Differentiation of natural and anthropogenic contaminant sources using isotopic and microbial signatures in a heavily cultivated coastal area

Hydrogeochemical and multiple isotope data for groundwater samples were obtained and interpreted to discriminate anthropogenic and natural contaminant sources in a coastal aquifer underlying a heavily cultivated watershed in Hwaseong, South Korea. The local aquifers are vulnerable to contamination, due to high anthropogenic N inputs and the location close to the ocean facilitating seawater intrusion. Thus, to effectively control the groundwater quality in the study area, it is necessary to differentiate between anthropogenic and natural contaminant sources. The concentrations of NO3-N in the groundwater ranged between 0.14 and 45.6 mg/L in August 2015 and 0.2-39.6 mg/L in March 2016. High concentrations of Cl- (388-1107 mg/L) and a high electrical conductivity (1027-2715 mu S/cm) were observed in the study area, suggesting that the groundwater was affected by seawater intrusion. Furthermore, delta N-15-NO3-, delta S-34-SO42- values and Sr-87/Sr-86 of groundwater were determined to reveal the origins of the natural and anthropogenic contaminants and the groundwater mean residence times (MRT) and Sr-87/Sr-86 ratios were used to assess the hydrogeochemical processes along the flow path in the study area. Young groundwater was affected by an anthropogenic contamination source with contributions of 26-46% adding nitrate to the aquifer, whereas old groundwater was impacted by mixing with seawater with contributions of 10-20% with low concentrations of NO3-N, but elevated concentrations of chloride and sulfate. Recently recharged uncontaminated groundwater showed oxic conditions with a diverse microbial community structure, whereas young groundwater contaminated by anthropogenic sources showed a less diverse microbial community structure. The results of this study suggest that multiple isotopes combined with groundwater MRT and microbial data can be applied to distinguish natural and anthropogenic contaminant sources in a groundwater system. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study conducted in Hwaseong, South Korea, used hydrogeochemical and multiple isotope data to differentiate between anthropogenic and natural contaminant sources in a coastal aquifer, highlighting risks of seawater intrusion and high anthropogenic nitrogen inputs. By distinguishing between different sources of contamination, effective groundwater quality control measures can be implemented to mitigate the impacts of pollution on the groundwater system.