Nutrient dynamics in stream water and groundwater in riparian zones of a mesoscale agricultural catchment with intense seasonal pumping

Abstract

Water and soil in agricultural areas are usually enriched with nutrients from excessive fertilizations. Riparian zones, the interfaces between terrestrial and aquatic systems, play crucial roles in delivery of the nutrients to groundwater and surface water. We evaluated spatiotemporal variations of water flow and nutrients in a mesoscale agricultural catchment along the riparian zones of the main stream for dry and rainy season by hydrologic monitoring and measurements of hydrochemical parameters and water stable isotopes. Gaining stream conditions were dominant in the mountainous upper reaches with steep topographic gradient while groundwater level in the lower basin considerably varied seasonally due to intense groundwater pumping to aid in heating greenhouses in winter season, which resulted in losing stream conditions during the dry season. Compared to shallow groundwater, deep groundwater had higher nitrate concentration derived from dry farmland in hilly terrains and induced recharge due to the groundwater pumping. Nitrate concentration was significantly higher in groundwater than that in stream water, indicating that groundwater is a major source of nitrate in stream water considering mainly gaining stream conditions. Compared to stream water, dissolved phosphorus concentration was higher in shallow groundwater recharged from paddy fields and floodplain areas with reducing conditions, evaporation signature of water stable isotopes, and lower nitrate concentration. Nutrient flux estimates for the catchment revealed that nitrate in stream water is considerably contributed by groundwater discharge and shallow groundwater in reducing conditions is a significant source of dissolved phosphorus in stream water, which was more pronounced in the rainy season. This study demonstrated that intense agricultural activities in riparian zones strongly affect stream-aquifer interactions and nutrient delivery to the stream, as well as groundwater. These findings can contribute to understanding hydrological and biogeochemical processes of nutrients in agricultural catchments and establishing an effective management of water use and nutrient application.