Multi-proxy (CFCs, Cl, δ18O and δD) assessment of the origin, residence
time and recharge of groundwater in the Bilate River basin (BRB) of
southern main Ethiopian rift (SMER)
Abstract
The volcanic aquifer of Bilate river basin (BRB) that situated on the
southwestern side of the MER serve as sole reservoir of water supply for
urban and rural people found in the basin. This study investigates
residence time and groundwater recharge process in the Bilate river
basin. For the first time, this research work delivers information on
the resident time and groundwater recharge on the basis of multi tracer
approach (CFC’s, Cl, δ 18O and δD). The δ
18O and δD value of groundwater found in the range of
-4.9 to -1.1‰ and -31 to 5.3‰, respectively. It is shown that stable
isotopic value of groundwater drawn along or close to AAMWL and GMWL,
indicates that groundwater recharge is of meteoric origin. The
groundwater of BRB volcanic aquifer comprises substantial amount of CFCs
concentration. It is observed from CFC data that the oldest component of
groundwater was recharged prior 1950, while the younger component
recharged in different time as of 1950. The relation found between CFC’s
indicates that groundwater comprise mainly of binary mixture with young
groundwater in the age of 20-25-year-old and that of groundwater older
than 60 years. The fraction of young water in the groundwater ranges
from 0.134 to 0.89 and shows decreasing trend from highland towards the
rift. CFC-113 is conservative in the system and used to determine
groundwater lifetime in the Bilate river basin that ranges from 26 to 63
years. The conservative chloride method i.e. chloride mass balance
employed to Bilate river basin calculated yearly average groundwater
recharge to be 219 mm/y which accounted for 21% of the mean annual
areal rainfall. This research can help us better understand how
groundwater circulation and recharge mechanism works, as well as how
long groundwater stays underground in the complex hydrogeological
setting of rift volcanic aquifer.