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 characteristic be used to optimize the siting of 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 the plume. The capture zone can also exceed the depth of the plume, although a cross-sectional view of this is not depicted here. 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. However, multiple pumping wells oriented perpendicular to non-pumping, ground-water flow direction can also be used to split a plume. 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. Even when 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 increasing depths within the water table or confined system.

When a recovery well is pumped all water comes from the upgradient direction. As the pumping rate is increased water comes from greater distances to the pumping well. This water comes from greater lateral distances and greater depths within the water table or confined system, but still from an upgradient position relative to the pumping well. When a well is pumped at increasing rates, the ground water does not just come from greater lateral distances to the recovery well, but also from greater depths within the water table or confined system. It may be able to achieve the required depth of capture at a lower pumping rate than is necessary to achieve the required width of capture. Therefore, contaminant concentrations are decreasing at a faster rate in the vertical component of ground-water transport than in lateral component of ground-water transport. This water either comes from upgradient, but laterally 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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