Wisconsin Resources Protection Council

by Al Gedicks
Wisconsin Resources Protection Council

Aquila’s Communications Manager Dan Blondeau has claimed that the tailings management facility at the Back Forty is not a traditional upstream raised tailings dam design and that it is “significantly different” from the failed Brumadinho tailings dam in Brazil. Blondeau says that “Our facility was designed to mitigate the known risks of common upstream raised tailings facilities.”

According to Blondeau, the entire perimeter of the Back Forty tailings facility will be constructed of waste rock, forming a sloped outer wall at least 108 feet thick. In contrast, traditional facilities use coarse tailings to construct their wall, which makes them prone to deterioration.

Blondeau’s description of the tailings impoundment is misleading

However, Blondeau’s description of the perimeter of the impoundment does not correspond to the Dam Safety Permit Application 2L and the tailings facility cross section in Figure 12 of the October 30, 2018 Golder report , Tailings Management Facility, Waste Rock Facilities, Ore Storage Areas and Overburden Stockpile: Permit Support Design, Back Forty Project, Michigan.

As Dr. Chambers has pointed out in his review of the Back Forty Mine Permit Amendment Application, “the tailings facility cross section is that of the upstream design; that is the tailings themselves form the support for a significant part of the impoundment. These tailings will nominally be dry after closure, but it is possible for lenses of saturated tailings to exist because the tailings are not mechanically compacted after placement. (Center for Science in Public Participation, February 11, 2019). Saturated tailings can generate high saturation of the embankment and a breach in the tailings dam.

The waste rock in the embankment can produce acid mine drainage

Blondeau says waste rock will be used to build the embankment around the tailings dam. However, in Aquila’s mine permit application, the company admits that 75% of the waste rock is expected to be potentially acid-generating. Dr. Chambers asks: how will Aquila ensure that the embankment itself does not contribute to acid and metal leaching?

According to the Environmental Protection Agency, construction materials to build the embankment “must be chemically stable, “so potentially acid-generating waste rock is not suitable for embankment construction, particularly in drainage systems” (see “Technical Report: Design and Evaluation of Tailings Dams, USEPA, Office of Solid Waste, Special Waste Branch, Washington, DC, August 1994. EPA530-R-94-038, p 23).

Blondeau’s description of tailings is misleading

Blondeau says that, unlike the Brumadinho, Brazil tailings, which he describes as “a wet, muddy slurry,” the Back Forty tailings “will be high density solids” and therefore less at risk of liquefaction, when seemingly solid material can abruptly become a murky liquid, flowing downhill.

However, as Dr. Chambers has noted in his review of the Aquila’s mine permit application, the tailings will only be dewatered to 81% solids. Aquila considers this feasible, although it does not consider dewatering to 86% to be feasible because “filtering and placement costs would be high” (Mining Permit Applications before 6.0). According to Chambers, “It does not seem that the expense of dewatering to an additional 5% would be that onerous. It may be more a matter of the cost of ‘placement’ – pumping 81% solid tailings versus trucking 86% dewatered tailings to the management facility. I would question whether 81% solids is pumpable, particularly in cold weather.”

Dr. Chambers recommended a comparison of the costs of a dry stack (86% dewatered) and the proposed 81% dewatering should be provided. He also recommended that examples should be provided that indicate tailings that are 81% solids can be pumped in cold climates.

Aquila’s philosophy throughout the entire permitting process is to construct a mine at the lowest cost possible. That is why Aquila has chosen the upstream design for its tailings dam construction. If Aquila was truly concerned with minimizing “the known risks of common upstream raised tailings facilities” they would have chosen the dry stack (86 % dewatered) option. If Aquila’s assumption about the feasibility of pumping 81% solid tailings in cold weather is mistaken, there is no guarantee that they wouldn’t choose a lower cost option to pump lower density tailings to avoid the added expense of trucking higher density tailings to the management facility.

Choosing low relative density of the tailings to save costs increases the risk for water mismanagement to generate high saturation of the embankment and subsequently creating liquefaction induced flow of the tailings.

“It is not clear from the Dam Safety Permit Application, MPAA, or the EIAA” says Dr. Chambers, “ why the cost savings gained with upstream impoundment construction is more important than the increased long-term risk to the public of impoundment failure.”
The higher cost, but safer downstream construction design was never considered by Aquila.

Blondeau’s assurance about “mitigating the known risks of upstream tailings facilities “ is not supported by the evidence in their dam safety permit application

According to the EPA, “Tailings embankments constructed using the upstream method generally have a low relative density with a high water saturation. This combination can result in liquefaction of the tailings embankment in the event of seismic activity…Therefore, upstream construction is not appropriate in areas with a potential for high seismic activity” (Technical Report: Design and Evaluation of Tailings Dams, USEPA, Office of Solid Waste, Special Waste Branch, Washington, DC, August 1994. EPA530-R-94-038, p 23).

Dr. Chambers has noted in his review of Aquila’s dam safety permit application that seismic analysis for tailings dams typically require both probabilistic and deterministic analyses to determine the largest ground motion that the structure could experience.
“Neither a probabilistic or deterministic analysis was done for the Back Forty project.”

Instead, Aquila’s subcontractor, Golder Associates, used U.S. Geological Survey seismic hazard maps not intended for tailings dams but are intended for engineers and planners who design buildings. By using inappropriate seismic hazard data, “the size of the seismic event the tailings facility could experience is being significantly underestimated.”
The use of this data and the lack of a probabilistic seismic analysis, “are viewed as unacceptable for tailings impoundment design in most regulatory jurisdictions.”

Aquila has seriously underestimated the risks of upstream dams from heavy rains

James Kuipers, principal consulting engineer at Kuipers and Associates LLC states that a heavy downpour can rapidly increase the weight of the material inside the dam and liquefy relatively dry mine waste that can then spill out, overwhelming and drowning people in its path. Kuipers is a consultant with the EPA and state governments on tailings dams (see Stephen Lee, “Brazil Dam Catastrophe Sounds Alarm for U.S. Waste Ponds, Bloomberg Environment and Energy Report, February 4, 2019).

In his review of Aquila’s Mining Permit Amendment Application, Stephen Hoffman, Senior Environmental Scientist with SHoffman Consulting LLC, notes that the tailings perimeter external sump and ditch were designed to meet a l00 year 24 hour event. “Such a design is not adequate to reflect current rainfall/snow events in the area.” Aquila minimizes the potential for external erosion of the tailings dam from the runoff of rainwater by using 18-year-old data on the severity of storms. This defies science and common sense. Heavy rain has been implicated in 25 percent of global and 35 percent of European tailings dam failures (see M. Rico et al, 2008. “Reported tailings dam failures: a review of the European incidents in the worldwide context,” Journal of Hazardous Materials, 152, pp. 846-852).