Hydrologic source identification using stable isotope ratios of bottled drinking water in Egypt

Abstract

A convenient way to understand hydrologic source identification is to make use of a suitable tracer. Isotopes of oxygen and hydrogen provide that opportunity since the isotope ratios of the primary source, precipitation, can be modified by post precipitation processes. This approach has been tested by carrying out a comprehensive isotope ratios investigation of domestic and imported bottled water sourced and distributed across Egypt. Our collection of bottled water includes 33 samples distributed under 27 distinct brands. We compare the isotope ratios of the bottled water with the isotope ratios of the reported natural water source of this water. The bottled water isotope ratios preserve information about the water sources from which they were derived. The measured oxygen and hydrogen ratios of the studied bottled water samples range from −11.51‰ to 2.94‰, with an average value of −1.89‰ for δ18O and from −81.69‰ to 23.94‰, with an average value of −9.54‰ for δ2H. The observed variability of isotope ratios in the bottled water falls within the span of the natural groundwater resources in Egypt, such as shallow and deep groundwater aquifers as well as claimed non-Egyptian sources. In general, the bottled water samples in the data set can be divided into four distinct groups based on their isotope ratios. One group represents imported bottled water brands, and three groups include domestic brands. The first group represents the imported bottled water brands. Their measured isotopic compositions are similar to those of their claimed sources. The second group has the lowest δ18O and δ2H values and likely originated from the Nubian aquifer. The third and fourth groups appear to have been sourced from the Nile Valley and Delta aquifer. The third group isotopic content and d-excess values indicate a higher degree of mixing with evaporated return flow water. The results provided in this study indicate the diverse hydrogeology of the local bottled water brands. This promises extending the approach to several sites globally.