Subsurface hydrological processes and groundwater residence time in a coastal alluvium aquifer: Evidence from environmental tracers (δ18O, δ2H, CFCs, 3H) combined with hydrochemistry

Tianzheng Cao; Dongmei Han; Xianfang Song; Dennis Trolle

doi:10.1016/j.scitotenv.2020.140684

Subsurface hydrological processes and groundwater residence time in a coastal alluvium aquifer: Evidence from environmental tracers (δ¹⁸O, δ²H, CFCs, ³H) combined with hydrochemistry

Sci Total Environ. 2020 Nov 15:743:140684. doi: 10.1016/j.scitotenv.2020.140684. Epub 2020 Jul 7.

Authors

Tianzheng Cao¹, Dongmei Han², Xianfang Song³, Dennis Trolle⁴

Affiliations

¹ Key Laboratory of Water Cycle & Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100049, China; Sino-Danish Centre for Education and Research, Beijing 10019, China.
² Key Laboratory of Water Cycle & Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100049, China; Sino-Danish Centre for Education and Research, Beijing 10019, China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China. Electronic address: handm@igsnrr.ac.cn.
³ Key Laboratory of Water Cycle & Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China; Sino-Danish College, University of Chinese Academy of Sciences, Beijing 100049, China; Sino-Danish Centre for Education and Research, Beijing 10019, China; College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China.
⁴ Aarhus University, Department of Bioscience, Vejlsøvej, Silkeborg, Denmark.

PMID: 32758828
DOI: 10.1016/j.scitotenv.2020.140684

Abstract

As an important part of the water cycle, the hydrologic process and chemical compositions of groundwater have changed significantly due to the joint influence of climate change and human activities. Groundwater salinization becomes a serious threat to water security in coastal areas. In order to assess the relationships between surface water, groundwater and seawater in the coastal plain, we performed a synthesis study based on hydrochemical-isotopic data, hydro-dynamical records and environmental tracers. Deuterium and oxygen isotopes and water chemical indicators were used to identify pollution status, salt sources and migration processes. Radioactive isotopes and gaseous tracers were used to obtain reasonable groundwater age. With the help of multi-tracer approach, the surface-groundwater interaction, salinization of groundwater and nitrate pollution were identified in the Yang-Dai River plain, northern China. The estimated groundwater ages determined from chlorofluorocarbons (CFCs) and tritium (³H) ranges from 18 to 41 years in this area, suggesting a modern groundwater circulation. The spatial distribution of the groundwater age varies significantly due to horizontal hydrogeological heterogeneity. The total dissolved solids (TDS) content of the groundwater near the Well Field (average: 970 mg/L) was higher than the TDS values in samples derived from places located at an equivalent distance to the coastal line (average is 708 mg/L), which resulted from the vertical seawater intrusion through river channels and pollutant inputs from agriculture activities. The nitrate concentrations in groundwater were elevated up to 271 mg/L and increased with increasing groundwater age, which was another water environment problem that should be solved urgently but lacks sufficient attention for years. This study provides a conceptual model with a number of comparable hydrochemical information, which is significant for regional pollution control and water resources management.

Keywords: Coastal aquifer; Groundwater age dating; Hydrochemistry; Seawater intrusion; Stable isotope.