State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, Field Scientific Observation & Research Base of Karst Eco-environments at Nanchuan in Chongqing, Ministry of Natural Resources, School of Geographical Sciences, Southwest University, Chongqing, 400715, China; Crawford Hydrology Laboratory, Department of Geography and Geology, Western Kentucky University, Bowling Green, KY, 42101, USA.
State Cultivation Base of Eco-agriculture for Southwest Mountainous Land, Field Scientific Observation & Research Base of Karst Eco-environments at Nanchuan in Chongqing, Ministry of Natural Resources, School of Geographical Sciences, Southwest University, Chongqing, 400715, China.
Environ Pollut. 2020 Oct;265(Pt B):114835. doi: 10.1016/j.envpol.2020.114835. Epub 2020 Jun 1.
Nitrate is one of the most common pollution sources in groundwater, particularly in highly vulnerable karst aquifers. The potential for nitrification and denitrification within karst aquifers varies in different settings depending on the extent of anthropogenic inputs, so that accurate identification of nitrate sources can be difficult. Geochemical data and dual nitrate isotopes were measured in this study, incorporating a Bayesian isotopic mixing model, and used to identify nitrate sources, nitrification and denitrification, and quantitatively determine nitrate sources under different extents of anthropogenic inputs in three karst catchments within Chongqing Municipality, SW China: Laolongdong (an urbanized area), Qingmuguan (a suburban village), and Shuifang Spring (a protected natural area). At the Laolongdong catchment, the groundwater was in a reducing condition and enriched in δN (averaging 18.9 ± 6.9‰) and δO (averaging 8.5 ± 4.6‰). Manure and sewage waste were the main contributing nitrate sources. A slope of 1.8: 1 of the dual isotopes suggested a denitrification process occurring in anaerobic conduit flow. Within the Qingmuguan catchment, groundwater had average δN and δO values of 9.7 ± 3.5‰, and 1.9 ± 3.4‰, respectively. The data showed evidence for nitrification, and the contribution of soil organic nitrogen was 52.1%, followed by a contribution of 44.8% from manure and wastewater. At the Shuifang Spring catchment, the mean δN and δO values in groundwater were 8.8 ± 2.9‰, 2.3 ± 4.6‰, respectively. Nitrification was the dominant process and most of the nitrate was derived from soil organic nitrogen. This study suggests that karst underground rivers overlain by urban land use undergo denitrification, while the suburban and relatively pristine karst aquifers are dominated by nitrification, allowing development of a conceptual model for nitrate sources and transformations in karst aquifers from the categories of land use (i.e., urban, suburban, and pristine areas). MAIN FINDING: Anthropogenic activities can change biogeochemical nitrogen dynamics of vulnerable karst aquifers, such that the groundwater overlain by an urban settlement has undergone denitrification, while suburban and pristine areas have been dominated by nitrification.
硝酸盐是地下水最常见的污染源之一，尤其是在高度脆弱的岩溶含水层中。岩溶含水层中硝化和反硝化的潜力因人为输入的程度而异，因此准确识别硝酸盐的来源可能很困难。本研究测量了地质化学数据和双重硝酸盐同位素，并结合贝叶斯同位素混合模型，用于识别硝酸盐来源、硝化和反硝化，并定量确定在重庆市三个岩溶流域（老龙洞（城市化地区）、青杠关（郊区村庄）和 shuifang 泉（受保护的自然区））不同人为输入程度下硝酸盐的来源。在老龙洞流域，地下水处于还原状态，δN（平均 18.9±6.9‰）和δO（平均 8.5±4.6‰）富集。粪肥和污水废物是主要的硝酸盐来源。双同位素的斜率为 1.8:1 表明在厌氧管道流中发生了反硝化过程。在青杠关流域，地下水的平均δN 和δO 值分别为 9.7±3.5‰和 1.9±3.4‰。数据表明硝化作用的证据，土壤有机氮的贡献为 52.1%，其次是粪肥和废水的贡献 44.8%。在 shuifang 泉流域，地下水中的平均δN 和δO 值分别为 8.8±2.9‰和 2.3±4.6‰。硝化作用是主要过程，大部分硝酸盐来自土壤有机氮。本研究表明，受城市土地利用影响的岩溶地下河发生反硝化作用，而郊区和相对原始的岩溶含水层则以硝化作用为主，从而为岩溶含水层中硝酸盐的来源和转化提供了一个概念模型，从土地利用类别（即城市、郊区和原始地区）。主要发现：人为活动会改变脆弱的岩溶含水层的生物地球化学氮动态，使受城市定居点覆盖的地下水发生反硝化作用，而郊区和原始地区则以硝化作用为主。
