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Abstract

Coal mining activities release nitrogen compounds into the environment. Nitrogen from undetonated and spilled explosives has been considered the primary source of nitrogen in waste rock. However, recent findings indicate a potential for geogenic nitrogen to contribute significantly to loadings downstream of coal mines. This study investigates nitrogen potential sources using small-scale kinetic tests and full-scale historical water quality monitoring downstream of a coal mine waste rock spoil in northeastern British Columbia, Canada. Leachable geogenic nitrogen was estimated from drill core samples because this sampling method avoids explosives residuals in contrast to sampling spoils. Total nitrogen in leachates from drill core humidity cell tests showed both stable and declining trends at the end of 60 weeks of monitoring. Cumulative total nitrogen leached indicated a nitrogen content ranging from 2.9 to 25 mg-N/kg, likely existing as exchangeable ammonia in the sedimentary waste rock. Field barrels filled with mine core have been monitored for over 10 years. Nitrogen leached from the barrels, in the form of ammonia and nitrate, continues to be detected. These small-scale tests support the growing recognition of geogenic nitrogen as a contributor to total nitrogen loading from sedimentary waste rock. The small-scale results support the analysis of over 25 years of water quality monitoring data from downstream of a coal mine waste rock spoil. Waste rock in the spoil would contain both geogenic nitrogen and nitrogen from explosives. Mine-site monitoring indicates that total nitrogen concentrations downstream of the spoil have re-stabilized to pre-mining levels after 20 years, exhausting both potential sources of nitrogen. A mass balance of total nitrogen released to downstream of the spoil indicates that up to 29% of nitrogen could be sourced from waste rock, and the remaining fraction is leached from explosive residuals.