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Liu, S.; Gao, M.; Hou, G., and Jia, C., 2021. Groundwater characteristics and mixing processes during the development of a modern estuarine delta (Luanhe River Delta, China). Journal of Coastal Research, 37(2), 349–363. Coconut Creek (Florida), ISSN 0749-0208. The Luanhe River Delta (LRD) is divided into two parts, the ancient LRD and the modern LRD (MLRD), and has formed since 7000 calibrated years before present (cal yr BP). The MLRD developed from 2500 cal yr BP. Influenced by paleoclimatic changes and human activity, its groundwater environment is complex. In this study, groundwater monitoring methods, hydrochemistry, and isotopes are used to determine the groundwater characteristics and mixing processes during MLRD development. The groundwater dynamics show seasonal variations. The groundwater salinity distribution features vertical zones and is the same as the stratal distribution. The saline groundwater formation involves evaporation, condensation, hydrolysis, dissolution of evaporated salts, and mixing of groundwater with different qualities and hydrochemical compositions. Brackish water and saline water are the result of mixing between fresh and highly saline waters in deep groundwater based on the hydraulic conditions and the dispersion effect. The formation of the MLRD, which can be described as natural reclamation, provides good groundwater flow and mixing channels. Based on hydrochemical data, the mixing model, and the hydrochemical facies evolution diagram, salinity in shallow groundwater is influenced by seawater intrusion and saline water intrusion. The concentrations of sodium and chloride can indicate the intrusion degree. Precipitation and other freshwater inputs provide the main recharge sources that lead to freshening of the shallow groundwater. Close to the sea, the water exchange between groundwater and seawater is intense, which can lead to similar hydrochemical characteristics of groundwater and local seawater. Saline water intrusion in deep groundwater is more serious than that in shallow groundwater because there is no other freshwater recharge to deep groundwater. [ABSTRACT FROM AUTHOR]