Standard Practice for Volatile Contaminant Logging Using a Membrane Interface Probe (MIP) in Unconsolidated Formations with Direct Push Methods

Importancia y uso:

5.1 The MIP system provides a timely and cost effective way for delineation of many VOC plumes (for example, gasoline, benzene, toluene, solvents, trichloroethylene, tetrachloroethylene) with depth (1, 2, 4, 8, 9). MIP detector logs provide insight into the relative contaminant concentration based upon the response magnitude of detector and a determination of compound class based upon which detectors of the series respond of the bulk VOC distribution in the subsurface but do not provide analyte specificity (1, 2, 7). DP logging tools such as the MIP are often used to perform expedited site characterizations (10, 11, D5730) and develop detailed conceptual site models (E1689). The project manager should determine if the required data quality objectives (D5792) can be achieved with a MIP investigation. MIP logging is typically one part of an overall investigation program.

5.2 MIP logs provide a detailed record of VOC distribution in the saturated and unsaturated formations and assist in evaluating the approximate limits of potential contaminants. A proportion of the halogenated and non-halogenated VOCs in the sorbed, aqueous, or gaseous phases partition through the membrane for detection up hole (1).

5.3 Many factors influence the movement of volatile compounds from the formation across the membrane and into the carrier gas stream. One study has evaluated the effects of temperature and pressure at the face of the membrane on analyte permeability (12). Formation factors such as degree of saturation, clay content, proportion of organic carbon, porosity and permeability will also influence the efficiency of analyte movement from the formation across the membrane. Of course, the volatility, concentration, molecular size and mass, and water solubility of each specific VOC will influence movement across the membrane and rate of transport through the carrier gas line to the detectors.

5.4 High analyte concentrations or the presence of Non-Aqueous Phase Liquid (NAPL) in the formation can result in analyte carry over in the MIP log (8, 13). This is a result of high analyte concentrations within the membrane matrix requiring time to diffuse out of the membrane into the carrier gas stream. This effect can lead to tailing of detector peaks on the MIP log to deeper intervals. Use of appropriate detectors and detector sensitivity settings can reduce this effect (14). Experience with log interpretation also helps to identify analyte carryover. Of course, targeted soil or groundwater sampling (D6001, D6282) should be performed routinely to verify log results and assist with log interpretation and site characterization (subsection 1.4).

5.5 Some volatile contaminants are composed of multiple analytes of different molecular mass, size and volatility (e.g. gasoline). A detailed study was performed using a gas chromatograph (GC)-mass spectrometer system to assess the delay in movement of several components of gasoline from the membrane face, up the trunkline, to the MIP detectors (15). The larger, more massive analytes were found to be delayed in reaching the detectors. This effect means that some analyte mass will be graphed on the MIP log at a depth below where it entered the membrane. This “dispersion” effect is difficult to overcome. However, knowledge of the site-specific analyte(s) and experience with log interpretation can help the user assess these effects on log quality and contaminant distribution. Of course, targeted soil or groundwater sampling (D6001, D6282) should be performed routinely to verify log results and assist with log interpretation and site characterization (subsection 1.4).

5.6 One of the important benefits of MIP logging is that the number of samples and laboratory analyses required to effectively characterize a VOC plume and source area can be greatly reduced, thus reducing investigative time and costs. Reduction of the number of samples required also reduces site worker exposure to hazardous contaminants. The data obtained from the MIP logs may be used to guide and target soil (D6282) and groundwater sampling (D6001) and the placement of long-term monitoring wells (D6724, D6725, D5092) (2, 7, 8) to more effectively characterize and monitor site conditions.

5.7 Typically, only VOCs are detected by the MIP system in the subsurface. Use of specialized methods and/or detector systems may allow for detection of other gaseous or volatile contaminants (for example, mercury). Detection limits are subject to the selectivity and sensitivity of the gas phase detectors applied, the analytes encountered, and characteristics of the formation being penetrated (for example permeability, saturation, sand, clay and organic carbon content).

5.8 Correlation of a series of MIP logs across a site can provide 2-D and 3-D definition of the of the primary VOC contaminant plume (7, 8). When lithologic logs such as EC, HPT, or CPT are obtained with the MIP data, contaminant migration pathways (7, 8) as well as storage and back diffusion zones (16) may be defined.

5.9 Some investigations (8, 17-21) have found the MIP can be effective in locating zones where dense nonaqueous phase liquids (DNAPL) may be present. However, under some conditions, especially when inappropriate detectors and methods are used (22, 23), analyte carryover (15) can mask the bottom of the DNAPL body (9, 13, 24). These limitations can be minimized by use of appropriate methods and detectors (14, 23).

5.10 While the conventional MIP system does not provide quantitative data or analyte specificity some researchers have modified the system with different sampling or detector systems in attempts to achieve quantitation and specificity (21, 25, 26). These methods typically reduce the speed of the logging process in order to provide improved quantitation and analyte specificity for a limited group of analytes.

5.11 MIP data can be used to optimize site remediation by knowing the vertical and horizontal distribution of VOCs as well as obtaining information on the soil type and permeability where contaminants are held by using tandem lithologic sensors such as EC, HPT, or CPT. For example, materials injected for remediation are placed at correct depths in the formation based upon the detector responses of contaminants and the proper type of injection is performed based upon the formation permeability.

5.11.1 This practice also may be used as a means of evaluating remediation performance. MIP can provide a cost-effective way to evaluate the progress of VOC remediation. When properly performed at suitable sites, logging locations can be compared from the initial pre-remedial investigation to logs of the VOC contaminants after remediation is initiated.

Note 1: The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the equipment and facilities used. Practitioners that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing/sampling/inspection/etc.. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors. Practice D3740 was developed for agencies engaged in the testing and/or inspection of soils and rock. As such, it is not totally applicable to agencies performing this practice. However, users of this practice should recognize that the framework of Practice D3740 is appropriate for evaluating the quality of an agency performing this practice. Currently there is no known qualifying national authority that inspects agencies that perform this practice.

Subcomité:

D18.21

Referida por:

D6286_D6286M-20, D6725_D6725M-16R24E01, D8571-24, D6724_D6724M-16R24E01, D8037_D8037M-16, D6001_D6001M-20

Volúmen:

04.09

Número ICS:

13.080.05 (Examination of soil in general)

Palabras clave:

CPT; direct push; electrical conductivity (EC); hydraulic profiling tool (HPT); membrane interface probe (MIP); soil investigations; volatile organic contaminants (VOCs);