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Modelling tool for predicting and simulating nitrogen concentrations in cold-climate mining ponds
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.ORCID iD: 0000-0002-2720-6442
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
2018 (English)In: Ecological Modelling, ISSN 0304-3800, E-ISSN 1872-7026, Vol. 380, p. 40-52Article in journal (Refereed) Published
Abstract [en]

A nitrogen model was developed with the aim to trace nitrogen cycling in a cold climate-mining pond at the Aitik copper mine in northern Sweden. The model contains 10 state variables and 19 nitrogen cycling reactions. The model also includes sediment and physical properties of the pond, such as evaporation, freezing and thawing. The model was written in Mathworks MATLAB and was calibrated and validated using environmental monitoring data for the clarification pond at the Aitik mine. The data used comprised monthly values of nitrogen speciation, phosphorous and water flow. The model accurately predicts ammonium (r2 = 0.84) and nitrate (r2 = 0.82) concentrations in a time series from February 2012–August 2014. The model did not accurately predict nitrate concentrations (r2 = 0.11), presumably due to high oxygen concentration in the pond water that prevented denitrification in the water column. The transport of organic material to the sediment was also limiting denitrification in the sediment. When allowing denitrification in the water column as well as increasing the rate of transport of organic material to the sediment the nitrate prediction capacity increased to a satisfactory level (r2 = 0.54). A sensitivity analysis for the system showed that the most sensitive reactions for the water column were oxic mineralisation as well as the nitrification rate.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 380, p. 40-52
National Category
Geochemistry
Research subject
Applied Geochemistry
Identifiers
URN: urn:nbn:se:ltu:diva-68688DOI: 10.1016/j.ecolmodel.2018.04.006ISI: 000434750500005Scopus ID: 2-s2.0-85046670027OAI: oai:DiVA.org:ltu-68688DiVA, id: diva2:1205031
Note

Validerad;2018;Nivå 2;2018-05-09 (andbra)

Available from: 2018-05-09 Created: 2018-05-09 Last updated: 2018-10-25Bibliographically approved
In thesis
1. Nitrogen-cycling tracing methods: Case studies at cold-climate mine sites in northern Sweden
Open this publication in new window or tab >>Nitrogen-cycling tracing methods: Case studies at cold-climate mine sites in northern Sweden
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Spårningsmetoder för kvävecykeln : fältstudier vid gruvor i norra Sverige
Abstract [en]

High nitrogen effluents from mine sites is an environmental issue which has received relatively little attention historically. In recent years a number of studies have showed the environmental effects of high nitrogen discharge to natural water bodies, which include local eutrophication, high risk of acute reducing conditions and changed species composition in the receiving waters. Apart from the direct environmental effects of high nitrogen discharge some forms of nitrogen can be directly toxic- ammonia and nitric gas for instance, and some can be indirectly toxic, for instance nitrate which causes methemoglobinemia in infants if ingested.

This thesis shows how the developed nitrogen tracing methods can be applied in complex water transport systems such as in a mining environment. Two main study sites were used in this thesis, the LKAB iron mine at Kiruna and the Aitik copper mine at Gallivare operated by Boliden Mineral AB. The nitrogen tracing methods used in this study are stable isotope analysis, combined nitrogen and oxygen isotope analysis, source partitioning using linear combination of sources, numerical analysis using kinetic nitrogen reaction rates, numerical model verification using stable nitrogen isotopes, and Bayesian statistical models to estimate nitrogen concentrations. The study also demonstrates an experimental method of reducing nitrogen concentrations in the mining pond at the LKAB Kiruna site. The nitrogen reducing method was tested in a small scale mesocosm experiment at the Kiruna site in northern Sweden, where a nitrogen reduction rate of around 0.25 - 0.5% total nitrate per day could be achieved.

The nitrogen treatment method consists of fertilising the pond system with small amounts of bioavailable phosphorus. Phosphorus is generally the limiting factor for primary production and in the studied mining systems which have high nitrogen concentrations the phosphorus is assimilated into organic matter almost immediately after fertilisation. The phosphorus is assimilated into phytoplankton (algae) which then settles and is used as a carbon source during anaerobic decomposition (denitrification). The denitrification reaction reduces nitrate into nitrogen gas. This would reduce the nitrogen release from mine sites significantly, since nitrate is the dominant form of nitrogen at the two studied mine sites.

Concluding the thesis is a 2.5D model which couples a numerical kinetic nitrogen model with a hydrodynamic model. The hydrodynamic model was the Shallow Water Equations (SWE) model that incorporates wind turbulence, inlets, and outlets as source terms for the water velocities. The two models are coupled via velocity, where the nitrogen model couples via chemical mixing and fluxes are calculated from the water velocities in each model cell.

The results of this thesis suggests that nitrogen release from the the Kiruna clarification pond could be reduced significantly via the use of phosphorus fertilisation. This is due to an increased denitrification rate in the pond, and the fact that much of the discharge water from the pond system is recirculated back into the mineral processing plants. The recirculation essentially means that the nitrate in the mine water will have quite a long retention time before being passed out the receiving waters.

Although the presented nitrate reduction approach showed promising reduction rates, the potential risk of eutrophication in the receiving waters is high, as the discharge water will contain high levels of nitrogen and potentially also phosphorus. The transport pathways and possible natural attenuation of phosphorus must be thoroughly investigated before the presented nitrate reduction method can be implemented at full scale in mine ponds.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2018
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Geochemistry
Research subject
Applied Geochemistry
Identifiers
urn:nbn:se:ltu:diva-71342 (URN)978-91-7790-246-1 (ISBN)978-91-7790-247-8 (ISBN)
Public defence
2018-12-14, E632, Luleå University of Technology, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-10-26 Created: 2018-10-25 Last updated: 2019-01-07Bibliographically approved

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Nilsson, LinoWiderlund, Anders

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