Have you ever submitted a piezometric map to a regulatory agency and received a comment like, "the piezometric map does not show a cone of depression capturing the plume." The fact of the matter is the plume may be captured by the pumping well(s); however, the "scale" that is being mapped may not show enough detail to draw the groundwater flow lines. Depending upon the hydraulic conductivity of the aquifer, the gradient may be so gentle (flat) it's not really measurable (some aquifers are less than a foot a mile [0.0002]). To illustrate this, let's take an extreme example. What would be the gradient of a well pumping in a lake or pond be at 1000 gpm? Probably not even measurable; however, all the water in the pond will eventually travel to the pump.
The amount of groundwater that needs to be captured for a given plume, per day (Q) can be calculated as follows: