Environmental engineers are increasingly using onsite chemical surveys as field screening techniques to pinpoint source areas or approximately delineate the extent of existing contaminant plumes. The use of onsite chemical surveys optimizes the number of samples taken by more expensive intrusive techniques and sent to the laboratory for confirmatory chemical analysis. Several techniques are available for volatile and nonvolatile organics as well as for inorganic compounds.
Onsite chemical screening techniques vary from qualitative chemical analyses using indicators such as organic vapor analyzers (OVAs) or HNU meters to more quantitative soil-gas surveys using gas chromatography and mass spectrometry (GC/MS).
Generally, environmental engineers use these field screening techniques to collect preliminary site information and guide future and more intrusive field investigations. Engineers can measure the pH of the soil, waste, or ground-
water in the field with a pH meter and use the results of these measurements to characterize the subsurface environment or classify the corrosivity of waste materials. They can also electrometrically measure the Eh of groundwater in the field using a platinum electrode and a reference electrode (Holm, George, and Barcelona 1986; Ritchey 1986). Then, they can use the results of the measurements to characterize oxidation-reduction conditions in the subsurface and evaluate the potential for mobility of heavy metals in groundwater.
OVAs, photo ionization detectors (PID/HNU meter), flame ionization detectors (FIDs/OVAs), argon ionization detectors (AIDs), and combustible gas indicators (EDs) are all total organic vapor survey instruments that locate source areas of volatile compounds within the vadose zone or track these compounds within groundwater (U.S. EPA 1993b).
Test kits are commercially available for preliminary field screening of many inorganic compounds (Hatch kits) and some organic compounds (Handy kits). These kits are based on the principles of colorimetry. Colorimetry involves mixing the reagents of known concentrations with a test solution in specified amounts. This mixing results in chemical reactions in which the color of the solution is a function of the concentration of the analyte of interest (Davis et al. 1985; Fishman and Friedman 1989).
Environmental engineers use soil-gas surveys to locate source areas of volatile compounds within the vadose zone, track plumes of volatile compounds in groundwater, identify migration patterns of landfill gases, and optimize the number and location of more expensive and intrusive monitoring points such as soil borings and groundwater monitoring wells.
Soil-gas surveys are based on several in situ soil sampling techniques such as headspace analysis, surface flux chambers, downhole flux chambers, surface accumulators, and suction ground probes. The most commonly used techniques, however, are the surface accumulators and the suction probes.
Surface accumulators involve the passive sampling of soil gas by trapping volatile organic compounds (VOCs) onto an adsorbent contained within an inverted glass tube (Zdeb 1987). The inverted glass tube is buried in the soil for a few days to weeks. The adsorbent consists of a ferromagnetic wire coated with activated charcoal and is contained in an inverted test tube. The adsorbent passively collects diffusing VOCs which adsorb onto the activated charcoal. After a few days or weeks, the glass tube is sealed and taken to the laboratory for VOC analysis.
Ground probe sampling techniques for soil gas involve inserting a tube into the ground and pumping the soil gas with a vacuum pump. Engineers then analyze the extracted gas in the field for VOCs using portable analytical instru ments. The probes can be manually or pneumatically driven or installed in boreholes. Grab samples can be taken at the same depth (or at different depths) at several locations for areal (or vertical) characterization of soil-gas concentrations.
The vertical and horizontal spacing of the probes can be affected by many factors such as soil moisture and organic matter content, presence of perched water, depth to groundwater, permeability of the subsurface materials, and the Henry's Law constant of the VOC in question (Silka 1986). The upward diffusion of vapors is usually blocked by soil strata containing a finer grained soil with a higher moisture content or higher organic carbon content.
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