Nitrogen and phosphorus interactions and transformations in cold-climate mine water recipients

Abstract: Process water discharged from mine sites may contain elevated concentrations of nitrogen (N) and phosphorus (P), which both are nutrients for phytoplankton and macrophytes. Thus, discharge of nutrient rich mine water can result in algal blooms, eutrophication, oxygen deficiency and changed species composition in the recipients. This thesis is focused on the speciation and transformation processes of N and P in streams and lakes receiving mine effluents from the Kiruna and Boliden mine sites. The thesis also aimed at evaluating N removal capacity of these aquatic systems. Research methods in the thesis included collection of field data, laboratory and field experiments and computer simulations. The question of limiting nutrient for production of phytoplankton and macrophytes in these mine water recipients was investigated. For this reason, total nitrogen (TN), total phosphorus (TP) and TN:TP ratios in water, sediment and macrophytes were analysed and evaluated. Depending on the ammonium concentration in the effluent at the Boliden site, TN:TP-ratios of the water column shifted from being >22, indicating P-deficiency for phytoplankton, to between 9-22, indicating a transition from N to P deficiency (co-limitation). However, water column TN:TP ratios at the Kiruna site always indicated P deficiency. On the other hand, the TN:TP ratios of macrophytes revealed that both sites may vary from N to P limitation. These aspects have implications for assessing the environmental influence of nutrient-rich mine effluents. A downstream decrease in inorganic N (NH4+ and NO3-) as well as lower concentrations during summer was observed in the receiving streams and lakes. To identify and quantify the major N transformation and removal processes responsible for these changes, a dynamic biogeochemical model was developed, calibrated and validated using hydrological and water chemistry data for the clarification pond Nya Sjön (Boliden). The model calculates concentrations of six N species and simulates the rate of 16 N transformation processes occurring in the water column and sediment as well as water-sediment and water-atmosphere interactions. The calibrated model rendered coefficients of determination (R2) of 0.93, 0.79 and 0.86 for the inorganic nitrogen species ammonium, nitrate and organic nitrogen, respectively. When applying the model in the downstream Lake Bruträsket, the corresponding R2 values were 0.86, 0.76 and 0.54. Model simulations in the two systems suggested that nitrification controlled the reaction rate of the coupled nitrification-denitrification process and that approximately 60 – 65% of permanent removal occurred though denitrification, followed by burial in the sediment (~30-35 %) (May - October). A stable nitrogen isotope (15N) was employed to trace N cycling in the various plant parts of common reed (P. australis) growing in the littoral zone of Lake Bruträsket. Isotope enrichment data indicated a significantly more effective assimilation of N in the roots. Maximum tracer uptake rates of 0.25 µg g-1 min-1 (NO3-) and 1.4 µg g-1 min-1 (NH4+) are similar to model simulated rates of macrophyte N uptake. Simulation results and results from the tracer study indicated that direct N removal through N uptake in macrophytes and phytoplankton may be of minor importance relative to nitrification and denitrification. A sediment incubation experiment using lake water and sediment from Lake Bruträsket (Boliden) resulted in a sedimentary flux of soluble reactive phosphorus (SRP) of 1.1 mg SRP m-2 d-1. Field measurements suggested that oxidation of organic matter and inorganic mining related chemicals (e.g. NH4+ and thiosalts) may result in increased internal SRP flux. These findings point to a possible interaction between the cycles of N (oxygen consumption) and P (flux from sediment) that may be important for nutrient regulation in mine water recipients. Sediment proxy data (δ13C, δ15N, C/N ratios) was used to reconstruct historical changes in organic matter (OM) accumulation in lakes receiving nutrient-rich mine waters in the Boliden and Kiruna mine sites. Sediment accumulation rates increased upwards in all cores, which correlates with an increase in suspended load in the mining effluents discharging to the systems. Similarδ15N values in dissolved inorganic N (DIN) and surface sediments most likely reflect biological assimilation of DIN and subsequent settling of phytoplankton and macrophyte organic detritus. The improved knowledge on N and P dynamics in mine water recipients can be used in selection of mine water management strategies that may lead to reduced N discharge.