CSC conducted soil sampling investigations at several sites impacted by the Woolsey Fire near Malibu, California. Various metals and other toxic materials can accumulate at elevated levels in shallow soil resulting from burning of houses and other structures. Contaminants often include lead from paint, vanadium from reinforcing steel components and arsenic or chromium from pressure treated wood.

Sampling was conducted by field sampling professionals following protocols set forth by the County of Los Angeles in its fire cleanup guidance document. The main variable in the process is determination of number of samples. The statistically valid number of samples is dependent upon the size of the ash footprint and is commonly coincident with the location of the eaves of the burned structure. Building foundations may be reused subject to testing by a structural engineer but are typically removed prior to regrading of the lot.

Five-part composite samples were collected around each predetermined sampling location. Samples were collected using stainless steel trowels at a depth of zero to three inches below ground surface (bgs). Samples were analyzed for the 17 listed metals identified in the California Title 22 Code of Regulations. Upon receipt of laboratory results, CSC evaluated the reported concentrations by comparing them with the screening levels contained in the County guidance document.

If sample concentrations exceeded screening levels, CSC provided recommendations to mitigate the condition. Actions taken typically included removing additional soil or in many cases simply removing excess ash residue. The areas were then resampled by CSC to confirm that metal contamination had been sufficiently addressed. For some metals, especially arsenic, natural background commonly exceeds the published screening level. In which case, CSC collected additional samples outside the ash footprint to establish background levels and/or used background levels found at other sites in the area. CSC provided technical memorandum to the owner documenting the investigation procedures and results. Owners were required to include the CSC technical memorandum in the rebuilding permit application package submitted to the County for final approval.

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