Isolating the well screen from the rest of the well casing sharply reduces the required volume to be removed. Instead of using expensive and difficult inflatable packers to isolate the screened interval of a well, the BESST Retro ZIST uses a series of stacked conformable washers to seal the inside of the casing directly above the well screen with the support on the well bottom by a perforated support post. Standard Retro ZIST systems are designed for 2” and 4” diameter wells, with custom sizes available. Shown at the right is a 2” Retro ZIST system with two displacement barrels for illustration. Barrels stack to fit the length of the well screen.
Maximizing purge volume reduction is a key goal of the Retro ZIST technology. There are two aspects to this concept. First, there is the casing volume, which is addressed by using the stacked sealing washers to isolate the well screen from the above casing. Second, there is the well screen volume itself (including the surrounding filter pack), which the Volume Displacement Stem minimizes while also supporting the isolation washers and receiver. There are two deployment options available. Option #1 uses a screened PVC support post composed of stacked Displacement Barrels and Option #2 uses a sand-filled solid support inner well screen. These options displace 65-85% of the water volume stored inside the well screen section of the well.
Water level measurements are simply accommodated. The pump is lifted up a few inches off the docking station allowing groundwater within the well screen to communicate with the atmosphere and adjust to hydrostatic pressure over time. The tubing slack (from the slightly lifted pump) is attached via a tubing clip located on the underside of the well suspension cap. When the operator opens the well, the first task typically is to obtain a water level measurement. The pump is then reset into the conical docking station by lowering it a few inches until it seats – thereby allowing the well screen section to be purged and sampled.