The origin of water leaks in Saskatchewan potash mines
Wittrup, Mark B.
Recent mine-level floods at some Saskatchewan potash mines have demonstrated the need to identify accurately the source(s) of water leaks so as to assist in remedial or preventative action. Chemical compositions of the water are inadequate tracers of these leaks because the concentrations of most of the elements change during the migration of the fluids. In marked contrast to most of the chemical constituents dissolved in the water, the stable isotopes of oxygen and hydrogen are conservative elements of water. Within the Elk Point Basin, Î´D and Î´ 18O values of basinal waters normally increase with depth because of mixing between trapped formation waters, which have isotopic compositions near those of seawater, and local surface waters, which have much lower Î´D and Î´18O values. Thus, the water in each aquifer within the basin generally has a unique isotopic composition in any given vertical section. This unique value, when compared to the flood or leak waters, allows an assessment to be made as to the origin of the flood waters. This comparison can be made irrespective of the changes that occur in the chemical composition during the migration of the fluid. Flood waters in the potash mines of Saskatchewan have chemical and stable isotopic compositions that indicate three different origins for mine level fluids. These are, 1) "normal", halite and sylvite saturated, basinal brines from Devonian formations directly above the deposits, which are a combination of meteoric waters and connate waters mixing within the basin, 2) "short circuited", undersaturated waters of predominantly meteoric origin, from stratigraphically higher aquifers such as the Cretaceous Mannville Group, and 3) Ca-rich brines that may represent ancient fluids associated with recrystallization of the evaporites. Short circuiting probably occurs when relatively "fresh" formation waters are directed to lower stratigraphic units through a collapse structure or related feature. Mining may reactivate the structures through which these "short circuited" waters flow.