The site slopes from the south to the adjacent Flint River, with an elevation drop of greater than 30 feet. The southern portion of the site is covered with thick, low-lying vegetation, and the northern portion next to the Flint River is covered by concrete slabs from former Building 5.

During U.S. EPA’s initial site assessment, oil was detected in 14 of the 18 site monitoring wells. The thickness of oil in the monitoring wells ranged from 0.38 feet to 14.53 feet.  Based on the well gauging, the average oil plume could be as thick as 6 feet across 3 acres; however, capillary action is likely causing oil to accumulate in the wells disproportion to its actual thickness.

As documented by GM, the City of Flint, and U.S. EPA, oil from the Chevy in the Hole Parcel A Site is discharging to the Flint River. The Flint River is a navigable waterway of the United States. Based on site conditions and the estimated volume of oil on the site, oil will continue to discharge to the Flint River unless a removal action is taken.

Summary of the investigation results

The purpose of the preliminary investigation was to define the vertical and horizontal extent of Nonaqueous-Phase Liquid (NAPL) at the site, estimate aquifer properties, estimate NAPL recovery rates, refine existing volume and cost estimates, and determine the waste characterization of the NAPL, soil and groundwater for treatment and/or disposal.

To determine the horizontal and vertical extent of NAPL, NAPL and soil samples were collected,  ultra-violet optical screen tool (UVOST®)-laser-induced fluorescence (LIF) was conducted at 25 locations, Roto-Sonic soil borings were conducted at 21 locations, OIL-IN-SOIL™ screening test kits were used, and visual observations and instrument results were collected.  The results of the NAPL data were provided to the U.S. EPA ERT and was input into a three-dimensional model and a solid surface representing the possible shape and volume of the NAPL was created.

Two NAPL samples were collected and 24 soil samples were collected, analyzed, and the results were compared to NAPL sample results.  This comparison was used to determine whether NAPL was present at the sampled locations based on constituent similarity and concentrations.  A total of 25 UVOST® LIF borings were advanced on-site. Seven of the 25 UVOST® LIF had a response of greater than 150 percent (% ) reference emitter indicated the potential for NAPL.

 The NAPL properties are consistent with mineral oils and hydraulic oils that are stable in high temperature and pressure environments.  The NAPL exists throughout the property in the small pore spaces of the predominant sandy silt and appears at different elevations and locations within these tight formations. 

Ten of the soil boring/UVOST® LIF locations indicated the presence of NAPL.  NAPL appears to be present throughout the site with the greatest extent and thicknesses located in the center to the northwest section of the site.  NAPL thickness ranged between 2 to 5 feet across the Site with the greatest NAPL thickness located near the center of the Site at boring SB28.

A total of eight slug tests, three NAPL recovery tests, a single well pumping test, and a single step-drawdown test were completed as part of this field effort to evaluate removal options involving NAPL recovery and groundwater extraction.  The aquifer testing results indicate that the hydraulic conductivities and transmissivities on-site range from low to very low.  Therefore, NAPL recovery will be slow.

The investigation results show that a zone of higher hydraulic conductivity exists along the northern portion of the site and lower hydraulic conductivity soils are present throughout the rest of the site and to the south as indicated in historical reports.  However, the higher hydraulic conductivity soils to the north are still lower permeability soils consisting of variable sands, gravels, and silts.