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Validation of Membrane Diffusion Sampler

Project Title: Validation of Membrane Diffusion Sampler
Project Number: 2454AVN
Project Chief:
Project Start Date: 01-OCT-2002
Project End Date: 30-SEP-2003

Project Objectives

This proposed effort will demonstrate a regenerated cellulose dialysis membrane diffusion bag sampler (hereafter referred to as the "RCDM diffusion sampler"), recently designed by the USGS, which can be used to sample for soluble inorganic and all organic (volatile/nonvolatile) contaminants in groundwater. It consists of commercially available flexible regenerated-cellulose dialysis tubing that can be configured to fit small-diameter monitoring wells at low cost, similar to the LDPE sampler. The primary objectives are:

  1. to determine if a broad range of soluble groundwater contaminants (e.g., metals, VOCs, and semivolatile organics) and natural cations and anions (e.g., MNA parameters) will readily pass through the RCDM under varying groundwater geochemical conditions (e.g., differing pH, conductivity, redox potential, and temperature); and
  2. to determine the shortest appropriate equilibration period for the tested parameters.

Statement of Problem

Collection of groundwater samples for long-term monitoring or to assess remediation at contaminated DoD sites is very costly in terms of manpower, time, and equipment requirements. The standard technique for groundwater collection is currently the USEPA low-flow purging procedure using a variable-speed submersible pump with disposable discharge tubing (Puls and Barcelona, 1995). The low-flow technique often requires a monitoring well to be pumped for long periods of time (hours) at a low-flow rate (<500 ml/min), until field parameters stabilize and samples are collected. Following sample collection, the pump and its sampling components must be decontaminated before deployment in another well. Normally, the purged groundwater and wash water are drummed and must be transported to treatment facilities offsite for disposal. Groundwater turbidity, often created by pump installation or bailing, is an important field parameter that is seldom allowed to stabilize in the purge step. If turbidity exceeds 5-10 NTU, serious bias can result for many contaminants that sorb with the suspended particulates (Gibs et al., 2000). This introduces uncertainty in the assessment of inorganic and organic contaminant concentrations in groundwater, which can result in incorrect conclusions concerning site water quality or remediation status.

Strategy and Approach

This project will provide demonstration and validation of the RCDM diffusion sampler for monitoring groundwater at DoD sites. Important volatile, semivolatile and highly soluble organic and inorganic constituents that are commonly measured at DoD sites to assess MNA or other remediation techniques will be assessed with the RCDM diffusion sampler. RCDM sampling effectiveness and cost will be compared to that of the USEPA standard low-flow purging technique and with the well-characterized and accepted LDPE diffusion sampler. The project will be undertaken in three parts:

  1. initial field site assessment and groundwater collection for bench-scale testing (year 1);
  2. bench-scale tests (year 1); and
  3. field demonstrations (year 2). Both the bench-scale tests and field demonstrations will be conducted in groundwater from the same field sites and monitoring wells. Three field sites, including approximately three wells per site, are presently planned for RCDM diffusion sampler evaluation. The objective of the bench-scale tests is to evaluate the samplers initially in static groundwater (that cannot rapidly change over the experimental timeframe) and determine the general equilibration periods for the diverse group of contaminants and important natural parameters under more controlled conditions. For example, poor field data comparisons may be due to confounding field-specific factors rather than failure of the samplers to accurately measure soluble groundwater parameter concentrations in the vicinity of the diffusion samplers. These factors include (a) changes in site groundwater stratification over the monitoring period, (b) unknown changes in groundwater flow induced by comparative conventional sampling methods, or (c) analytical interference (e.g., from particulates in the sample). Such problems are frequently indicated during use of the extensively tested LDPE samplers in field monitoring.

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