Fate and transport of nitrate in karst groundwater using high resolution sensors and stable isotope measurements

Nitrate generated and leached from agricultural soils to karst aquifers can lead to water quality problems at and downstream of karst springs. Karst groundwater systems remain particularly under-studied in terms of nitrate fate due to difficulty in collecting measurements and modelling the morphology of subterranean caves and fracture networks. Our research goal was to use high-resolution nitrate sensors and the isotope signature of nitrate, δ15NNO3 and δ18ONO3, to gain knowledge of nitrate source, fate and transport in karst basins. To do so, we use a multi-year high resolution (15 minute) submersible ultraviolet nitrate analyzer measurements and a multi-year dataset of δ15NNO3 and δ18ONO3 collected from agriculturally impacted groundwater basins in the inner Bluegrass region of central Kentucky USA. Time series analyses of nitrate sensors shows 5 mg/L N peaks in January-February and 2 mg/L N lows in July-August. Analyses of δ18ONO3 shows a similar trend to that of nitrate concentration with peaks of 4‰ in July-August and lows of 0‰ in January-February. Time series analyses shows δ15NNO3 peaks around 9‰ in September-October and δ15NNO3 low periods of 6‰ in March. The nitrogen isotopic signature peaks occur when nitrate delivery to the stream is low and soils are dry, allowing the enriched denitrified nitrate to be slowly leached from soil into the watershed. The spatial variability of soil properties produces conditions where nitrate can be at different stages of its oxidation-reduction potential and therefore differs in both the amount of available nitrate as well as its isotopic signature. Future work includes further data analysis and a process-based numerical model that includes stable isotope subroutines to elucidate the fate and transport of nitrate in karst groundwater and streams.