Abstract
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.
Highlights
Water 2019, 11, 2591 water balance leading to increased activity of groundwater discharge [6,8,9], (c) Irrigation return water from the adjacent farms joining the lake via sub surface paths [2,10] and (d) occasional accidental and unreported direct spill of excess irrigation water from farms through surface drains as a result of faulty drains arising from breakage and siltation related clogging [10]
Pattern in the 222 Rn has been used for example to interpret the role of the lake bed geology in governing 222 Rn pattern [22] or in localizing sites of groundwater discharge [16,24]
The few cases of 222 Rn use in lake studies focus on methodological development as well as on quantifying groundwater inflow to lakes
Summary
Water 2019, 11, 2591 water balance leading to increased activity of groundwater discharge [6,8,9], (c) Irrigation return water from the adjacent farms joining the lake via sub surface paths [2,10] and (d) occasional accidental and unreported direct spill of excess irrigation water from farms through surface drains as a result of faulty drains arising from breakage and siltation related clogging [10] Most of these hypotheses do not clearly identify the source of the water versus the mechanism that has led to increase in discharge of the water. The same authors indicated the source of the water that has led to the increment in thermal water discharge is unknown and is not related to the irrigation water
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