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DECREASING CONCENTRATIONS WITH INCREASED PUMPAGE
 
By the earthDr!
 
When a recovery well is initially pumped, one of three things can be observed with respect to water quality over time or changing pumping rate.
The concentration of contamination in the pumped ground water will either increase, decrease, or stay relatively the same. In the first example figure on this page, the contaminant concentration decreases at increasing rates of ground-water pumpage. In this example figure, take note that contaminant concentrations drop off at an increasing rate when the pumping rate increases above 10 gallons per minute (gpm) and the rate of contaminant decrease slows above approximately 20 gpm. Overall the contaminant concentration is decreasing with increasing rates of ground-water pumpage. But how is this possible? And, what is the significance of decreased contaminant concentrations with increasing ground-water pumping rates? Is this relationship a negative for ground-water remediation? Can this characterization be used to in the optimization of siting pumping wells and in the adjustment of ground-water pumping rates?

Assume, that a single pumping well has been located so that it is screened along the long-axis of the plume where higher contaminant concentrations generally can be found. As the ground-water pumping rate is increased, as indicated by this next figure, the water that is produced by any of these depicted wells, while still originating from the upgradient direction, is coming from greater and greater lateral or vertical distances from the pumping well. Since these wells are constructed along the long-axis of the
plume, where the higher contaminant concentrations can typically be found, increased pumpage must necessarily draw less contaminated ground water to the recovery well as the transverse-lateral extent of the capture zone starts to approach the plume width and depth. It is not only the width of capture that increases, but the depth of capture increases, also. If the contaminant concentration decreases with depth, the contaminant concentration in the pumped ground water decreases as the pumping rate is increased. This second figure illustrates that the capture zone can extend beyond the width of capture, as can the depth of capture. Obviously, the contaminant concentration is further diluted when the recovery well is pumped at a rate that can produce a capture zone more extensive than the transverse-lateral extent of the plume.

In examples on preceding webpages of this website, plume splitting was introduced as well as partial capture of the plume. Plume splitting can be achieved by the pumping of a single well or by the pumping of multiple wells oriented perpendicular to non-pumping, ground-water flow direction. Provided that there is water in storage of the formation and the permeability is sufficient, increasing widths and depths of capture can be achieved by simply increasing the pumping rate of a single recovery well. Although, a well can yield sufficient water to achieve the needed width and depth of capture, often times the contaminant concentrations are decreased due to diluting the plume with clean ground-water originating from upgradient locations both laterally and at increasing depths within the water table or confined system. Other times, a well does not have the capacity to yield a sufficient volume of water to achieve the necessary width or depth of capture, but increased rates of pumpage still result in decreasing contaminant concentrations. If either, yield or decreasing contaminant concentrations are a limitation, then it is often necessary to install multiple recovery wells oriented perpendicular to non-pumping, ground-water flow conditions.
 
 
 
 
 
 
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