CBS Notification of Borings at IC Springs

November 29, 1999

As you requested, prior to proceeding with the exploratory borings we plan to do at the Illinois Central Spring emergence, we are providing you with the following information:

1. The reason and purpose for drilling exploratory borings near the Illinois Central spring emergence.
2. The construction details of the exploratory borings.
3. Location of the exploratory borings.
4. Estimation of the effect of the borings.on the spring emergence.

Item 1: The reason and purpose for the borings.

1. To identify pathways of PCB transport nearer to the landfill with the goal of reducing the total amount of stormwater needed to be treated during peak PCB transport periods. The nearer to the landfill that such conduits are intercepted the less uncontaminated stormwater from other parts of the subterranean drainage basin will need to be treated. -
2. To identify source areas of the high PCB storm pulses and evaluate their potential to be remediated.
3. To aid in the placement and re-routing of drainage around the landfill area to minimize PCB transport and maximize post-excavation stability of the remaining consolidated landfill material.

CBS is employing several geophysical methods in the attempt to locate the major conduit(s) that transport the high PCB pulse from the landfill area. Because we know that Illinois Central Spring is the first resurgence of the Groundwater carrying, that PCB pulse and that it is the final convergence point for the conduits that convey that Groundwater, it would be logical to gain some knowledge of the transport system and its characteristics at this known point. The approach will be to perform the geophysics in the area of the spring, then drill and intersect the conduit. CBS can then compare the geophysical signatures of a'tsuccessful boring", i.e. one that intersects the PCB carrying conduit, and then look for those distinctive geophysical characteristics in the surveys that have been conducted nearer to the landfill. Drilling near the q\_^ Dive information on the elevation of those conduits and their,probable maximum dimensions. There is also the potential that the PCB carrying conduit may be able to be tracked back to its source area from the spring. Again, this will necessitate an exploratory boring to determine which conduit is carrying PCBs, since the geophysics may be able to identify conduits, but obviously not which ones carry PCBs. In summary, the borings are exploratory in nature and are not intended as recovery wells or even, necessarily, as monitoring wells, although some initial sampling will, of course, be conducted.

Item 2: Construction details.

Item 3: Location of borings.

Figure 15 is a resistivity pseudo-section through the L-20 line, that is, it is a cross-section based on resistivity imaging. The section shows a prominent anomaly at -90 to -92.5 location, which is the main anomaly we intend to drill. As a backup, CBS will set surface casing at -72 and - 107 feet on the L-20 line. These borings may be drilled if a conduit is not successfully located on the first try at the -92 location to be used as cross-hole tomagraphic stations. If they are not drilled, they will eventually be grouted and abandoned Figure 17 is a similar cross-section along the L-80 line. It shows resistivity anomalies at - 2 and -97 locations that CBS also intends to drill. Attached is a modification of EarttiTech's C-5 drawing which shows the proposed drill locations.

Item 4: Estimation of effect of drilling.

The use of geotechnical borings to investigate karst formations is standard procedure for most projects in karst and CBS is unaware of any special precautions used by investigators in such situations. Nor is CBS aware of any situations that resulted in substantial collapse due to such borings. In fact, even in situations where very large diameter holes (up to 10 feet) have been placed in conduits, such as at Mammoth Cave National Park, no substantial collapses occurred.

In the unlikely event that a large-scale collapse did occur, the spring waters may still remain within the existing area and the EPA intake structures may not effected. If the EPA intakes did become less effective, relatively minor excavation and clearing of the spring area should alleviate any problems.

74I would be glad to meet with you and review this letter and the locations mentioned or discuss any questions you might have at your earliest possible opportunity. We hope to resume these activities next week to take advantage of current weather conditions.