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ADVISORY — ACTIVE SOIL GAS INVESTIGATIONS 3) Annular Seal. The composition of the annular seal is a function of the long-term use of the soil gas well, as follows: Temporary Soil Gas Wells. For wells that will be sampled for less than one year, the annular seal can be hydrated bentonite or other materials, as appropriate. Studies have shown that bentonite annular seals in the vadose zone desiccate readily and will not rehydrate once damaged (Lackey et al., 2009). Hence, bentonite-only annular seals are discouraged for long-term use. The integrity of a bentonite annular seal can be increased by adding sand to the grout mixture as indicated by Lackey (2009). Permanent Soil Gas Wells. For wells that will be used for longer than one year, the annular seal should be neat cement with bentonite. Typically, the bentonite content is 1 — 5 percent. The integrity of a neat cement annular seal can be increased by adding sand to the grout mixture as indicated by Lackey (2009). When using a bentonite seal on a temporary soil gas well, the bentonite should be hydrated at the surface in a container and then slowly poured or pumped into the borehole. If the borehole is deeper than 15 feet, a tremie pipe should be used to place the hydrated bentonite.' It is important to have a good annular seal to prevent "short circuiting" of air from the upper portions of the borehole into the sample, yielding contaminant concentrations not indicative of the targeted interval. Follow a similar procedure for deep well construction with multiple probe depths, in that one foot of dry granular bentonite should be emplaced on top of the sand pack encasing each probe, followed by the annular sealing material. The annular seal should continue until the next sand pack, as shown on Figure 1. Accordingly, soil gas probe construction should be consistent with California Department of Water Resources Bulletin 74-90 (California Well Standards) (DWR 1991) and the annular seal should be appropriate for the local climate, subsurface conditions, and permanence of the soil gas well. 4) Probe Support Rod. A down-hole rod should be used to support the well tubing in the borehole. A down-hole rod is a support device used in boreholes deeper than 15 feet to ensure that probe tips are placed at appropriate depths. A small diameter PVC pipe that is capped at both ends is the most commonly used material for probe support. The probe support is created by attaching the probe tip and tubing to the exterior of the PVC pipe. Other materials may be used, such as metal, as long as the support rod is free of contaminants. Depending on the depth, centralizers may be necessary to ensure the support rod is in the center of the borehole. Alternative probe support designs with accompanying descriptions may be proposed in the project workplan. Justification should be 4 The protocol for the placement of the hydrated bentonite is similar to the approach in USEPA (2014). July 2015 10