Well Restabilization Discussion

Most of the recent discussion of 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 fluid when it is placed in the well. The two week period frequently mentioned in the literature was developed because it provides a large margin of safety and it is also the amount of time needed for all contaminants to equilibrate in the standard polyethylene passive diffusion bag. In general, the longer the HydraSleeve or any sampler equilibrates, the better. There is no maximum time for the HydraSleeve -- a new one can be left in the well immediately after sampling for the next event, expediting future sampling and assuring restabilization.

Studies using a colloidal borescope have recommended that 24 hours be allocated for a well to restabilize. These studies were conducted in the early 1990's in conjunction with the development of low flow sampling protocol. The purpose of the borescope studies was to determine how long a submersible pump must sit in the well after placement before the well returns to ambient flow conditions. It should be noted that both the colloidal borescope and submersible pumps displace a much larger amount (at least 10 times the volume) of water compared to an empty HydraSleeve.

Additional Notes:

• Well restabilization period for the HydraSleeve is still undefined. It seems fairly obvious, based on dye studies and HydraSleeve field feedback, that the 2-week criterion is overly conservative for most wells. Users have reported deploying HydraSleeves in the morning and recovering them in the afternoon with analytical results comparable to their prior sampling method.
  
• GeoInsight has conducted numerous tests deploying a standard 2" HydraSleeve in a clear 2" I.D. test well through a 1 foot layer of dye at the top of the water column. As the empty HydraSleeve passed through, the dye was pulled down about 6-12" (visual observation). The test well was 8' deep. Conducting the same test with a 2” HydraSleeve in a clear 4" I.D. test well showed a drag down of < 6". In short, the larger the well diameter compared to the size of the HydraSleeve, the less disturbance and therefore a shorter well restabilization period.
  
• An empty 2" HydraSleeve with 8 oz. weight displaces ~75 ml, less than two VOA vials. It is not certain that a standard HydraSleeve used in a 4-inch well with a 10-foot or longer saturated screen would require any well restabilization period at all. Any mixing of stagnant casing water would be confined to the top foot of the well screen, far above the zone being sampled.
  

HydraSleeve No-Purge Sampling  •  Passive Ground Water Sampling
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