Formatted Title

Comparison of Chlorinated VOC Field Screening and Laboratory Results to Support a Vertical Aquifer Profiling

Background/Objectives

EA utilized a field-based screening method which uses colorimetric gas detector tubes to detect low concentrations of chlorinated volatile organic compounds (CVOCs) in groundwater or soil slurry samples. The screening method provides information on the presence or absence of CVOCs in the sample. The primary goal of the screening method is to provide quick, low-cost data that can help determine sampling frequency and coverage to identify contaminated zones within soil and/or groundwater. The screening method is designed to provide data that, in conjunction with laboratory analytical methods, can help identify CVOC hot spots.

At the project site, groundwater has been impacted by a plume of CVOCs, contaminating municipal wells. Since 2004, multiple sampling events have identified TCE, cis-1,2-DCE, and vinyl chloride as contaminants of concern (COCs), though a source has yet to be identified. Pump-and-treat systems have been installed to address CVOC contamination of the municipal water supply but are insufficient for long-term treatment. EA is performing a Remedial Investigation using vertical aquifer profiling (VAP) techniques to fill data gaps, determine the nature and extent of groundwater contamination, and identify potential flow pathways within and between aquifers in unconsolidated material and bedrock. This study aims to identify the accuracy of field screening results by comparing data collected in the field to laboratory data in order to understand the benefits and drawbacks of using the field screening method to target discrete sampling intervals during VAP investigations.

Approach/Activities

The investigation is using sonic drilling methods to collect groundwater samples from five aquifers, two within unconsolidated material overlying bedrock and three within bedrock. During the initial phase, cores were advanced to top of bedrock, encountered from 216 to 298 feet (ft) below ground surface. Further phases will include a bedrock investigation, monitoring well installation, and sampling of newly installed wells. As part of VAP activities, a field screening tool was used to target specific groundwater samples for lab analysis. The screening system involves volatilizing grab samples in the field to provide an estimated range of total CVOC concentrations within the sample. Results of the field screening were used in conjunction with geological assessment of the vertical aquifer profile to select samples for full lab analysis. Generally, at least one lab sample was collected within each transmissive zone, regardless of screening results. For transmissive zones greater than 10 ft in depth, sample screening was conducted within each 10-ft interval of the zone. Lab samples were collected from intervals that showed detections with the field analysis. Additional samples were collected from above and below the detection interval to delineate the vertical extent of contamination, where possible.

Results/Lessons Learned

Out of 51 lab samples, 32 had results within the range estimated by the screening results (success rate of 63%). For each of the 19 cases where the results fell outside of the range, lab results indicated higher concentrations than estimated by the screening. The field screening gave no false positive detections. In nine samples with field readings of 0 (estimated total CVOC concentration of 0-5 µg/L), total CVOCs were greater than 5 µg/L according to lab results; 12 samples with field readings of 0 had lab results with an exceedance of health risk limits for one or more of the primary COCs. Though further analysis is still being performed, the initial comparison of field screening results to lab results shows that the screening method may underestimate total CVOC concentrations within a sample. Care should be taken when making decisions based on screening results, especially involving COCs whose health-based criteria fall within the ultra-low detection range of the screening method. Additional data should be collected to determine the benefits and potential drawbacks of using the screening method to make field determinations of sampling frequency.