Mine water discharges are the most significant reason for mine-derived adverse impacts on the environment. Treated mine effluent is usually released into nearby rivers or lakes. Mine water discharge from sulphidic metal ore mines is typically saline due to elevated levels of solutes such as sulphate. Sulphate has been shown to be one of the primary indicators for detecting mine-derived pollution in rivers, even before detecting changes in acidity and pH (e.g. Parker and Carey 1980; Rikard and Kunkle 1990). Furthermore, sulphate has been accounted for as the main contributor to the adverse impacts of Kittilä mine in Finland on freshwater ecosystems in the nearby Seurujoki River (e.g. Pöyry 2008; Palmer et al. 2015). In most countries, however, there are no water quality guidelines for sulphate concentrations in surface waters. Sulphate also correlates well with the electrical conductivity (EC) of waters. In contrast to mine water, EC in natural rivers tends to remain within a rather constant range. Thus, EC is a useful general measure of river water quality that can be applied as a baseline for comparing and detecting the influence of contamination sources in rivers (e.g. Younger 1995; Banks et al. 1997).