SOIL MANAGEMENT
discarded debris (e.g., abandoned vehicles/debris), grubbing the area if there is significant overgrowth, or abating asbestos- containing flooring if sampling is to be performed through a building slab foundation. If drilling is planned, subsurface utilities and obstructions will need to be located and marked. All these tasks need to be considered in the planning phase.
After the site has been prepared, sample collection can com- mence. If drilling for subsurface soil samples or groundwater is required, it’s important to consider the driller’s availability and factor that into the project timeline. Larger projects that require significant field time – especially continuous field time – are more difficult to schedule with the limited number of crews available in Hawaii. Drillers may not be available to support the desired construction schedule as they are often booked weeks in advance.
MI sampling also requires more field time than discrete sampling. Instead of one sample location for one result (dis- crete sampling), the objective of MI sampling is to provide an average concentration of a contaminant over a volume of soil, called a decision unit (DU). To do this, small amounts of soil – increments – are collected from 30 to 100 locations within the DU and combined into a single soil sample (the bulk sample) before submitting it to the lab. The lab then air dries the soil, sieves it, and collects 30 increments (or more, if requested) that are then combined as the analytical sample. All of that for a single analytical result (Figure 2a).
While MI sampling requires more field time and cost, the analytical costs for a given volume of soil are typically much lower when compared to discrete sampling. Furthermore, the result provides a more representative picture of the actual con- taminant concentrations in the area than traditional discrete sampling. The MI sampling and analytical technique addresses the non-homogeneous distribution of contaminants in the envi- ronment, thereby eliminating both the inter-sample (Figure 2b) and intra-sample (Figure 2c) variability commonly seen in discrete sampling results. Most importantly, if planned prop- erly – and specifically if the plan was reviewed and approved by the regulatory agency – confident decisions can be made on a DU-by-DU basis, allowing for quantification of the amount of soil that requires special handling and/or disposal.
The number of DUs needed for an investigation is based on the site history, releases, dispersal patterns, migration, and exposure pathways, intended depth(s) of soil disturbance, and the planned future use of the site. The number of increments collected for each DU depends on both the contaminants sus- pected and the project objectives. Contaminants that are more likely to occur in “nugget” form (e.g., PCBs and lead-containing paint chips) require more increments than contaminants that tend to be more evenly distributed, such as sprayed-on pesti- cides or aerial fallout downwind of a smokestack.
Based on the analyses to be performed, some samples are faster to collect than others.
The depth of the investigation can also significantly impact the project timeline, as can the type of contaminants suspected to be present. The deeper you go, the longer it takes.
Figure 1. Infographic depicting the typical Phase II ESA project flow dependent on the presence or absence of contamination.
www.aipg.org
Once samples are collected, they (usually) need to be shipped to the continental United States for analysis. Once at the lab, samples are dried, prepped using an MI sampling technique, and analyzed for the specified contaminants. Upon receipt of the results, the consultant evaluates the data, adjusts it as
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