A GIS BASED DRASTIC MODEL FOR ASSESSING GROUNDWATER VULNERABILITY IN TONGI, GAZIPUR DISTRICT OF BANGLADESH

Abstract

Groundwater resources in Tongi are currently threatened by many garments, dyeing, paper and pulp, wood, tannery, dairy, poultry, fishery, chemical and textile industries. Many of these industries release untreated heavy metals, solid and liquid wastes consisting of cadmium (Cd), chromium (Cr), and lead (Pb), threatening water resources. A model-based groundwater pollution risk assessment was conducted to address these issues. The most sophisticated among the vulnerability assessment techniques is the GIS-based DRASTIC model. The model considers parameters like depth to the water table, net recharge, aquifer and soil media and hydraulic conductivity, which were collected from Bangladesh Water Development Board (BWDB), Geological Survey of Bangladesh (GSB). This study determined the net recharge from specific yield using bore log data and water table fluctuation. The average net recharge has been calculated as 359 mm/year and varied from 105 to 614 mm/year. Determination of the DRASTIC index involves multiplying each parameter weight by its site rating and summing up the total. The higher index values symbolise a greater risk of GW pollution or greater aquifer vulnerability. Finally, for the year 2013 and 2019, two groundwater vulnerability map were developed for each year for the study area using the DRASTIC Index and GIS mapping. One of the maps was developed following typical DRASTIC weightage suggested in the model and the other map was developed by modifynig weightage for the study area. In 2013, the DRASTIC index values vary from 104 to 156 according to model suggested weightage and it was reclassified into two groups of moderate, and highly vulnerable zones. About 21% of the area is highly vulnerable and 79% area is moderately vulnerable to groundwater contamination. On the other hand, modified weightage DRASTIC index values vary between 113 and 183 and it was reclassified into two groups of moderate and high vulnerable zones. About 3.5% area is moderately vulnerable, 96.5% area is highly vulnerable. In 2019, the DRASTIC index values vary from 91 to 137 according to model suggested weightage. The whole study area is susceptible to moderate vulnerablility to groundwater contamination. On the other hand, modified weightage DRASTIC index values vary between 95 and 164 and it was reclassified into two groups of moderate and high vulnerable zones. About 20.6% area is moderately vulnerable, 79.4% area is highly vulnerable. The modified weightage map has more vulnerbility compared to typical map. Thus, the model provides important information that could be useful to city planners and decision-makers for the establishment of ETPs and putting restrictions on further industrialisation in the vulnerable zones.