| Summary: | Hydrogeochemical tracing is an important tool to identify groundwater flow systems.However, the complexity of regional hierarchically nested groundwater flow systems makes it more difficult to explain chemical compositions of groundwater.This study takes the Hutongchahan groundwater flow system in the lakes-controlling discharge area in the northern Ordos Basin as a typical example, and applies a cluster and principal component analysis to explore the hydrochemical data of groundwater sampled from different depths in both dry and wet seasons, aimed to reveal spatial and temporal patterns of groundwater chemistry and its controlling mechanism and to examine the reliability of identification of hierarchically nested groundwater flow systems.The groundwater samples were classified into three clusters.The C1 is the deep groundwater characterized by Na-HCO3 type, more negative δD(<-70‰) and δ18O(<-9‰) and little NO3- concentration, whereas the C2 is the shallow groundwater characterized by Ca-HCO3, more positive δD(>-70‰) and δ18O(>-9‰) and high NO3- concentration.The C3 is characterized by mixing of deep and shallow groundwater, with no dominant cations, δD and δ18O having wide range but significant linear correlation.The groundwater samples of C2 and part of the C3, which are mainly distributed along the north-south strip in the Subei-Hutongchahan lake drainage area, showed seasonal variations in chemical compositions.This study verified the spatial distribution of the shallow local and deep regional groundwater flow system controlled by the topography and lake discharge, and identified the Subei-Hutongchahan drainage area as the zone influenced by shallow and deep circulation.The lumped cluster analysis is proved as an effective method to understand spatial and temporal patterns of groundwater geochemistry and hierarchically nested flow systems.
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