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1.6 Groundwater 11
Groundwater sources, too, have an intake or catchment area, but the catch, or recharge,
is by infiltration into soil openings rather than by runoff over its surface. The intake
area may be nearby or a considerable distance away, especially when flow is confined
within a water-bearing stratum or aquifer (from the Latin aqua, “water,” and ferre, “to
bear”) underlying an impervious stratum or aquiclude (from the Latin aqua, “water,”
and cludere, “to shut” or “to close out”).
The maximum yield of groundwater is directly proportional to the size of the intake
area and to the difference between precipitation and the sum of evapotranspiration and
storm runoff. Laterally, flow extends across the width of the aquifer; vertically, it is as
deep as the zone of open pores and passages in Earth’s crust and as shallow as the ground-
water table. When the water surface rises and falls with seasonal changes in recharge,
flow is unconfined or free, and the groundwater table slopes downward more or less par-
allel to the ground surface. Flow then moves at right angles to the water table contours. If
a porous stratum dips beneath an impervious layer, flow is confined as in a pipe dropping
below the hydraulic grade line. When this kind of aquifer is tapped, artesian water rises
from it under pressure, in some geological situations, even in free-flowing fountains. In
other geological formations, water is perched on a lens of impervious material above the
true groundwater table.
Groundwater reaches daylight through springs: (a) when the ground surface drops
sharply below the normal groundwater table (depression springs); (b) when a geological
obstruction impounds soil water behind it and forces it to the surface (contact springs); and
(c) when a fault in an impervious stratum lets artesian water escape from confinement (also
contact springs). A cutoff wall carried to bedrock will hold back subsurface as well as sur-
face flows behind an impounding dam and so put the full capacity of the catchment area to
use unless there is lateral leakage through the sides of the reservoir or around the abut-
ments of the dam.
The rate of flow through the substantially vertical cross-section of ground at right an-
gles to the direction of flow is not great. Because of the high resistance of the normally nar-
row pores of the soil, the water moves forward only slowly, traveling about as far in a year
as stream flow does in an hour. Natural rates of flow are seldom more than a few feet per
hour (or meters per hour); nor are they less than a few feet per day (or meters per day) in
aquifers delivering useful water supplies. However, if a well is sunk into the ground and
the level of water in it is lowered by pumping, water is discharged into the well not only
from the direction of natural flow but from all directions. That is why wells can be spaced
many times their own diameter apart and yet intercept most of the water once escaping
through the intervening space.

1.6.1 Springs
Springs are usually developed to capture the natural flow of an aquifer. In favorable
circumstances their yield can be increased by driving collecting pipes or galleries,
more or less horizontally, into the water-bearing formations that feed them. Pollution
generally originates close to the point of capture. It is prevented (a) by excluding shal-
low seepage waters through encircling the spring with a watertight chamber penetrat-
ing a safe distance into the aquifer and (b) by diverting surface runoff away from the
immediate vicinity. Some springs yield less than 1 gpm (3.78 L/min); a few yield more
than 50 MGD (189 MLD). Some are perennial; others are periodically or seasonally
intermittent.