Fig. 1

Location of (a) the North China Plain, (b) the study piedmont plain, and (c) the groundwater sampling sites. The map was created via ArcGIS 10.2 (https://www.esri.com/en-us/arcgis/products/arcgis-desktop/resources) based on the ASTER GDEMV2 data from Geospatial Data Cloud site, Computer Network Information Center, Chinese Academy of Sciences (http://www.gscloud.cn).