Both shallow and deep groundwater flow mediates a variety of geologic processes. In the discharge zones of the nested groundwater flow systems, saltwater often emerges due to high evaporation (in endorheic drainage basin), tide surge, or marine transgression and regression (in coastal areas) or salt pollution (in streams). However, to our best knowledge there are limited studies that consider the impact of density flow in the discharge zone on the nested groundwater flow systems. In this study, nested groundwater flow systems are analyzed with saltwater infiltration in their discharge zones. To quantify the effects of saltwater concentration on the flow systems, seven scenarios with different saltwater concentrations in the discharge zones are modeled. It is found that the flow systems are most sensitive to the saltwater concentration of the discharge zones when the concentration is between 2.23 and 4 g/L, and the threshold saltwater concentration that starts to affect the flow systems is about 1.35 to 2.23 g/L for the specific aquifer configuration selected for this study. The results also show that the local flow systems retreat upward and the overall groundwater velocity of the entire flow systems is decreased with the increase of the saltwater concentration. This study may shed light on the control of salinization, evolution of saline lake basins, and seawater intrusion from a perspective of nested groundwater flow systems.

Nutrient inputs through submarine groundwater discharge (SGD) play a significant role in sustaining primary productivity and nutrient cycling in coastal areas. Currently, various geochemical isotopes are used to trace the SGD processes. However, mass balance models of stable water isotopes (2H and 18O) are seldom used in SGD estimating. In this study, mass balance models of 18O and radium isotopes were used to estimate the water age and SGD in Laizhou Bay, China based on the isotope data sampled in August 2017. The water age ranged from 23.5 to 50.0 days with an average of 32.1 ± 16.3 days. The SGD flux ranged from 1.29 × 108 m3 d-1 to 2.84 × 108 m3 d-1 with an average of (2.07 ± 1.04) × 108 m3 d-1. The sensitivity analysis revealed that estimated results of the water age and SGD are very sensitive to the 18O value in evaporation, as well as 18O and 228Ra values in groundwater end-members. Based on the isotope method, the proportion of the Yellow River discharging into Laizhou Bay was estimated to be less than 27% of the total discharge. Furthermore, combining water and salt mass balance models, the submarine fresh groundwater discharge (SFGD) flux ranged from 0.54 × 107 m3 d-1 to 1.31 × 107 m3 d-1 with an average of (0.93 ± 0.46) × 107 m3 d-1. This study reveals that stable and radium isotopes can be effectively combined to estimate the water age and SGD, which may be applied to coastal areas elsewhere.