Sources and seasonal variations of nitrate in the coastal multiple-aquifer groundwater of Beihai, southern China

Abstract

Elevated nitrate (NO₃⁻) loadings in groundwater may cause health effects in drinking water and nutrient enrichment of aquatic ecosystems. To reveal the sources and seasonal variations of NO₃⁻ in the coastal groundwater of Beihai, southern China, we carried out hydrochemical and isotopic (δ¹⁵N-δ¹⁸O in NO₃⁻) investigations in the summer and winter, respectively, concerning multiple-aquifer groundwater, rainwater, seawater, and surface water. The sources of the main elements present in the waters were interpreted by ionic ratios. NO₃⁻ sources were identified by combined use of the δ¹⁵N values and δ¹⁸O values or NO₃⁻/Na⁺ molar ratios, with estimations of the proportional contribution by the Bayesian stable isotope mixing model. Denitrification was interpreted along the flow paths. The results show groundwater main elements are originated primarily from silicate weathering, and secondarily from anthropogenic inputs and carbonate dissolution. Its qualities are largely affected by seawater intrusion along the coastline. Because of difference in the predominant minerals within the aquifers and in scale and extent of seawater intrusion, the groundwater displays distinct ionic ratio characters. NO₃⁻ concentrations are up to 33.9 mg/L, with higher loadings in the plains relative to along the coastline. Soil N, domestic sewage, rainwater, chemical fertilizers, and algae are NO₃⁻ sources, with average proportional contributions of 0.255, 0.221, 0.207, 0.202, and 0.116, respectively. In relation to the winter, higher production of NO₃⁻ from nitrification of soil N- and algae-derived ammonium induced by higher temperatures in the summer accounts for increases in groundwater NO₃⁻ loadings. In the rural areas, elevated loadings of NO₃⁻ in the winter may be due to larger infiltration fractions of sewage. Seasonal variations of atmospheric NO₃⁻ deposition and farming may also cause the dynamics. Our results improve the understanding of sources and seasonal dynamics of NO₃⁻ in coastal groundwater.