6.1  Unconfined Aquifers

William W. Woessner; Eileen P. Poeter

6.1 Unconfined Aquifers

An unconfined aquifer, or water-table aquifer, is an aquifer with the water table as the upper boundary. The fluid pressure of water at the water table is equal to atmospheric pressure and the hydraulic head at the water table is equal to the elevation of the water table. The triangle in Figure 44 indicates the elevation of the water table.

**Figure 44 –** Schematic of an unconfined aquifer. The upper surface is the water table (dashed black line) and the bottom boundary is typically a lower permeability unit (black bar). The water at the water table is at atmospheric pressure, h_p = 0, so the elevation head, z, represents the hydraulic head. Water levels in wells penetrating to depths just below the water table are shown. Recharge occurs to the water table from infiltration at the land surface (blue dashed arrows). The long solid blue arrows show general groundwater flow directions.

The term unconfined indicates there is free communication between the land surface and the water of the aquifer (Figure 44). These aquifers receive recharge from rainfall and melting snow; they are not overlain by a lower permeability unit that confines the movement of water within the aquifer. Water moves through the vadose zone and crosses the water table to become groundwater. The elevation of the water table changes in response to smaller or larger amounts of recharge, changes in rates of discharge, and the introduction or extraction of water (e.g., infiltration of surface irrigation water and pumping). When a borehole is advanced into the earth, the first groundwater encountered will be part of an unconfined aquifer. The water table may be many 10s to 100s of meters below the ground surface, especially in upland areas and arid regions. In lower areas, the water table will be near the surface where springs, rivers, wetlands and lakes occur.

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