SULPHUR-35 AS A HYDROLOGIC TRACER: USES AND LIMITATIONS

Sulphur-35, a naturally produced radioisotope of sulphur with an 87 day half-life, has received limited use in hydrologic studies. The limitations on the use of S-35 in hydrologic situations are primarily the result of sulphates interacting with the biosphere and lithosphere. Natural S-35 has been used in a series of different watersheds to determine its suitability as a hydrologic tracer. The watersheds include glacial and alpine systems in the Rocky Mountains, a series of watersheds in the Eastern United States, and the Jezeri watershed in the Czech Republic which is highly impacted from acid deposition. Results from these studies indicate that S-35 is useful in watersheds with little or no soil and vegetation development. In glacial settings, S-35 is very effective in determining the fraction of new water in runoff. In the Loch Vale watershed on the eastern slope of the Rocky Mountains in Colorado, S-35 was used to determine that about half of the sulphate deposited in the watershed had a residence time of less than one year, indicating a system that was very susceptible to changes in atmospheric deposition. In the Ned Wilson watershed (western Colorado), which lies in a system of fractured basalt with minimal soil development, S-35 was able to give an estimate of the percentage of water (and sulphate) entering the lake system each year, as well as estimate rates of biological recycling of 2-10 micrograms/L/day within the lakes during summer. However, in watersheds where significant soil development occurs, it is evident that the atmospherically deposited sulphate exchanges rapidly and S-35 is rarely measured in stream runoff. Even in the Jezeri watershed, where the system was presumably loaded with excess sulphate, no S-35 is detected in runoff. Thus, S-35 appears to be suited to alpine and fractured rock watersheds where little exchange with soil or biological systems can occur.