Page 410 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch11_357-397.qxd 8/2/10 9:01 PM Page 370

370 Chapter 11 Hydrology: Rainfall and Runoff

Soil water is near enough to the
Under- surface to be reached by the roots Ground surface
saturated of common plants. Some soil water
zone remains after plants begin to wilt.
Soil
or Stored or pellicular* water adheres
to soil particles and is not moved
Zone by gravity.
of
aeration Gravity or vadose** water moves
down by gravity throughout zone.

Capillary water occurs only in the
Suspended Capillary fringe
capillary fringe at bottom of the
water
zone of aeration.
Water table
Free water occurs below the water
table. Movement controlled by the Free water
slope of the water table.
Saturated
zone Confined or artesian water occurs
beneath a confining stratum.
Ground Moves laterally as water in a Aquiclude or
water pressure conduit. confining bed
Aquifer
or Fixed groundwater occurs in
Confined water
subcapillary openings of clays,
Phreatic † silts, etc. Not moved by gravity.
water Confining bed
‡
Connate water entrapped in rocks Fixed groundwater
at the time of their deposition. Connate water
* Latin pellis skin ** Latin vadere going
† Greek phreas a well. ‡ Latin connatus born together.
Figure 11.5 Occurrence and Distribution of Subsurface Water

surface. Pollution spreads out along the water table and is lifted into the fringe. There it is
trapped and destroyed in the course of time. Hydraulically, an aquifer dipping beneath an
impervious geologic stratum has a piezometric surface, not a groundwater table.
How much rain filters far enough into the ground to become groundwater is quite
uncertain. Governing factors include the following:
1. Hydraulic permeability. Permeability, not merely pore space, determines the rate
of infiltration of rainfall and its passage to the groundwater table. In winter, the
permeability is usually reduced by freezing.
2. Turbidity. Suspended matter picked up by erosion of tight soils clogs the pores of
open soils.
3. Rainfall patterns and soil wetness. Light rainfalls have time to filter into the
ground, heavy rainfalls do not. Wet soils are soon saturated; dry soils store water in
surface depressions and their own pores. Some stored water may reach the
groundwater table eventually. Heavy rains compact soil, and prolonged rains
cause it to swell. Both reduce surface openings. Air displacement from soils
opposes filtration; sun cracks and biological channels speed it up.