ANALYSING ARSENIC CONTAMINATED AQUIFERS OF GOPALGANJ DISTRICT BANGLADESH USING MODFLOW

Abstract

Groundwater has been used as a safe potable water source and more than 80% of rural and urban areas population in middle-income countries like Bangladesh is majorly dependent on this subsurface resource. Due to continuous increasing demand, and its wasteful use and contamination by numerous anthropogenic activities, groundwater resources have been under increasing threat. Considering these facts, it is quite challenging to keep the groundwater resources safe for human use under varying environmental conditions. Efficient management practices and proper water use planning are therefore needed to tackle these challenges. Amongst various polluting elements of groundwater resources, presence of arsenic in subsurface formations is one of the ubiquitous issue particularly in south central Asia including Bangladesh. Use of groundwater in Bangladesh and West Bengal state of India have become unsafe due to the massive arsenic pollution. In general, groundwater of deeper depths has been considered free from arsenic. But recently arsenic is observed in the deeper aquifers of Bangladesh at alarming concentration. According to the latest survey, out of 64 districts of Bangladesh, groundwater resources of 61 districts are effected by arsenic pollution. Particularly groundwater resources of Gopalganj district of Bangladesh, the study area of this dissertation, is severally contaminated by arsenic. Apart from these presence of arsenic is being detected in new areas of Bangladesh posing around 35 million Bangladeshi people at high health risk. Arsenic transport in subsurface largely depends on the physical, chemical, and microbial process of the aquifer media. Mathematical modeling is a powerful tool that can provide crucial information about the movement and spreading of arsenic in the subsurface under existing site conditions. Therefore, a modeling framework has been used here for Gopalganj district of Bangladesh, one of the most affected area of Bangladesh by arsenic. Movement of arsenic over time in various aquifers and aquitard of the study area is analysed using USGS survey modular Visual MODFLOW-2000 along with MT3D and MODPATH simulation modules. The study area is comprised of two aquitards and three aquifers. Thus the subsurface system is comprised of five layers situated from top to bottom as: 1) aquitard of 15 m depth, 2) shallow aquifer of 75 m depth called as aquifer unit 1, 3) main aquifer of 115 m depth called as aquifer unit 2, 4) aquitard of depth 12 m and 5) deep aquifer up to 83 m depth. To investigate the arsenic movement in future, characteristic arsenic contaminated pockets are considered in lower part of aquifer unit 1 based on present arsenic concentration in groundwater of the study area. It is to be noted here that a high arsenic concentration has been reported at a depth of 55- iv 90 m in subsurface of Gopalganj district, thus hypothetical zones of are taken at similar depths for simulating arsenic spreading with progression of time. Results obtained from the model runs depict that if the present groundwater utilization pattern from the main aquifer present at 145-205 m depth continued over time, arsenic level in will exceed the WHO permissible limit of drinking water in the next 50 years and it will cross the Bangladesh standard of safe drinking water after 100 years. The unutilized deep aquifer which is presently free from arsenic and located at deeper depth (217-300 m), will be affected by arsenic. The projection shows that the concentration of arsenic will cross the WHO standard of safe drinking water in this aquifer after the 100 years. Insignificant movement of arsenic is predicted in horizontal/lateral directions as compared to the vertical spreading. An increased amount of groundwater extraction (10%) is further elevating arsenic concentration in shallow aquifers as compared to its present withdrawal rate. Results of this study are of direct use in planning groundwater extraction patterns of arsenic polluted subsurface resources, particularly Gopalganj district of Bangladesh. A suitable remediation strategies can also be framed based upon the projected patterns of arsenic containing plumes.