3.3.8 Stockpile Decision Units

Stockpiles can offer easy access for sample collection. Data collected from stockpiled soils are often used to make decisions on disposal or reuse of the soil. Soil that is known or suspected to be contaminated should be segregated from soil that is presumed to be clean prior to sampling. Stockpiles should be subdivided into volume-based DUs for sampling based on the target contaminants, potential environmental hazards associated with the soil, and the desired use of the soil. Stockpiles should be subdivided into volume-based DUs for sampling.

Example stockpile decision units

Physical sampling is best carried out as the stockpile is being created (e.g., collecting a specified number of increments from individual dump trucks or backhoes). The shape of the stockpile can bias characterization of the DU by limiting access to soil within the pile (ASTM 2006, HDOH 2009). Therefore, the stockpile should be flattened to a thickness of approximately 3 feet or less before sampling to help ensure equal access to all soil and ensure that the samples collected are representative of the DU as a whole. Other means to ensure equal access to all soils during sampling may also be used, such as moving the pile from one location to another or digging into the stockpile with a backhoe or hand tool.

Figure 3-10 depicts example DUs for a large stockpile. The stockpile contains approximately 25,000 cubic yards of soil excavated from an agricultural settling pond. The soil is being considered for reuse in a new, residential development project. Each lot will be approximately 1 acre in size; therefore, the exposure areas DUs are designated as 1 acre. Each lot will be covered with approximately 6 inches of fill material from the stockpile. This plan results in an exposure area DU volume for each lot of approximately 800 cubic yards. The stockpile has been flattened to a thickness of approximately 3 feet in preparation for sampling. The stockpile is subsequently divided into thirty 800-cubic-yard DUs of soil, and each one is tested with one or more ISM samples. This method helps provide equal access to all soil in the stockpile. Samples should be collected from the surface as well as the interior of the stockpile.

Example of stockpile DUs.

Figure 3-10. Example of stockpile DUs.