Introduction to Basic Ground-Water Flow
By the earthDr!
Changing the Ground-Water Flow Direction through the Pumping of Ground Water
view of the water table, under pumping conditions, is depicted in the figure
below, but without equipotential lines or equipotential surfaces. Since
equipotential lines (see the second figure to refresh your recollection)
are not included, it is necessary to include ground-water flowpaths to illustrate
ground-water flow direction. The black arrows indicate ground-water flow
that follow either the contour of the surface of the water table or the
vertical gradient. Under pumping conditions, normal ground-water flowpath
directions change in response to changes in the water table gradient induced
by the pumpage of ground water from the well. When ground water is pumped
from a well, the water levels decline the most in those soils closest to
the well. These unequal changes in water levels cause unequal changes in
the surface of the water table resulting in a gradient or a bowl-like depression
of the water table, centered about and radiating out from the pumping well
as illustrated in the figure. Remember that the water table is a dynamic
system. Even without the operation of a pumping well, the ground-water is
flowing from a higher total head to a lower total head as depicted in the
second figure. Whenever a well is being pumped, these unequal declines in
water level are superimposed upon the natural (non-pumping) gradient of
the water table.
Under non-pumping conditions, all the ground water that flows past the recovery well comes from the upgradient direction where the water levels are higher. More ground-water flows to the recovery well as pumping is increased. Where does the water originate that supplies the additional water pumped from this well? Note that this figure indicates that all the water pumped from the recovery well comes from the upgradient direction. Typically, the way that more water flows to the recovery well is not by influencing the additional water to flow to the well from the downgradient direction. The additional water pumped by the well still comes from the upgradient direction as illustrated by the figure. However, now the water comes from a bigger upgradient swath. The greater the volume of water that is pumped from the well, the bigger is the area of this swath. (The origin of the ground-water flowpaths that contributes all the water to the pumping well comes from an area called the line source, which typically is in the upgradient direction. The line source will be defined later and its practical significance explained.)
The blue line depicted in this figure depicts the boundary of the capture zone. All free water contained within the capture zone will ultimately be captured by the recovery or supply well. For all practical purposes, no ground-water from outside the capture zone is contributed to the pumping well. The ground- water flowpaths immediately outside the zone of capture while not captured by the pumping well will be diverted (influenced) to follow alongside the outside perimeter of the capture zone. These flowpaths can be used to advantage to exhaust contaminants by flushing the soils with clean water. Ground-water flowpaths are influenced to a lesser extent as the distance to the recovery/supply well increases.
Remember that this first figure is a sectional view of the subsurface and the capture zone. A sectional view of the water table makes it possible to display the boundary of the capture zone; the ground-water flowpaths entering the well; and, the ground-water flowpaths being influenced to change direction, but not being captured by the well. However, the displayed capture zone only shows half of the capture zone. The capture zone actually would encompass this volume plus the mirroring of this volume to the left. The capture zone is somewhat parabolic-like in shape.