Spatio-temporal variations of nitrate pollution of groundwater in the intensive agricultural region: Hotspots and driving forces

Nitrate (NO3- ) pollution of groundwater is a persistent and widespread problem worldwide, particularly in intensive agricultural regions with high nitrogen (N) surplus. Identifying spatio-seasonal variations, drivers and sources of NO3- in groundwater is key to controlling this pollution. In this study, we monitored 175 wells in areas with different irrigation practices (dryland, well irrigation and canal irrigation) in an intensive apple-planting region over a year (September 2020-October 2021) in the southern Loess Plateau, China. The integration of hydrochemical analysis, deep soil profiles, NO3- isotopic composition and a Bayesian isotope mixing model (SIAR) was used to identify the hotspots and hot moments of groundwater NO3- pollution, and main NO3- sources. The results showed that average NO3- concentrations of three regions gradually decreased from north to south, following the order of dryland region (9 mg NO3 L-1) < well irrigation region (23 mg NO3 L-1) < canal irrigation region (94 mg NO3 L-1), and orchard > residential area & AP;cereal land. In the study region, 44% of groundwater exceeded the drinking standard of the World Health Organization (50 mg NO3 L-1). The intensive apple planting in the canal irrigation regions has caused groundwater NO3- pollution, and changed hydrochemical types from HCO3- Ca & BULL;Mg to SO4 & BULL;Cl & BULL;NO3-Ca & BULL;Mg. In the canal irrigation region, NO3- in the vadose zone has migrated to groundwater. The hotspots of groundwater NO3- pollution (NO3- vulnerable zones) were identified at low-altitude loess tableland and alluvial plains with canal irrigation. Irrigation and precipitation accelerated soil NO3- deep migration. The hot moments of NO3- pollution in the irrigated region was the period from the wet season to the dry season; and the "hidden reactive N pool" in the deep vadose zone (>2 m) caused a time lag of NO3- reaching into groundwater. Chemical N fertilizer and manure N applied in apple orchards were the main contributing sources of groundwater NO3- pollution in the apple-planting region. Our study highlights the significant effect of apple-planting industry development on groundwater quality.