• 标题：Nitrate sources and biogeochemical processes in karst underground rivers impacted by different anthropogenic input characteristics
• 关键词：Nitrate isotopes,Anthropogenic sources,Denitrification,Biogeochemical processes,Karst catchments
• 作者：Pingheng Yang, Yuyang Wang, Xinyu Wu, Longran Chang, Brian Ham, Lisheng Song, Chris Groves

Nitrate is one of the most common pollution sources in groundwater, particularly in highly vulnerable karst aquifers. The potential for nitrifification and denitrifification within karst aquifers varies in different settings depending on the extent of anthropogenic inputs, so that accurate identifification of nitrate sources can be diffificult. Geochemical data and dual nitrate isotopes were measured in this study, incorporating a Bayesian isotopic mixing model, and used to identify nitrate sources, nitrifification and denitrifification, 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 d15NNO3 (averaging 18.9 ± 6.9‰) and d18ONO3 (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 denitrifification process occurring in anaerobic conduit flflow. Within the Qingmuguan catchment, groundwater had average d15NNO3 and d18ONO3 values of 9.7 ± 3.5‰, and 1.9 ± 3.4‰, respectively. The data showed evidence for nitrifification, 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 d15NNO3 and d18ONO3 values in groundwater were 8.8 ± 2.9‰, 2.3 ± 4.6‰, respectively. Nitrifification 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 denitrifification, while the suburban and relatively pristine karst aquifers are dominated by nitrifification, 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 fifinding: Anthropogenic activities can change biogeochemical nitrogen dynamics of vulnerable karst aquifers, such that the groundwater overlain by an urban settlement has undergone denitrifification, while suburban and pristine areas have been dominated by nitrifification.

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