How long should I leave the HydraSleeve in the well before I retrieve it? How long does it take for the well to restabilize after the HydraSleeve has been deployed? The short answer to these and similar questions is: "It all depends on the amount of water displaced when placing the HydraSleeve in the well."
Studies have shown that any sampling device made of plastic needs to equilibrate with the fluid being sampled for a period of time. The original studies were done on material used in purge systems, and determined that measurable concentrations of VOC's were being lost in discharge tubing and plastic pump components. This also occurs with plastic no-purge devices, although to a lesser degree; less surface area to volume of fluid exists. If you want to be sure that the plastic material has equilibrated with the fluid being sampled it has been recommended that a 48 hour contact period be used. Of course, this is never done with discharge tubing or bailers used to purge wells, so comparisons to no-purge devices that have equilibrated would add another variable.
Most of the recent literature for well restabilization has been based on the assumption that a diffusion-type sampling device is being used, and that the device will displace a substantial amount of fluild when it is placed in the well. The two week criteria frequently mentioned was developed because it is safe and it is also the amount of time needed for all contaminants to equilibrate into the standard PE passive diffusion bag. Studies using colloidal borescopes have recommended that 24 hours be allocated for a well to restabilize. These studies were conducted in the early 1990's in conjuntion with developement of low flow sampling protocol. The purpose was to determine how long a submersible pump should set in the well after placement for the well to return to ambient flow conditions. Both the colloidal borescope and submersible pumps displace a huge amount of water compared to an empty HydraSleeve.
- Well restabilization period for the HydraSleeve is still undefined. I have a hunch, from what I've seen in dye studies and what has been reported to me from users, is the 2-week criteria is way too conservative for most wells. Users have reported they deployed HydraSleeves in the morning and pulled them in the afternoon with comparable results with their prior sampling method.
- We have deployed (numerous times) a standard 2-inch HydraSleeve in a clear 2-inch test well through a 1-foot layer of dye in the top of the water column. As the empty HydraSleeve passed through the dye it pulled the dye down about 6 inches to one foot. (Visual observation). The test well was 8-feet long. Conducting the same test but in a 4-inch clear test well showed a drag down of less than 6 inches. In short, the larger the well compared to the size of the HydraSleeve, the less disturbance thus a shorter well restabilization period. An empty 2-inch HydraSleeve with an 8-ounce weight displaces ~75 mls, less than two VOA vials. I'm not sure that using a standard HydraSleeve in a 4-inch well with a 10-foot saturated screen would require any well restabiliazation period at all. All of the mixing would be confined to the top foot of the well screen, well above the zone being sampled.
- If the water table is located within the screened interval there is probably no need to allow the well to restabilize. You can't mix fluid in the casing with the screen interval if there is no water in the casing.
- Conversely, if the well screen is long and completely saturated, let's say a 15-foot screened interval, well restabiliaztion with a single standard size HydraSleeve is not critically important. This is because any mixing of the casing water with the "live" water in the screen will be confined to the top of the screened interval. The sample will be collected from a substantial distance below the mixing zone.