|Grand Plaza Site Investigation Using the Triad Approach and Evaluation of Vapor Intrusion September 2006
This document provides a detailed report about a field study conducted by Environmental Quality Management, Inc., and it subcontractor, URS Corp., on behalf of EPA’s National Risk Management Research Laboratory, to characterize the subsurface contamination of volatile organic compounds at a commercial brownfield site.
The Triad approach was implemented to characterize the extent of soil, ground water, and soil gas contamination. These data were used to assess impact on indoor air due to vapor intrusion. Seventy-seven soil samples, 28 ground water samples, and 10 soil gas samples were collected from Geoprobe borings and analyzed on site using EPA Method SW-846 8265 direct sampling ion trap mass spectrometry. Additional SW-8260b and TO-15 analyses were performed on approximately 10 percent of the samples by off-site laboratories. Tetrachloroethylene (PCE), trichloroethylene, and cis-1,2-dichloroethylene were detected in all media, with PCE as the prevalent compound.
The on-site analyses for PCE were 22 percent higher than the off-site analyses for methanol extracts from soil samples. For the shallow soil gas samples, the on-site results for PCE agreed with the off-site analyses within about one order of magnitude for the sample pairs where PCE was present at concentrations greater that 10 parts per billion by volume (ppbv). The off-site results for the sub-slab soil gas samples were several orders of magnitude higher than the on-site results, perhaps due to limitations in the on-site sampling and analytical approach at these high concentrations. The geology was interpreted from the boreholes and logs from previously drilled ground water monitoring wells. All data indicated a small PCE hot spot that was roughly 40 feet by 40 feet (12 meters by 12 meters). The hot spot was shallow (fewer than 10 feet [3 meters] below ground surface [bgs]) on top of a low-permeability clay under the southwestern edge of the building where a dry-cleaning facility was once located.
Canister samples of indoor air were collected in April and August of 2005. The results were compared with shallow soil gas and sub-slab soil gas results to assess the impact of this contamination on the indoor air. PCE concentrations in the five indoor air samples ranged from 3.7 to 16 ppbv, with four of five results between 10 and 16 ppbv. For comparison, the ambient air contained 0.11 ppbv. The six samples of shallow soil gas collected at a depth of 5 feet (1.5 meters) bgs directly within or near the building had from 39 to 780 ppbv of PCE. The highest of the three sub-slab soil gas samples had 2,600,000 ppbv of PCE. The time-averaged indoor air concentration of 12 ppbv corresponds to a cancer risk of 2E-05, based on an inhalation unit risk of 3.0E-06 per micrograms per cubic meter and an occupational exposure scenario of 8 hours per day, 7 days per week, 50 weeks per year for 25 years.
The productivity of the aquifer was evaluated at several monitoring wells, using two methods: the slug test and a constant discharge test. The results of both types of tests demonstrate that site well yields are significantly greater than the 150-gallons-per-day criterion used by the Texas Commission on Environmental Quality to differentiate between Class 2 (potential) and Class 3 (nonpotential) ground water resources. Therefore, the shallow ground water zone at this site is designated a Class 2 ground water resource.