Abstract
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.
Highlights
It is estimated that around 40 % of the world’s population live in coastal areas (UN Atlas, 2010)
The brackish and low-total dissolved solids (TDS) saline groundwaters, which vary from CaHCO3 to Na-HCO3 and Na-Cl types, occur mainly in the upper aquifer of the delta area, while the lower part contains Na-Cl-type saline and brine groundwaters with high TDS
We used a range of isotopic–geochemical methods to analyze the groundwater recharge and salinity sources in the Luanhe River delta
Summary
It is estimated that around 40 % of the world’s population live in coastal areas (UN Atlas, 2010). The study area is located in northeastern Hebei Province, China, on the west coast of Bohai (Fig. 1a). It consists of an alluvial fan and coastal delta bounded by a Holocene maximum transgression line (Xue, 2016). The thickness of Quaternary sediments in the study area is about 400–500 m According to their lithology and hydrogeological characteristics, there are four distinct Quaternary aquifers (Fig. 1b). The second Late Pleistocene aquifer (Q3), the third Middle Pleistocene aquifer (Q2), and the fourth Early Pleistocene aquifer (Q1) have bottom depths of 120–170, 250–350, and 350–550 m, respectively They are confined aquifers primarily made of medium sand and gravel (Niu et al, 2019). The deep groundwater in the second, third, and fourth aquifers is mainly recharged by a surrounding mountain range and mainly discharged by human pumping (Ma et al, 2014)
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