Origin and Transformation of Nitrate in Karst Cave Groundwater in the Middle Reaches of the Qingjiang River

Abstract

Nitrate pollution is a severe threat to the fragile ecosystems in karst regions. However, our knowledge of the sources and transformations of nitrate in karst cave groundwater is still limited. This study aimed to investigate the temporal and spatial patterns of nitrate dynamics in the underground water of karst caves located on the south bank of the Qingjiang River in central China, through a comprehensive application of multiple approaches, such as hydrochemistry, nitrogen and oxygen isotope compositions of nitrate, and a Bayesian isotope mixing model (SIMMR). During the sampling period (from December 2018 to December 2019), the nitrate concentration did not show an apparent temporal variation; meanwhile, no water samples in this study had a nitrate concentration higher than the limit for drinking water, but the nitrate concentration in karst underground rivers is significantly higher than that in surface water. The results of the comprehensive analyses revealed that the predominant nitrate sources included nitrification in soil and chemical fertilizer, which had mean percentages of 43% and 32%, respectively. The source contribution varied in the outlet water among different caves. The soil-derived nitrate in the underground water from the Shizi Cave accounted for the highest proportion (49%), while chemical-fertilizer-derived nitrate in the underground water from the Mishui Cave accounted for the highest proportion (36%). The dualisotope signatures of nitrate supported a major influence on nitrogen dynamics in the cave underground from nitrification. These findings suggest that nitrate carried by underground rivers in karst caves should be alerted when making the nitrate balance in rivers flowing through karst areas.

Keywords

karst area / cave underground water / river / nitrate / source

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