Application of radium isotopes to estimate seawater intrusion rate in coastal aquifers

Coastal aquifers, as the interface zone between ocean and land, are highly sensitive and vulnerable to seawater intrusion. This study presents the behavior of radium isotopes and their application to estimate the rate of seawater intrusion in a typical coastal city (Qinhuangdao, China). The hydrochemical results indicated that the coastal aquifers have Na–Cl type water with the lower Na/Cl ratios. The analysis revealed that the formation process of the chemical composition of groundwater was mainly the mixing of seawater and groundwater, while that of inland groundwater was dominated by mineral dissolution and cation exchange and adsorption. Groundwater of intruded aquifers contain significantly higher activities of radium than those of nearshore seawater and inland groundwater. The rates of seawater intrusion were estimated by using equation of the fate and transport of radium in 1–D transient groundwater flow system. In the calculation process, we ignored dissolution and co-precipitation and mainly considered mixing, alpha recoil and decay contribution for radium activity as sources and sinks terms of radium in coastal groundwater based on the characteristics of radium activity and ²²⁴Ra/²²⁸Ra ratio in groundwater. The recoil (P) was determined by the activity ratio of ²²⁴Ra to Th (²³²Th/²³⁰Th), while the retardation (R_f) was calculated from the steady-state radium equilibrium equation. The seawater intrusion rates were obtained by integrating ²²⁴Ra and ²²⁸Ra activities under different thorium ratios (²³²Th/²³⁰Th). The results show that the rate of seawater intrusion varies in a wide range at different locations, which are 0.17–1.03 (²³²Th/²³⁰Th = 0.67) and 0.11–1.01 (²³²Th/²³⁰Th = 1.25) m/d. The method of calculating seawater intrusion rate with radium isotope can obtain the rate of different parts of heterogeneous anisotropic aquifer, which also extends the application of radium isotope in hydrogeology.