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Open AccessArticle

Use of 222Rn and δ18O-δ2H Isotopes in Detecting the Origin of Water and in Quantifying Groundwater Inflow Rates in an Alarmingly Growing Lake, Ethiopia

by Seifu Kebede 1,* and Samson Zewdu 2
1
Center for Water Resources Research, School of Agricultural Earth and Environmental Sciences, University of KwaZulu Natal, Pietermaritzburg 3201, South Africa
2
School of Earth Sciences, Addis Ababa University, POBOX 1176 Addis Ababa, Ethiopia
*
Author to whom correspondence should be addressed.
Water 2019, 11(12), 2591; https://doi.org/10.3390/w11122591
Received: 16 September 2019 / Revised: 25 November 2019 / Accepted: 26 November 2019 / Published: 9 December 2019
Dual Radon (222Rn) and δ18O-δ2H isotopes were 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 area expanded from 2 km2 to 50 km2 over the last 60 years, causing serious engineering and socio-economic challenge (inundation of urban utilities, irrigation farms, railways and roads; ecological changes in the lake; and threatening water salinization for water users downstream). Commensurate with the changes in volume, there was a change in salinity of the lake from a hypersaline (TDS 50 g/L) to a near freshwater (3 g/L) condition. 222Rn is powerful in pinpointing sites of groundwater inflows and determining groundwater inflow rates in lake systems with non-hydrologic steady-state conditions. The 222Rn method is complemented by the use of the stable isotopes of water (δ18O-δ2H pair). The δ18O-δ2H isotopes were used to discriminate the source of the water responsible for the expansion of the lake. The results show that the main source of water responsible for the expansion of the lake is the irrigation of excess water joining the lake through subsurface flow paths. The fast and voluminous flow is aided by a dense network of faults and by seismically induced modern ground-cracks that enhance the transmissivity of the aquifers to as high as 15,000 m2/day. The 222Rn mass balance shows the groundwater inflow rate is estimated at 4.6 m3/s. This is comparable with the 4.9 m3/s annual seepage loss from three large farms in the area. This work adds to the meager literature in the use of 222Rn in lake-groundwater interaction studies by demonstrating the capability of the method in addressing a practical engineering and socio-economic challenges. View Full-Text
Keywords: 222Rn; δ18O-δ2H; lake-groundwater interaction; expanding lake; Ethiopia 222Rn; δ18O-δ2H; lake-groundwater interaction; expanding lake; Ethiopia
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Kebede, S.; Zewdu, S. Use of 222Rn and δ18O-δ2H Isotopes in Detecting the Origin of Water and in Quantifying Groundwater Inflow Rates in an Alarmingly Growing Lake, Ethiopia. Water 2019, 11, 2591.

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