Laserfiche WebLink
Section No. 8 <br /> Revision No. 1 <br /> Date: January, 1997 <br /> 8.0 Soil Gas Survey and Evaluation Page 1 of 13 <br /> 8.1 Introduction <br /> Soil gas monitoring in the vadose zone is a method used to directly measure <br /> characteristics of the soil atmosphere that are frequently used as an indirect indicator of <br /> processes occurring in and below a sampling horizon. Soil gas monitoring is used as a <br /> method to suggest the presence, composition, and origin of contaminants in and below <br /> the vadose zone. <br /> A soil gas survey is very effective as a rapid and relatively inexpensive method of <br /> detecting volatile contaminants in the vadose zone. Soil gas measurements can be used <br /> to both determine the presence or absence of a volatile material in the subsurface and to <br /> determine relative concentrations. Volatile materials diffuse through unsaturated soils to <br /> the surface, achieving an equilibrium with other soil gases. Their equilibrium <br /> concentration will be a function of their source concentration (separate phase product, <br /> soils, and groundwater), their volatility, and the distance they have to travel. Once the soil <br /> gas over a contaminant source has equilibrated, its composition is fairly stable as the <br /> only active exchange of soil gas with the atmosphere occurs in the top 12 to 18 in. of soil. <br /> Thus sampling of the soil gas below this depth can provide useful information about <br /> subsurface contamination. <br /> 8.2 Purpose <br /> A soil gas survey (SGS) is a simple procedure that can provide a qualitative indication of <br /> the presence and delineation of volatile subsurface contaminants. The purpose of this <br /> document is to provide standard procedures for conducting soil gas surveys in the field. <br /> 8.3 References <br /> The following ASTM Standard was consulted in the preparation of this SOP: <br /> D 5314-93 Standard Guide for Soil Gas Monitoring in the Vadose Zone. <br /> 8.4 Overview of Methods <br /> Soil gas surveys are generally used to indicate presence and horizontal delineation of <br /> subsurface volatile contaminants. It is a survey method which is qualitative, not <br /> quantitative. The quantity of data is more important than the quality in that an individual <br /> soil gas reading is not reliable by itself, but becomes a reliable indicator when supported <br /> by surrounding readings. The more data generated, the more reliable the delineation will <br /> be. In a soil gas survey the concentrations of specific compounds is irrelevant; it is the <br /> relative levels of a general class of contaminant (e.g., gasoline, chlorinated organics, <br /> etc.) that are important. One should therefore choose the simplest system that provides <br /> adequate confirmation of the type of contaminant being monitored. <br /> Six basic soil gas sampling systems have been identified: <br /> 1. Collection of soil gas using a whole-air active approach: <br /> - involves the forced movement of bulk soil atmosphere from the sampling <br /> horizon to a collection or analyzing device through a probe; and <br />