Groundwater quality and aquifer vulnerability assessment of several northern districts of Bangladesh

Abstract

The quality of ground water depends on various chemical constituents and their concentration. The present study area covers several northern districts of Bangladesh and includes 28 upazilas under Dinajpur, Thakurgaon, Panchagarh and Joypurhat districts. The main objective of the study is to determine the baseline water quality and aquifer vulnerability assessment. Drinking water quality of the study area is assessed based of World Health Organization guideline (WHO, 2004) and Bangladesh Standards (DoE, 1997). DRASTIC method is used for the aquifer vulnerability assessment. The seven parameters considered in the evaluation of the vulnerability of aquifer are Depth of water table, Recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone and Hydraulic conductivity. 22 DTWs and 21 STWs groundwater quality data is collected from IWM for baseline water quality assessment. According to DoE, 1997 guideline the study area groundwater both from STW and DTWis safe from arsenic contamination. Only one DTW sample at Birampur upazila of Dinajpur district exceeds the WHO, 2004 guideline for arsenic. For Manganese concentration 24 % shallow groundwater and I % deep groundwater sample exceed WHO, 2004 guideline. For Boron concentration 33% shallow groundwater sample exceedWHO, 2004 guideline. Groundwater in the study area is suitable for livestock conswnption and irrigation use. BWDB groundwater table data is used to produce groundwater depth map of the study area. The minimum groundwater depth in the wells ranges from 0.75m to 5.85m. For the estimation of groundwater recharge groundwater fluctuation method is used. The fluctuation of groundwater level is taken as the difference between the highest groundwater depth and the lowest groundwater depth of the recharge period in a water-year. Collected horelog data reveal that aquifer media in the study area is composed of mainly medium and coarse sand with little gravel. Aquifer type for the study area is divided on sorting of the aquifer materials and the presences of interbeded clay and fme materials. SRDI soil association map is geo-referenced and digitized with Arc View 3.2 to produce soil map. Soil type of the study area is simplified in four categories like loam, sandy loam, clay and silt. The slope percentage for the study area is derived from surface slopes obtained from Digital Elevation Model available in IWM. Analysis of lithologs reveal that the vadose zone is composed mostly clay, silty clay, sandy clay, fine sand and fine to medium sand. Hydraulic conductivity values are collected from IWM to prepare conductivity map. Hydraulic conductivity of the study area ranges from 20 m/day to 100 m/day. Depending on the relative significance on pollution rating is assigned for the seven DRASTIC parameters. Since extensive agricultural practices exist in the study area pesticide weightages is used to calculate DRASTIC Index. Summation of the product of rating and weight for each parameter give the final DRASTIC Index. Upazila wise DRASTIC Index is calculated. The Aquifer Vulnerability Map is prepared by using the DI value. DRASTIC Index for the study area varies from 147 to 204. The vulnerable zone is divided into three categories. The study shows that 29% area is highly vulnerable, 32% area is moderately vulnerable and remaining 39% area is low vulnerable to groundwater pollution. The output of the DRASTIC method indicates that high vulnerable areas are mainly in the North-West of the study area. Among the seven DRASTIC parameters the depth to groundwater table has the highest average contribution (28%) on the total DI and the impact of the vadose zone contributes the lowest average percentage (6%) in total DI. Although depth to water table has the higher influence on total DI, the impact of the vadose zone and soil type are the main parameters which divide the area as high vulnerable area. A positive correlation exists between DI and concentration of ammonium, potassium and nitrate of STW samples. There is no positive correlation exists between DI and iron, boron and manganese concentration. The high concentration of these parameters for STW sample is due the soil chemical properties, which is not considered in DRASTIC analysis.