Common practice for soil sampling is to collect a discrete sample (soil from a known location and depth) from a few points and compare the analytical data to exposure criteria. In particular situations, discrete sampling is biased and not reproducible. For instance, if a soil sample is analyzed for arsenic and the small portion of soil that was processed contains an arsenic-bearing mineral, results from that soil sample would not necessarily be representative of site conditions. Incremental sampling method (ISM) attempts to remove some of the variability associated with discrete sampling by combing small amounts of soil collected from many locations across a site and essentially capturing the average contaminant concentration.
Prior to implementing a sampling plan that utilizes ISM, developing a thorough conceptual site model (CSM) is a key component to identifying potential impacted areas at a site. Once the acceptable exposure level has been identified, the site is divided into decision units (DUs). A set of DUs over a site can be designated as equal areas that are based upon exposure (exposure area) or irregular areas in and around a known source area (source area). Dependent upon the necessary number of sample increments (minimum of 30 to 50), a sample grid is generated over each DU. Soil increments collected from each grid node across a DU are combined and sent to a lab where the combined sample is further processed and sub-sampled.
Envirologic was recently involved with a residential redevelopment project where ISM was employed as a unique tool to address site concerns. Soil analytical results from discrete samples, collected during a Phase II Environmental Site Assessment, identified metals in excess of direct contact criteria. Two potential sources include fill material with slag that was used to surface the site to the current grade or through aerial deposition from a nearby iron smelting facility. Prior to collecting ISM samples, the site was split into DUs based upon future residential lots, with each lot representing a DU. Results from the ISM samples indicate that the direct contact pathway is not of concern at most, but not all, of the future residential lots. Envirologic has proposed remedial options for the single lot where the direct contact pathway is potentially complete, with the goal of allowing for redevelopment across the entire site.
ISM is commonly used to evaluate exposure concerns stemming from metals in soil at or near the ground surface; however, this method can also be implemented to address issues associated with volatile organic compounds or contaminants encountered below the ground surface. Due to less variability associated with the analytical data, when compared to discrete sampling, ISM allows for a higher degree of confidence when evaluating risks associated with an exposure pathway. While ISM may incur additional set-up time due to grid generation and layout, lower analytical costs allow for ISM to be financially competitive with traditional discrete sampling.
To read the first post in our sampling series, Vapor Pin and Vapor Well Sampling, click here.
Image credit www.caslab.com.
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incremental soil sampling • ism • soil sample • conceptual site model • decision units • soil contamination