Isotopic study of sulfate sources and residence times in a subalpine watershed

Stable sulfur and oxygen isotope ratios and naturally occurring S-35(SO4) activities were used to examine sulfate sources, address the role of sulfur dynamics, and estimate residence times of atmospherically derived sulfate in Loch Vale Watershed, Colorado. In 1996, surface water samples from small streams flowing through talus, forest, and wetland areas had delta(34)S(SO4) values ranging from 1.8 to 3.7parts per thousand. Values of delta(18)O(SO4) at the three sites ranged from -1.3 4 to 3.7parts per thousand. Average delta(34)S(SO4) and delta(18)O(SO4) values in Loch Vale precipitation (1991-1999) are higher (5.2 and 13.6, respectively) than surface water values, indicating that some of the deposited sulfate is transformed and/or mixed with other sulfur sources in the watershed (e.g. mineral and organic sulfur). Sulfate ages determined by S-35(SO4) activities support this and show that deposited sulfate may be stored on a timescale of 1 year or more prior to being released to surface waters.