Engineering geology
Groundwater flow


Water table (= groundwater surface) is the level in the rocks below which all voids are water-filled; it generally follows the surface topography, but with less relief, and meets the ground surface at lakes and most rivers. Vadose water drains under gravity within an aerated aquifer above the water table.
Phreatic water flows laterally under hydrostatic pressure beneath the water table; it is the resource for all high-yield wells; there is less at greater depths and pressures, and most rocks are dry at depths >3 km.
Capillary water rises above the water table by surface tension, by very little in gravels, by up to 10 m in clays.
Hydraulic gradient is slope of the water table, created by pressure gradient required to overcome frictional resistance and drive phreatic flow through aquifer rock.
Water table is steeper where permeability is low or flow is high; typical gradient is 1:100 in good aquifer.
Groundwater flow is in direction of water table slope, identified in wells that are not pumped.
Rivers normally have water table sloping towards them, with groundwater flow into them. Ephemeral rivers lie above water table, and leak into the aquifer.
Perched aquifer lies above the regional water table. Unconfined aquifer has vadose zone in upper part.
Confined aquifer has artesian water held beneath an overlying aquiclude, with a head of artesian pressure to drive the water above the aquifer, perhaps to rise to ground level; artesian water is common in sand/clay alluvial sequences, and in complex landslides.


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