Closure technologies / Monitoring

Lauri Solismaa, Geological Survey of Finland, PO Box 1237, 70211 Kuopio, Finland; lauri.solismaa@gtk.fi

This section, describing Closure technologies related to post-closure monitoring, is an overview of different monitoring technologies that can be used in the area impacted by a closed mine site. Technologies are presented in the context of a certain monitoring area and in more detail in the form of checklists. Checklists are divided into different types of monitoring to be done during and after successful closure of a mine site. Some pros and cons of certain chosen technologies are also evaluated.

Monitoring obligations in Finland include:

  • The obligation to monitor the state of the environment at the mine site is set out in the environmental permit (EPA §46).
  • The monitoring is required to last as long as the activities on the site will have impacts on the environment.

A general list of post closure monitoring (Heikkinen et al. 2008) includes the following issues:

  • Geotechnical and visual monitoring of mined out areas and structures remaining in the area.
  • Filling in of mined out areas and monitoring of the quality of water forming in the filled voids.
  • Monitoring of the quantity and quality of water forming and flowing in the area (in particular in the waste storage area).
  • Monitoring of the performance of the water treatment systems.
  • Monitoring of how successful the revegetation and landscaping of mined out areas, built areas (concentrating plant, loading areas, roads) and the waste storage areas have been.

According to MLWB & AADNC (2013) different monitoring activities at different parts of a closed mine site may include the following:

  • Underground mine workings
  • Open Pit Mine Workings
  • Waste Rock and Overburden Piles
  • Tailings Containment Areas
  • Infrastructure and transportation routes at site

The following discussion is mainly based on MLWB & AADNC (2013):

Underground mine workings

Monitoring frequency of underground workings is based on the stability of the monitoring target. The monitoring frequency can be decreased upon the establishment of stable conditions. Monitoring may include visual inspections of reclaimed openings to see if there are signs of physical deterioration and checking the openings and surrounding areas for signs of underground failure (subsidence). Water quality and quantity need to be monitored underground and from controlled discharge points of workings to ensure water quality is as predicted. Also unanticipated mine-related drainage discharge points needs to be identified and monitored and passive treatment systems must be inspected for maintenance requirements. If underground workings have pressurized water inside they will need special monitoring provisions such as visual inspection, piezometers, seepage measurement weirs, and sampling to check water quality parameters.

Open Pit Mine Workings

At open pits, mine monitoring should include the monitoring of physical and geotechnical stability of the remnant pit walls, inspecting the integrity of the infrastructure (roads, fences etc.) and monitoring the interaction of wildlife with the barriers. Also aquatic habitats in flooded pits should be inspected where applicable. The water level of the pit and the quality and quantity of the water must be monitored at controlled pit lake discharge points. Identifying and subsequent testing of unanticipated areas, where water management could be an issue due to water quality or seepage quality, should be included in the monitoring program. Levels of open pit dust can be monitored also if needed.

Waste Rock and Overburden Piles

Physical, chemical and biological monitoring at waste rock and overburden piles include, at a minimum, observations of the stability of the facilities, the performance of the cover materials (including water level observations in the case of water covers), the conditions of ditches and berms, measurements of dust levels, and observations of wildlife and its behaviour at the site. The quality of water emanating from the facilities should be tested and its volume measured from controlled discharge points of workings to confirm that the drainage quality is as predicted, in compliance with the environmental permit, and not adversely affecting the environment. Unanticipated water discharge areas should be identified and the volumes and quality of such seepages measured. Technical and aesthetical aspects of revegetation activities should be monitored. Revegetation should maintain the physical stability and blend aesthetically to surroundings. Alien species may be monitored if applicable. In addition, vegetation must not have negative impact to the effectiveness of selected closure activities or become a source of metals due to uptake.

Tailings Containment Areas

The local climate conditions should be monitored (rain, precipitation, permafrost etc.) as well as their potential impact on the tailings (flooded or covered). Monitoring of tailings containment areas should also include physical stability and safety reviews of dams, spillways and other structures that remain after closure. Inspections of seepage collection systems for water flows and assessment of seepage water quantity and quality should be made periodically. Revegetation should contribute to the physical stability of the facility and blend aesthetically to the surroundings. In addition, vegetation must not have negative impact to the effectiveness of selected closure activities (e.g. roots penetrating the cover layer in an unplanned manner/degree) or become a source of metals due to uptake. The dispersion of dust and its potential risks to the surrounding environment should also be monitored.

Infrastructure and transportation routes at the site

General inspections of the area include surface observations, because parts of the old infrastructure might be buried and remnants of buried material can migrate to the surface as a result of frost-heave. Stability of slopes and formation of breaches must also be monitored. The impact of stream-crossing remediation and any degradation associated with decommissioned roads, such as erosion or ponding of water, should be checked and the quality of water downstream of remediated areas of contamination needs to be monitored. Old transportation routes can be acid producing and potential sources of harmful substances if gangue has been used as a building material or if dust from ore trucks has contaminated the routes. Monitoring program of old infrastructure and transportation routes also should include observations of the movement of wildlife and people in the area. Vegetation should be monitored to determine whether the related closure objectives are or will be met.

Checklists for monitoring:

References

Environmental Protection Act, section 46 (Finland)

Heikkinen, P.M. (ed.), Noras, P. (ed.), Salminen, R. (ed.), Mroueh, U.-M., Vahanne, P., Wahlström, M., Kaartinen, T., Juvankoski, M., Vestola, E., Mäkelä, E., Leino, T., Kosonen, M., Hatakka, T., Jarva, J., Kauppila, T., Leveinen, J., Lintinen, P., Suomela, P., Pöyry, H., Vallius, P., Nevalainen, J., Tolla, P. & Komppa, V. 2008. Mine closure handbook. Espoo: GTK; VTT; Outokumpu Oyj; Finnish Road Enterprise; Soil and Water Ltd. 169 p. http://en.gtk.fi/informationservices/publications/publications/latest/publication/EJ74.html

MLWB & AADNC 2013. Guidelines for the Closure and Reclamation of Advanced Mineral Exploration and Mine Sites in the Northwest Territories. Mackenzie Valley land and Water Board & Aboriginal Affairs and Northern Development Canada 88 pp. http://mvlwb.com/sites/default/files/documents/wg/WLWB_5363_Guidelines_Closure_Reclamation_WR.pdf