A long legacy of coal mining in the Sydney Coalfield which ended in 2001 has left a complex and extensive network of abandoned underground workings. With the shutdown of mining operations and the end of active pumping, the mines began to flood. This rising water is typically of poor quality, and when it surfaces contact with air promotes the precipitation of dissolved metals such as iron. The result is red ochre staining sometimes referred to locally as “copperous” water.
In the past, acid mine drainage (AMD) and associated environmental impacts were an accepted part of mining operations. Recently, however, methods to control these problems are better understood, and many of these new approaches have been implemented in the New Victoria Mine Water Treatment Plant.
Located in the heart of the Sydney Coalfield, this “active” treatment plant is a high density sludge system. It uses hydrated lime to reduce the acidity of the mine water, raises the pH and promotes the precipitation of dissolved metals. The precipitated metals are then collected as a solid residue and are disposed of in an on-site containment area. After the mine water undergoes active treatment it is directed for final polishing to a passive treatment system, which consists of a settling pond and constructed wetland, before it is discharged into the Atlantic Ocean.
New Waterford and Sydney Mines pools treated at one plant
Enterprise Cape Breton Corporation retained the services of CBCL In January 2010 to design and implement a mine water treatment scheme for the New Waterford mine pool only. In the process of refining the concept design, the CBCL team identified an opportunity to access the Sydney Mines mine pool water along with the New Waterford mine pool water at a single surface location.
The significant benefits of this revised approach included:
• the elimination of the need for a second active treatment plant, reducing both capital and long term operating costs;
• the elimination of two existing acid mine drainage seeps;
• an optimized treatment plant location, relative to sludge disposal;
• the management of sludge generated from one, not two, separate treatment facilities;
• an optimized location for possible future passive marine discharge; and
• the opportunity to use artesian flow to access mine water (reducing pumping requirements).
Mine water management requires a detailed understanding of mine workings interconnections. The large body of water within one or more interconnected workings is referred to as a mine pool, and the movement of mine water within the pool is dependent on how the collieries are connected. The Sydney Mines and New Waterford mine pool systems cover an area of approximately 50 km2 with approximately 42 km2 located under the Atlantic Ocean.
The location of the plant in New Victoria is at the site of a former airshaft tunnel that connects to the Sydney Mines mine pool. This is a critical component of the system, because from this centrally located site, both the New Waterford and Sydney Mines mine pools can be accessed.
The transmission of mine water from the two pools to the new plant involves complex mine water hydraulics using pumping wells and pipelines.
Establishing the dynamics of the mine water flows
To understand the dynamics of the individual mine pools, a complex and challenging drill program into the old workings was completed to collect data on the mine pool water elevation and chemistry. Subsequent borehole monitoring, a study of the historic mine plan and modeling of the seam structure were done to establish the inflow rates and the quality of the mine pool water. This baseline data was critical in establishing the rate of mine water level rise, the estimated timelines before equilibrium levels would be reached, and the volume and quality of water requiring treatment.
Given the presence of variable flow rates and the uncertain chemistry of the mine water, the treatment plant was designed for two phases of operations with different flow rate ranges. It was also designed for deteriorating mine water quality, and the suspected poor quality of water in the inaccessible submarine areas of the deep Sydney Mines pool.
The plant control system not only provides local control of the treatment system, but also acts as the central control point for remote pumping systems that control water levels in various underground mines. The comprehensive program addresses not only the direct impacts of acid mine drainage but also related concerns such as ground subsidence, the potential infiltration and contamination of groundwater aquifers, and the displacement of mine gases. The social and economic impacts on both private property and public infrastructure could be catastrophic if mine water from historic coal mining is left uncontrolled.
A geothermal heating and cooling system was incorporated into the facility to take full advantage of the huge geo-thermal energy source that is the mine pool water itself.
The construction and final commissioning of the plant was completed in February 2013 before the mine water levels reached critical levels. cce
Project name: New Victoria Mine Water Treatment Plant, Cape Breton, N.S.
Award-winning firm (prime consultant): CBCL, Sydney, N.S. (Aaron Baillie, P.Eng., Lorna Campbell, P.Eng., Donnie Arsenault, P.Eng., Robert Dickson, CET, Mark MacNeil, CET, Glenn MacLeod, P.Geo., Kevin MacPherson, Brad
Kennedy, P.Eng., Kori MacPherson, P.Eng., Richard Morykot, P.Eng.)
Client/owner: Enterprise Cape Breton Corporation
Other key players: Atkins International (water quality modelling); BGC Engineering (geotechnical investigation); Conestoga-Rovers and Associates (mine workings mapping).