The Critical Zone, depicted in Figure 2.1 , comprises the outermost layers of the continental crust that are strongly affected by processes in the atmosphere, hydrosphere, and biosphere. The upper boundary of the Critical Zone includes the land surface and its canopy of lakes, rivers, and vegetation, as well as its shorelines and shallow marine environments. On land, the shallower part is the vadose zone, in which unconsolidated Earth materials are intermixed with soil, air, and water. This porous medium is a host for many chemical transformations mediated by radiant energy, atmospheric deposition, and biological activity. Its storage capacity influences runoff and groundwater recharge, affecting both the flow and the quality of surface and subsurface waters. Within the vadose zone is the pedosphere, a collective term for soils at the land surface. The characteristic layering of the soil profile reflects the strong interaction of climate and biota in the upper portion and the accumulation of weathering, leaching, and decay products below. The water table marks the transition from the vadose zone to the deeper groundwater zone, where the pore space is filled by water. Like the vadose zone, the groundwater zone is a chemically reactive reservoir. The lower limit of the Critical Zone generally corresponds to the base of the groundwater zone, a diffuse boundary of variable depth extending a kilometer or more below the surface. The volume of water in the upper kilometer of the continental crust is an order of magnitude larger than the combined volume of water in all rivers and lakes. ¹ The Critical Zone is perhaps the most heterogeneous and complex region of the Earth and the only region of the solid Earth readily accessible to direct observation.