The role of the hyporheic zone in the nitrogen dynamics of a semi-arid gravel bed stream located downstream of a heavily polluted reservoir (Tafna Wadi, Algeria)

Amina Taleb , Nouria Belaidi , José-Miguel Sanchez-Pérez (1) , Philippe Vervier (1) , Sabine Sauvage (1) , James Gagneur (1)

1 LEFE - Laboratoire Ecologie Fonctionnelle et Environnement

Nitrogen retention was measured along the Tafna wadi downstream of a heavily polluted reservoir in North-West Algeria to understand the role of the hyporheic zone (HZ) in nitrogen dynamics. Nutrient concentrations were measured monthly for 2 years within the bed sediments of a 300m reach located 20 km downstream from the dam. Due to strong hydrological fluctuations hyporheic water was analysed during natural low and high water (HW) periods, and during water reservoir releases. Nutrient concentrations in surface water (SW) increased during water releases and in the HZ during the low water (LW) periods. Surface/hyporheic water interactions were characterized by determining the vertical hydraulic gradient (VHG) and the chemical signature of the ground water (GW). The latter was obtained from regional GW monitoring. Hyporheic chemistry was strongly influenced by patterns of surface flow. Hyporheic and SWs had similar chloride concentrations during high flow when they were significantly lower than those of the regional GW. GWwas generally richer in nitrates and nitrites, but was lower in ammonium concentrations than interstitial and river waters. Nitrates decreased significantly from upstream to downstream within the HZ throughout the hydrological period even though temporal fluctuations were high. Ammonium concentrations in interstitial water (IW) were significantly higher than in SWand generally increased from upstream to downstream. This study demonstrates the importance of the HZ in altering the dissolved inorganic nitrogen composition and concentrations of heavily polluted arid streams. The study is of interest because it documents a large ‘natural experiment’ that being the effect of periodic water release from a reservoir with serious water quality problems on the water quality dynamics (particularly nitrogen) of subsurface and SWs downstream.

Mots clés: Nutrients; Reservoir; Hyporheic zone; Pollution; Semi-Arid stream; Ecology

Domaines: Chimie organique; Hydrologie

Amina Taleb, Nouria Belaidi, José-Miguel Sanchez-Pérez, Philippe Vervier, Sabine Sauvage, et al.. The role of the hyporheic zone in the nitrogen dynamics of a semi-arid gravel bed stream located downstream of a heavily polluted reservoir (Tafna Wadi, Algeria). River Research and Applications, 2008, 24 (2), pp.183-196. ⟨10.1002/rra.1039⟩. ⟨hal-03476955⟩