Stable isotopes in rain and cloud water in Madeira: contribution for the hydrogeologic framework of a volcanic island

Madeira is a volcanic island whose economy is highly dependent on groundwater. Previous studies suggest that, besides precipitation, cloud water interception contributes to groundwater recharge. As such, the isotopic characterization of the sources of recharge is useful for the hydrogeological framework of the island. The δ 18O and δ 2H content of rain and cloud water was analyzed during three campaigns. Rain plots over a local meteoric water line (LMWL) whose equation is δ 2H = 7.92 δ 18O + 10.49 (R 2 = 0.97, n = 43). The volume-weighted average meteoric line (WLMWL) is given by δ 2H = 7.97 δ 18O + 10.57 (R 2 = 0.98, n = 20). Both are similar to the global meteoric water line which indicates, along with the d-excess values (10.8 and 11 ‰ for the LMWL and WLMWL, respectively), that rain originates, essentially in the Atlantic. Seasonal variations were also observed and rain in Campaign II was isotopically enriched and had a higher d-excess (δ 18Omean −3.63 ‰; δ 2Hmean −16.3 ‰; d-excess 12.7 ‰), than rain in Campaign III (δ 18Omean −7.01 ‰; δ 2Hmean −46.5 ‰; d-excess 9.5 ‰). Campaign I stood in between. These variations were probably the result of differences in temperature, water vapor source areas and rain forming processes. Rain was found to become increasingly depleted with altitude (δ 18O), at a rate of −0.15 ‰/100 m and −0.11 ‰/100 m in the windward and leeward sides, respectively. Cloud water was always isotopically enriched when compared with rain at the same altitude. This was related to constant water vapor replenishment of orographic clouds, to the fact that it usually represents an early stage condensation from air moisture and to the smaller size of cloud water droplets when compared to rain droplets. These results will be useful to refine Madeira’s hydrogeological conceptual model. Future extended sampling during more years, with smaller sampling periods and single rainfall events should be useful to support the conclusions of this study.