Groundwater-surface water exchange affects nitrogen and phosphorus exports from tideless rivers to a ria coast in the sea of Japan

Abstract

• Groundwater discharge in the downstream area of six tideless rivers was quantified. • Riverbed slope affected the relative contribution of groundwater to surface water. • Groundwater-derived nitrogen and phosphorus were critical for the coastal ecosystem. Groundwater discharge in downstream regions along the river continuum is considered a significant source of water and nutrients. However, the influence of groundwater discharge on the net export flux of surface water and nutrients from tideless rivers to coastal seas has not been thoroughly assessed. We conducted field surveys in the downstream region of six tideless rivers with different watershed properties along the ria coast of Wakasa Bay in the Sea of Japan during the summer. Mass balance models of water, salt, and 222 Rn were constructed to determine groundwater fluxes and associated fluxes of nitrogen and phosphorus. The model revealed that groundwater discharge ranged from 0.08 to 3.89 m d −1 , accounting for 1.3–14.8% of the export flux. This variability among the rivers correlated with the riverbed slope in each river, indicating that the local topographic gradient was a significant factor controlling groundwater-surface water exchange in tideless rivers. Groundwater discharge accounted for 0.8–16.7% of the nitrogen export and 0.9–27.4% of the phosphorus export to coastal seas. We concluded that the groundwater inflow driven by the hydrological and geographic properties of the watershed in the downstream region of tideless rivers was nonnegligible for nutrient dynamics and biological production in the coastal seas.