Index-based groundwater vulnerability and water quality assessment in the arid region of Tata city (Morocco)

Within the Tata city region of the Moroccan Anti-Atlas Mountains groundwater is the only source for drinking and irrigation water supply. Nevertheless, a moderate groundwater quality was already outlined. In order to understand the possible extent of groundwater pollution and to develop sustainable groundwater management strategies, a groundwater vulnerably assessment was required. In this context the overlay index model DRASTIC (depth of water, net recharge, aquifer media, soil media, topography, impact of vadose zone, and hydraulic conductivity) was implemented using specific ratings and weightings for each of the parameters. In the end, areas of greatest potential for groundwater contamination were identified based on geological and hydro geological parameters as well as on human impact (land use). The DRASTIC Index revealed that most of the study site is characterized by a low geological and hydro geological vulnerability. After implementing the land use parameter, a moderate vulnerability was observed, indicating a greater groundwater pollution risk caused by human hazards. Areas of greatest vulnerability occur along agricultural areas and villages. A single-parameter sensitivity analyses resulted in a resealing of the different DRASTIC parameters. Based on this resealing process most of the study site is of high vulnerability. In a second step, hydrochemical groundwater data was used as an actual pollution indicator to verify the vulnerability obtained from DRASTIC. To this end, a new Water Quality Index with respect to drinking and irrigation purposes was developed, based on four key parameters (specific electrical conductivity, chloride, nitrate, and ammonia). Thereby, nitrate revealed to be the most important chemical parameter reducing the degree of usage. Elevated nitrate concentrations are restricted to urban areas and derived from manure and septic waste.