Factors controlling stable oxygen, hydrogen and carbon isotope ratios in regional groundwater of the eastern United Arab Emirates (UAE)

Increasing groundwater salinity and depletion of the aquifers are major concerns in the UAE. Isotopes of oxygen, hydrogen, and carbon concentrations in groundwater were used to estimate evaporation loss using the isotopes of oxygen and hydrogen, and using a carbon isotope to trace inorganic carbon cycling in two main aquifers in the eastern part of the United Arab Emirates. The δD-δ¹⁸O of groundwater samples plotted on a line given by: δD = 4 δ¹⁸O + 4.4 (r² = 0.4). In comparison, the local meteoric water line (LMWL) has been defined by the line: δD = 8 δ¹⁸O + 15. In order to better understand the system investigated, samples were separated into two groups based on the δD-δ¹⁸O relationship. These are (1) samples that plot above the LMWL (δD = 6.1 δ¹⁸O + 12.4, r² = 0.8) and which are located predominantly in the north of the study area, and (2) samples that plot below the LMWL (δD = 5.6 δ¹⁸O + 6.2, r² = 0.8) and which are mostly distributed in the south. Slopes for both the groups are similar and lower than that for LMWL indicating potential evaporation of recharging water. However, the y-intercept, which differs between the two groups, suggests evaporation of return flow and evapotranspiration in the unsaturated zone to be more significant in the south. This is attributed to intense agricultural activities in the region. Samples from the eastern Gravel Plain aquifer have δ¹³C and dissolved inorganic carbon (DIC) values in the range from -10 to 17%, and 12-100 mg CA, respectively, while the range for those from the Ophiolite aquifer is from -11 to -16.4‰, and 16-114 mg CA respectively. This suggests the control of C-3 and C-4 plants on DIC formation, an observation supported by the range δ¹³C of soil organic matter (from - 18.5 to -22.1‰.)