Photo 1 |
As part of the effort to clean up or isolate historic waste disposal areas in the Bear Creek Valley, two oil disposal waste ponds were closed in 1989. The ponds had historically received waste oils that were also contaminated with PCBs and they were unlined which led to leaching of contaminants into the groundwater, which surfaced into a small tributary (NT7) and thus into Bear Creek. The extent of contamination of soils was so great that merely removing the waste pond would not be sufficient to eliminate continued leaching of PCB contaminated oils into the surface waters. Thus, it was decided to pump out the oil and cap the pond and NT7 tributary with plastic and a thick layer of clay. |
To compensate for the loss for the natural stream channel a new channel was designed to flow alongside the cap and be connected to Bear Creek (Photo 1). The new stream used the existing natural headwaters of NT7 and had approximately 0.5 km of reconstructed channel. Construction of the restored stream was affected by hydrologic concerns related to the cap. To ensure that heavy rains did not erode the cap, the channel was designed to handle a 200-yr flood. The channel was lined with large (30 cm diameter) rip-rap rocks, and underlain by an impervious plastic fabric (Photo 1).
These requirements resulted in a wide deep channel which under normal flow conditions created a shallow stream with intermittent surface flow (much of the water traveled through the rip-rap gravel substrate, just below the surface). However, the design did try to mimic other features of natural stream channels. The engineering included berms to create a pool:riffle structure, a winding layout that provided some sinuosity to the channel, and a mixture of smaller gravel sizes was added to the rip-rap (Photo 2). After construction, trees were planted, although outside of the immediate riparian zone, and the recovery of the remaining vegetation was by natural succession. As far as the impact on the natural biotic communities, the BMAP initiated pre- and post-construction surveys to compare the fauna and flora in the natural channel versus the artificial channel. |
Photo 2 |
Surveys were made in the natural stream and riparian zone in 1989 (Photo 3), which had to be treated as a hazardous waste area, requiring proper gloves and chemical suits, as well as a de-contamination process. Surveys included a complete fish community analysis, stream habitat analysis and riparian vegetation. After construction, these surveys were continued over the next decade to evaluate how successful the stream reconstruction had been. Results of the surveys follow.
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Last Modified: April 17, 2001