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

Northland, New Zealand has been affected by natural hot water spring system s depositing elevated concentrations of mercury and arsenic over the past 5 million years. Due to the different erosion levels of these hot water syst ems, four principal types of mercury and arsenic occurrences are found: act ive hto springs; layered surface deposits (sinters) deposited by hot spring s; 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. Mer cury is irregularly distributed through limonitic cements formed during oxi dation. Arsenic occurs as a minor impurity (<1 wt%) in phosphate minerals a nd iron sulfides, particularly marcasite. Arsenic is also variably disperse d through limonite, but not necessarily with mercury. Decomposition of marc asite constitutes the most significant source of mercury and arsenic pollut ion from the studied sites. Release of mercury and arsenic into the environ ment from marcasite, phosphates and limonite is enhanced by acidification o f the sites (down to pH of 2), caused by oxidation of iron sulfides. Mercur y and arsenic concentrations of up to 100 parts per billion should be expec ted in waters near the deposits; these concentrations are in excess of reco mmended drinking water levels.