Using environmental isotopes for estimating the relative contributions of groundwater recharge mechanisms in an Arid Basin, Central Saudi Arabia

Stable isotope compositions of water (2H and 18O) were utilized to estimate the relative contributions of diffused recharge (DR) through the soil matrix from direct precipitation and concentrated recharge (CR) through coarse-textured ephemeral streambeds to groundwater in the arid granitic basin of Alquway, central Saudi Arabia. Groundwater ranged in its stable isotope composition from -2.51% to -2.03% from delta 18O and from -12.7% to -10.1% delta square H. Local groundwater line (LGWL) is defined by: delta square H = 4.72 delta 18O-0.57.Average stable isotopic content of CR is -2.58% for delta 18O, and -12.37% for delta 2H. Local CR line (LCRL) is given by: delta square H =5.28 delta 18O + 1.24. Its slope indicates that CR experiences significant kinetic fractionation resulting in a greater relative enrichment of 18 O than 2H. DR is more enriched in the heavy isotopes with means of -0.38% for delta 18O and -9.8% for delta square H. Local DR line (LDRL) is given by : delta square H = 2.01 delta 18O -9.03. Its slope is much lower than that of LCRL, indicating that DR goes through more stable isotopic fractionation than CR, due to experiencing more evaporation form unsaturated soils. Recharge-inducing rainfalls in this basin were estimated to be relatively depleted in the heavy isotopes, suggesting that groundwater recharge in this basin originates only from heavy downpours. Using mean values of delta 18O in a mass-balance equation, the relative contributions of CR and DR, to ground water were estimated to be 84% and 16% respectively. (author)