Reply on RC1

Abstract

Since the Quaternary Period, palaeo-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 Bohai Sea, China) is characterized by the distribution of saline, brine, brackish and fresh groundwater, from coastline to inland, with a wide range of total dissolved solids (TDS) between 0.38–125.9 g L−1. Meanwhile, previous studies have revealed that this area was significantly affected by Holocene marine transgression. In this study, we used hydrochemical, isotopic, and sedimentological methods to investigate groundwater salinization processes in the Luanhe River Delta and its links to the palaeo-environmental settings. The isotopic results (2H, 18O, 14C) show that deep confined groundwater was recharged during the Late Pleistocene cold period, shallow saline and brine groundwater was recharged during the warm Holocene period, and shallow brackish and fresh groundwater was mainly recharged by surface water. The results of hydro-geochemical modeling (PHREEQC) suggest that the salty sources of salinization are seawater and concentrated saline water (formed after evaporation of seawater). The 18O–Cl relationship diagram shows that saline and brine groundwater are formed by three end-member mixings (seawater, concentrated saline water and, fresh groundwater). In contrast, brackish groundwater is formed after the wash-out of saline groundwater by surface water. Using palaeo-environmental data from sediments, we found that palaeo-seawater intrusion during the Holocene marine transgression was the primary cause of groundwater salinization in the study region. Seawater was found to evaporate in the lagoon area during the progradation of the Luanhe River Delta; the resulting concentrated saline water infiltrated into the aquifer, eventually forming brine groundwater due to salinity accumulation. Surface water recharge and irrigation, on the other side, would gradually flush the delta plain's saline groundwater. This study provides a better understanding of saline groundwater evolution in other similar coastal zones.