Discrimination of sulfur sources in pristine and polluted New Zealand river catchments using stable isotopes

Abstract

An analysis of the S and O isotopic compositions and concentrations of dissolved S0 4 in river-and lake-water from 7 major catchments of the North and South Islands, New Zealand, allows the distinction between natural (geological, geothermal and volcanic) and anthropogenic S sources. The Buller and the Wairau, relatively pristine rivers in the South Island, show two end-member mixing between 34S- and 18O-rich rain-water S0 4 (relatively enriched isotope values) and relatively depleted S0 4 from oxidation of bedrock sulfide. Tertiary sediments contribute the isotopically most depleted S (down to δ 34S CDT−15‰ ) to the river-water S0 4, whereas Mesozoic greywacke contributes S with slightly positive δ 34S values. River-water S0 4 δ 18O SMOW values range from 0 to + 5‰ most probably depending on the micro-environment of the oxidising zone. South Island rivers with S0 4 δ 34S> + 5‰ have low S0 4 concentrations (< 3 mgl −1) and are dominantly composed of rain-water S0 4 which is principally sea-water derived. In the North Island, the Hutt River S0 4 samples also lie on an isotopic mixing trend from “greywacke bedrock” to rain-water S0 4, the latter with δ 34S and δ 18O values up to + 16 and + 6‰ respectively and a So 4/SO 4 + Cl fraction of 0.15 (sea-water is 0.12. Although dominated by greywacke, some samples in the Wairarapa area have relatively enriched δ 18Sand δ 34S values and elevated S0 4 concentrations (up to 16 mgl −), together with higher SO 4/SO 4 + Cl fraction ratios. This suggests input of fertilizer S0 4 ( δ 34S+ 17.2‰andδ 18O+ 12.7‰ ) in the rivers of this agricultural area. The fertilizer loading of the Ruamahanga river can be estimated by its graphical offset from a deduced baseline for bedrockrainfall derived S0 4 on a S versus O isotope plot. The fertilizer loading represents about 20% of the S0 4 in the river. Extrapolation of this figure to the annual river discharge indicates that approximately 18% of the amount applied within the catchment is lost to the river. The source of the Whangaehu river is the Ruapehu crater lake (active volcano) with high S0 4 concentrations and very enriched S0 4 isotopic signatures ( δ 34S> + 17‰andδ 18O> + 12‰ ). Downstream this water is diluted by tributaries with lower S0 4 concentration and isotope signatures of Tertiary sediments similar to the rivers in the South Island. Both geothermal and rain-water S0 4 inputs to the streams flowing into Lakes Taupo and Rotorua were identified isotopically; in particular waters flowing out from Lake Rotorua have a higher geothermal derived S0 4 content than the inflows, indicating that there must be a considerable underwater geothermal input to the lake.