Surface run‐off from mineralised road aggregate, Puhipuhi, Northland, New Zealand

Abstract Roads in the Puhipuhi area of Northland, New Zealand, have been surfaced with road aggregate derived from hydrothermally mineralised rocks, which contain iron sulphides (pyrite and marcasite). Oxidation of the iron sulphides results in localised acidification, typically to pH 3, on roads and in source quarries. This low pH is at least one pH unit more acidic than typical background pH of 4–5.5 in forested areas through which the roads pass. Regional background pH is near neutral, and this may be in part an anthropogenic result of development of farmland in the Puhipuhi area. Lower natural pH is partly because of variable degrees of mineralisation of the underlying rocks on the 100 m scale. Arsenic (As) is the best metallic indicator of mineralisation, and mineralised rocks commonly have As contents between 100 and 1000 ppm above a regional background of 5–20 ppm (fresh greywacke) and 0.2–5 ppm (basalt and clay‐altered greywacke). Road aggregates have As between 20 and 200 ppm, locally lower than ...