Moisture source in the Hyblean Mountains region (south-eastern Sicily, Italy): Evidence from stable isotopes signature

Here the authors present results of an isotope study on precipitation collected during a 2-a period from a rain-gauge network consisting of 6 stations located at different elevations in the Hyblean Mountains (HM) region, in south-eastern Sicily. The slope of the local meteoric water line (delta D = 6.50 delta O-18 + 9.87) obtained for the region suggests that precipitation is affected by evaporation during rainfall events. The main variations in rainwater isotope composition are due to seasonal effects and elevation. An average H-2 excess value of +21.2 parts per thousand was found for precipitation events less affected by evaporation (i.e. when the rainfall was > 65 mm/month). The spatial distribution of O isotope composition of precipitation shows a negative gradient from east and south to the inner areas. The depositional rate of Cl, used as a tracer of the origin of air masses, is highest at the coastal rain-gauges (SR and MRG stations) and lowest on the northern flank of the HM region (SC station). Based on these findings, a model is proposed for the origin of precipitation in the HM region, which assumes that a Mediterranean-derived component is the main source of moisture in the studied area. D/H and O-18/O-16 ratios of inferred meteoric recharge waters were also compared with the isotope composition of waters collected from the main local springs and wells. The best linear fit of the delta O-18 vs delta D relationship for Hyblean groundwater is delta D = 4.85 delta O-18-2.01. The enrichment of heavy isotopes in Hyblean groundwater is probably due to evaporation occurring after precipitation events or to a recharging contribution from surface waters (lakes or rivers) enriched in heavy isotopes. (c) 2006 Elsevier Ltd. All rights reserved.