Spatial And Temporal Variations In Deuterium Excess Of Tap Water Across China And Their Significance For Water Source

As natural tracers,stable hydrogen and oxygen isotopes,have been widely applied in water source identification for different environments.Compared with the raw measurements of stable hydrogen or oxygen isotope ratios(?²H or?¹⁸O),the second-order parameter deuterium excess(d=?²H-8×?¹⁸O)combines the two stable isotopes above-mentioned and thus shows the advantage in understanding evaporation process and water source.Tap water is a kind of water intensely exposed to human society.The knowledge of spatial pattern and temporal variation of deuterium excess in tap water as well as the environmental significance are helpful to understand the source and sink of water resources,which can provide the scientific basis for regional water management policies and promote the sustainable development of human-environment system.At present,the factors controlling nationwide patterns of deuterium excess in tap water across China is still unclear,and more concern on the significance of water source is needed.Therefore,based on the nationwide data of stable isotopes in tap water,a comprehensive analysis of spatiotemporal pattern of deuterium excess in China's tap water is presented;considering deuterium excess in precipitation and surface water,the significance of deuterium excess in tap water for water source identification is discussed.The results show:?1?Regarding the seasonal variations in deuterium excess of tap water across China,the median value of deuterium excess is generally high in summer and low in winter for each subregion.The median deuterium excess of tap water in North China is usually lower than that in other subregions?Northwest China,Tibetan Plateau and South China?.According to the spatially interpolated deuterium excess of tap water across China,the high values mainly occurs at the western and middle parts of Northwest China,northeastern part of Tibetan Plateau and central part of South China,which is consistent for each season.The correlation between deuterium excess and?¹⁸O in tap water presents spatial discrepancy for different subregions,indicating environmental controls that is not identical for deuterium excess and?¹⁸O.?2?Considering that precipitation is the fundamental source of surface water and groundwater?and then tap water?,the isotopic difference between tap water and precipitation is analyzed.The difference between deuterium excess of tap water and precipitation?tap water minus precipitation?is usually positive in summer,especially in Northwest China;that is,deuterium excess of tap water is usually larger than that of local precipitation.On seasonal and annual basis,a vast area with low difference between deuterium excess of tap water and precipitation can be seen in North China,especially in autumn and winter with relatively limited precipitation.Checked with the statistical yearbooks,the proportional contribution of groundwater?instead of surface water?to total water supply is high for these areas.In contrast,in South China with low contribution of groundwater?high contribution of surface water?,the difference between deuterium excess of tap water and precipitation is usually positive.In Northwest China,the high difference during summer maybe related to the water transport from the high-altitude precipitation and meltwater to low-lying oasis.The regional characteristics shows that deuterium excess in tap water can be applied as an indicator for water source identification of groundwater and surface water.Several cities are selected to detect the relationship between deuterium excess of tap water and precipitation,and great diversity can be seen for these cities.The inconsistency of deuterium excess in tap water and precipitation can be attributed to the different water sources as well as the evaporation conditions under various hydrogeological conditions.