WORK PLANS FILE 2 - Laserfiche WebLink

Home Browse Search

Regardless of the design of the sampling equipment, there are a number of important topics to address in order to obtain reliable soil vapor concentration data. These topics are discussed below. B. Field activities prior to sampling/documentation Written documentation of field conditions during sampling is recommended. This includes weather conditions (temperature, barometric pressure, wind direction and speed, humidity, degree of cloudiness); surface soil conditions (presence of standing water, wet soil, irrigation activities, etc.) and groundwater elevations. Do not perform soil vapor sampling during or immediately after a significant rain event (e.g., >_ 0.5 inch; California EPA, 2003) due to the reduced effective diffusion coefficient and decrease in relative vapor saturation in the unsaturated zone. Maintain detailed field records of all activities, conditions, and sampling processes, including names of field personnel, dates and times, etc. It is important to maintain consistency in sampling activities between sampling events (e.g., purging volume and purge rate, sampling volume, leak testing methods, equipment used). Carefully plan all sampling activities to maintain consistency between sampling events and to avoid errors that can affect soil vapor concentrations. C. Equilibration time The installation of soil vapor sampling probes can introduce oxygen into anaerobic portions of the vadose zone. An equilibration time is required to account for the effects of soil vapor probe installation; this allows for equilibration of vapor component concentrations between the probe and subsurface. Soil vapor samples should not be obtained until after the equilibration time is reached. Probes installed using augers should be allowed at least 48 hours of equilibration time while probes installed using direct-push techniques should be allowed at least 30 minutes of equilibration time (California EPA,2003). D. Purging Prior to collecting a soil vapor sample, the stagnant air in the sampling tubes must be removed. This ensures that the soil vapor sample that is collected is representative of actual soil vapor concentrations. Field notes containing information about the above- ground sampling equipment and below-ground tubing length and inner diameter should be used to calculate the "dead volume" to be purged. The volume of the probe and borehole sand pack is not included in the calculation as it is assumed that the soil vapor concentrations in the probe and sand pack are equilibrated with the surrounding soil. The volume of a sample container, such as a Summa® canister (which is not used during purging), should not be included in this calculation. A battery powered pump can be used to purge the system. Check to make sure that all connections, fittings, etc. are tightly fit in the sampling equipment prior to purging. ETC Soil Vapor Sampling Technical Toolkit 17 Version 1.4,September 7,2005