Geochemical and isotopic evidence of shallow groundwater salinization in a reclaimed coastal zone: the Yellow River Delta, China

This paper aims to improve the current knowledge of groundwater salinization processes of a young coastal aquifer in the Yellow River Delta (YRD) and to assess the impact of both the transfer of irrigation water from the Yellow River and seawater intrusion, using a combination of hydrochemical–isotopic measurements. The major ion/Cl⁻ ratios generally closely follow the mixing line between freshwater end-member (the Yellow River) and saline water end-members (salt groundwater or seawater), indicating the importance of mixing under natural and anthropogenic influences. The examination of stable isotope in groundwater (ranging from −9.0 to −3.5 ‰ for δ¹⁸O and from −73 to −29 ‰ for δD, respectively) and chloride concentrations (~2 to 629 meq/L) confirms that the saline end-member is associated with saltwater rather than present seawater, and most groundwater samples are plotted on mixing lines between the Yellow River and saltwater. The similar range in stable isotopes (δ¹⁸O and δD) and Cl⁻ compositions between saltwater and confined aquifer supports the hypothesis that upward seepage of confined saline water intruded the shallow aquifer. Moreover, the deviations in some cases from mixing line imply water–rock interaction is also important in dominant control on groundwater chemistry, such as cation exchange, SO₄ ²⁻ reduction, and NaCl solution. The brackish water was characterized by slight excess of Na⁺ and deficit of Ca²⁺ compared to non-reactive mixing line. This indicates that the coastal aquifer is generally freshening with respect to extensive irrigation network, as well as the transfer from the Yellow River for ecological recharge into wetland.