The origin of NO ₃ ^- and N ₂ in deep subsurface fracture water of South Africa

Deep (>0.8km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO ₃ ^-, up to 50mM N ₂ (90 times air saturation at surface) and 1 to ~400μM NH ₃/NH ₄ ⁺. 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 NO ₂ ^-, NO ₃ ^- and NH ₃/NH ₄ ⁺ concentrations of the pore water and fluid inclusion leachate recovered from the low porosity quartzite, shale and metavolcanic units were ~10 ⁴ times that of the fracture water. The δ ¹⁵N-NO ₃ ^- and δ ¹⁸O-NO ₃ ^- of the pore water and fluid inclusion leachate, however, overlapped that of the fracture water with the δ ¹⁵N-NO ₃ ^- ranging from 2 to 7‰ and the δ ¹⁸O-NO ₃ ^- ranging from 20 to 50‰. The δ ¹⁵N-NO ₃ ^- of the mining water ranged from 0 to 16‰ and its δ ¹⁸O-NO ₃ ^- from 0 to 14‰ making the mining water NO ₃ ^- isotopically distinct from that of the fracture, pore and fluid inclusion water. The δ ¹⁵N-N ₂ of the fracture water and the δ ¹⁵N-N from the cores ranged from -5 to 10‰ and overlapped the δ ¹⁵N-NO ₃ ^-. The δ ¹⁵N-NH ₄ ⁺ of the fracture water and pore water NH ₃/NH ₄ ⁺ ranged from -15 to 4‰. Although the NO ₃ ^- concentrations in the pore water and fluid inclusions were high, mass balance calculations indicate that NO ₃ ^- accounts for ≤10% of the total rock N, whereas NH ₃/NH ₄ ⁺ trapped in fluid inclusions or NH ₄ ⁺ present in phyllosilicates account for ≥90% of the total N.Based on these findings, the fluid inclusion NO ₃ ^- appears to be the source of the pore water and fracture water NO ₃ ^- rather than paleometeoric recharge or mining contamination. Irradiation experiments indicate that radiolytic oxidation of NH ₃ to NO ₃ ^- can explain the fluid inclusion NO ₃ ^- concentrations and, perhaps, its isotopic composition, but only if the NO ₃ ^- did not attain isotopic equilibrium with the hydrothermal fluid 2billion years ago. The δ ¹⁵N-N, δ ¹⁵N-N ₂ and δ ¹⁵N-NH ₄ ⁺ suggest that the reduction of N ₂ to NH ₄ ⁺ also must have occurred in the Witwatersrand Basin in order to explain the abundance of NH ₄ ⁺ throughout the strata. Although the depleted NO ₃ ^- concentrations in the fracture water relative to the pore water are consistent with microbial NO ₃ ^- 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.