Whangaehu River, New Zealand: geochemistry of a river discharging from an active crater lake

During 3 surveys in 1992, water and sediment samples were collected from the length of the Whangaehu River and above the confluence points of major tributaries. The waters were analysed for major cations (Ca, Li, Mg, Na), anions (Cl, SO4), metals (Al, As, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn), SiO2, pH, conductivity and temperature. The sediments were analysed for some metals. During 1992, the volcanic Crater Lake of Mt Ruapehu intermittently discharged very acid waters into the Whangaehu River. For example in February, during an intense period of snow melt and Crater Lake overflow, the pH of the river water was < 5 for most of its length and < 3 in the estuary. For most of the year, conductivity, anion and metal concentrations decreased from the outlet of the Crater Lake to the estuary as tributaries diluted the river water. During periods of high overflow from Crater Lake (> 2001/sec), metal concentrations were similar in the total recoverable and dissolved metal fractions of the water. At times of lower overflow (< 35 l/sec), tributary waters diluted Whangaehu River water to the extent of raising pH to 6. Under these conditions, downstream sediment contained more leachable heavy metals than upstream sediment. The more acid upstream waters prevented adsorption of metals onto the surfaces of fine suspended particles and bottom sediments. In contrast, the chemistry of the diluted downstream waters allowed dissolved metals to transfer to particulate surfaces.