The Illinois Environmental Protection Agency (IEPA), prepared this fact sheet to address concerns about groundwater contamination originating from the former Monterey Coal Mine #2, now known as ExxonMobil Coal USA, Inc. (ExxonMobil), and potential effects on the surrounding private wells.
ExxonMobil began surface reclamation at the Mine #2 site early in 2005 and completed work in early 2007. After the required surface grading was completed in June 2005, a clay slurry wall was placed into the Pearl Aquifer along the southern edge of the refuse disposal areas (RDAs). The placement of the clay slurry wall was chosen because the groundwater in the Pearl Aquifer flows in a southwesterly direction in the vicinity of the RDAs. The slurry wall functions as an impermeable barrier to redirect contaminated water in the aquifer to the west, where it is pumped out by extraction wells and treated in a series of vegetative drainage swales on-site before being discharged to the Kaskaskia River. After installation of the slurry wall, ExxonMobil installed two new monitoring wells, MW-19 and MW-20 in 2006, south of the slurry wall, but on the ExxonMobil property.
Both wells MW-19 and MW-20 contained high levels of iron and manganese, which can come from coal mine waste. MW-20 also contained sulfate, which when found at this location, pointed to the mine as the source of the contamination. These results were confirmed by two samples from each well.
As part of ExxonMobil’s Corrective Action Plan approved by IEPA in 2002, ExxonMobil is required to conduct further investigation if significant increases in existing contamination or new areas of contamination are found. ExxonMobil is required by the Corrective Action Plan to conduct a study of the nature and extent of contamination, and propose actions to IEPA that will correct the problem.
Yes. A number of temporary monitoring wells and permanent monitoring wells were installed on the ExxonMobil property line, south of the RDAs. Because the results of this study showed that the contamination had left the ExxonMobil property in one area, additional monitoring wells were constructed on adjoining property. The monitoring wells on the adjoining property did not contain mine related contaminants. Therefore, it was determined that the mine contaminants extend a few hundred feet off the ExxonMobil property at this time. As an added precaution, IEPA and ExxonMobil’s consultants sampled four private wells near the area where contamination had left ExxonMobil’s property. The samples were analyzed by two different laboratories. The test results from both sets of samples were similar. The private wells tested have not been contaminated by the mine.
When MW-19 and MW-20 were tested, it was found that MW-19, which was closest to the RDAs, did not contain high levels of sulfate. However, MW-20 which was further from the RDAs, did contain high levels of sulfate. Area groundwater must flow past the MW-19 location before it reaches MW-20. Therefore, any plume of contamination originating from the RDAs should be detected in MW-19 before contamination in reaches MW-20. Since these wells have not shown mine-related contamination, it has been determined that contamination at MW-20 has not been caused by a leak in the slurry wall.
The groundwater to the south of the RDAs used to be controlled by extraction wells. These wells pumped groundwater to prevent contamination from spreading south of the RDAs. During slurry wall construction, the extraction wells had to be turned off so that large construction equipment could operate safely around power lines. A likely scenario is that a slug of contamination may have escaped during the time the wells were turned off, but before the slurry sealed the aquifer flow in this direction in the vicinity of the RDAs.
As required by the Corrective Action Plan, ExxonMobil must propose a solution, subject to IEPA approval, that will address groundwater contamination. In discussions between IEPA and ExxonMobil, the company anticipates their consultants will have a report containing the Company’s proposal complete in May 2007. IEPA will approve a remedy and keep the public informed.
The IEPA plans to sample 3-5 additional private wells to the south and west of the ExxonMobil property to assure residents of the safe quality of the groundwater should they choose to drink it, or use it for other domestic and agricultural purposes. Though no specific dates have been set, the IEPA plans to sample selected private wells in early summer 2007, provided wells are accessible and permission is given by well owners.