Frequently Asked Questions

1. Can recirculation be demonstrated?

This question often comes up because density-driven convection (DDC) technology relies on multiple treatment steps (multiple passes through the well) to achieve a high degree of treatment.  For example, at the 10th Street Site in Columbus, Nebraska, the theoretically calculated recirculation ratio was 3.10.  That is, a typical water molecule entering the treatment zone of the well from upgradient would pass through the well and the treatment zone 3.10 times (on average) before escaping downgradient. 

Since the stripping efficiency in the well was 75% (measured twice, at 74.6% and 75.3%) recirculation was critical to achieve a higher overall removal efficiency.  Monitoring results from a well-designed system of monitoring wells indicated that the overall removal efficiency actually achieved across the treatment zone (upgradient concentrations of TCE versus downgradient concentrations) was 90.1%.  The only way to get an overall reduction greater than the single-pass stripping efficiency is for the water to pass through the well, on average, more than once.  These reduction rates indicate an average of 3.02 trips (versus the 3.10 predicted from theoretical calculations) through the DDC well for each water molecule entering the treatment zone.  Thus, a typical water molecule would enter the treatment zone and pass through the DDC well, and then return to the DDC well twice before escaping downgradient.  These data demonstrate recirculation in the 10th Street Site DDC system.

For most projects, demonstration of recirculation is hampered by lack of a sufficient monitoring system.  Only pilot studies have sufficient wells in a well-designed configuration to allow demonstration of recirculation.  has performed three pilot studies with adequate monitoring systems.  The three pilot studies were at the Former Naval Ammunition Depot (NAD) in Hastings, Nebraska, at the 57th & N. Broadway  Site in Wichita, Kansas, and at the 10th Street site mentioned above.  In all three cases, recirculation was demonstrated.  At the NAD and the 57th & N. Broadway sites, dye tracer studies were used to demonstrate recirculation.

DDC, Blowerless Air Sparging, and Blowerless In-Well Stripping technologies utilize the several passes the water makes through a recirculating well to increase overall effectiveness.

n-Situ Groundwater Remediation, Floating Product Removal, and Dissolved Oxygen Enhancement technologies do not rely on recirculation to achieve adequate removal of dissolved contaminants.  For ISGR and FPR systems using granular activated carbon treatment, for example, removal of the contaminants typically is complete (to non-detect levels) in a single pass through the well.  DOE technology generally adds sufficient oxygen to the water on a single pass to support complete mineralization of the contaminants (up to 3.5 ppm hydrocarbons) to CO2 and H2O, but can saturate the water with oxygen on each of the several passes through the well.

2. Since a recirculating well establishes a closed-loop zone of circulation around the well, how can water approaching the treatment zone from upgradient enter the well? Why doesn’t the water from upgradient simply go around the recirculating well’s zone of circulation?

This question comes up because flowlines do not cross, and it would seem that the three-dimensional pattern of flow lines established around a recirculation well would effectively bar the flow of “new” water from upgradient entering the treatment zone of a recirculation well.

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Figure 1 - Vertical Cross-Section of Treatment Zone - Click to enlarge

The water exiting the upper screen flows outward in all directions from the recirculating well, including upgradient.  The resultant obstruction to flow at the upper level of the aquifer can be seen in the plot on the left.  The water approaching and entering the treatment zone from upgradient cannot flow through the recirculating zone; but, it can flow under it, as it is pulled toward the well by the low pressure induced at the inlet screen.  This is the manner in which the “new” water enters the treatment zone, is mixed with other (recirculating) water in the well, and begins its recirculation.

3. Is rebound a problem with recirculating well technologies?

Rebound happens when a remediation technology fails to remove or degrade all of the contamination, as when a reagent such as potassium permanganate, or a biological agent such as a particular strain of bacterium, or a nutrient such as molasses, or a physical removal method such as air sparging, fails to reach all parts of an aquifer.  Residual contamination then diffuses out of un-remediated zones once the remediation technology is turned off or ceases to function.  Any technology that leaves residual contamination in the aquifer is likely to suffer rebound.  Also, any site in which there are continuing sources of contaminants, either in source areas upgradient, in the vadose zone, in low-flow zones within the aquifer, or in LNAPL or DNAPL layers or zones within the aquifer, will experience rising concentrations once a remediation system is turned off or is no longer functioning (e.g., exhaustion of a nutrient).

At most sites where we have installed recirculating well systems, it was not the role of the systems to deal with concentrated source areas.  Rather, our projects have tended to be in downgradient portions of the plume, dealing exclusively with dissolved contamination.  In this “gate-keeper” function, a remediation system can only remediate the contamination that reaches it.  As long as contaminants continue to arrive at the recirculating well system from upgradient, the system will still be needed and will have to be operated continuously. If the system is shut off with contaminants still moving into the area from upgradient, concentrations are going to rise.

In cases where there is residual contamination in the vadose zone that is still leaching downward into the groundwater, no amount of remediation of the groundwater (removal of dissolved constituents from the groundwater) is going to remove the source in the vadose zone.  Until the source is addressed, by soil vapor extraction, excavation, soil heating, or other means, the continuing release of contaminants to the groundwater will necessitate continuing operation of the recirculating well system.

In cases where there is LNAPL or DNAPL contamination, groundwater remediation will have to continue until the LNAPL or DNAPL is removed.  Floating Product Removal (FPR) technology can be used to remove floating product and other floating contaminants, while treating the groundwater to non-detect levels.  However, if an FPR system is shut off before all the floating product is removed, rebound is quite likely.

Many sites have contaminants that are trapped in silt layers, clay layers, and other low-flow regimes within the aquifer.  The contaminants entered those zones perhaps years before, when contamination levels were higher, and are slowly diffusing out of those zones into the higher flow regions that are now at lower concentrations.  One of the difficulties with pump-and-treat systems and air-sparging systems is that they tend to treat water only from those layers and zones of the aquifer that yield water most readily, the higher conductivity zones.  Yet the higher conductivity zones are often the least contaminated.  Removing water from the lower-flow, more stagnant zones of the aquifer would be preferable, once the permeable pathways have been cleaned, but there is no way to redirect flows with pump-and-treat or air sparging technologies. 

Pump-and-treat systems, in effect, preferentially remove water from the least contaminated zones of an aquifer and continue to do so even after those zones have been cleaned.  These systems rely primarily on diffusion to remove contaminants from the more stagnant zones in the aquifer.  While diffusion eventually will move the contaminants to the higher conductivity zones, where pumping can remove them, diffusion processes are very slow when compared to convective flow processes. While waiting on diffusion, it is necessary to pump and treat and release enormous quantities of water, at very low contamination levels, over many years.  This problem is exacerbated for contaminated slit and clay layers that are relatively thick.

Recirculating well systems circulate water through all zones of the aquifer by inducing vertical gradients and flows over a large treatment area.  By removing water from one level of the aquifer and releasing it to another level of the same aquifer, recirculating wells induce vertical gradients.  The vertical gradients induce vertical flows through the more stagnant parts of the aquifer and flush contaminants into higher flow zones.  By pushing water through the more stagnant zones of the aquifer, recirculating wells do not rely on diffusion alone to move trapped contaminants.  Convection moves contaminants faster than diffusion, often orders of magnitude faster.

Because recirculating well technologies move all of the water in an aquifer, they can remove contaminants that pump-and-treat and air sparging systems must trust to diffusion to move.  In this way, recirculating well technologies can more rapidly remove these trapped contaminants that will, if not removed, lead to rebound when the remediation is ended.

4. How was the projected capture width or radius of influence calculated for my site? Has this theory ever been demonstrated in the field?

The calculations are performed according to a method developed several years ago, and that is commonly used with recirculating wells.  The calculations require about three hours to perform with a pencil and calculator.

At the 10^th Street Site in Columbus, Nebraska, a transducer study was performed to measure the actual capture width of the pilot well, so it could be compared to the predicted capture width. The theoretical calculations indicated an expected capture width of 192 feet.  The transducer study results were used to determine the gradient imposed by the pumping of the recirculating well.  The imposed gradient and the natural gradient, were entered into a proprietary particle-tracking model developed by to permit modeling using actual aquifer pressures generated by a recirculating well system.  The results of the particle-tracking model are presented in the figure below.  The result was that the capture width of the recirculating well, was 216 feet.

Figure 2 - Particle Tracking Model Result - Click to enlarge