5:15 PM - 7:15 PM
[AHW30-P07] Estimation of the origin of nitrate-nitrogen in Lake Kawaguchi based on nitrogen and oxygen stable isotope ratios
Keywords:nitrate nitrogen, stable isotope, lake, river
Nitrate nitrogen (NO3-N) is a major water pollutant that contributes to lake eutrophication through the inflow of polluted river water and groundwater. Thus, it is essential to determine the source of NO3-N to prevent deterioration of lake water. In Lake Kawaguchi, a volcanic dammed lake located at the northern foot of Mt. Fuji in Yamanashi Prefecture, eutrophication has rapidly progressed since the 1960s as the results of an increased influx of anthropogenic nitrogen from the 1960s to the 1980s. Although nutrient concentrations in the inflow river water of Lake Kawaguchi increase during the spring, suggesting potential input from agricultural discharge, most of the lake water is thought to be derived from groundwater and underflow water, making the origin of nitrogen unclear. In this study, we analyzed NO3-N concentrations and the stable nitrogen (δ15NNO3) and oxygen (δ18ONO3) isotope ratios of nitrate in inflow rivers and lake water to determine the origin of NO3-N in Lake Kawaguchi.
Water samples were collected monthly from April 2022 to March 2023 in the downstream areas of the major inflow rivers of Lake Kawaguchi (the Terakawa and Okukawa rivers). Samples were filtered through 0.2 µm membrane filters, and NO3-N concentrations were measured by ion chromatography. The nitrogen (δ15NNO3) and oxygen (δ18ONO3) stable isotope ratios of NO3-N were also measured using the denitrification method. Lake water samples were also collected at 2 m intervals from the surface at the center of Lake Kawaguchi from April 2023 to December 2023 and analyzed for NO3-N concentration, δ15NNO3, and δ18ONO3.
The δ15NNO3 values in the Terakawa River ranged from 1.4‰ to 6.8‰, with higher values observed in the May and October–November. Enriched δ15NNO3 was associated with increased NO3-N concentrations, suggesting an increase in anthropogenic nitrogen input during these months. In contrast, the δ15NNO3 values in the Okukawa River (–1.6 to 2.1‰) were lower than those in the Terakawa River. The NO3-N concentration and δ15NNO3 increased in October and November following typhoons, suggesting that surface water inflow likely increased due to heavy rainfall. Meanwhile, the δ15NNO3 (–0.1‰ to 8.2‰) and δ18ONO3 (–0.9‰ to 11.6‰) values in lake water samples differed significantly from those in the Terakawa and Okukawa river samples, except for near-bottom samples in June, which showed similar values to those in the Okukawa River. These results indicate that most NO3-N in lake water during the circulation period originated from the decomposition of organic matter and subsequent nitrification rather than direct riverine input. Additionally, NO3-N concentrations were elevated in near-bottom samples in June, suggesting that NO3-N was likely introduced via groundwater inflow associated with the heavy (~240 mm) rainfall three to four days before sampling.
Water samples were collected monthly from April 2022 to March 2023 in the downstream areas of the major inflow rivers of Lake Kawaguchi (the Terakawa and Okukawa rivers). Samples were filtered through 0.2 µm membrane filters, and NO3-N concentrations were measured by ion chromatography. The nitrogen (δ15NNO3) and oxygen (δ18ONO3) stable isotope ratios of NO3-N were also measured using the denitrification method. Lake water samples were also collected at 2 m intervals from the surface at the center of Lake Kawaguchi from April 2023 to December 2023 and analyzed for NO3-N concentration, δ15NNO3, and δ18ONO3.
The δ15NNO3 values in the Terakawa River ranged from 1.4‰ to 6.8‰, with higher values observed in the May and October–November. Enriched δ15NNO3 was associated with increased NO3-N concentrations, suggesting an increase in anthropogenic nitrogen input during these months. In contrast, the δ15NNO3 values in the Okukawa River (–1.6 to 2.1‰) were lower than those in the Terakawa River. The NO3-N concentration and δ15NNO3 increased in October and November following typhoons, suggesting that surface water inflow likely increased due to heavy rainfall. Meanwhile, the δ15NNO3 (–0.1‰ to 8.2‰) and δ18ONO3 (–0.9‰ to 11.6‰) values in lake water samples differed significantly from those in the Terakawa and Okukawa river samples, except for near-bottom samples in June, which showed similar values to those in the Okukawa River. These results indicate that most NO3-N in lake water during the circulation period originated from the decomposition of organic matter and subsequent nitrification rather than direct riverine input. Additionally, NO3-N concentrations were elevated in near-bottom samples in June, suggesting that NO3-N was likely introduced via groundwater inflow associated with the heavy (~240 mm) rainfall three to four days before sampling.