Distribution and mineralogical controls on ammonium in deep groundwaters

Compositional data from published sources, environmental monitoring and new analyses demonstrate that for a wide range of water types (oilfield water, coal mine water, landfill leachate) NH₄⁺ is present in amounts up to 2200 mg/L. Oilfield waters from Alberta, Canada contain 1–1000 mg/L NH₄⁺, coal mine water (UK) surface discharges 1–45 mg/L NH₄⁺, and landfill leachates (UK) up to 2200 mg/L NH₄⁺. Ammonium contents generally show a positive correlation with K, and increase with increasing salinity. Geochemical modelling of sufficiently complete data using SOLMINEQ88 demonstrates that NH₄⁺ activities vary systematically, and are consistent with a mineralogical control. Sodium–K exchange divides the entire sample suite into at least 4 groups, controlled by reaction temperature and reaction with either albite/K-feldspar or illitic clay minerals. In contrast, comparison of NH₄⁺ and K divides the sample suite into 2 groups. On the basis of geological setting, these correspond to K–NH₄⁺ exchange involving illitic (illite-muscovite) clays (and possibly feldspars) for samples from natural sources, and to exchange involving smectitic clays for samples from landfill sites. This study demonstrates the importance of NH₄⁺ as a constituent of natural groundwaters, requiring that this reservoir of N is taken into account in detailed discussion of hydrological components of the N cycle.