.

At scales of tens or hundreds of meters, Darcy’s Law based approaches are expected to gain utility and cost-effectiveness. As with the single borehole methods, larger scale problems require a larger number of monitoring points, i.e., wells or piezometers, to ensure the variability of the aquifer is represented in the ultimate data set. Nevertheless, in regional scale studies, wells may be placed kilometers apart. Large inter-well spacings tend to reduce the apparent variability in flow, which can be appropriate if a large-scale picture of flow patterns is the goal of the work. If such averaging is of concern — at any scale — then the single borehole methods could provide data that are complimentary to the Darcy-based methods, especially for cases where small and intermediate scales of investigation are of interest.