Application of multi-isotope ratios to study the source and quality of urban groundwater in Metro Manila, Philippines

Abstract

Abstract To characterize water quality in terms of dissolved elements and to investigate both the origin of the water and the source and behavior of groundwater contaminants in Metro Manila, 33 water (groundwater and surface water) samples were analyzed for ion and element concentrations, H and O isotope ratios (δD-H2O and δ18O-H2O), SO 4 2 - isotope ratios (δ34S- SO 4 2 - and δ18O- SO 4 2 - ), and Sr isotope ratio (87Sr/86Sr). The chemical measurements showed that the primary important environmental concerns within Metro Manila are groundwater salinization for both shallow and deep aquifers, and As concentrations (up to 22.5 μg/L) in shallow groundwaters. Comparison of SO 4 2 - and Sr isotope values with possible source materials revealed that contamination by man-made materials such as fertilizers and detergents are present in some shallow groundwaters. Shallow groundwater having higher δD-H2O and δ18O-H2O values (av. −44‰ ± 5.6‰ and −6.8‰ ± 0.6‰, respectively, n = 15) than deep groundwater (av. −48‰ ± 4.4‰ and −7.4‰ ± 0.7‰, respectively, n = 7) suggests that the origins of H2O in both groundwaters are different from each other. Since the mixing of shallow and deep groundwater does not commonly occur under Metro Manila, the contaminants in the shallow aquifers are unlikely to be transported into the deep aquifers. Sulfate reduction by bacterial activity was observed for some groundwaters, resulting in a maximum elevation in δ34S- SO 4 2 - values of around 10‰. By using SO 4 2 - isotope ratios as an indicator of changes in redox conditions and Sr isotope ratio as a source indicator, it was shown that As was dissolved from unconsolidated sedimentary deposits of volcanic origin which enclose shallow unconfined aquifers, but was not the result of changes in redox conditions. The study demonstrated that multi-isotope ratios are useful for evaluating water quality problems in urban groundwaters.