4 The importance of knowing groundwater velocity

The spread of dissolved pollutants from a source area by groundwater movement typically produces a contaminated zone referred to as a plume. The greater the groundwater velocity, the faster the plume grows. Plumes may be generated in many different shapes and sizes, but they share the common attribute of growing primarily in the net downstream direction of groundwater flow. In flow systems with a single predominant flow direction, and an aquifer comprising sediments lacking geologic complexity, such as preferred or channelized flow, plumes will develop into long, thin zones, resembling a sausage shape. It is interesting to note that this notion of plumes is contrary to earlier thinking, where, in the 1980’s, it was common to see conceptual models assuming substantial lateral spreading of dissolved solutes due to transverse (horizontal or lateral) dispersion. Since then, high resolution groundwater sampling and tracer studies have led to the ‘weak dispersion’ view of plumes. Site characterization efforts are commonly designed to identify the boundaries of a plume. However, the number of monitor wells (hence expense) required to outline the shape of a long, thin plume with sufficient fidelity to confidently document plume growth or attenuation can be impractical.

To overcome this difficulty, attempts may be made to instrument the plume along its centerline as shown in Figure 14 (McNab and Dooher, 1998). Finding a plume centerline is not straightforward. A possible approach is to first characterize the plume with high-resolution depth-discrete groundwater sampling using one-time sample collection devices (e.g., hydropunch™ methods or Waterloo APS™ Vertical Aquifer Profiling Technology, and others). By aligning closely spaced depth-discrete sampling locations aligned in transects perpendicular to the plume axis, the plume can be characterized in three dimensions, and the location of the centerline defined with reasonable accuracy. The advantage of this approach is that, with relatively few monitoring wells, data can be gathered that establish the greatest extent of the plume in the predominant direction of growth, and the resulting profile of concentrations can be used to infer plume spreading rates and attenuation rates. The advantage of this approach is also its weakness; the use of few wells translates to low confidence that the actual centerline is being monitored with clear implications for the reliability of the subsequent interpretations.

With the above conceptualization of plumes in mind, the average hydrogeologist would likely advocate the use of groundwater velocity measurements first and foremost to predict the earliest arrival times of contaminants at various receptors (e.g., water wells and surface water bodies). In today’s climate of risk-driven corrective action for groundwater contamination, that objective is very reasonable but incomplete. Since real-world plumes exhibit more complexity than the simple case shown in Figure 14, the case can be made that without a proper — i.e., more detailed — understanding of groundwater velocity, or flux, at a site (note that Darcy flux measurements provide the same essential information for the examples presented below and may be substituted for seepage velocity in many situations), very little else can be known with much certainty.

To illustrate the fundamental importance of velocity, consider the following characteristics of contaminants and aquifers that are commonly estimated without a detailed consideration of groundwater velocity: transformation rate constants (parameters from which the apparent rates of chemical reactions can be calculated), oxidation or reduction capacity (the capacity of the dissolved chemicals to gain or lose an electron to one another or to aquifer solids), groundwater mixing, residence times, and contaminant mass flux across a boundary.