Coupled hydrogeological modeling and nitrate transport modeling to assess vulnerability pollution in an anthropized watershed, case study of the lower Soummam valley (Bejaia Northeast ofAlgeria)

Abstract

Abstract Anthropogenic activities as waste water discharges and fertilizers use were the main factors controlling nitrate concentration in the lower Soummam valley, situated in the North-East of Algeria. The objectives of this study are to investigate nitrogen pollution, evaluate the groundwater vulnerability using DRASTIC-method and simulate nitrate mass transport coupled with a hydrogeological model. Hydrogeochemical results demonstrate a large seasonal variation between the high water period (May 2019 and 2021) and the low water period (September 2019- November 2020 and 2021). Temperature and electric conductivity were increased in the low-water period. However, pH, dissolved oxygen and nitrate decreased in the high-water period and finally nitrites and ammonium showing irregular fluctuation. DRASTIC vulnerability issues illustrate that the alluvial aquifer of lower Soummam presents high to very high contamination. A hydrogeological model was performed using PMWIN and coupled with a solute transport model. Calibration was achieved under steady state conditions during the period of 2011–2021. It was validated by the comparison with measured piezometric levels and nitrate concentrations in May 2021. Three scenarios were established under transient conditions in order to simulate variation of both groundwater levels and nitrate concentrations from 2011 to 2050. Simulation issues show that river-aquifer exchanges and dependent head boundary present the main terms in water inflow and restituted respectively about 44% and 38% of the total inflow. However the head constant presented approximately 85% of the total outflow. Nitrate model illustrates a slight variation in 27 wells of a total of 29. Substantial increase in Remila well located in the upstream, a raise was observed at 14.6 mg/l when the pumping volumes reached 45.8 Mm3/years, 16.5 mg/l according to a slight increase in pumping volume 49.19 Mm3/years and finally, an increase was calculated at 27.3 mg/l when the pumping volume increases to 98.38 Mm3/years in 2050. In Saidj well situated in the downstream, a significant reduction was observed at 19.9 and 24.4 mg/l respectively in 2021 and 2030, however, a sharp increase was estimated at 16.8mg/l in 2050. The river and groundwater head boundaries are the main factors controlling the hydrodynamic aquifer system and nitrate transport in the alluvial aquifer of the lower Soummam valley.