The general principles behind Passive Diffusion Bag (PDB) sampling and groundwater sampling using the HydraSleeve (HS) are very similar. Both devices collect a discrete sample from within the screened interval of the monitoring well without purging, and rely on ambient flow and diffusion of contaminants from the aquifer into the well screen. Unlike traditional purge and sample methods or low flow sampling, neither sampler actively pulls water from the formation into the well during sample collection.
The primary difference between the PDB and the HS is how the sample enters the device. PDB samplers rely on diffusion of specific volatile organic contaminants (VOC) across a membrane, usually polyethylene, which is fabricated into a long, thin bag and filled with deionized water. Over a period of time, typically 8 to 14 days, VOC concentrations adjacent to the PDB will pass through the membrane and equilibrate with the water in the bag. In contrast, the HydraSleeve physically collects a core of water from within the well screen, with minimal displacement and disturbance of the water column. Once full, the sample is sealed within the sleeve and isolated from overlying, extraneous fluids. These very different collection methods result in a few considerations about the use of the two samplers.
- Use of the PDB is limited to contaminants that will diffuse across the selected membrane; the water sample collected by the HydraSleeve can be analyzed for all contaminants.
- PDB samplers require varying time periods for different contaminants to equilibrate across the membrane. The resulting sample is a time integrated average. When activated, HydraSleeve provides an instantaneous sample of contaminant concentrations in the well.
- Both devices sample from a discrete interval. Normally, the length of the PDB sampler defines its sample interval. By using different activation methods, a single HydraSleeve can be used to collect samples from a range of intervals (from a few feet to over 8 feet), depending on sampling objectives.
- Because the HydraSleeve goes into the well empty and flat, it displaces much less water than the PDB, resulting in less disturbance and much faster equilibration of the water column in the well.
Noting that material cost, deployment, field sampling effort, and expense for both devices are very similar, the differences described above can be used to help choose the optimum sampler for a particular site and project.