McClellan AFB comparison of 6 no-purge groundwater samplers


Courtesy of EON Products Inc.

This report describes the activities and results of a field demonstration of six different diffusion and grab groundwater sampling devices at the former McClellan Air Force Base (McClellan), located in Sacramento, California. Analytical results from these samplers are compared to ‘baseline’ analytical results from samples collected using conventional (low-flow and three-casing-volume purge) techniques for all analytes. As described at the beginning of Section 6, conventional techniques represent baseline data only in the sense that they are the commonly-used sampling methods that are generally accepted by the regulatory community. They do not necessarily represent the correct answer (only a different answer).

This demonstration project included an assessment of diffusion and grab samplers (i.e., no-purge samplers) for collection of groundwater samples to be analyzed for VOCs, metals, and selected contaminants listed as California emergent chemicals (California Regional Water Quality Control Board [CRWQCB], 2003), including 1,4 dioxane and hexavalent chromium.

The six sampling devices demonstrated were classified as either diffusion or grab samplers depending on the predominant operative mechanism of the sampling device. The group designated as diffusion samplers was comprised of the PDBS, a rigid porous polyethylene sampler (RPPS), a polysulfone membrane sampler (PsMS), and a regenerated cellulose sampler (RCS). The group designated as grab samplers included the Snap Sampler manufactured by ProHydro, Inc. and the HydraSleeve manufactured by GeoInsight (Distributed by EON Products, Inc). It should be noted that the membrane pore size of the RPPS and PsMS may be sufficiently large to permit some limited advection of water molecules through the sampler wall. However, diffusion is believed to be the dominant mechanism for transport of dissolved constituents into these samplers. All of the diffusion and grab samplers tested at McClellan are “no-purge” sampling devices in that they are intended to be used to collect groundwater samples without prior purging of the well.

The diffusion and grab sampling devices tested are relatively new approaches to groundwater sampling that eliminate the need for well purging. Typically, a capsule (e.g., diffusive membrane or self-sealing “grab” container) is deployed at a specified position within the screened interval of a well. Depending on the type of sampler, the capsule may either be filled with purified water and sealed at the surface prior to deployment (e.g., PDBS, RPPS, PsMS, RCS), or it is deployed empty and filled with groundwater and sealed upon retrieval (e.g., Snap Sampler and HydraSleeve).

With the PDBS, RPPS, PsMS, and RCS, the constituents in the groundwater enter the sealed sampler through the process of diffusion, and the water quality inside the sampler reaches equilibrium with groundwater quality in the surrounding well. The sampler is subsequently retrieved from the well, and the water in the sampler is transferred to a sample container and submitted for laboratory analysis.

The grab samplers are empty when deployed and, following an equilibration period, they are either closed remotely to trap ambient groundwater (Snap Sampler) or they are filled and sealed during the retrieval process (HydraSleeve). Potential benefits of using diffusion or grab sampling methods include reduced sampling costs and reduced generation of investigation-derived waste (i.e., purge water).

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