As a part of the coastal groundwater system, fresh submarine groundwater discharge (SGD) is an important source of freshwater discharge into the ocean. Since SGD is inherently difficult to obtain, understanding the hydrological connection between SGD and coastal groundwater is a useful and effective method to clarify the dynamic changes of fresh SGD. We selected a well-studied coastal alluvial fan in central Japan, whereas the hydrochemical composition of fresh SGDs was determined at the two sites in 2002. The geochemistry, hydrogeology, stable isotopes, and a 10-year hydrochemical monitoring data of groundwater were used to reveal the hydrological evolution and quality of fresh SGD. The fresh SGD and the coastal shallow groundwater are the same water type with intensively hydrological connection. The fresh SGDs were dominated by the groundwater on top of the clay layer in the ancient river courses aquifer. However, the SGD at 22 m under sea level was generally a combination of fresh groundwater and re-circulated seawater which was controlled by hydraulic gradient in the adjacent aquifer. The water quality of groundwater and fresh SGDs is good and stable. The confined groundwater beneath the clay layer has high salinity as a result of freshwater–seawater mixing. A better understanding of SGD and coastal groundwater is urgent and important to clarify the global environmental changes.

作为沿海地下水系统的一部分，淡水海底地下水排放（SGD）是淡水排放到海洋的重要来源。由于SGD本质上难以获得，了解SGD与沿海地下水之间的水文联系是阐明新鲜SGD动态变化的有用和有效方法。我们选择了日本中部一个经过充分研究的沿海冲积扇，而 2002 年在这两个地点确定了新鲜 SGD 的水化学成分。地球化学、水文地质、稳定同位素和地下水的 10 年水化学监测数据被用于揭示新鲜 SGD 的水文演变和质量。淡水SGD与近岸浅层地下水为同一水型，水文联系密切。古河道含水层黏土层顶部的地下水以新的SGDs为主。然而，海平面以下 22 m 处的 SGD 通常是淡水和再循环海水的组合，受相邻含水层的水力梯度控制。地下水和新鲜SGDs的水质良好且稳定。由于淡水-海水混合，粘土层下方的承压地下水具有高盐度。更好地了解 SGD 和沿海地下水对于阐明全球环境变化至关重要。地下水和新鲜SGDs的水质良好且稳定。由于淡水-海水混合，粘土层下方的承压地下水具有高盐度。更好地了解 SGD 和沿海地下水对于阐明全球环境变化至关重要。地下水和新鲜SGDs的水质良好且稳定。由于淡水-海水混合，粘土层下方的承压地下水具有高盐度。更好地了解 SGD 和沿海地下水对于阐明全球环境变化至关重要。