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 SO4 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 S-34- and O-18-rich rain-water SO4 (relatively enriched isotope values) and relatively depleted SO4 from oxidation of bedrock sulfide. Tertiary sediments contribute the isotopically most depleted S (down to delta(34)S(CDT)-15 parts per thousand) to the river-water SO4, whereas Mesozoic greywacke contributes S with slightly positive delta(34)S values. River-water SO4 delta(18)O(SMOW) values range from 0 to +5 parts per thousand most probably depending on the micro-environment of the oxidising zone. South Island rivers with SO4 delta(34)S > +5 parts per thousand have low SO4 concentrations ( < 3 mg l(-1)) and are dominantly composed of rain-water SO4 which is principally sea-water derived. In the North Island, the Hutt River SO4 samples also lie on an isotopic mixing trend from ''greywacke bedrock'' to rain-water SO4, the latter with delta(34)S and delta(18)O values up to +16 and +6 parts per thousand 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 delta(18)O and delta(34)S values and elevated SO4 concentrations (up to 16 mg l(-1)), together with higher SO4/SO4 + Cl fraction ratios. This suggests input of fertilizer SO4 (delta(34)S + 17.2 parts per thousand and delta(18)O + 12.7 parts per thousand) 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 bedrock-rainfall derived SO4 on a S versus O isotope plot. The fertilizer loading represents about 20% of the SO4 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 SO4 concentrations and very enriched SO4 isotopic signatures(delta(34)S > +17 parts per thousand and delta(18)O > +12 parts per thousand). Downstream this water is diluted by tributaries with lower SO4 concentration and isotope signatures of Tertiary sediments similar to the rivers in the South Island. Both geothermal and rain-water SO4 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 SO4 content than the inflows, indicating that there must be a considerable underwater geothermal input to the lake. (C) 1997 Elsevier Science Ltd.