Cartography of the vulnerability to pollution of groundwater : Application to the Gharb’s plain (Morocco)
Here we present a method for the evaluation and mapping of groundwater vulnerability to pollution. For this purpose, we introduce a vulnerability index, Iv, as a linear function of three parameters: 1) time of transit, T, via the unsaturated zone, 2) degree of purge, C’p/Cp (the ratio of the pollutant concentration, C’p, reaching the water table and the initial pollutant concentration in the aquifer, Cp) and 3) the degree of recharge R’/R (the ratio of the effective recharge and the potential recharge). The vulnerability index thus takes into account the weighted sum of all three parameters, which characterize the transfer of a pollutant from the soil surface to the water table. In our study, the transit time and the degree of purge were estimated by the infiltration rate, purge power and thickness, which characterize the various layers of the unsaturated zone. The degree of recharge was estimated by the water balance. The weighting coefficients of the above parameters were estimated by sensitivity tests and analyzed by linear regression using pilot wells. This method was applied to the Gharb plain in Morocco, known for its complex hydraulic system and intense agricultural activity. Intrinsic vulnerability and parametric maps were established for this area using a Geographical Information System.