College of Eco-Environmental Engineering, Guizhou Minzu University, Guiyang 550025, China.
Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin 300072, China.
J Environ Manage. 2023 Jul 1;337:117751. doi: 10.1016/j.jenvman.2023.117751. Epub 2023 Mar 16.
Karst groundwater provides drinking water for a quarter of Earth's population. However, in intensive agricultural regions worldwide, karst water is commonly polluted by nitrate (NO), particularly in the valley depression areas with well hydrological connectivity. The valley depression aquifers are particularly vulnerable to anthropogenic pollution because their pipes and sinkholes respond quickly to rainfall events and anthropogenic inputs. Identifying nitrate sources and transport paths in the valley depression areas is key to understanding the nitrogen cycle and effectively preventing and controlling NO pollution. This study collected high-resolution samples at four sites including one surface stream-SS, two sinkholes-SH and reservoir-Re, during the wet season in the headwater sub-catchment. The chemical component concentrations and stable isotopes (δN-NO and δO-NO) were analyzed. The stable isotope analysis model in R language (SIAR) was used to quantitatively analyze the contribution rate of NO sources. The results showed that the down section site (Re) has the highest [NO-N], followed by SH and the lowest SS. The sources calculation of SIAR demonstrated that, during the non-rainfall period, soil organic nitrogen was the primary source of the down section site, followed by fertilizer and the upper reaches sinkholes. During the rainfall period, fertilizer was the primary source of the down section site, followed by soil organic nitrogen and from upper reaches sinkholes. Rainfall events accelerated fertilizer-leaching into the groundwater. Slight denitrification may have occurred at the sampling sites but the assimilation of Re and SH could not occur. In conclusion, agricultural activities were still the primary influencing factor of [NO-N] in the study area. Therefore, the focus of NO prevention and control in the valley depression areas should consider the methods and timing of fertilization and the spatial distribution of sinkholes. To reduce nitrogen flux in the valley depression area, effective management policy should consider, e.g., prolongation of water residence time by wetland, and blocking nitrogen loss paths by sinkholes.
岩溶地下水为全球四分之一的人口提供饮用水。然而，在世界范围内集约化农业地区，岩溶水通常受到硝酸盐（NO）的污染，特别是在水文连通性良好的山谷洼地地区。山谷洼地含水层特别容易受到人为污染的影响，因为它们的管道和落水洞对降雨事件和人为输入的反应非常迅速。确定山谷洼地地区硝酸盐的来源和输移路径是了解氮循环并有效预防和控制 NO 污染的关键。本研究在雨季于河源子流域的四个地点（包括一条地表溪流 SS、两个落水洞 SH 和一个水库 Re）采集了高分辨率样本。分析了化学组分浓度和稳定同位素（δN-NO 和 δO-NO）。使用 R 语言中的稳定同位素分析模型（SIAR）定量分析了 NO 源的贡献率。结果表明，下游地点（Re）的 [NO-N] 最高，其次是 SH，最低的是 SS。SIAR 的源计算结果表明，在非降雨期，土壤有机氮是下游地点的主要来源，其次是肥料和上游落水洞。在降雨期，肥料是下游地点的主要来源，其次是土壤有机氮和上游落水洞。降雨事件加速了肥料向地下水的淋溶。采样地点可能发生了轻微的反硝化作用，但 Re 和 SH 没有发生同化作用。总之，农业活动仍然是研究区 [NO-N] 的主要影响因素。因此，山谷洼地地区 NO 防控的重点应考虑施肥的方法和时间以及落水洞的空间分布。为了减少山谷洼地地区的氮通量，有效的管理政策应考虑通过湿地延长水停留时间和通过落水洞阻断氮损失途径等方法。
