Groundwater Contaminant Transport Modeling and Aquifer Vulnerability Assessment of Gazipur District, Bangladesh

Abstract

Gazipur District is located in the Dhaka Division of central Bangladesh, covering an area of approximately 1,741 square kilometers, of which 17.53 km 2 is river and water bodies, and 273.42 km 2 is forest area, while the rest includes rural and urban settlements, agricultural lands, and industrial areas. As per the 2021 census the total population of the district is approximately 3.4 million, one of the most populous districts in the country. Gazipur is situated north of the capital city, Dhaka, and shares its borders with Mymensingh, Tangail, Kishoreganj, Narsingdi, and Narayanganj. It has a tropical monsoon climate characterized by high temperatures and heavy annual rainfall. The average annual temperature in Gazipur ranges from 25 to 30°C, with the highest temperatures occurring between April and June. The average annual rainfall in the district is around 2,200 millimeters, with the highest precipitation occurring during the monsoon season from June to September, while the dry season from December to February typically experiences little to no rainfall. Gazipur District is divided into five administrative Upazila or sub-districts, viz., Gazipur Sadar, Kaliganj, Kapasia, Sreepur, and Kaliakair. Due to a wide range of economic and industrial activities, and fast-growing job market, movement of people form the neighbouring rural areas resulted into a continuous rising trend of population, that doubled during the last two decades. It is a major industrial city, a hub for the country’s textile industry with almost 1500 industries. About one-third of the export-oriented ready-made garment factories of the country is located in the district, resulting in significant urbanization over recent years, with drastic changes to infrastructures. The district has been facing a significant increase in groundwater extraction over the years, due to population growth, urbanization, and industrialization. Almost all the drinking water supply in the area comes from groundwater sources. Rapid urbanization and industrialization caused sharp rise in abstraction of groundwater alongside the preexisting usage for irrigation. The highest groundwater consumption is in the urban (85%) and industrial settings (15%). Groundwater reserves are dwindling due to the continuous increase of uncontrolled abstraction alongside gradual decrease in recharge rate due to change in land use an land cover types. Higher abstractions and lower recharge result in an average annual drop of >2 meters in the groundwater levels of the underlying aquifers. This research aimed to determine the impact of rapid urbanization and increasing industrialization on groundwater in the Gazipur District; and relate contamination levels of groundwater with growing land cover and land use changes. To meet the increased demand for water, there has been a surge in abstraction, which raised challenges in managing water resources and caused sustainability challenges.

Electrical conductivity (EC) and Total Dissolved Solids (TDS) were measured throughout the district between 2018 to 2021. Over the years, the average high EC value increased from 1071 μS/cm to 1781 μS/cm, higher values in urban and industrial areas of the District. A similar comparative increasing trend can be observed with historical measurements. Contaminants introduce additional ions into the water causing an increase in EC values, indicating contamination. This increase can be attributed to the heavy metal from industrial waste and domestic effluents into groundwater, observed within the main urban and industrial settings of the district. A detailed sampling plan was prepared with a target to cover the district's main urban settlements, industrial hubs, growing areas, forests, and agricultural areas. A Total of 143 groundwater samples were collected from the district and analyzed; thirteen parameters were considered for WQI calculation: pH, TDS, sodium, potassium, calcium, magnesium, iron, manganese, bicarbonate, chloride, sulfate, nitrate, and fluoride. The computed WQI shows that 48% of the water sample falls in excellent and 48% in good water categories. Spatially, WQI values exceed the limit in areas with high urbanization and industrialization setups. Significantly high values were found in the eastern part of Kaliakair, the central part of Gazipur Sadar, the northern part of Sreepur, the eastern part of Kapasia, and the northern part of Kaliganj within the growing urban and industrial areas of the district. Urbanization and industrialization lead to an increased demand for water, affecting quality and sustainability of groundwater. The DRASTIC method has been modified to assess groundwater vulnerability by incorporating population density, an outcome of urbanization and industrialization. The new assessment methodology of groundwater vulnerability is termed as DRASTIC-P. According to the new produced DRASTIC-P Map, urbanization and industrialization have been found to be hazardous activities impacting the district’s groundwater resources. According to the vulnerability map most part of the district is impacted, with minimum impacts in the southeastern part. Industrial processes often use large amounts of water, and the growing population in urban areas also requires more water for domestic use. This increased demand lead to the over-extraction of groundwater, causing depletion of aquifers and lowering of the water table. The solute transport model predicts spreading contaminants will spread to the neighboring regions in less than ten years. Flow is more rapid in the regions with high abstraction rates. This study indicated the limitations of modeling using hypothetical data and generalized information. Though MODFLOW will give a generalized flow pattern and contamination transport, yet lack of data can make the observations flawed. Fundamentally, this study indicates that the cone of depression may expand outside the district area; hence, further work should concentrate on a more precise measurement of in-situ hydrogeological parameters.