Sources and sinks of sulphate dissolved in lake water of a dam reservoir: S and O isotopic approach

Understanding the cycling of biophilic elements is crucial for successful management of water quality in lowland dam reservoirs. This study aimed to determine the sources and sinks of SO42- in lakewater using S and O stable isotope analysis. Water samples were collected from 5 sampling points along the Sulejów Reservoir (central Poland) on 5 separate sampling campaigns (spring 2005, summer 2005, winter 2006, spring 2006 and summer 2006) applying a diel sampling strategy. Pronounced spatial, seasonal and diurnal variations were found in SO42- concentration (from 10.3 to 36.2 mg L−1) and in their S (δ34S from 2.1‰ to 5.4‰) and O isotopic composition (δ18O from 2.8‰ to 10.6‰). Sulphate isotopic signature has been applied to define the fluxes of SO42- by means of the isotopic mass balance and thus identify “missing” fluxes. A significant seasonal input of SO42- from the direct catchment was found during the spring, whereas this input appeared to be fairly negligible in summer. Moreover, an additional SO42- flux from mobilisation of reduced S compounds stored in the sediments was identified, especially during periods of elongated water retention time. Pronounced diurnal variation was observed for δ18O(SO42-) (up to 4‰) in all seasons except winter, which is probably caused by dynamic biochemical red-ox reactions and intensive mixing processes of SO42- originating from different sources.