Drought induced pulses of SO sub(4), from a Canadian shield wetland: use of delta super(34)S and delta super(18)O in, to determine sources of sulfur

• Published in: Applied geochemistry 2005-04, Vol.20 (4), p.691-700
• Main Authors: Schiff, S L, Spoelstra, J, Semkin, R G, Jeffries, D S
• Format: Article
• Language: English
• Subjects: Freshwater
• ISSN: 0883-2927
• DOI: 10.1016/j.apgeochem.2004.11.011

Following summer drought periods, pulses of elevated SO sub(4) concentrations are frequently observed in streams draining forested catchments that contain wetlands. Delays in the recovery of freshwater streams and lakes in eastern Canada from historically high levels of acidic precipitation have been partially ascribed to these periodic pulses of SO sub(4). Climate in eastern Canada has also changed within the last 25 a, with a documented increase in summer dryness and duration of droughts. In small forested catchments in the Turkey Lakes Watershed (TLW), SO sub(4) concentrations in streams draining wetlands can be elevated by up to a factor of 7 during post-drought discharge events compared to the annual average. Two neighbouring catchments, one with a series of cascading wetlands and one without any wetlands, were selected for comparison. Stable S and O isotope ratios were analyzed in samples of bulk precipitation, streams, and groundwaters to examine sources of SO sub(4) in post-drought pulses. super(34)S-SO sub(4) in the streams and groundwaters show that SO sub(4) is retained in the wetland via SO sub(4) reduction and stored in the upper peat profile. Nitrate is elevated in soil and groundwaters at TLW due to high rates of nitrification in forest soils and the presence of NO sub(3) can be used to identify piezometers unaffected by SO sub(4) reduction. super(18)O-SO sub(4) shows that higher concentrations of SO sub(4) in deeper groundwater are likely due to oxidation of organic S and not a geologic source of reduced S. Following drought, the low super(34)S-SO sub(4) in streams is consistent with wetland retention by SO sub(4) reduction and much lower than SO sub(4) released by weathering in deep glacial till and bedrock. High SO sub(4) groundwaters and geologic sources do not contribute to the SO sub(4) pulses in streams. Isotopic patterns over 6 a were similar. Pulses of SO sub(4) in the wetland catchments following drought are a result of the oxidation of S previously reduced and stored in the wetland.