Stable Isotopes Reveal Sources Of Chinese Tap Water

How much do you know about your tap water? Does your tap water come from groundwater or surface water sources? Actually, it is not an easy task to identify the water sources using common chemical and biological methods.

In the past decade, the stable hydrogen and oxygen isotope ratios in tap water have aroused attention in water resource studies, such as regional water use and source diagnosis, which is not always obvious from traditional water management data. Additionally, environmental information preserved in tap water isotopes have also been important in ecology, food, and forensic sciences.

Dr. Shengjie Wang, associate professor of hydrology from the College of Geography and Environmental Science, Northwest Normal University, China, and collaborators established a nation-wide network to collect monthly tap water across China. More than 2000 samples were collected across China during 2014-2016. Volunteers who participated in this network were instructed to collect cold tap water on a monthly basis.

All the samples were delivered to Dr. Wang’s laboratory and then analyzed for stable hydrogen and oxygen isotopes. This nation-wide isotope database provides a good platform to understand the spatial pattern and seasonal variation of stable isotopes in China’s drinking water and the signatures of water supply as well as water management.

Generally, the spatial distribution of annual mean tap water isotope ratios is consistent with that of local precipitation across China. However, seasonal and monthly variation in tap water isotope ratios at individual sites shows contrasting patterns that appear to be related to the types of water sources exploited.

Yearbook data suggest that water supplies in South China are almost totally supplied by local surface water, and those in the remote western provinces are also dominated by surface water. In contrast, the contribution of groundwater in some provinces of North China is very high. The correlation between isotope ratios in tap water and precipitation in South China suggests that seasonal isotope variation in precipitation may be transferred through surface water systems to tap water.

To test the plausibility of this finding, the researchers evaluated output from an isotope-enabled climate model. The simulated stable isotopes in runoff exhibit a similar spatial pattern to that of precipitation across China. On a seasonal basis, correlations between precipitation isotope ratios and those of runoff are strong across most of South China. This suggests that in South China, where the climate is more humid and the residence time of water within drainage basins may be lower, transfer of seasonal precipitation isotope variation through surface water systems to tap water is a plausible mechanism to explain tap water isotope variability.

The relatively invariant tap water isotope ratios elsewhere in China, which are not correlated with seasonal variation of precipitation isotope ratios, can be attributed to use of groundwater or water from drainage basins with longer storage times. In these locations, long-term limitations on precipitation recharge are likely the primary risks to water supply sustainability.

The tap water isotope signatures identified in this work could be widely applied to characterize water supplies and associated sustainability challenges in different regions of the world. At least, the scientists in the isotope laboratory might know your tap water sources if they acquire your tap waters.

These findings are described in the article entitled Water source signatures in the spatial and seasonal isotope variation of Chinese tap waters, recently published in the journal Water Resources Research. This work was conducted by Shengjie Wang and Mingjun Zhang from the Northwest Normal University, Gabriel J. Bowen from the University of Utah, and other collaborators.

References:

1. Wang S, Zhang M, Bowen GJ, Liu X, Du M, Chen F, Qiu X, Wang L, Che Y, Zhao G. (2018). Water source signatures in the spatial and seasonal isotope variation of Chinese tap waters. Water Resources Research, 54(11): 9131–9143. https://doi.org/10.1029/2018WR023091