漓江流域岩溶与非岩溶农业小流域水体硝酸盐源解析

为研究岩溶与非岩溶小流域水循环和氮循环的差异，该研究选取漓江流域青狮潭水库灌区内以农业为主的睦洞岩溶小流域和金龟非岩溶小流域为研究对象，利用水的氢氧同位素和硝酸盐的氮氧同位素结合贝叶斯同位素混合模型（Bayesian Model Stable Isotope Analysis in R，SIAR），对比两个小流域水体硝酸盐的污染源以及迁移过程。结果表明：水体硝酸盐污染的时空分布受流域水文地质及其伴生的地形地貌、土壤特性和土地利用类型的影响，以耕地为主且土壤层分布不连续的睦洞小流域水体硝酸盐的浓度均值（0.19～15.84 mg/L）、水的氢氧同位素（分别为-44.08‰～-19.63‰和-7.24‰～-1.55‰）以及硝酸盐的氮氧同位素（-1.65‰～53.98‰和0.04‰～13.74‰）值域都宽于以林地为主且土壤层相对较深厚的金龟非岩溶小流域；空间分布上，睦洞小流域北部补给区裸露岩溶石山果园下渗的氮肥和中部密集村庄下渗的动物粪便/生活污水在中部构造盆地区分散排泄不畅，造成G4～G7采样点井水的NO3￣浓度均值（11.96～15.84 mg/L）以及氮氧同位素丰度值（分别为0.65‰～28.23‰和2.46‰～14.43‰）偏高；时间尺度上，睦洞小流域碳酸盐岩风化形成的浅薄土壤层持水能力差且渗透性强，导致旱季土壤N和雨季动物粪便/生活污水对水体NO3ˉ的贡献率小于同季节的金龟小流域。研究结果可为岩溶与非岩溶区水环境治理提供科学依据。

Nitrate source apportionment of water in karst and non-karst agricultural sub-basins in the Lijiang River Basin of Guilin, Guangxi, China

Abstract: Spatiotemporal variation of nitrate (NO3ˉ) and isotope components were investigated in the karst and non-karst sub-basins, to clarify the influence of hydrogeological conditions and underlying surface properties on the source and migration of nitrate in water. The research areas were selected as the Mudong and the Jingui sub-basin in the Qingshitan Reservoir Irrigation District of Lijiang River Basin in southwest China. Both small karst watersheds were also dominated by agricultural activity. Multiple isotopes (δD-H2O, δ18O-H2O, δ15N-NO3ˉ, and δ18O-NO3ˉ) and the Bayesian Model Stable Isotope Analysis in the R (SIAR) model were applied to compare the multiple potential nitrate sources in the water. The results indicated that the spatial and temporal variation of nitrate in water depended on the hydrogeological conditions and the associated topography, soil characteristics, and land use types of the sub-basin. Total nitrogen (TN) was mainly formed by NO3ˉ and NH4+ in the river, where the proportion of NO3￣ was slightly higher while decreasing along the river. TN was overwhelmingly dominated by NO3￣ in the shallow groundwater. Nitrification was the dominant process of NO3ˉ formation and transformation. The Jingui sub-basin included the woodland with a thicker soil layer, while the Mudong sub-basin was mainly agricultural land with the karst development and the discontinuous thin soil layer. The ranges of mean NO3ˉconcentration (0.19-15.84 mg/L), and the isotope values of δD-H2O (from -44.08‰ to -19.63‰), δ18O-H2O (from -7.24‰ to -1.55‰), δ15N-NO3ˉ (from -1.65‰ to 53.98‰), and δ18O-NO3ˉ (from 0.04‰ to 13.74‰) in the Mudong sub-basin, were all greater than those in the Jingui sub-basin. In terms of spatial distribution, the mixing of nitrogen fertilizer infiltration from the orchard of exposed karst stone mountain in the north recharge area and manure & septic waste infiltration from the village groups were not smoothly dispersed and discharged in the central tectonic basin region, resulting in the high concentration of NO3ˉ(the mean concentration range from 11.96 to 15.84 mg/L) and the high isotope value of δ15N-NO3ˉ (from 0.65‰ to 28.23‰) and δ18O-NO3ˉ (from 2.46‰ to 14.43‰) in the water of G4-G7 wells. The contributions of NO3ˉsourced from the soil nitrogen in the dry season, as well as the manure and septic waste in the wet season in the Mudong sub-basin, were less than those in the Jingui sub-basin. The reason was the low capacity of water storage in the thin soil layer that was formed by the weathering of carbonate rocks, together with the strong permeability of the shallow soil layer in the Mudong sub-basin. Consequently, spatial and temporal variations of nitrate pollution and migration in water were achieved under karst hydrogeological conditions and underlying surface properties. The finding can provide decision support for the control of agricultural non-point source pollution and the improvement of the water environment in karst and non-karst areas.