Antecedent rainfall and land use controlling the fate of nitrogen in karst urban rivers, elucidated by an isotopic approach

Abstract

Combined with human activities, rainfall events have become an important factor affecting the urban ecological environment, particularly in karst areas, where the ecosystem is fragile and more affected by pollution due to the frequent water exchange. To understand the fate of nitrogen in karst urban rivers, samples were collected under various rainfall events classified in four different hydrological conditions. The concentration of nitrogenous species, water isotopes and nitrate dual isotopes were measured. The results showed that nitrate was the primary inorganic nitrogen (>70%) in most of the samples. The nitrate isotopic signature and hydrochemistry suggested that volatilization and denitrification enriched the isotopic composition during low flow conditions (20.0 ± 8.0‰), while a high isotopic signature during high flow conditions suggested an anthropologic source, e.g. domestic waste. A response between accumulated rainfall and nitrate concentration in the studied urban karstic river suggested that antecedent conditions and sources accumulated in an urban area can affect N pollution in an urban area. This study highlighted that careful management in urban areas is required to understand the knowledge of transport flow paths of pollutant during rainfall events.