Richard N. Downer

Re: Review of "A Critical Evaluation of Slope Stability and Drainage of Surface Water and Groundwater at the Elizabeth Mine Site." by Richard W. McGaw, P.E.

The title of Mr. McGaw's paper suggests that he has done a critical review of the slope stabilities of TP-1, TP-2 and TP-3. However, I am not aware that sufficient geotechnical data yet exists to make a definitive analysis of the stability of the slopes. The EE/CA process is an opportunity for the community to make suggestions and express concerns to EPA. Mr. McGawÕs comments should be forwarded to EPA.

It is still my opinion that future operation and maintenance costs can best be reduced by covering the tailings, cutting off seepage and by stabilizing the slopes. And, that a great deal of site specific testing still needs to be done before final design begins. Only after the testing is done will a final design emerge.

With respect to TP-3 and the North Cut, Mr. McGaw writes, " All piles, including the walls and floor of the north cut, are clearly stable against gravity and against the action of runoff of rain and melting snow." True, the piles have not moved yet. But, that does not mean they are stable. Experience has taught the engineering profession that constructed slopes steeper than 3H : 1V are not always stable. Thus, extensive geotechnical studies of TP-3 are warranted to prove beyond a reasonable (high factor of safety) doubt that the slopes will always be stable.

"No major water courses have been formed, although two or three riverlets converge to form Copperas Brook." True, there has been no deep incisement above Copperas Road (Class IV). However, it is clear that surface erosion is taking place. The piles of TP-3 are well rounded and the riverlets that Mr. McGaw mentioned show no evidence of fine-grained materials in their bottoms. All the fine-grained materials are being washed away.

Just below Copperas Road, the brook bed has eroded down to the bedrock and the timber cribbing. In one or more places the brook has changed course as it down-cuts. Out-wash depositional fans are evidence of erosional material movement.

A large depositional fan just above Mine Road is further evidence that TP-3 is eroding. This is the area where EPA proposes to construct a 1 acre detention pond to settle out much of the material migrating down from TP-3.

With respect to Copperas Brook and TP-2, Mr. McGaw presents a convincing scenario for the failure mode of the decant pipe. His recommendation for closing off the erosional gully of TP-2 with rock fill and a pipe certainly deserves consideration by EPA. His suggestion for finishing off the face of TP-2 seems feasible and practical, "The toe of the entire face of TP-2 would be protected by a sloping rock fill placed to the same height as in the gully. . . . Particular attention would be given to the newly placed material in the area of the gully, possibly providing some form of internal reinforcement at the face."

I do not agree that infiltration into the top is negligible. Until geotechnical tests are done, I will continue to advocate for a cap or cover over TP-2 to reduce future seepage and pore water buildup which could contribute to the global instability of the mass. With respect to TP-1 and Lower Copperas Brook, Mr. McGaw presents a reasonable argument for separating the flow off TP-2 from the flow off TP-1. EPA proposed treatment cells (ALDÕs, SAPÕs or SRBÕs) could easily be modified to accept the separated flow. His suggestion for placing a pipe at the toe of TP-2 and diverting the flow in a northly direction toward the gully on the north west side of TP-1 deserves investigation.

Mr. McGaw's proposal for isolating only the pond area of TP-1 is intriguing. Whether the scheme is feasible depends on whether the currently installed segmental concrete pipe is adequate to pass the projected flows and whether it can be made absolutely leak-proof. Reinforce concrete pipe is notorious for separating at the joints due to ground forces. Again, Mr. McGaw has suggested a fresh approach that should be investigated as a part of the design phase. Hydrologic and hydraulic computations will determine the adequacy of the decant pipe, while a thorough internal inspection should find any structural deficiencies. It may be possible to line the pipe with one of several new polymer liners.

Without data it is presumptuous to suggest factors of safety for the stability of TP-1. I would prefer to wait until EPA has done extensive geotechnical testing and analysis before stating the face of TP-1 will always be stable.

However, I do agree that McGaw's recommendation for a rock face on TP-1 seems reasonable. "It is recommended that a permanent rock fill be constructed across the entire toe of the front face of TP-1 to carry a roadway wide enough to accommodate trucks and heavy equipment such as a crane. A sloping zone of rock rip-rap could then be placed against the face to a height of an additional 15 or 20 ft. The rock fill and rip-rap would serve to protect the toe of the front face against further erosion that might have a destabilizing effect. A similar rip-rap should be utilized for the lateral face of TP-1."

Mr. McGaw's call for additional site specific chemical testing of TP-3 is wise and may support his suggestion for segregation of the flows emanating from TP-3. Segregation of the flows may reduce the size of the passive treatment systems.

However, a conservative and prudent approach should address the seepage flow from infiltration. Average annual precipitation is about 35 inches per year, whereas average annual evaporation is only 19 inches per year, most of which occurs between May and October. This translates to 16 inches that either infiltrates or becomes surface runoff. Vermont is described as having a humid climate because 47% (16/35) of its precipitation ends up as flow in the rivers. It is the water that keeps Vermont green and maintains our groundwater levels. But, it is also the water that leaches the minerals from TP-3. It is the water that degrades the water quality of Copperas Brook and parts of the West Branch. Any adopted cleanup option must address the treatment of the seepage from TP-3.

Both Options 2B and 2C attempt to preserve TP-3 while addressing the continuing contamination. It seems prudent to ask EPA to adjust their future testing protocols to determine whether Mr. McGaw's suggestions have merit.

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