Advantages Of Density-Driven Convection
Over Pump & Treat

In-well stripping is frequently compared to pump-and-treat systems because both involve pumping the contaminated groundwater through a treatment step and because pump-and-treat systems often use air stripping-based treatment. The two technologies are actually more different than similar.

In-situ versus ex-situ.

DDC is an in-situ treatment process that does not involve pumping the groundwater to the surface. Pump-and-treat is an ex-situ treatment process.

Water Discharge Problems.

Because DDC does not involve pumping groundwater to the surface, there is no treated water to release. Pump-and-treat systems create a large volume of water that must be released, typically through an NPDES permit or to a municipal treatment plant. Such releases must be monitored, with periodic reporting. In some settings, it would not be possible to release the water, often because the local streams or the treatment plant could not handle the increased flows. A polishing step, often with aqueous phase carbon, is sometimes required after stripping to meet the discharge requirements. Release is especially problematic if there are other contaminants or natural components of the groundwater (e.g., salt, radioactive constituents) that could not be released, either to streams or to a treatment plant. It is often desirable to remove volatile contaminants through in-well stripping and leave the groundwater with all of its other constituents in the aquifer.

Point of Stripping.

DDC accomplishes the stripping within the well and then releases the stripped water to the aquifer to recirculate and bring more contaminants back to the treatment well.

Pump-and-treat systems accomplish the stripping step (or other treatment step) outside the well, usually in an aboveground treatment system specially designed for the site. For volatile compounds, the initial treatment step often is air stripping in a stripping tower or tray stripper. To avoid damage during the winter, the above-ground treatment system must be protected from freezing.

Pumping and groundwater circulation.

DDC pumps water through the well, drawing water from one level of the aquifer, treating it, and releasing it to another level of the same aquifer. Because the water moves vertically as well as horizontally through the aquifer, under the influence of strong vertical gradients induced by the draw down and mounding created by the pumping and release, the water moves outward from the well and then back in a torroidal circulation pattern. The torroidal treatment zone has a radius of influence that is similar to pump-and-treat capture zones. The radius of influence has to be determined for each site, but often is 2 to 5 times the distance between the inlet screen and the outlet screen. For a 50-foot depth of contamination, well spacings of 300 to 500 feet are theoretically possible, though more conservative designs (200+ feet) are typically used.

Pump-and-treat systems remove the water from the aquifer by drawing it into a pumped well and conveying it to the surface. Extraction preferentially removes water 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. 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.

More vigorous remediation.

DDC circulates 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 through a recharge screen, in-well stripping induces vertical gradients and flows. The vertical gradients induce flows through the more stagnant parts of the aquifer and flush contaminants to higher flow zones. By pushing water through the more stagnant zones of the aquifer, DDC does not rely on diffusion to move trapped contaminants. Convection moves contaminants faster than diffusion, often orders of magnitude faster.

Pump-and-treat approaches 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 enormous quantities of water over many years. This problem is exacerbated for contaminated lower conductivity zones that are relatively thick.

Faster.

Pump-and-treat and DDC operate on different principles: pump-and-treat passively draws to its extraction wells whatever water will flow most easily under the horizontal gradients induced by the pumping, and relies on diffusion to remove contaminants from lower permeability zones; while DDC, through strong vertical gradients, vigorously circulates and treats all of the water within the treatment zone. Because DDC operates on this different principle, it can be relatively quicker in remediating a site. At a site in Wichita, Kansas, cleanup to the remediation goal was achieved in most of the DDC wells in less than one year.

Cheaper.

DDC systems are built using standard, off-the-shelf well components. Capital expenditures for above-ground equipment are slightly less than pump-and-treat, since no stripping tower is necessary. In-well stripping avoids the need for an NPDES permit or discharge to a treatment plant and the attendant monitoring and reporting costs.

During its life cycle, any remediation system requires oversight, monitoring, maintenance, and periodic reporting. With life cycles extending to decades, pump-and-treat approaches incur large costs for continuing these efforts throughout a remediation.

Thus, Density-Driven Convection technology is less expensive to install, cheaper to operate each month, and completes the remediation in a fraction of the time. All of this adds up to much lower costs than pump-and-treat.