Environmental isotope and hydrochemical characteristics of groundwater in central portion of Mekelle sedimentary outlier, northern Ethiopia

This study investigated the source and hydrogeochemical processes responsible for variation in groundwater chemical characteristics in multi-layered aquifers in the central portion of Mekelle Mesozoic Sedimentary Outlier. Groundwater samples from Aynalem and Ilala sub-catchments were collected and analyzed for major and trace elements and stable isotopes (δ²H, δ¹⁸O and δ¹³C). The findings showed significant difference in concentrations of dissolved ions in the groundwater in response to residence time and varying degrees of water-rock interaction. The groundwater exhibited a pH between 7.85 and 7.38 (average of 7.59) and electrical conductivity from 2430 to 650 μS/cm (average 1336 μS/cm) and water types that were characterized by the dominance of Ca-HCO₃/SO₄ and Ca-SO₄. Most of samples from Aynalem aquifer displayed relatively low dissolved ions due to rapid infiltration of groundwater through fractured zones. The increased concentrations in groundwater are attributed to significant degree of solubility in aquifer where the groundwater flow is slow. The geochemical analysis was supported by the ²H/H, ¹⁸O/¹⁶O ratios that revealed the effect of evaporation prior to infiltration in groundwater samples from shale dominated aquifer. The δ¹³C_DIC values ranged from −10.3 to −6.6‰, with a mean value of −8.8‰ suggesting both carbonate dissolution and soil CO₂ are major sources in the groundwater system. While rapid recharging water maintains minor isotope exchange, extended water-rock interactions in low permeability aquifers may result in heavier δ¹³C_DIC values as infiltration water had sufficient time to equilibrate. The study integrates hydrochemistry and new isotope tracers to gain a better understanding on source of dissolved ions and hydrogeochemical evolution that influence the groundwater chemical variability in multilayered carbonate aquifers.