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 naturally mineralised rocks and soils. Mineralised rocks have 1–100 ppm mercury (Hg) above regional background of 0.1–0.25 ppm (fresh greywacke) and 0.04 to 0.14 ppm (basalt and clay‐altered greywacke). Soils have up to 10‐fold variation in As and Hg within the soil profile, with high metal levels in the C horizon associated with iron oxyhydroxides. Base metals, copper (Cu), lead (Pb), and zinc (Zn) are generally near to regional background throughout the Puhipuhi area, including road aggregates. Neither As nor Hg are significantly mobilised in solution in run‐off from road aggregate, but the low pH signature of road run‐off can be traced for c. 50 m into forest soil, or more than 300 m in clay‐based watercourses. Sediments carried with the road run‐off may release As and Hg if they are transported into higher pH environments such as major rivers or farmland. The chemical effects of road run‐off are likely to be less than those from the natural mineralised rock occurrences that underlie many parts of the Puhipuhi area.