Saline groundwater evolution in the Luanhe River delta (China) during the Holocene: hydrochemical, isotopic, and sedimentary evidence

Abstract

Since the Quaternary Period, paleo-seawater intrusions have been suggested to explain the observed saline groundwater that extends far inland in coastal zones. The Luanhe River delta (northwest coast of the Bohai Sea, China) is characterized by the distribution of saline, brine, brackish, and fresh groundwater from the coastline inland. The groundwater in this region exhibits a wide range of total dissolved solids (TDS): 0.38–125.9 g L−1. Meanwhile, previous studies have revealed that this area was significantly affected by Holocene marine transgression. This study used hydrochemical, isotopic, and sedimentological methods to investigate groundwater salinization processes in the Luanhe River delta and its links to paleo-environmental settings. Isotopic results (2H, 18O, 14C) allowed old groundwater recharge to be distinguished from new groundwater recharge. Hydrochemical analysis using the PHREEQC code indicated that the salt in saline and brine groundwater originates from a marine source. The 18O–Cl relationship diagram yields three-end-member groundwater mixing, and two mixing scenarios are suggested to explain the freshening and salinization processes in the study area. When this was interpreted along with data from paleo-environmental sediments, we found that groundwater salinization may have occurred since the Holocene marine transgression. The brine is characterized by radiocarbon activities of ∼ 50–85 pMC and relatively depleted stable isotopes, which are associated with seawater evaporation in the ancient lagoon during delta progradation and mixing with deeper fresh groundwater, which was probably recharged in the cold Late Pleistocene. The brackish and fresh groundwaters are characterized by river-like stable isotope values, where high radiocarbon activities (74.3–105.9 pMC) were formed after the washing out of the salinized aquifer by surface water in the delta plain. This study presents an approach that utilizes geochemical indicator analysis with paleo-geographic reconstruction to better assess groundwater evolutionary patterns in coastal aquifers.