Mapping of vulnerability to groundwater pollution by the application of the DRASTIC method and GIS, case of the Hennaya alluvial water table (Tlemcen wilaya, North-West Algeria)

Résumé

The plain of Hennaya (North-West Algeria) agricultural vocation par excellence continues to be directly influenced by human activity (urban and industrial discharges, chemical fertilizers, irrigation treated wastewater from the Ain station El Hotz, etc.). This plain contains an alluvial layer with an area of 29 km2 and a capacity of about 22 million m3 which constitutes the first source for irrigation. The degradation of the water quality of this aquifer and their protection has become a major concern for water managers and users.
To address these concerns, a methodological approach has been proposed. It is based on the determination of the vulnerability of the water table to pollution. Groundwater protection methods, based on vulnerability maps, characterize an area according to its ability to self-protect. The assessment of the groundwater pollution vulnerability of the Hennaya water table was based mainly on the integral analysis of the intrinsic parameters acting directly on the propagation of a pollutant and recommended by the DRASTIC method (Aller et al. , 1987) coupled to a GIS. The data necessary for this study are the six factors specific to the DRASTIC model which are: the depth of the aquifer, the recharge, the lithology of the aquifer, the topography (slope), the lithology of the unsaturated zone and finally the conductivity hydraulic. Due to the lack of pedological data of the plain, we did not use the soil type parameter "S".

For each critical parameter, a coefficient ranging from 1 to 10 and a weight ranging from 1 to 5 were assigned. The vulnerability index is obtained by adding the products (coefficient * weight). The resulting map shows three classes of vulnerability; a strong vulnerability covers 8.15% of the total area of the aquifer, an average vulnerability of 61.7% in the central part and a low vulnerability occupies 30.14% in the southern part. High and medium vulnerability areas are characterized by significant agricultural activity, where groundwater nitrate concentrations are in the order of 108 mg / l in 2011. These results provide a guide for zone protection decision-makers. Deemed to be sensitive to pollution, as well as the identification of sites that will subsequently house socio-economic infrastructures.