ISOTOPE COMPOSITION OF RAIN WATER, WELL WATER AND FUMAROLE STEAM ON THE ISLAND OF VULCANO, AND THEIR IMPLICATIONS FOR VOLCANIC SURVEILLANCE

Water samples from eight wells in the Vulcano Porto area and fumarole steam from the crater of Vulcano (Southern Italy) were collected at monthly intervals over a two-year period from February 1987 to January 1989. Samples of bulk precipitation were also collected in three points at Vulcano Porto and Vulcano Piano (island of Vulcano) and Castroreale (Tyrrhenian coast of Sicily), again at monthly intervals. The weighted mean isotopic composition of rain water on Vulcano is delta-D = -32 parts per thousand, and delta-O-18 = -6.4 parts per thousand. Results for well waters tend to support the existence of at least two water bodies: one of purely meteoric water and the other of brackish thermal water. An intermediate water body, probably the result of mixing of the other two, was also recognized. The isotope composition of fumarolic steam from the crater suggests the mixed contribution of both sea and rain waters, although mixing is not the only process occurring. The existence of a deep marine-brine body is inferred, characterized by delta-D and delta-O-18 values more positive than those of Mediterranean sea water. A close relationship, indicating common source effects, is shown by temporal isotopic variations at some wells and the fumarolic steam, wich may be used for volcano survey at distance from the active crater.