The Groundwater System

Groundwater is best thought of as a system of many different parts, some of which act as conduits and reservoirs, and others that serve as offramps and onramps into the groundwater system.

Recharge areas are where water enters the groundwater system, and discharge areas are where water leaves the groundwater system. In humid climates recharge areas encompass nearly the land's entire surface (except for streams and floodplains), whereas in desert climates recharge areas consist mostly of the mountains and alluvial fans. Discharge areas consist mainly of streams and lakes.

The level of the water table changes with different amounts of precipitation. In humid regions it reflects the topographic variation, whereas in dry times or locations it tends to flatten out to the level of the streams and lakes. Water flows faster when the slope is greatest, so groundwater flows faster during wet times. The fastest rate of groundwater flow observed in the united states is 800 feet per year (250 m/yr).

Aquifers include any body of permeable rock or regolith saturated with water through which groundwater moves. Gravel and sandstone make good aquifers, as do fractured rock bodies. Clay is so impermeable that it makes bad aquifers, and typically forms aquicludes that stop the movement of water.

springs are places where groundwater flows out at the ground surface. They can form where the ground surface intersects the water table or at a vertical or horizontal change in permeability, such as where water in gravels on a hillslope overlays a clay unit and the water flows out on the hill along the gravel/clay boundary.

Water wells fill with water simply because they intersect the water table. The rocks below the surface are not always homogeneous, however, which can result in a complex type of water table known as a perched water table. Perched water tables result from impermeable bodies in the subsurface that create bodies of water at elevations higher than the main water table.

BEDROCK HILL

ALLUVIAL HILL

Spring on top of impermeable layer

Perched water table

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Spring on top of impermeable layer

Perched water table

Creaming Curve

through fractures

through fractures

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Impermeable layer

Groundwater flow lines

Permeable layer

G Infobase Publishing

G Infobase Publishing

Schematic diagram of the groundwater system. Water enters the system on hillslopes and emanates lower on hills as springs and in effluent streams.

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