Radium and nitrogen isotopes tracing fluxes and sources of submarine groundwater discharge driven nitrate in an urbanized coastal area

Abstract

The quantitative evaluations of nutrients delivered by submarine groundwater discharge (SGD) have been widely conducted worldwide, but sources of nutrients in the discharged submarine groundwater remain unclear. Identifying these sources of nutrients is essential to the protection and management of marine ecological environments. This study aims to evaluate the magnitudes of SGD and the associated nitrate in the Guangdong-Hongkong-Macao Greater Bay Area (GHM Greater Bay Area), China, and identify the sources of SGD-driven nitrate in this region using radioactive radium (Ra) isotopes (223Ra, 224Ra, and 228Ra) and stable nitrogen (N) and oxygen (O) isotope composition of nitrate (δ15N-NO3− and δ18O-NO3−). The results of the Ra mixing model show that the estimated SGD and the associated nitrate fluxes into the Greater Bay Area are (9.15 ± 1.26) × 108 m3/d and (3.77 ± 0.52) × 107 mol/d, respectively, both of which are comparable to the contributions from the Pearl River. Combing NO3− dual isotopic signatures of sampled coastal groundwater and five kinds of potential nitrate sources, we found that ammonium (NH4+) fertilizer and natural soil N are the two main sources of nitrate in discharged submarine groundwater and rivers. No anthropogenic inputs from manure or sewage waste were identified. This study provides significant insights into the establishment of effective management strategies for controlling SGD-nutrients into the bay and protecting the marine ecological environment.