Interaction between groundwater and seawater in bedrock islands

The hydrogeological settings of bedrock islands in northern China are unique. Climate change and human activities can affect the interaction between groundwater and seawater around the islands to varying degrees, but there is a lack of quantitative understanding of such hydrological processes, including seawater intrusion (SWI) and submarine groundwater discharge (SGD). In this study, we analyzed the precipitation, groundwater levels, and water quality data (2012-2016) for identifying the characteristics and controlling factors of the interaction between groundwater and seawater in bedrock islands by using mathematical statistics, spatial interpolation, and hydraulics methods. The results show that precipitation and groundwater pumping are the main factors affecting the groundwater-seawater interaction. The variations of groundwater levels lag behind the rainfall events by about 10 days. There occurred no SWI in most areas on the northeastern and southern coasts of the South Island, with relative stable marine groundwater discharge. The average SGD velocity was 0.2 m/d, and the average NO₃-N flux to the sea was 81.8 mmol/(m²·d). The southeast area of the North Island has been seriously threatened by SWI, and the groundwater levels have been below sea level for a long time and falling year by year. The average SWI rate is 0.3 m/d, and the average NO₃-N flux towards the land is 69.6 mmol/(m²·d). The SGD rates of the South and NorthIslands were estimated for dry season (April 2014) and wet season (September 2013), respectively. It was 3.5×10⁴−4.5×10⁴ m³/d for the North Island, and 0.4×10⁴−1.1×10⁴ m³/d for the South Island. The results can provide important reference for coastal groundwater resource management and ecological environment protection in bedrock islands.