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On-Site Wastewater Treatment and Disposal Systems - Forced ...

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FIGURE 7-17<br />

UNDERDRAINS USED TO LOWER WATER TABLE<br />

le- Above High<br />

@ Water Table<br />

Underdrains<br />

a. Subsurface Drainage Problems<br />

Y Drainage Pipe /<br />

There is an unlimited variety of subsurface drainage problems but the<br />

most common ones can be grouped into four general types (26). These<br />

are: (1) free water tables, (2) water tables over artesian aquifers,<br />

(3) perched water tables, <strong>and</strong> (4) lateral groundwater flow problems.<br />

Free water tables typically are large, slow moving bodies of water fed<br />

by surface waters, precipitation, <strong>and</strong> subsurface percolation from other<br />

areas. In the lower elevations of the drainage basin, the groundwater<br />

is discharged into streams, on the ground surface in low areas, or by<br />

escape into other aquifers. The groundwater elevation fluctuates<br />

seasonally. The slope of a free water table surface is usually quite<br />

gentle.<br />

262

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