Dissolved anthropogenic nitrogen at Macraes mine and contributions of rock nitrogen to Otago ecosystems, southern New Zealand

ABSTRACT Metasedimentary Otago Schist rocks contain up to 0.1 wt% nitrogen, mostly substituting for potassium in the muscovite mineral structure. This study examines the rates and amounts of nitrogen leached by water from schist in and around the Macraes gold mine, to quantify the amounts of rock nitrogen that may be contributing to terrestrial and aquatic ecosystems. Quantification of this dissolved rock nitrogen requires quantification and exclusion of mine‐related nitrogen sources, principally cyanide decomposition and explosive residues. The most effective leaching of rock nitrogen has occurred in kilometre‐scale waste rock stacks, where schist debris is freshly fractured with abundant surface area available for water‐rock interaction. In these settings, discharging shallow groundwater has dissolved rock nitrogen progressively increasing up to 10 mg/L on a decadal time scale, in parallel with increasing concentrations of other rock‐derived constituents including potassium and sulphate. In contrast, groundwater in schist basement has dissolved rock nitrogen from <0.1 to ∼1 mg/L. Use of water emanating from waste rock piles for irrigation of surrounding farmland can contribute minor nutrient N. When soil is absent, leaching of rock nitrogen from schist debris can also contribute this nutrient to nitrogen‐poor natural ecosystems on a regional scale.