Influences of the stream groundwater hydrology on nitrate concentration in unsaturated riparian area bounded by an intermittent Mediterranean stream

Abstract

Stream aquifer hydrology and nitrate removal were studied, over a period of 2 years, in an unsaturated riparian zone, bounded by an intermittent Mediterranean stream, (Fuirosos, northeastern Spain). The riparian groundwater system is characterized by drastic hydrological changes and by mixing of stream water with hillslope groundwater. The hillslope groundwater flowed through a medium with low hydraulic conductivity (9.6 10 - 3 < k s < 0.1 m d - 1 ) and low specific discharges (1.7 10 - 3 < q h l l < 15 10 - 3 m d - 1 ). In contrast, stream water infiltrated through the near stream porous medium with relatively high hydraulic conductivity (4.8 < k s < 19 m d - 1 ) and variable specific discharges (i.e., 0.03 < q s t < 1.5 m d - 1 ). An intense and short stream discharge period occurred in autumn, when stream water infiltrated a maximum of 10 m into the riparian zone. Nitrate concentration and nitrate removal spatial rates (η N O 3 ) showed wide spatial heterogeneity. Higher nitrate concentrations (3.4 NO 3 -N mg L - 1 ) and effective nitrate removal (η N O 3 = 0.098 ′ 0.04 m - 1 ) were found in the deep groundwater of hillslope zone associated to low water fluxes. In contrast, in the stream edge zone (with higher water fluxes), nitrate release predominated over depletion (η N O 3 = -0.13 ′ 0.04 m - 1 ) during the stream discharge period. This opposite pattern of nitrate removal observed in the study area suggests that the depletion of diffuse nitrate inputs in riparian zones bounded by intermittent streams requires careful consideration.