Development of a groundwater model for new Vaal colliery

Abstract

New Vaal Colliery, is an open cast mine in the northern Free State along the meandering Vaal River. The Colliery is a historically bord and pillar underground mine and the sole provider of coal to the Lethabo Power Station. The underground mine voids were flooded by water prior to open cast operations starting in 1985 as a result, dewatering forms an integral part of the mining activities. In 2011, the mine received its approved water use license which contained the condition that the all the dams on site had to be lined, including the main pollution control dam, the Maccauvlei dam. A combination of a high rainfall season, declining capacity in the Maccauvlei dam and limited options for the disposal of the mine impacted water resulted in mobile reverse osmosis water treatment plants being installed to treat water from Maccauvlei dam and the brine discharged back into the dam. Discussions with the DWA triggered the need to re-evaluate the water management strategy on site in order to illustrate that the mine was not impacting on the underlying dolomitic aquifer. Given that the site has been collecting groundwater data since the mine’s initiation, it was assumed that there are sufficient data to develop a conceptual groundwater flow model. The process of developing a groundwater conceptual model was used to understand subsurface flow dynamics at the mine. Water levels in the shallow artificial mine indicated a synchronous connection between the aquifer and the main pollution control dam, Maccauvlei dam. Furthermore, the water levels indicated that the groundwater gradient is from the mine towards the Vaal River. Water levels in the dolomitic aquifer confirmed findings from previous studies conducted at the site. The dolomitic monitoring borehole water levels remained generally above the elevation at which the Vaal River is being managed therefore confirming the confined to semi-confined nature of the aquifer. A single dolomite monitoring borehole located south of the mine also confirmed the confined nature of the aquifer. Water levels in the borehole are consistently above those of the boreholes monitoring the underground mine voids. It is therefore more likely that the dolomite aquifer is contributing water into the mine rather than the mine discharging water into the aquifer. A comparison of groundwater levels between the New Vaal Colliery and Cornelia Colliery indicated a groundwater water gradient from Cornelia towards New Vaal. The conceptual model is able to illustrate that the mine is not likely to have an impact on the dolomitic aquifer. This is in spite of the identified data limitations which include the skewed distribution of the boreholes and lack of site specific hydraulic parameters provide a basis on which further studies can be conducted.