Characterizing groundwater and surface water interaction using geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index methods for part of the Upper Awash and the adjacent Blue Nile Basin, Ethiopia

Abstract

Characterization of groundwater-surface water interaction using a combined approach of geological, environmental tracers (222Rn, EC, δ18O, and δ2H) and baseflow index in a complex geological environment was carried out for part of the Upper Awash and adjacent Blue Nile Basins. In this work, total measurements of 59 water samples were used (52 from different reaches of main Rivers and tributaries, 4 from springs, and 3 from boreholes) for 222Rn analysis in dry and wet seasons, along with recent 8–20 years of daily stream flow records from 11-gauge stations. For sub-surface inference, regional geological structures and lineament density zones were determined using Kriging interpolation techniques. 222Rn values for the river and streams show 2.7–468 Bq/m3 during the rainy season and 17.1–1467 Bq/m3 for the non-rainy season. The credible justification related to moderate to high seasonal 222Rn values (100–400 Bq/m3) in the upstream parts of the two Basins, including areas falling along the surface water divide between the two adjacent Basins, shows a link between groundwater and surface water systems. In the same area, a very high mean annual baseflow index (BFI >0.5) is dominantly associated with high lineament density (0.9 < LD < 3.0), regional faults, and geologic units of highly permeable scoriaceous basalt, rift basalt, and fractured ignimbrite units. Relatively, low 222Rn (<100 Bq/m3) in both seasons and low baseflow index (BFI <0.35) values mainly in the central part of the Upper Awash basin are attributed to existing impervious superficial deposits (alluvial, tuff and ash). This, in turn, shows the poor connection between the groundwater and surface water systems in the central plain part of the Upper Awash Basin and, as also evidenced by the storage of floodwater for an extended time after the end of the rainy season, prove poor connectivity between groundwater and surface water systems. The very high 222Rn (400–1467 Bq/m3) with varied mean annual baseflow index (0.35 < BFI <0.5) at different stations in the extreme southern and southeastern parts of the Upper Awash Basin show Awash River is gaining from the aquifer system through highly fractured basalt and fractured ignimbrite geologic units. Evidence from analysis of measured electrical conductivity and stable isotopes (δ18O, δ2H) generally agree with the observed 222Rn value and baseflow analysis.