Assessing aquifer vulnerability to sea water intrusion in coastal southwest region of Bangladesh

Abstract

The use of groundwater in the coastal districts is affected by salinity. Any increase in salinity will reinforce the existing problems and create fresh problems in many places. A reduction in groundwater gradient (e.g. by excessive pumping) will cause the denser saline water to replace the freshwater space of the aquifer. A sea level rise will have a similar effect on the salinity. Depending on the land use activities, different parts will be subject to different degrees of vulnerability to saltwater intrusion. This study mapped the spatial distribution of the vulnerability of southwest coastal aquifers (in Satkhira, Khulna and Bagerhat districts) to potential seawater intrusion using GALDIT index method. The GALDIT method uses six parameters, namely groundwater aquifer type, hydraulic conductivity, height of groundwater level above sea elevation, distance from the shore (or high tide), impact of the existing seawater intrusion, and thickness of the aquifer. A numerical ranking system to assess seawater intrusion potential in hydrogeologic settings is devised using GALDIT factors. The findings showed that there are declining trends in groundwater levels in Kalaroa, Sadar and Tala Upazilas of Satkhira district and Dumuria and Daulatpur Upazilas of Khulna district. However, the trends are not considerable enough to infer that groundwater lowering has caused an imbalance between fresh and saline water. Salinity, in general, increases from north towards south except a few pockets in northern part of the study area. Salinity is also found to increase with depth at a few locations. Lithology analysis indicated that lithology layers are very complex, and follow no general patterns. The upper ‘composite’ aquifer and the ‘main’ aquifer are hydraulically connected, and hence can be considered as one complex aquifer. The GALDIT index map indicates that most of the areas of Satkhira district have high vulnerability to seawater intrusion (Kalaroa Upazila and about half of Sadar and Tala Upazilas of the district lies in the moderate vulnerability zone). In Khulna district, however, area under moderate vulnerability (Phultala, Terokhada, Dighalia, Rupsa, Batiaghata, and part of Dacope) is slightly higher than the area under high vulnerability (Koyra, Paickgacha, and parts of Dumuria and Dacope). Most of the Upazilas of Bagerhat are under moderate vulnerability, including Mollahat, Chitalmari, Fakirhat, Bagerhat Sadar, Rampal, and parts of Mongla. Area under high vulnerability includes Sarankhola, Morrelganj and Kachua Upazilas. Satkhira Sadara and Kala Upazilas, located in the moderate vulnerability area, experienced the highest irrigation development in Satkhira district, and also were found to be exhibiting declining trends in minimum groundwater levels. Hence, further expansion of irrigation warrants careful consideration. The finding that all areas in Khulna are under high or moderate vulnerability to seawater intrusion implies that further development needs careful consideration. This is particularly true for Dumuria Upazila, where irrigation development has been the highest in Khulna district and also where minimum groundwater levels were found to have a declining trend. The three Upazilas that have experienced growth in irrigation in Bagerhat district are Fakirhat and Mollahat. Further expansion in irrigation in Fakirhat and Mollahat also need to be carefully monitored given that they lie within the moderate vulnerability zone.