FIGURE 2.1 The hydrologic cycle as applied to basins.

Recharge occurs whenever precipitation or surface water infiltrates downward through the soil to the water table. Recharge can also result from subsurface lateral flows that reach the aquifer. Recharge may occur naturally, and natural recharge can be augmented by artificial recharges (as outlined in a recent study, National Research Council, 1994a). Surface water is usually viewed as a renewable resource, since it derives from rainfall and snowmelt, which recur periodically. Natural ground water supplies may be either renewable or nonrenewable, depending upon whether recharge occurs at rates similar to those of withdrawal.

The rate of recharge may be influenced to a large extent by whether the aquifer is confined or unconfined. Aquifers may have upper and lower boundaries, termed "confining layers." These boundaries normally comprise layers of unconsolidated material or rock that have a much lower permeability than the materially lying immediately above or below. Confined aquifers have a confining layer both above and below, while an unconfined aquifer has no confining layer on top. Since unconfined aquifers tend to be found uppermost in a ground water system, they are frequently called surficial aquifers. Unconfined aquifers are the first to receive water infiltrating from the surface. This means that the depth to water or the water table frequently fluctuates in such aquifers. It also means that such aquifers tend to contain higher concentrations of dissolved materials of anthropogenic origin than do lowerlying, confined aquifers. Indeed, water contained in many shallow, unconfined aquifers is often not used for drinking because of contamination.

Confined aquifers are protected to some degree by the presence of a confining, low-permeability zone between the surface (and the source of recharge water) and the ground water itself. While an unconfined aquifer is characterized by a water table or the depth to ground water, a confined aquifer is characterized by a piezometric, or potentiometric, surface, which results because the height of the upper surface of the aquifer is constrained by the confining layer. The potentiometric surface represents the height of rise of the water due to hydrostatic pressure when the constraint of the confining layer is removed, as illustrated in Figure 2.2.