Spatial variations in water chemical components in a coastal zone of northern China: Insights from environmental isotopes

Abstract

• Seawater intrusion and weathering determined water quality of coastal zone (CZ). • Dam regulation in CZ rivers affected water chemical components and nitrogen cycle. • N fate in the weak coastal hydrological zone rivers was driven by microorganisms. • Sewage and manure sources were major nitrate contributors in the studied CZ. Coastal zones (CZ) are ecologically and environmentally significant, and it is thus critical to study the interactions among different types of natural CZ waters for better understanding biogeochemical processes. Water samples (n = 101) were analyzed for multiple stable isotopes (δ 18 O-H 2 O, δD-H 2 O, δ 15 N-PON, δ 15 N-NO 3 − , and δ 18 O-NO 3 − ) in seawater, river water, reservoir water and groundwater at Tianjin CZ to demonstrate the spatial variations in the CZ aqueous environment and to identify key influencing processes factors of the N geochemical cycle. This study confirmed that weathering dominated upstream water properties, whereas seawater intrusion determined the downstream and midstream water chemical components, where the seawater fraction was 44% and 11%, respectively. Evaporation and precipitation processes were active across the whole CZ area, but their impacts on water chemical components were less significant than seawater intrusion and weathering. Dam regulation in CZ rivers has significantly affected water chemical components and the nitrogen cycle. The isotopic evidence indicated that bacterioplankton and phytoplankton were the primary forms of PON. The dual nitrate isotopes revealed that animal manure and industrial sewage contributed the leading nitrate to river water. The spatial variation of their contribution was quantitated (from upstream to midstream and downstream: 62%, 91% and 83%, respectively). Considering the potential isotopic fractionation of nitrification reduction from upstream to downstream, 39%, 61% and 57%, respectively. This study proposes a quantitative framework for detecting CZ areas with similar hydrodynamic conditions and climate characteristics found at the Tianjin CZ, which has important implications for CZ water management and environmental protection policies.