Anthropogenic activities alter the hydrological process of submarine groundwater discharge in a coastal area of Shanghai City, China

Abstract

Submarine groundwater discharge (SGD) is recognised as a crucial factor influencing coastal ecosystems and biogeochemical processes. Anthropogenic activities not only discharge a large amount of nutrients and pollutants into oceans but also alter the hydrological processes of SGD. Changes in the recharge source and flow pathway of submarine groundwater under anthropogenic activities were revealed in this study by investigating the spatial and temporal isotopic, geochemical and hydrometric characteristics of various water types. The interaction between salt water and fresh water in the intertidal zone was also explored based on the law of isotopic mass conservation and the model of end-member mixing. Our results indicated that the shallow groundwater at depths of 4 m and 13 m is mainly recharged by local precipitation. The fresh submarine groundwater, river water, lake water and terrestrial groundwater at depths of 23 m and 60 m are largely recharged by the Yangtze River. The fresh submarine groundwater is mainly affected by artificially recharged with water from the Yangtze River, and its recharge pathway is relatively deep due to the impact of dam construction. Seawater contributes to more than 60 % of the SGD in most intertidal zones through the discharge processes of mixed salt water and seawater in the upper saline plume. Our study contributes to an improved understanding of anthropogenic impacts on SGD and has crucial implications for accurately tracing and quantifying the hydrological processes of SGD.