No abstract

Mine water discharges can cause negative effects on recipient rivers. The magnitude of the effects depends on how quickly and efficiently contaminants dilute in the river. Electrical conductivity (EC) is linked to the water quality and can be utilized as a general tool to compare and detect the contaminant sources and concentration changes derived from mine water discharge. Hydrological and flow velocity profiling devices were tested to assess the impact as well as mixing and dilution of mine effluent discharge in a river next to a gold mine in Finland. Additionally, flow rate and velocity profiles of the cross-sections were measured. Recently, the greatest ecological impacts had been detected in the vicinity of the dewatering discharge point, which has the highest sulphate concentrations in rivers. According to EC measurements of this study, these were the same locations, where the mine effluent did not dilute and mix efficiently due to lower flow velocities and lesser turbulence. Further, EC values displayed a significant positive correlation with sulphate, magnesium, potassium, sodium, and calcium, whereas a lower correlation was observed with the trace elements. The tested study method revealed how changes in the river morphology and flow velocity affect behaviour, mixing, and dilution processes. Mixing and dilution of contaminants depended on the discharge location and method as well as on the density differences between the mine water discharge and fresh river water. This study highlights the importance of detailed hydrological and flow rate measurements when designing the location of mine water discharge to recipient rivers.

The behaviour of arsenic (As) derived from tailings was investigated at the Yara Siilinjärvi apatite mine and industrial site in eastern Finland. The study assessed factors influencing the migration and fate of As and compared the anthropogenic As load to the natural geogenic background. Environmental risks related to As were assessed by examining the As concentrations in humus, glacial till, aquatic sediments, groundwater, and surface water. The occurrence and fractionation of As and the presence of secondary precipitates and geochemical transformations in the tailings and in the ambient soil and sediment were evaluated by selective extraction. The water-derived emissions were evaluated by field measurements, hydrogeochemical analysis, and modelling. Results indicate elevated environmental risks due to dust and seepage emissions from the tailings since the concentrations and mobility of As and other potentially harmful elements (PHEs) such as Co, Ni, and Zn were elevated relative to the geogenic background. These elements were mainly associated with Fe (oxy)hydroxides in the soil and their mobility was closely linked to Fe biogeochemistry. Additionally, although the concentrations of As and PHEs were high in the tailings pond and seepage water, they decreased in ambient groundwater and surface water, indicating Fe (oxy)hydroxide stability. This was supported by hydrogeochemical modelling, which indicated precipitation of Fe oxides and hydroxides. According to speciation modelling, As was present mainly as toxic trivalent arsenious acid (H 3 AsO 3 ) in groundwater and as the less toxic pentavalent As acid (H 2 AsO 4 -and HAsO 4 2-) in surface water.

Mining operations disturb the natural hydrological cycle at a local scale by creating new pathways for water flow and conveying large amounts of water, both for use in ore beneficiation and through the dewatering of mining workings. Sulfide minerals exposed to water and oxygen are oxidized through chemical and microbial-enhanced reactions, significantly altering the quality of water (acid mine drainage). Stable isotopes may reveal important information concerning water origin and transfer, as well as chemical processes and solute fluxes within the aqueous environment. Several case studies illustrate the use of stable isotope techniques for clarifying aspects related to water circulation in mining areas within Romania and Finland. Hydrogen and oxygen stable isotopes coupled with total dissolved solids have been used to characterize mine water and for assessing water sources, flow paths and mixing processes at some Romanian mining sites. At Talvivaara mine in Finland, water isotopes and chemical composition were used to determine the extent of waste water contamination in groundwater systems after a leakage at gypsum ponds.