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.
