Environmental mercury and arsenic sources in fossil hydrothermal systems, Northland, New Zealand

Abstract

Northland, New Zealand has been affected by natural hot water spring systems depositing elevated concentrations of mercury and arsenic over the past 5 million years. Due to the different erosion levels of these hot water systems, four principal types of mercury and arsenic occurrences are found: active hot springs; layered surface deposits (sinters) deposited by hot springs; highly fractured rock zones formed immediately beneath hot springs; and chemically altered and mineralized rock from the deeper roots of hot spring systems. Mercury occurs principally as cinnabar and as a minor impurity (<1 wt%) in phosphate minerals and iron sulfides, particularly marcasite. Mercury is irregularly distributed through limonitic cements formed during oxidation. Arsenic occurs as a minor impurity (<1 wt%) in phosphate minerals and iron sulfides, particularly marcasite. Arsenic is also variably dispersed through limonite, but not necessarily with mercury. Decomposition of marcasite constitutes the most significant source of mercury and arsenic pollution from the studied sites. Release of mercury and arsenic into the environment from marcasite, phosphates and limonite is enhanced by acidification of the sites (down to pH of 2), caused by oxidation of iron sulfides. Mercury and arsenic concentrations of up to 100 parts per billion should be expected in waters near the deposits; these concentrations are in excess of recommended drinking water levels.