Estimating groundwater discharge into Minjiang River estuary based on stable isotopes deuterium and oxygen-18
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Through analyzing the characteristics of deuterium and oxygen-18 composition in precipitation, surface water and groundwater, this paper will reveal the isotopic effects of precipitation, the origin and evolution of groundwater, and estimate the mixing processes between various water bodies and groundwater discharge into the Minjiang River estuary. The hydrogen and oxygen isotopic compositions of precipitation are more depleted in heavy isotopes during summer, showing the so-called amount effect. The 未18O versus 未D plots for two types of water samples are on the Fuzhou local meteoric water line (LMWL). The two water types are the fissure groundwater and the shallow groundwater respectively collected from a mountainous area in the north bank of the Minjing River estuary and a piedmont plain and hilly land. By contrast, the shallow groundwater samples collected from a piedmont plain and hilly land in the south bank of the Minjiang River estuary mostly fall in the lower right side of LMWL. The intersection points of the two fitted lines on LMWL are so close to the weighted average of isotopic composition of meteoric water during the local agricultural irrigation period from May to September. The result shows that the groundwater in the north bank is mainly recharged from meteoric water, while the groundwater in the south bank is simultaneously recharged from both irrigation water and meteoric water accompanied by a different degree of evaporation during the infiltration process. In addition to the groundwater from both sides of Minjiang River estuary, fracture water from the fracture zone also locally recharge into the estuary. The linear end member mixing model, the digital elevation model and the underground hydrologic analysis are combined to quantitatively study the groundwater contribution to the estuary and the mixing processes among various water sources. The modeling results show that the maximum mixing ratio of groundwater is up to 8.8% in the freshwater zone of the estuary including 0.4% of the fracture water. In the saltwater zone of the estuary, the ratio of freshwater (river water and groundwater) to seawater is 53:47, which includes approximately 1.7% of groundwater. The conservative estimate of groundwater discharge into the Minjiang River estuary is 87.0 m3/s which accounts for 12.8% of the Minjiang River runoff during the dry season.