5 Summary

The study and characterization of contaminated sites, usually with the aim of deciding upon, designing, and later implementing remedial actions — or settling on the natural attenuation mechanisms to achieve cleanup — depends on a good understanding of the mechanisms that cause pollutant transport and attenuation. This has led to much emphasis being placed on processes involving microbiological activity and abiotic chemical reactions in the subsurface, including those that merely retard contaminant movement without transforming the chemicals. However, none of these factors is sufficient to predict contaminant spreading in the ground unless they are combined with a realistic understanding of the site-specific flow system. Typically, Darcy’s Law has been the basis for describing flow systems. The approach is to measure water levels in at least three wells, and from this determine a hydraulic gradient and flow direction. Hydraulic conductivity is then estimated either by laboratory or field techniques — and usually with a notable uncertainty attached to it — and a Darcy flux is calculated. Seepage velocities are subsequently estimated with the additional consideration of the effective porosity. Furthermore, the spacing between wells in these types of investigations often leads to spatial averaging of the flow variations in the subsurface. This methodology has proven effective enough over several decades that alternative methodologies have been used only sparingly. Our modern understanding of contaminant hydrogeology has revealed the ubiquity and importance of subsurface heterogeneity. There is gaining appreciation that aquifers are challenging to characterize with conventional methods that are usually applied at scales of tens of meters or greater. In a field that depends on the identification and treatment of pollutants in concentrations as low as parts per trillion, a very detailed knowledge of the prevailing flow system is highly advantageous if cleanup efforts are to have a chance of succeeding. Perfect knowledge of biodegradation rates, mineral reactions, and sorption are all insufficient to predict contaminant fate and transport, mixing rates, mass discharges, or consumption of aquifer buffering capacities unless groundwater flow velocities are also well known.

The technologies developed to compliment conventional Darcy-based studies tend to rely on tracers to infer the nature of flow. These may be implemented using multiple wells, as in inter-well tracer tests, or single wells, such as is the case with point dilution methods, passive flux meters, heat pulse flowmeters, passive flux meters, colloidal borescopes, or in-well point velocity probes. A subset of technologies advocate deployment of instruments in direct contact with the aquifer material, within dedicated boreholes, to avoid complications and flow distortions associated with filter packs and well screens. The ‘direct contact’ requirement of these instruments currently limits their use to aquifers that will collapse against them, i.e., those comprising unconsolidated, non-cohesive porous media. Two examples of such technologies are the In Situ Permeable Flow Sensor (ISPFS) and the Point Velocity Probe (PVP).

Many of the tools under development for small scale velocity measurements in porous media may be adaptable for use in fractured media as well. Fractured media pose a variety of special challenges for aquifer characterization studies. These include the identification and characterization of important conduits for flow, and the determination of flow directions that can deviate substantially from those predicted by water level maps. Traditionally, the Darcy approach is reliable if an EPM assumption is justified. Otherwise, tracers are effective to gain large scale pictures of where water flows and the lengths of transit times. Tracers can also be effective in smaller scale studies, but here they compete with single borehole tests that may be less expensive and arguably more controlled. The single borehole techniques that can be adapted to measure small intervals in a borehole are the ones best positioned to shed light on transport in fractured media. These technologies range from instruments that measure flow in single fractures to those that can characterize an entire borehole in a single operation.

Contaminant hydrogeology is an applied field that depends on many sciences and coaxes them to “play together” nicely. The overarching rules that ensure these playmates are harmonious are those that govern where, and how fast, the groundwater moves. The future promises to provide us with tools that will make observations of flow systems in time and space more detailed and affordable than ever before. This prognosis bodes well for the future of hydrogeology and our ongoing endeavors to reclaim contaminated aquifers.