Selected Depth of Soil Gas Probes for Environmental Investigations (5 and 15 feet bgs)
The soil gas probes at the site will have the purpose of to investigating the presence of VOCs in shallow soil, and (2) to monitor the subsurface for vacuum. We selected a A probe at approximately 15-feet with the intent of closely monitoring the depth interval closest to the foundation. The work plan is being reviewed by the regional board and we’ve requested a meeting to discuss our approach. If the regional board is not in agreement, I anticipate that they will request changes or conditionally approve the workplan. In terms of implementation, we’d like to closely monitor the vacuum at 5 and 10 feet, and if possible, adjust the SVE system and well data using vacuum data collected overtime. The DTSC guidance you listed makes multiple statements, among other, that there should be two probes between 5 and 25 feet to assess shallow soil (one at ‘approximately’ 15 feet).
Selected Depth of Soil Gas Probes for Monitoring VOCs and SSDS Performance Monitoring (5 and 10 feet bgs)
Selected Depth of Soil Gas Probes at 5 and 10 feet bgs (SSDS Performance Monitoring) – The soil gas probes installed at 5 and 10 feet bgs at the site will have a dual-purpose: (1) to monitor the known presence of VOCs in shallow soil, and (2) to monitor the subsurface for vacuum. We selected a 10-foot depth rather than exactly 15-feet with the intent of closely monitoring the depth interval closest to the foundation. If the regional board is not in agreement, I anticipate that they will request changes or conditionally approve the workplan. In terms of implementation, we’d like to closely monitor the vacuum at 5 and 10 feet, and if possible, adjust the SVE system and well data using vacuum data collected overtime. The DTSC guidance you listed makes multiple statements, among other, that there should be two probes between 5 and 25 feet to assess shallow soil (one at ‘approximately’ 15 feet).
Monitoring Shallow Soil for SSD Influence
With regards to the Radius of Influence of SVE wells, the actual area cannot be determined based on the design or immediately after startup, and we do not warranty specific performance; however, we often operate with the SVE well radius (and area) of influence noted in the design. We often operate with a much larger area of influence after we’ve been able to induce a higher vacuum and flow. The final area of influence will depend on vacuum, sustained flows, and site lithology. We use soil gas probes to gather performance data, assess the performance of the system and assess the subsurface conditions in terms of vacuum and VOC concentrations. That is an approach that the regional board and DTSC have often agreed with.
Indoor Air Sampling with an Existing SSDS
When VOCs are present in soil vapor below or near residential and commercial buildings, our preference is to install shallow soil gas probes around the perimeter of the building, and sample those probes to assess the VOC concentrations and then monitor for vacuum. We have often used vacuum measurements at the probes to assess if indoor air is protected. We will need an access agreement to monitor the probes outdoors, but we will reduce the disturbance of an occupant or a tenant.
• Indoor Air Sampling – Our preferred approach is not to collect indoor air samples. Collecting representative data inside an industrial building is requires that we follow an indoor air testing protocol. Among other aspects removing the occupant, performing a chemical inventory at the site, removing possible sources, and then collecting indoor air samples. Indoor air sampling is very disruptive for a potential tenant and has a high potential of reporting a false positive.
• Subslab Sampling – As with indoor air sampling, our preference is to avoid entering any structure to collect samples. Our preference will be to collect subslab samples, rather than sample indoor air for an industrial building.