The origin of NO3− and N2 in deep subsurface fracture water of South Africa

Deep (> 0.8 km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40 μM of NO3−, up to 50 mM N2 (90 times air saturation at surface) and 1 to ~ 400 μM NH3/NH4+. To determine whether the oxidized N species were introduced by mining activity, by recharge of paleometeoric water, or by subsurface geochemical processes, we undertook N and O isotopic analyses of N species from fracture water, mining water, pore water, fluid inclusion leachate and whole rock cores.The NO2−, NO3− and NH3/NH4+ concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite, shale and metavolcanic units were ~ 104 times that of the fracture water. The δ15N–NO3− and δ18O–NO3− of the pore water and fluid inclusion leachate, however, overlapped that of the fracture water with the δ15N–NO3− ranging from 2 to 7‰ and the δ18O–NO3− ranging from 20 to 50‰. The δ15N–NO3− of the mining water ranged from 0 to 16‰ and its δ18O–NO3− from 0 to 14‰ making the mining water NO3− isotopically distinct from that of the fracture, pore and fluid inclusion water. The δ15N–N2 of the fracture water and the δ15N–N from the cores ranged from − 5 to 10‰ and overlapped the δ15N–NO3−. The δ15N–NH4+ of the fracture water and pore water NH3/NH4+ ranged from − 15 to 4‰. Although the NO3− concentrations in the pore water and fluid inclusions were high, mass balance calculations indicate that NO3− accounts for ≤ 10% of the total rock N, whereas NH3/NH4+ trapped in fluid inclusions or NH4+ present in phyllosilicates account for ≥ 90% of the total N.Based on these findings, the fluid inclusion NO3− appears to be the source of the pore water and fracture water NO3− rather than paleometeoric recharge or mining contamination. Irradiation experiments indicate that radiolytic oxidation of NH3 to NO3− can explain the fluid inclusion NO3− concentrations and, perhaps, its isotopic composition, but only if the NO3− did not attain isotopic equilibrium with the hydrothermal fluid 2 billion years ago. The δ15N–N, δ15N–N2 and δ15N–NH4+ suggest that the reduction of N2 to NH4+ also must have occurred in the Witwatersrand Basin in order to explain the abundance of NH4+ throughout the strata. Although the depleted NO3− concentrations in the fracture water relative to the pore water are consistent with microbial NO3− reduction, further analyses will be required to determine the relative importance of biological processes in the subsurface N cycle and whether a complete subsurface N cycle exists.

► We examined subsurface NO3− and N2 sources using stable isotopes.
► Trace NO3− in deep fracture water is isotopically distinct from that of mining water contamination.
► Trace NO3− is isotopically identical to NO3− in pore water and fluid inclusions.
► NO3− can be generated by radiolysis of NH3 in anaerobic water.