Injection and distribution of treatment reagents and re-injection of treated groundwater are two of the most challenging tasks undertaken by remediation professionals. When we began to recognize the limits on transverse dispersivity in aquifers, the prescribed reagent injection volumes increased dramatically, eliciting questions of feasibility from practitioners and concerns regarding lateral displacement of contaminated groundwater, by site owners, regulators and practitioners, alike. Figure 13.1 shows the volume-radius relationship for injected fl uids, based on values for the mobile porosity, θm. For the range of typical mobile porosities, the volume required to reach a prescribed radius of coverage is quite large. The technical challenge can be distilled to the question: How does an aquifer accommodate injected fl uid? The simple answer is: The aquifer volume expands. A more complete answer is developed for Section 13.1.