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Effects of Pumping Wells
Groundwater is accessed for use either by pumping from wells drilled into the aquifer in the subsurface (Figure 33), or by developing natural springs, where the potentiometric surface intersects the land surface (Big Spring in Bellefonte, PA is one example of a relatively large spring that is used for municipal supply). Although springs are relatively inexpensive to develop, they are not always present, nor are the flow rates always sufficient to support demand. As a result, most groundwater extraction occurs by pumping at wells, or in many cases from "fields" of wells concentrated in a small area.
Source: Philippe Alès (Own work) CC-BY-SA-3.0, via Wikimedia Commons (top)
Source: USGS Nevada Water Science Center, photo by Lindsay R. Burt (bottom)
Cones of Depression: Pumping at a well, or at a wellfield, pulls water toward the well from all directions – in other words, it induces radial flow. In doing so, pumping causes a reduction in hydraulic head, known as drawdown, and generates a cone- or funnel-shaped depression (Figure 35). This perturbation to the potentiometric surface is called a cone of depression. The reduction in hydraulic head is, in fact, what drives groundwater flow to the well (in the down-gradient direction), as shown in the example from Long Island in Figure 36.
Source: USGS: Ground water in the Great Lakes Basin: the case of southeastern Wisconsin
Both the width and the depth of the cone of depression scale with the rate of pumping, the aquifer permeability and storativity, and the duration of pumping. In general, larger cones of depression result from larger pumping rates, higher permeability or lower storativity, and longer elapsed time. If the cones of depression from two separate pumping wells grow large enough that they overlap, it is known as well interference. When well interference occurs, the drawdown is additive. The result is that drawdown is accelerated when cones of depression from multiple extraction wells interact; this is generally not desirable, and is one important consideration in the design, permitting, and operation of wells.
Not only does the cone of depression draw water to the well, but if the pumping rate is large enough or pumping is sustained for a long time, it can reverse the natural hydraulic gradient hundreds of meters to several tens of km away from the well(s). In some cases, this may result in interception of groundwater that would normally feed a stream or river as baseflow, and even in interception of streamflow itself by inducing infiltration in the stream bed or banks (Figure 35B). In other cases, large cones of depression (up to a few miles wide!) associated with industrial or municipal well fields may reverse regional topographically-driven hydraulic gradients and lead to problems like saltwater intrusion (Figure 35B).