Groundwater Vulnerability Assessment Using Modified DRASTIC Index, the Case of Doornfontein Area (Johannesburg)

Groundwater is an essential resource for the domestic, agricultural, and industrial sectors. Nevertheless, groundwater quality has declined due to the increased use of pesticides and industrial effluent being discharged into water systems. This study aims to assess groundwater vulnerability by using the modified DRASTIC model around Doornfontein, Central Business District (CBD) of Johannesburg. Land use types are considered as a model parameter to produce a modified DRASTIC model, where the result varies between 103 and 219 (low to very high, vulnerable zones). Approximately 5.6% of the study area is covered by very high vulnerable zones and distributed in southern region. High, moderate and low vulnerability zone covers the area of 55.3%, 28.8% and 10.3%, respectively. The study uses a nitrate concentration level to validate the accuracy of the model. The correlation analysis between nitrate level and DRASTIC model shows R2 = 0.548 and significant correlation with the modified DRASTIC model R2 = 0.56. The single parameter sensitivity analysis reveal that land use, aquifer media, soil media, and topography have a prominent role in assessing vulnerability. The study output shows the area is highly vulnerable to contamination, necessitating the relevant authorities' careful attention and protection of the area's groundwater resources.