Présentations Colloques

Oral Presentation
8.04
Session 8.04: Isotopic and residence time tracers
Kebede Seifu
Use of 222Rn and 18O-2H isotopes in detecting origin of water and in quantifying groundwater inflow rates in an alarmingly growing Lake
Dual Radon (222Rn) & 18O-2H isotopes have been utilized to a) detect the origin of water, b) pinpoint groundwater inflow zones and c) determine rates of groundwater inflows in an expanding lake in central Ethiopia. The lake expanded from 2 km2 in to 50 km2 over the 60 years causing serious engineering & socio economic challenges (inundation of urban utilities, inundation farms, inundation of railways and roads, ecological changes, and threatening water quality salinization for water users downstream). Commensurate with the changes in volume there quality changed from a hypersaline (TDS 50g L) to a brakish water (3 g L) condition. **Presence of multiple interplay of mechanism (deformation related to sesmisity, irrigation, neo-fractures, change in river stage) that could increase water volume make it difficult to take management measures. Radon isotope is powerful in pinpointing sites of groundwater inflows and determining groundwater inflow rates in a system with non-hydrologic steady state. However radon was not capable in detecting the source of the groundwater as shallow or deep, local or regional, cold or geothermal. This limitation in the capability of the radon method is complemented by use of 18O-2H. The 18O-2H has been used to discriminate the source of the groundwater responsible for expansion of the Lake. The results show that the main source of water responsible for expansion of the lake is irrigation excess water joining the lake through subsurface flow paths. The fast and voluminous flow is aided by dense network of faults and by seismically induced modern ground-cracks that enhance the transmisivity to as high as 10 000 m2 day. The radon mass balance shows the groundwater inflow amounts to 5.4 m3 s. This is comparable with the 4.9 m3 s annual seepage from three large farms. This work adds to the growing literature in use of radon in lake-groundwater interaction studies by demonstrating the capability of the method in solving a practical engineering challenge. Proposed engineering measures shall be discussed.**
Ethiopia