Application of hydrogen and oxygen isotopes to trace groundwater circulation in a typical groundwater exploitation reduction area,north China plain
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Abstract:
The north China plain is one of the hotspots of groundwater depletion in China.The long-term groundwater over-exploitation has caused numerous geological and environmental issues such as large-scale groundwater depression cones,downward leakage from upper saline water,seawater intrusion,deterioration of water quality,etc.Since 2014,the Chinese government has initiated the groundwater exploitation reduction program to restore the groundwater resources and alleviate associated environmental problems in the north China plain.Due to the implementation of this program,the hydrological processes are expected to significantly change,and the groundwater level in some areas has shown an upward trend.The Heilonggang plain in Handan City,where the groundwater pumping reduction program yields significant effects,was selected as the study area to identify the processes of the water cycle and determine the groundwater recharge sources.The results of this study will have important implications for groundwater sustainable management in the north China plain. Hydrogen and oxygen isotopes were used as tracers for the water cycle with groundwater-pumping reduction.Multivariate statistical methods and GIS techniques were used to interpret these isotopic data.Rayleigh fractionation equation was used to calculate the evaporation loss.The twocomponent method was used to reveal the interchange volume between surface water and groundwater. Results exhibited that the stable isotope values in the surface water ranged from -6.4‰ to -4.9‰ inδ(18O) with an average of -5.9‰,and from -47.5‰ to -40.2‰ inδ(D) with an average of -44.9‰ in the wet season,while these values changed from -9.1‰ to -3.6‰ in δ(18O） with an average of -7.2‰,and from -65.3‰ to -49.8‰ inδ(D) with an average of -58.4‰ in the dry season.The isotopic compositions in surface water varied greatly from enriched isotopes in the wet season to depleted isotopes in the dry season.The slope (3.8) and the intercept (-29.2) of the surface water evaporation line were much smaller than those of the Local Meteoric Water Line,indicating that the surface water was largely affected by evaporation.The average evaporation loss of surface water was about 40%.Hydrogen and oxygen isotopic compositions in groundwater had insignificant seasonal variations.The slope of the groundwater evaporation line was 6.5,similar to that of the Local Meteoric Water Line.This phenomenon showed that the groundwater was less affected by evaporation,with an average evaporation loss of about 24%.Groundwater interacts with the local surface water frequently.The recharge ratio of groundwater to Lao Zhanghe River was about 30%.The recharge ratio of the Laosha River to the groundwater in the lower reaches was about 20%. The groundwater and the surface water were mainly recharged by atmospheric precipitation while affected by evaporation.There was a close hydraulic relationship between surface water and groundwater.The isotopic composition of surface water was influenced by evaporation and the difference in water supply.The isotopic composition of surface water varied greatly with the input of different water sources.Phreatic water was closely related to the hydraulic relationship of confined groundwater.The -isotopic- enrichment in the confined groundwater indicated that the confined groundwater may be mixed with the downward flow from isotope-enriched unconfined groundwater.Additionally,the isotopic depletion of confined water in some areas may be associated with the lateral flow from old groundwater with depleted isotopes.
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