Assessing water pollution risks and groundwater vulnerability using spatial data. Case of the Sidi Bel Abbes plain (North-West Algeria)
Due to strong population growth and changing consumption patterns, we are faced with a rapid increase in the volumes of waste that create risks for water quality in the Sidi Bel Abbes plain. The study carried out concerns the assessment of the risks of pollution as well as the mapping of the vulnerability of the alluvial groundwater to pollution. A first step of the study was the use of ALSAT-2A high resolution (2.5 m) Algerian sensor spatial data for the identification and mapping of pollution sources from urban and industrial waters. These data offer the possibility to increase the speed and accuracy of this mapping. A second step involved the protection of water resources from sources of pollution. This preservation is especially necessary in semi-arid areas where groundwater is the major water source. The mapping of the vulnerability of the alluvial groundwater to pollution is useful to identify the areas that require protection. The mapping is done by applying the DRASTIC method, based on seven physiographic and hydrogeological parameters from remote sensing data (DTM) and exogenous data (lithological, pedological, meteorological, hydrogeological). The study area is the plain of Sidi Bel Abbes (1 166 km2) in Northwest Algeria. It is a Quaternary depression. The choice of the zone is linked to the existence of a hydrogeological unit (alluvial water table), a zone with high agricultural value and a multiplication of sources of water pollution. The results obtained translated the risk of pollution of water resources (alluvial water and Chorfa dam water, stream Mekerra) by various sources, urban and industrial, including landfills and industrial wastewater. The flow of polluted water towards the Chorfa dam degrades the quality of these waters. The waters retained by this dam in December 2016 were highly polluted and had a dry residue of 1 940 mg∙L-1, dissolved oxygen of 66.5%, NH4 of 2 mg∙L-1, nitrite of 0.22 mg∙L-1 and phosphorus of 0.51 mg∙L-1. Stream Mekerra waters analysed in 2010 were of very poor quality: biochemical oxygen demand (BOD5) of 41.296 t∙a-1, chemical oxygen demand (COD) of 123.465 t∙a-1, total nitrogen of 3 193 t∙a-1 and total phosphorus of 2 342 t∙a-1. Given the permanent connection between the stream and the groundwater, there is a deterioration in the quality of groundwater. In view of this degradation, a vulnerability map was drawn up to distinguish the zones of high vulnerability in order to take the necessary protective measures; these zones represent 34% of the surface of the alluvial sheet.