Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 8.0
FIELD SCREENING METHODS

8.0 FIELD SCREENING METHODS

The Hawai‘i Department of Health (HDOH) Hazard Evaluation and Emergency Response Office (HEER Office) supports the use of field screening methods to help streamline the site investigation or remediation process. This Section provides a general overview of selected field screening methods that have been approved by USEPA or other state environmental protection programs and is not intended to be comprehensive. A number of companies market field screening kits or instruments based on the methods described below or similar methods developed by a specific company. Alternative methods should be discussed with the HEER office on a site-by-site basis. Periodic updates on field screening methods will be included in this TGM Section. Detailed information is not provided on analytical systems used in conjunction with field screening on direct-push field platforms, including laser-produced fluorescence, membrane interface probes and fiber-optic chemical sensors. Refer to the USEPA link for Field Analytic Technologies noted in Table 8-2 for more detailed information on these methods.

The use of field screening methods is consistent with the “Triad” approach to site investigations promoted by the USEPA (USEPA, 2010e, Triad Resource Center, 2011). This includes the use of real-time field measurements to guide an investigation or remedial action, and optimize available resources as well as reduce the need for multiple remobilizations. Field screening should follow the same systematic planning steps outlined in Section 3 of the TGM. Site investigation objectives and decision statements should be developed to guide the investigation. This should include a thorough review of the site history to identify areas for targeted collection of field data, in addition to visual clues such as soil staining, obvious disposal areas, etc. Fixed laboratory data are generally collected for final decision making purposes. As described in this section, benefits of field screening include:

  • Identification of contaminants of potential concern;
  • Identification of areas of relative higher and lower contamination and the general magnitude of contamination in order to assist in initial site evaluation and Decision Unit (DU) designation for more detailed characterization;
  • Rapid identification for removal of heavily contaminated soil prior to confirmation sampling using DU and Multi-Increment Sample (MIS) investigation approaches;
  • Assessment of contaminant variability at the scale of individual sample points in order to estimate and optimize the mass and number of increments for Multi Increment (MI) sample collection;
  • Selection of samples for initial laboratory analysis;
  • Pre-screening of samples to optimize selection of laboratory analysis method (e.g. identification of low- or high-concentrations samples for the laboratory); and
  • Carry out health and safety monitoring to determine possible worker hazards or exposures (generally not overseen by HDOH).

Field screening methods generate qualitative data (e.g., presence or absence),semi-quantitative data (e.g., above or below a pre-specified concentration), or quantitative data (specific concentration of targeted chemicals), depending on the method employed. It is important to understand both the advantages and limitations of the methods discussed. Field screening can be carried out in situ, without disturbing the targeted soil, or ex situ, on samples collected from targeted areas. Ex situ screening has traditionally been carried out on discrete samples but may also be used to screen MI samples (e.g., field XRF). The time and cost required to meet necessary data quality requirements for final decision making purposes utilizing field screening methods should be considered as part of the site investigation planning and site data quality objectives.

Note that a single test on a small mass of soil (e.g., one to ten grams) is unlikely to generate a representative mean concentration for either the bulk sample collected or the soil immediately surrounding a sampling point (refer to TGM Sections 3 and 4). Screening data for individual sample points should be used in a semi-quantitative manner, with the understanding that concentrations of the targeted COPCs in immediately surrounding soils could vary by up to an order of magnitude or more. The objective of screening is not to identify the “maximum” concentration of the COPC in the bulk sample or DU, since this is entirely dependent on the volume/mass of soil tested. Testing a small number of the likely millions of potential small masses of soil at a site will not identify the maximum concentration of the COPC present, and doing so is not relevant to investigation objectives.

The quality of field screening data will depend on representativeness of the sample tested and the method used. Higher quality field screening that generates concentrations of targeted contaminants is sometimes referred to as is “Field analysis” (see Subsection 8.2). Field screening should always be carried out in terms of “DUs” and well-thought-out DU questions and objectives, even if MI samples will not be collected at this point in the investigation (refer to Section 3). Testing of small masses of soil from unprocessed discrete samples, for example, are highly prone to error due to random small-scale distributional heterogeneity and variability of contaminant concentrations within any given mass of soil, including individual samples (see Section 4.1.1). This and other factors discussed below (e.g., soil moisture, particle size differences) can lead to “false negatives” or erroneous “cold spots” that lead to an underestimation of the extent and magnitude of contamination. Processing samples prior to screening when practical (e.g., drying and sieving prior to field XRF analysis) can help address these types of errors. However, the data are still subject to small-scale variability and erroneous decisions regarding apparently isolated “hot spots” and “cold spots.” At the other end of the spectrum, field XRF analysis of processed MI samples with accompanying field QA/QC by trained and experienced persons can generate high-quality data that rivals or exceeds laboratory data (refer to case study in Subsection 8.4.1). High quality data can also be obtained using field Gas Chromatography (GC). This could include the preservation of MI samples from excavation sidewalls or targeted intervals within borehole cores in methanol and analysis in the field to guide additional remediation or testing.

While large-scale patterns might be discernable from grid point sample data, boundaries between areas of “clean” and “contaminated” soil can be difficult to accurately establish based on traditional soil data (see Section 4). The zone between clean and heavily contaminated soil in particular is typically marked by scattered seemingly isolated “cold spots” and “hot spots” at the scale of a discrete sample (refer to XRF case study in Subsection 8.4.1. It is highly probable that this reflects random, small-scale variability of contaminant concentrations in the soil. If a new and independent set of discrete samples was collected across the same area, a similar pattern of cold spots and hot spots may appear, but in different locations. It is therefore important not to over interpret individual “cold spots” and “hot spots” identified with discrete sample data. Removal of soil in the immediate vicinity of apparent “hot spots” identified by a single or even a small number of discrete sample points is unlikely to reduce the average concentration across the area as a whole (see Section 4 for more detail on discrete sample variability). Properly designated DU-MIS soil samples are necessary to confirm initial estimates of the extent and magnitude of contamination based on grid point screening data.

This same type of random, distributional heterogeneity can introduce error in attempts to collect “splits” of samples for comparison of field screening data and fixed-laboratory data. Laboratory data may not necessarily be more accurate or representative of the original bulk sample if the sample handling and processing are not conducted appropriately. A significant and random variability between field and lab data could simply reflect inadequate processing of a sample before the splits were prepared. Refer to TGM Section 4 for additional information on subsampling of bulk samples for separate analyses.

When understood in this context, even qualitative and semi-quantitative screening data based on testing of samples from grid points designated across a targeted area could still be useful for the initial estimation of “clean” and “contaminated” areas, and improve the efficiency of more detailed followup investigations. Use of a field XRF by experienced personnel as discussed below is one example. Multi Increment samples tested at a fixed-laboratory are recommended to confirm initial decisions (see Section 4). Final confirmation could in theory be accomplished with field screening data provided that the following criteria are met: 1) An adequate number of increments (for MI samples) or alternative types of samples (e.g., discrete) of proper shape and mass are collected (see TGM Section 4 discussion of increment mass and shape), 2) The samples are assigned to well-thought-out DUs that include consideration of risk to human health and the environment as well as potential removal or remedial actions (refer to TGM Section 3), 3) Field QA/QC methods are comparable to QA/QC methods that are used at a fixed laboratory, 4) An adequate correlation of field screening data versus fixed-laboratory data is accomplished, and 5) Data for a subset of DUs are verified by replicate/independent sets of samples (refer to TGM Section 4.2.7).

The USEPA maintains a detailed overview of Field Analytic Techniques (USEPA, 2007). Refer to EPA’s web page for additional information. New methods are constantly being developed and can be discussed with HDOH for site application. Field screening methods for site investigation and cleanup are typically followed by or used in conjunction with laboratory analysis testing for decision-making. Used in concert with laboratory analysis measurements, field screening data can serve to expedite site characterization or site remediation activities and reduce overall costs.