Groundwater vulnerability assessment using GIS based DRASTIC model in Aksum area, central Tigray, Northern Ethiopia

Abstract

The study area is located 220 km northwest of Mekelle, the capital of the Tigray Regional State in Ethiopia. The objective of this research is to produce a groundwater vulnerability map for Aksum and its surroundings using the GIS-based DRASTIC model to protect the groundwater quality of the area. The DRASTIC model uses seven parameters that influence and control movement of the contaminants from the surface to the subsurface. These are Depth to aquifer (D), net Recharge (R), Aquifer media (A), Soil media (S), Topography (T), Impact of vadose
zone (I), and Hydraulic conductivity (C). These factors were rated, weighted and overlaid to create vulnerability maps showing areas affected by varying levels of groundwater contamination. The ArcGIS 10.7 software was used to create the groundwater vulnerability map by overlaying the seven layers. The DRASTIC vulnerability index was calculated as the sum of the products of ratings and weights assigned to each parameter on scales of 1 to 10 and 1 to 5, respectively. The vulnerability index ranged from 79 to 165 and reclassified into three classes using the quantile classification method: low (79–120), moderate (120–130) and highly (130–165) vulnerable zone, and cover about 78.3 km2 (33.9%), 83.1 km2 (36%), and 69.5 km2 (30.1%) of the study area, respectively. The resulting DRASTIC index indicated a relative degree of vulnerability of the aquifer to pollution. The results were validated using nitrate concentrations in groundwater and the observed nitrate concentrations in wells are in accordance with the vulnerability map in most parts of the study area. The groundwater vulnerability map can be used as an initial tool at the planning, policy, and operational levels of the decision-making process concerning sustainable groundwater management and protection.