The dynamics of central Main Ethiopian Rift waters: Evidence from δD, δ~(18)O and ~(87)Sr/~(86)Sr ratios

摘要

Water samples from cold and geothermal boreholes, hot springs, lakes and rivers were analyzed for δD, δ~(18)O and ~(87)Sr/~(86)Sr compositions in order to investigate lake water-groundwater mixing processes, water-rock interactions, and to evaluate groundwater flow paths in the central Main Ethiopian Rift (MER) of the Ziway-Shala basin. Different ranges of isotopic values were recorded for different water types: hot springs show δ~(18)O -3.36 to +3.69 and δD -15.85 to +24.23, deep Aluto-Langano geothermal wells show δ~(18)O -4.65 to -1.24 and δD -12.39 to -9.31, groundwater wells show δ~(18)O -3.99 to +5.14 and δD -19.69 to +32.27, whereas the lakes show δ~(18)O and δD in the range +3.98 to +7.92 and +26.19 to +45.71, respectively. The intersection of the Local Meteoric Water Line (LMWL: δD=7 δ~(18)O+11.2, R~2=0.94, n=42) and the Local Evaporation Line (LEL: δD=5.63δ~(18)O+8, n=14, R~2=0.82) was used to estimate the average isotopic composition of recharge water into the basin (δD=-5.15 and δ~(18)O=-2.34). These values are depleted if compared with the modern-day average precipitation, presumably indicating paleo-groundwater components recharged during previous humid climatic phases. The measured stable isotope values indicate that the geothermal wells, some of the hot springs and groundwater wells mainly consist of meteoric water. The Sr isotopic signatures in all waters are within the range of the Sr isotopic composition of the rift basalts and rhyolites. The variability of Sr isotopic data also pinpoints complex water-rock interaction and mixing processes in groundwater and surface water. The ~(87)Sr/~(86)Sr ratio ranges from 0.70445 to 0.70756 in the hot springs, from 0.70426 to 0.70537 in two deep geothermal wells, and from 0.70673 to 0.70721 in the rift lakes Ziway, Langano, Shala and Awasa. The radiogenic composition recorded by the lakes indicates that the input water was predominantly affected by progressive interaction with rhyolitic volcanics and lacustrine sediments.There is a change in MER water composition between the recharge-infiltration zone (surrounding highlands) and the discharge areas, from depleted δD, δ~(18)O and less radiogenic Sr isotope compositions to enriched δD, δ~(18)O and more radiogenic Sr isotope compositions. Isotope analyses are therefore useful in providing further understanding of the hydrologeological processes occurring within a productive aquifer and the surface water system.