Water is the dominant wetting fl uid for aquifer systems and groundwater fl uid fl ow calculations are typically structured on that assumption. We encounter numerous fl uids other than water in remediation engineering applications and it can be quite diffi cult to describe or predict their movement through aquifers. The simplest fl uids to work with are aqueous solutions with fl uid properties similar to groundwater. Tracer solutions, for example, often include salts that increase the fl uid density, which can be a factor in their movement through aquifers. Oxidant solutions often have very high dissolved solids content and their densities and viscosity and the temperature increases associated with exothermic reactions all contribute to signifi cant modifi cations to their fl uid properties and temperature and density infl uences on hydraulic conductivity were examined in Chapter 3. When aquifers receive fl uids that are immiscible with water, the prediction of fl uid fl ows becomes much more challenging. These fl uids may form two-or three-phase systems, with groundwater, a gas phase and a non-aqueous liquid phase all occupying the pore spaces of an aquifer. Figure 4.1 illustrates the production of oxygen-dominated gas mixture associated with the application of Fenton’s Reagent in aquifer cleanup. The disproportionation of hydrogen peroxide generates oxygen, the oxidation process generates carbon dioxide and the heat generated by the reactions vaporizes organic mass, all of which contribute to the very high rate of gas production

seen in Figure 4.1. These gases displace groundwater and substantially decrease the water-phase (hydraulic) conductivity of the aquifer.