Study Of Groundwater Age And Patterns Of Groundwater Recharge And Flow In Ili River Valley Based On Multi Istopes

The Ili River Valley was the water-rich zone in the arid inland area of Northwest China due to the special geological,geomorphic and climatic conditions in the Ili River Valley.The groundwater recharge conditions and the transformation of different water in this area had attracted major focus of hydrogeological research.In addition,as a part of the arid areas in Central Asia,the rational exploitation and utilization of groundwater resources in the Ili Valley had attracted great attention at home and abroad.Therefore,a comprehensive understanding of the recharge,flow pattern of groundwater,and the transformation among different water in the Ili River Valley was the key factor to maintain the sustainable development of the ecological environment,and it was of great significance to the sustainable utilization of regional groundwater resources.According to the isotopic and chemical composition in groundwater and surface water with the characteristic of having abundant information on water cycle,the methods on the isotope hydrogeology,hydrogeochemistry,water dynamics and hydrogeology were used to understand the recharge processes of groundwater and surface water.On the strength of full understanding the regional geological condition,hydrogeological condition and the studies on the lli basin,this paper identified the groundwater age distribution and structure,determined the recharge patterns of groundwater and surface water,and conformed the quantitative transformation between groundwater and surface water along the Ili River and its tributaries.In this paper,the age distribution and age structure of groundwater in the Ili River Valley were systematically identified for the first time.The seasonal connection and transformation of groundwater and different rivers were quantitatively analyzed.The transboundary flow characteristics were analyzed based on the groundwater age for the first time.This paper was beneficial to understand the water cycle in arid areas,and the rational exploitation of groundwater and surface water in Ili Valley.The results are described as follows.(1)The groundwater age and its spatial distribution in the study area were determined.The groundwater ages in the phreatic aquifer were calculated by the traces of CFCs,⁸⁵Kr,and ³H.The results showed the groundwater age in the piedmont zones is less than 8a,and the groundwater age in central plain around the Ili River could reach up to 60a.The groundwater ages in the confined aquifer were calculated by the traces of¹⁴C.The residence time of confined groundwater ranged from hundreds of years to tens of thousands of years.The ages of confined groundwater increased with the groundwater flow process,and the maximum was distributed in the discharge area near Ili River.The age of groundwater increased horizontally and vertically,which indicated that the aquifers in the study area received vertical and horizontal recharge.(2)The recharge mechanisms of groundwater were identified.River water from mountain area,irrigation water,atmospheric precipitation and the groundwater from mountains were the main recharge sources of the groundwater in the plain.The recharge ways were shown as follows.1)The rapid infiltration of river water in the piedmont plain;2)the lateral inflow of groundwater from mountain areas;3)the infiltration of river water in the central plain;4)the infiltration of irrigation water in the irrigational areas;and(5)the infiltration of precipitation.The groundwater in the piedmont areas mainly consisted of the infiltration of the surface water from the mountain areas and the groundwater lateral inflow from the fissure aquifers,which accounted for nearly 50%and 40%of the recharge amount of groundwater in the piedmont plain,respectively.But the infiltration of precipitation only accounted for 10%of the recharge amount of groundwater in the piedmont plain.In the irrigation area of the central plain,the infiltration of irragation water accounted for 40?50%of recharge amount of groundwater,and the infiltration of precipitation only accounted for 10%.(3)Three groundwater flow systems were identified in the Ili Valley Basin:shallow local flow circulation system,intermediate flow circulation sytem,and deep circulation flow system.The shallow local flow circulation system mainly existed in the piedmont,and the groundwater age was less than 60a.In addition,the shallow local flow circulation system also existed in the irrigation areas of central plain.The intermediate flow circulation system existed below the local flow system,and the groundwater age was more than 60a,and the hydrochemical type was usually SO₄·HCO₃.The deep flow system existed in the deep confined aquifer,where the groundwater age was more than 1ka,even up to 20ka.The water type was usually SO₄·HCO₃.(4)The runoff compositions of the Ili River and the quantitative relationship between the groundwater and river water were calculated.In the eastern mountainous area where was the original recharge zones of Ili River,the main recharge sources of Ili River were the melting water from snow and the meteoric precipitation,which totally occupied more than 90%of the river runoff.And the contribution of groundwater from mountainous area could be negligible.When the river flowed into plain,the runoff of the Ili River was maintained mainly by the runoff from the upper reaches and the discharge of groundwater along the river,taking up 90%of the total river runoff.At the Sandaohe Station in Ili River,the river runoff was mainly recharged by the upstream runoff and the discharge of groundwater along the river,accounting for more than95%of the runoff at Sandaohe Station.