关键词: 浅层地下水, 硝酸盐来源, 同位素, 时空变化

Abstract:

Groundwater has become an important water source for production and daily life because of its wide distribution, stable volume, and good quality. However, due to the increasing intensity of intensive agricultural planting, acceleration of urbanization, and inadequate collection and treatment facilities for domestic sewage, various nitrogen pollutants have directly or indirectly entered groundwater, resulting in severe nitrate (NO₃⁻) pollution. This not only exacerbated the regional water supply crisis, but also posed a threat to human health. The significant spatiotemporal differences in the nitrogen concentration of groundwater were driven by seasonal precipitation or irrigation, land use, nitrogen fertilizer input, and soil type, which increased the complexity of NO₃⁻ sources in the groundwater. The main sources of NO₃⁻ in groundwater are the atmospheric deposition of nitrogen, soil organic nitrogen, animal manure, domestic sewage nitrogen, chemical fertilizer nitrogen, and industrial wastewater nitrogen. It is influenced by microbial-driven nitrogen cycling processes such as nitrification and denitrification. Therefore, accurately identifying the source of NO₃⁻ in groundwater and analyzing its contribution rate is of great significance for controlling groundwater NO₃⁻ pollution. On the basis of qualitatively identifying the sources of NO₃⁻ in groundwater using methods such as hydrochemistry and isotope tracing, the introduction of the stable isotope source analysis model (SIAR) not only improved the accuracy of source analysis, but also made it more convenient to quantify multiple NO₃⁻ sources. Yilong Lake is one of the nine major plateau lakes in the Yunnan Province. Farmlands, towns, or villages around the lake area are cross-distributed, with a high degree of agricultural intensification, a large vegetable planting area, a high crop replanting index, and high water, fertilizer, and pesticide input intensities. In addition, the lake area is mainly composed of river alluvial deposits or lake sediments with loose structure, strong permeability, and large and concentrated rainfall. These combined factors increase the risk of shallow groundwater pollution in lakes. Therefore, this study selected typical months representing the rainy season (August 2020) and dry season (May 2021) to collect 46 groundwater samples from 13 irrigation wells in the farmland area and 10 domestic water wells in the residential area around Yilong Lake using a groundwater sample collector. Simultaneously, 0‒100 cm soil profile samples were collected from the farmland where the groundwater samples were collected. Using methods such as hydrochemistry, δ¹⁵N and δ¹⁸O tracing, and SIAR models, the spatiotemporal changes of nitrogen in shallow groundwater around Yilong Lake were studied, the main sources of NO₃⁻ in shallow groundwater and the contribution rates of different sources to NO₃⁻ were identified, and the main driving factors affecting nitrogen pollution in shallow groundwater were determined. The results showed that the hydrochemical types in the shallow groundwater around Yilong Lake are complex, with significant spatial and temporal distribution differences. HCO₃⁻ and SO₄²⁻ are the dominant anions, whereas Ca²⁺ and Na⁺ are the dominant cations. The HCO₃-Na·K type was predominant in residential areas, whereas the SO₄-Na·Ca type was predominant in farmland areas. The HCO₃·SO₄-Na·Ca type was predominant in the dry season, while the HCO₃·SO₄-Ca type was predominant during the rainy season. Spatiotemporal variations in the nitrogen concentration of shallow groundwater around Yilong Lake were significant. The nitrogen concentrations and their forms in shallow groundwater were affected by land use and seasonal changes. The nitrogen concentration in shallow groundwater in farmland was higher than that in residential land, and it was higher in the dry season than in the rainy season. ON and NO₃⁻-N are the main forms of nitrogen in the shallow groundwater around Yilong Lake. 25% of the sampling points of in farmland area had groundwater ρ(NO₃⁻-N) levels exceeding the Class III groundwater quality requirements (GB/T 14848—2017) of 20 mg·L⁻¹. The water and soil environments had a significant influence on the nitrogen concentration of shallow groundwater in farmlands. The key factors reflecting and influencing the nitrogen concentration were EC, pH, DO, and water temperature (t) in the groundwater, whereas soil carbon and nitrogen indicators had a weak effect on the nitrogen concentration of the groundwater. Spatiotemporal variations in the characteristic values of δ¹⁵N-NO₃⁻ and δ¹⁸O-NO₃⁻ in the shallow groundwater around Yilong Lake showed significant differences. A total of 71.43% of groundwater samples from residential land and 75.00% of groundwater samples from farmland had δ¹⁸O-NO₃⁻ values less than 10‰, indicating strong nitrification of groundwater between different land types. There was a positive correlation between the concentrations of δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, and NO₃⁻ in shallow groundwater from both residential land and farmland, indicating that denitrification was not significant. These results further demonstrated that nitrification is the main nitrogen cycling process in shallow groundwater. By combining nitrogen and oxygen isotope tracing methods with the SIAR model, it was determined that the main sources of NO₃⁻ in the shallow groundwater in the study area were manure and sewage nitrogen, soil organic nitrogen, and chemical fertilizer nitrogen. Their contribution rates to NO₃⁻ in residential land and farmland were 46.02%, 25.83%, and 21.49% and 35.27%, 34.39%, and 24.57%, respectively. The use of hydrochemical methods has confirmed that most groundwater samples have moderate levels of NO₃⁻/Cl⁻ and Cl⁻, indicating that NO₃⁻ in shallow groundwater is a mixture of multiple sources. In addition, some of the sampling points had lower NO₃⁻/Cl⁻ ratios and higher Cl⁻ concentrations, indicating that manure and sewage were also main sources of NO₃⁻ in the shallow groundwater in the distribution area of these sampling points. Therefore, improving sewage collection and treatment facilities, rational stacking and application of manure, precise application of chemical fertilizers, and reduction of soil nitrogen leaching are important strategies for preventing and controlling NO₃⁻ pollution in the groundwater around Yilong Lake. These results provide scientific evidence for the protection of shallow groundwater and lake water quality in the Yilong Lake area.

Key words: shallow groundwater, NO₃^- source, isotope, spatiotemporal variation