Decant of sigma colliery

English: This study of Sigma Colliery was intended: • To determine the water quality of each aquifer associated with the mining area. • To determine the overall electrical conductivity profile of the mine to aid in the overall management of the mine. • To delineate possible decant positions with the help of water levels and to determine what the water quality of the possible decanting water will be. • To discuss the use of fly ash as a backfilling material in underground mines with the help of case studies. • To determine if ashfilling is a viable option for Sigma Colliery. From the study of Sigma Colliery, the following conclusions and recommendations could be made in this document: • A total of 12 samples were obtained from the shallow aquifer, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Only sample WW024D was classed as above the maximum allowable limit and not suitable for human consumption. Four samples were classed within Class 2, suitable for human consumption for a limited duration use only. A total of seven samples were classed as Class 1, suitable for human consumption. • A total of 23 samples were obtained from the intermediate aquifer, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Samples WW036, NW021, NW037 and UG001 were classed as above the maximum allowable limit and not suitable for human consumption. Three samples were classed as Class 2, suitable for human consumption for a limited duration use only while 16 samples were classed as Class 1, suitable for human consumption. • A total of 21 samples were obtained from the deep aquifer system, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Samples NW006, NW036, NW040, NW041, NW042, NW044, NW046, NW051, UG027D, UG071, UG072D and UG072M were classed as above the maximum allowable limit and not suitable for human consumption. Two samples were classed within Class 2, suitable for human consumption for a limited duration use only and seven samples were classed as Class 1, suitable for human consumption. • A total of six samples were obtained from the disturbed aquifer system, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Samples UG014 and UG023 were classed as above the maximum allowable limit and not suitable for human consumption. Two samples were classed within Class 2, suitable for human consumption for a limited duration use only and two samples were classed as Class 1, suitable for human consumption. • A total of 20 samples were obtained from the ashfill boreholes, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Samples UG033, UG034, UG044, UG069, 812/179, 812/1830, 812/183M and 812/183S were classed as above the maximum allowable limit and not suitable for human consumption. Only one sample (sample C316/47) was classed within Class 2, suitable for human consumption for a limited duration use only and 11 samples were classed as Class 1, suitable for human consumption. • A total of 19 samples were obtained from the mine boreholes, classified according to the SANS 241 :2006 drinking water standards and discussed in this document. Samples UG0130, UG024, UG0370, UG046, 812/530 and 812/53S were classed as above the maximum allowable limit and not suitable for human consumption. Five samples were classed within Class 2, suitable for human consumption for a limited duration use only and eight samples were classed as Class 1, suitable for human consumption. • All 94 boreholes were profiled and a three-dimensional image of the whole area was created with the use of the electrical conductivity profiles. From this image created, varies possible decant areas were identified and the water quality of these possible decant areas were discussed. • The ashfilling used turned out to be a viable option when there was still mine void space available before the mine was filled up with water from the flooding. • Since the mine voids were filled with water and the fine ash slurry was pumped into the mine voids, the conditions was disturbed in that the void space in the mine was decreased with some volume of water. • The storage of the strata above is not enough for the water that was pumped in with the ash and the water is therefore forced to decant. • The only place where decant and pollution of the shallow aquifer is evident is where ashfilling has been done. • The situation improves as soon as the ashfilling is ceased and this can be substantiated by the water level and water quality behaviour of borehole UG069. The water levels of borehole UG069 has been at decanting levels from September 2009 to March 2012 (Figure 15-3) after which the water level started to decrease again after the ashfilling was ceased. From the electrical conductivity profiling done of borehole UG069 (Figure 15-2) (after the ashfilling was ceased) the profile already indicates that the upper part of the water column is of a better quality than deeper down. • In this document it is therefore recommended that in the event that the company would like to continue with the ashfilling, they should pump water out of the mine to provide void space and prevent decant. • The ash slurry should simultaneously be injected into the mine with the water that is pumped out. This should be pumped in equal volumes, which will prevent other problems such as the collapsing of the mine roof.