地下水化学组成的时空聚类分析与多级嵌套水流系统识别

Lumped cluster analysis for understanding spatial and temporal patterns of groundwater geochemistry and hierarchically nested flow systems

水文地球化学是识别地下水流系统的重要方法,然而区域尺度上多级嵌套地下水流系统的复杂性使地下水化学组成的分析和解释难度增加。以鄂尔多斯北部盆地湖泊集中区典型的胡同察汗淖地下水流系统为例,基于丰水期和枯水期3个期次不同深度地下水样品的物理化学数据,应用时空聚类与主成分分析方法,揭示地下水化学组成的空间分布特征、变化规律及其作用机制,分析水化学时空聚类结果对多级嵌套地下水流系统划分的可行性。该聚类结果将地下水样品分为3类,其中C1为以Na-HCO_(3)型为主的深层地下水,具有偏负的氢氧同位素组成(δD<-70‰,δ^(18)O<-9‰)和极低浓度的NO_(3)^(-);C2为Ca-HCO_(3)为主的浅层地下水,具有偏正的氢氧同位素组成(δD>-70‰,δ^(18)O>-9‰)和高浓度的NO_(3)^(-);而C3呈无优势阳离子、δD和δ^(18)O变化范围大且显著线性相关等深、浅地下水混合特征。呈南北条带分布在苏贝淖-胡同察汗淖排泄区的C2和部分C3水化学组成有一定的季节变化。研究验证了研究区受地形和湖泊排泄控制的浅层局部和深层区域地下水流系统的空间分布,识别了苏贝淖-胡同察汗淖排泄区受浅循环和深循环共同影响的强烈作用带,证明了水化学时空聚类方法识别多级嵌套地下水流系统的可行性。

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-HCO_(3) type,more negative δD(<-70‰)and δ^(18)O(<-9‰)and little NO_(3)^(-) concentration,whereas the C2 is the shallow groundwater characterized by Ca-HCO_(3),more positive δD(>-70‰)and δ^(18)O(>-9‰)and high NO_(3)^(-) concentration.The C3 is characterized by mixing of deep and shallow groundwater,with no dominant cations, δD and δ^(18)O 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.