COMPLIANCE INFO - Laserfiche WebLink

Home Browse Search

ADVISORY - ACTIVE SOIL GAS INVESTIGATIONS the soil gas investigation, one or more continuously cored boring(s) should be installed at the first location to the proposed greatest depth of the soil gas investigation. If the soil gas data are to be used for human health risk assessment, geotechnical data may be needed. Geotechnical information needed for vapor intrusion risk assessment purposes can be found in DTSC's Vapor Intrusion Guidance (DTSC, 2011). Lithologic logs should be prepared for all borings, including soil matrix and geotechnical borings. Information gathered from the continuously cored borings may include lithologic descriptions, geotechnical data and contaminant data. Information collected from borings should be used to update the CSM. All boring logs generated during the soil gas survey should be provided to the regulating agency. 3.1.2 Sample Spacing Sample locations may be based on historical site use and potential contaminant release points. Initial spacing can be grid-based such as sampling on a 50- by 50-foot grid. Alternatively, initial sampling can be based on historical or suspected site use. When areas of contamination are identified, a more focused grid spacing or biased sampling approach may be employed. Use a close interval grid or radial or step-out sampling pattern such as 10- to 20-foot grid pattern and multi-level sampling at 5-, 10-, 15-feet vertically to delineate identified contaminant areas. If historical information for the area is unknown, a screening grid pattern, such as 100- by 100-foot, may be used. 3.1.3 Sample Depth All available information such as boring logs and field instrument readings from soil cuttings or cores should be used to select the correct depths to collect soil gas samples. Probes should be installed at depths with elevated vapor readings. Vertical sampling should continue until vapor-phase contaminants are no longer encountered. If vertical characterization to groundwater is needed, the deepest soil gas sample should be collected near the capillary fringe. Soil gas wells or probes should not be installed too close to the water table because low flow conditions might be encountered due to the high moisture content. Nested soil gas wells may be installed in the annular space of groundwater monitoring wells to serve as a dual-purpose well if both vapor and groundwater monitoring are required. Soil gas sample depths should be chosen to minimize the effects of changes in barometric pressure and temperature, breakthrough of ambient air from the surface, and to ensure that representative samples are collected. Soil gas samples collected at less than 5 feet below ground surface (bgs) may be subject to barometric pressure effects and prone to breakthrough of ambient air through the soil column and, hence, samples are not typically collected from depths of less than five feet unless site-specific conditions dictate otherwise. Consideration should be given to source location, types of chemicals of concern and the lithology encountered. Variation of sample depths and the need for deeper sample locations should be evaluated based on site-specific characteristics and DQ0s. When evaluating vapor intrusion, sampling soil gas immediately adjacent to a building's foundation is only appropriate if the samples are collected near the contaminant source. July 2015 8