Metal Mass Balance for Rävlidmyran Pit Lake, Northern Sweden : Implications for the metal transport to Lake Hornträsket

The industry of mining and metal production has always been in conflict with environmental issues. A problem of major concern is the contamination of freshwater around sulphide-bearing mine waste deposits. In northern Sweden many mine wastes originate from sulphide ores and thus generate acid drainage (AMD). The object of this study, Rävlidmyran pit lake, is an abandoned sulphide mine owned and operated by Boliden Mineral AB between 1951 and 1991. The aim of this study is to getter a better understanding of the aquatic system at Rävlidmyran. This is done by calculating a metal balance for the pit lake, determining the trace metal transport into the recipient Lake Hornträsket and reviewing how the water quality changes in the surface streams draining the area. Rävlidmyran pit lake is a meromictic flow-through water body with no surface inlets or outlets. Downstream of the pit lake, a waste rock dump acts as an embankment. The pit lake partially drains via seepage through the embankment and the outflowing water shows the characteristics of acid mine drainage (i.e. low pH and high levels of trace metals). Field data (pH, conductivity and temperature) were sampled at Rävlidmyran on a weekly basis from four groundwater pipes, five surface stream stations and from the surface water of the pit lake. Water samples were analyzed for total concentrations of Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Hg, Mg, Mn, Na, Ni, Pb and Zn once a month. Additional sampling was performed for this thesis in September and October 2010, when field data was measured at six locations downstream of the pit lake and a profile was measured in the water column of the pit lake (at the deepest point). The metal balance is calculated assuming steady state regarding the pit lake water balance. Inflowing fluxes of water used in the mass balance are: deep groundwater, shallow groundwater and precipitation. Outflowing fluxes are: groundwater and seepage through the embankment. Metal concentrations for the inflowing and outflowing water fluxes are mainly taken from the sampling data provided by Boliden Mineral AB. For the inflowing deep groundwater no data exist, so concentrations have been estimated from the bottom water of the pit lake. The mass balance is calculated per month for a period of one year between November 2009 and October 2010. Data does not exist for the full year, so two cases of the metal mass balance have been calculated, one with estimated maximum concentrations and one with estimated minimum concentrations. The metal transport to Lake Hornträsket is calculated by multiplying concentration data with measured water flow in the two surface streams draining the mine site. Results from the mass balance suggest that the pit lake continuously accumulates As, Ba, Cd, Co, Cu, Fe, Pb and Zn. For Mn and Ni there appears to be a net export from the pit lake. The largest uncertainties in the mass balance calculations are the metal concentrations for the inflowing deep groundwater. The importance of this parameter was judged by calculating a hypothetical deep groundwater concentration corresponding to a zero mass balance. The result from the zero-balance can be used as guideline values for concentrations of trace metals in the inflowing deep groundwater. It is recommended to install a deep groundwater well in the upstream area and compare measured deep groundwater concentrations with the calculated concentrations from the zero-balance. To further improve the mass balance sampling should continue until a one-year cycle is measured in all points. To install a permanent sampling station to measure concentrations and water flow for the seepage to the north is also considered to be a good approach for improving the credibility of the mass balance. From the vertical profiles of conductivity, temperature, pH and dissolved oxygen saturation measured in the pit lake in October 2010, a permanent stratification with a steep chemical gradient between 7 and 8 meters depth can clearly be observed. The conductivity in the monimolimnion during 2010 was higher than for earlier profile measurements from 2001 to 2004. This observation supports the result from the mass balance, suggesting that metals are continuously accumulating in the pit lake. It is suggested that this observation is followed up by measuring new depth profiles at regular intervals. The largest annual metal fluxes into Lake Hornträsket are observed for Zn (1300 kg), Mn (930 kg) and Fe (380 kg). The impact Rävlidmyran has on Lake Hornträsket is difficult to judge from these figures. One possible approach for getting a better view is to compare the calculated metal transport from Rävlidmyran with the metal transport from the nearby Hornträskviken mine. Another way of assessing the influence from Rävlidmyran is to directly observe the measured concentrations in the two outlets and compare them with local background values. The concentrations of Ba, Cd, Co, Fe, Mn, Ni and Zn are clearly elevated in the surface water streams that enter Lake Hornträsket. The only element that is measured to be higher in the reference point is As. It is possible that the station measuring local background values is influenced by mining activities and thus not representative as a reference point. To clarify this it is suggested that samples for metal analysis are collected further upstream from the reference point, and possibly also from other natural surface streams in the area. It should then be evaluated if the reference point needs to be relocated.Validerat; 20110220 (anonymous