Recirculating Wells

We work primarily with re-circulating well technologies, which remove contaminants within wells.  With such technologies, the water is never brought above-ground, but remains in the aquifer.  The groundwater is pumped more vigorously than in a pump-and-treat or air sparging approach, circulated several times through a treatment zone established around the recirculating well, and treated for removal of contaminants during each of several passes through the well.

While there are numerous possible configurations for a recirculating well system, each optimized for a different set of geologic conditions, the most basic approach is also the most commonly used.

• The well penetrates to the maximum depth of the contamination.
• An inlet screen, typically standard well screen, is installed across the upper or lower several feet of the contaminated thickness.  In some cases, the upper screen is the inlet screen.  In other cases the lower screen is the inlet.
• The well incorporates a second screen, an outlet screen, usually installed across the upper or lower several feet of the contaminated thickness.
• The inlet portion of the well is often separated from the outlet portion by a packer.
• Water is drawn into the well through the inlet screen, treated by one of several possible processes, and released through the outlet screen.  The water is usually pumped through the well either by air-lift pumping or by a submersible pump, or a combination of the two, depending on the treatment step or steps being used.  Other pumping methods can also be used to move the water through the well.
• The treated water is released through the exit screen, under gravity and/or applied pressure, and flows back into the aquifer through the sand pack and the aquifer materials.
• At the exit screen, higher than normal pressures are formed, resulting in higher head values near that portion of the well.
• The treated water flows outward from the well and either downward or upward under the influence of the vertical gradients created by the extraction process at the inlet screen and the mounding at the outlet screen.  Because aquifer materials are typically anisotropic, allowing horizontal flows more readily than vertical flows, the flows tend to be more horizontal (outward) than vertical (downward or upward).
• A recirculation zone is created that typically returns the majority of the treated water to the inlet 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 down gradient.  The treatment process does not have to achieve final cleanup levels in a single pass, since the water will return for additional treatment.  In a well using stripping as the treatment step, five passes through the well, even at only 85% removal on each pass, easily exceeds 97% removal.  In a well using activated carbon as an adsorptive medium, treatment is usually 100% in a single pass through the well.  Other treatment processes have differing levels of single-pass treatment efficiency.

While the basic configuration and process are straightforward, there are many considerations in designing and installing recirculating well systems.  Well diameter, optimal pumping rate, number of wells and well placement, length of the outlet screen, special development procedures, choice of treatment process, controls and instrumentation, in-well plumbing configuration, constructability, 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 and multiple rows of wells.

Given an aquifer, confined or unconfined, that produces at least one gallon per minute, a recirculating well treatment system can generally be developed for a site.  However, no technology solves every problem, and there are limitations that must be borne in mind when applying these technologies.

If you would like to consider recirculating well technology for a site, just fill in the Design Data Form and fax or mail it to us, or fill out the form on line and e-mail it to us.  It is not necessary to have all of the information requested on the form; some information can be estimated, or replaced with an assumed value.  We will provide a conceptual design and budget-level cost estimate, typically within one week.