Mapping groundwater zones contaminated by hydrocarbons in the Dammam aquifer in the Karbala–Najaf plateau, Iraq

Abstract

This study focused on delineating the groundwater contamination zones that contain hydrocarbons (heavy oil) in the Dammam aquifer in the Karbala–Najaf plateau, central Iraq, using two hybrid models, specifically, the analytical hierarchy process (AHP) and the technique for order preference by similarity to an ideal solution (TOPSIS) (AHP-TOPSIS and entropy-TOPSIS). Six factors were identified as causing the contamination of groundwater in the Dammam aquifer depending on data availability and the opinions of experts. These factors were the distance to the Abu Jir fault, fault density, aquifer hydraulic conductivity and thickness, borehole depths, and elevation above mean sea level. The AHP and entropy methods were used to derive the weights required to run the TOPSIS algorithm. The ranked values from TOPSIS were classified into five contamination hazard zones: very low, low, moderate, high, and very high. The results from the hybrid models were validated using the receiver operating characteristic curve and proved that the hybridization of entropy-TOPSIS with an area under the receiver operating characteristic curve equal to 0.80 performed better than that of AHP-TOPSIS with a value of 0.75. The direct comparison of groundwater-contaminated boreholes (validation dataset) with groundwater contamination zones also confirmed this result. The moderate–very high contaminated zones occupy 60% of the study area and are distributed in the northern and northeastern parts of the plateau, whereas the very low–low zones are concentrated in the southwestern and southern parts. The map of contamination level hazard zones from this study could be used by decision-makers and drilling workers as a guide to drill new boreholes and thus to avoid drilling-faulted contaminated boreholes.