Density Driven Convection

In-Well Stripping With Re-Circulating Wells

Density-Driven Convection (DDC) is a simple patented technology that removes volatile and many semi-volatile contaminants from groundwater within the well.  It does not pump the water to the surface or otherwise remove it from the aquifer.  The groundwater is pumped more vigorously than in a pump-and-treat approach, circulated several times through a treatment zone established around the treatment well, and stripped of contaminants during each of several passes through the well.

While there are numerous possible configurations, each optimized for a different set of geologic conditions, the most basic approach is also the most commonly used. Figure 1 shows the basic configuration.

• The well penetrates to the maximum depth of the contamination, with a normal inlet screen across the lower several feet of the contaminated thickness.

• The well incorporates a second screen, an outlet screen, usually at or near the normal water table.

• The functions of the two screens can be reversed, with the upper screen serving as the inlet screen and the lower screen serving as the outlet screen.

• An air line is located in the center of the well and extends to some distance below the normal water table.  The depth of the air line below the water table is called the submergence.

• The air line is connected, generally via underground piping, to a pressure blower.

• Air is forced down the air line.  The water is pumped by air-lift pumping to a level above the water table.  The air-lift pumping both pumps the water through the well and strips the volatile and semi-volatile contaminants from the water.

• The treated (stripped) water exits through the upper screen and flows back into the aquifer through the sand pack and the aquifer materials.

• At the water table, a mound is formed, resulting in higher head values near the well.

• The treated water flows outward from the well and downward under the influence of the vertical gradients created by the extraction process at the bottom of the well and the mounding at the water table.  Because aquifer materials are typically anisotropic, allowing horizontal flows more readily than vertical flows, the flows tend to be more outward than downward.

• A torroidal-shaped treatment zone is created that typically returns the majority of the treated water to the lower screen.  The shape and size of the treatment zone are largely determined by the treated thickness, the hydraulic conductivity, the anisotropy of the aquifer, and the pumping rate.

• The water cycles through the treatment zone several times, on average, before escaping downgradient.  The stripping process does not have to achieve final cleanup levels in a single pass, since the water will return to the well for additional treatment.  The process is rather like having several stripping towers in series, each achieving good stripping efficiency.  Five passes through the well, even at only 85% removal on each pass, easily exceeds 97% removal.

• The air travels up the well to the surface.  At the surface, it can be treated for contaminant removal (e.g., carbon adsorption) or released directly to the atmosphere.  If it is treated, it is typically recycled to the pressure blower and reused in a closed-loop mode for additional stripping.

While the basic configuration and process are straightforward, even for this simplest case there are many considerations in designing and installing density-driven convection systems.  Well diameter, optimal pumping rate and air-water ratio, number of wells and well placement, length of the outlet screen, special development procedures, blowers versus compressors, controls and instrumentation, calcite precipitation potential, pipe sizes, air flow velocities, and many other factors must be addressed in developing a complete design.  For more complex or challenging geology, there are additional considerations such as confined aquifer configurations, infiltration galleries, higher air-water ratios, and multiple rows of wells.

The major advantages of DDC technology are listed below. The specific advantages of DDC over air sparging and pump-and-treat are explained elsewhere on this web site.

Figure 1 - Schematic DDC Well - Click to see a larger image

Major Advantages Of Density-Driven Convection
With Re-Circulating Wells

Faster

• Faster than pump and treat or air sparging.  Much less than the 30 to 50 years commonly estimated for pump-and-treat.

• More vigorous than pump-and-treat approaches.  Pump-and-treat is passive in nature, extracting the water that is easiest to extract and waiting for contaminants to diffuse from lower permeability zones.  Re-circulating wells induce vertical gradients to vigorously circulate and treat all of the water in the aquifer multiple times.

Cheaper

• Lower initial capital costs, lower maintenance costs, and faster cleanups result in lower life-cycle costs.

• Fewer wells than air sparging.  Well spacing typically 2 to 5 times the depth of contamination.  At a site with 50 feet of saturated zone, well spacing can be 200+ feet.

• Lower pressures than air sparging systems, typically only 3 to 7 psi.  Equipment and energy costs are lower than for air sparging.

More Flexible

• Large well spacings allow great flexibility in placing wells.

• Tolerant of variable geology.  Rather than being impeded by thin silt lenses and discontinuous clay layers, re-circulation patterns are enhanced by these typical real-world features.

• Pumping rate and air/water ratio, the two essential system variables, can be independently adjusted after installation to match actual aquifer response.  Even well diameter and screen placement can be modified to meet changing conditions during cleanup.

• Does not affect adjacent plumes.  Because groundwater is not extracted, adjacent plumes are not drawn toward a re-circulating well.  Specific plumes or specific parts of a plume can be targeted. 

• Enhances bioremediation of biodegradable contaminants.  Saturates the treated water with dissolved oxygen.  Facilitates natural attenuation.

• Can be used to distribute nutrients in the groundwater to enhance bioremediation.   

• Compatible with soil vapor extraction systems.

Regulatory Advantages

• No extraction of groundwater.  Does not lower groundwater levels.  No re-injection problems.  Eliminates the need for water treatment at the surface. 

• No air emissions.  Re-circulating wells can be operated in a closed loop mode, with zero discharge.