Use of analysis of seasonal hydrochemical regime for better understanding of mine water genesis and more accurate estimate of its impact on stream water quality at flooded Rudňany ore mine

Seasonal hydrochemical regime of water flowing out of the Rochus Fe-Cu mine in the Spiš-Gemer Ore Mts. was monitored by discharge measurements and laboratory analyses of mine water chemical composition. Regression analysis of these data showed a statistically significant dependence of concentration of many important chemical elements on mine water discharge. The obtained data made it possible to better understand the processes of mine water chemical composition genesis, as well as to determine in more detail the degree of its environmental impact. Geochemical calculations and forward geochemical modelling suggest that chemical composition of mine water is formed in three stages. First phase takes place in the aeration zone of the mine, where ankerite dissolution, intensified by pyrite oxidation, dominates. In saturated zone of the mine, ankerite dissolution is probably controlled by CO2 input in open carbonate system. These two main geochemical processes take place permanently and at the time of low flow conditions they lead to relatively stable composition of water outflowing from mine. In time of higher flow conditions, concentration of SO4, Mg, Ca, Na and As significantly increases, probably as a consequence of mixing with stagnant water from deeper or hydraulically more isolated parts of the flooded mine. Water flowing out from mine enters the Rudniansky potok creek and contaminates it mainly by manganese. The limit for Mn content in surface water is exceeded here at the time of low flow conditions – usually for 180 days a year. Anomalous concentrations of SO4, Mg, As and Sb in mine water are sufficiently diluted in stream water were they do not exceed environmental limits.