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

An analysis of the S and O isotopic compositions and concentrations of dissolved S04 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 between34S- and18O-rich rain-water S04 (relatively enriched isotope values) and relatively depleted S04 from oxidation of bedrock sulfide. Tertiary sediments contribute the isotopically most depleted S (down to δ34SCDT−15‰) to the river-water S04, whereas Mesozoic greywacke contributes S with slightly positive δ34S values. River-water S04δ18OSMOW values range from 0 to + 5‰ most probably depending on the micro-environment of the oxidising zone. South Island rivers with S04δ34S> + 5‰ have low S04 concentrations (< 3 mgl−1) and are dominantly composed of rain-water S04 which is principally sea-water derived. In the North Island, the Hutt River S04 samples also lie on an isotopic mixing trend from “greywacke bedrock” to rain-water S04, the latter with δ34S and δ18O values up to + 16 and + 6‰ respectively and a So4/SO4 + Cl fraction of 0.15 (sea-water is 0.12. Although dominated by greywacke, some samples in the Wairarapa area have relatively enriched δ18Sand δ34S values and elevated S04 concentrations (up to 16 mgl−), together with higher SO4/SO4 + Cl fraction ratios. This suggests input of fertilizer S04 (δ34S+ 17.2‰andδ18O+ 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 S04 on a S versus O isotope plot. The fertilizer loading represents about 20% of the S04 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 S04 concentrations and very enriched S04 isotopic signatures (δ34S> + 17‰andδ18O> + 12‰). Downstream this water is diluted by tributaries with lower S04 concentration and isotope signatures of Tertiary sediments similar to the rivers in the South Island. Both geothermal and rain-water S04 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 S04 content than the inflows, indicating that there must be a considerable underwater geothermal input to the lake.