Statistical investigation of groundwater quality variables in north-western zone of Bangladesh

Abstract

Increased and prolonged use of chemical fertilizers and pesticides in agriculture causes runoffs and percolation that enter water courses and underground aquifers. These are likely to have adverse effect on aquatic lives, flora-fauna and human health. The emerging problem of groundwater contamination has resulted a need for information which can be supplied by properly designed groundwater quality monitoring programs. Over the last 35 years or so, a substantial amount of groundwater quality data has been collected in this country. The data is meant to be used to provide early warning of pollution events. This necessarily involves statistical analysis of data to detect changes in quality over time or space. In this study, a statistical analysis of existing groundwater quality data were conducted. Specifically the purpose of the study was to determine whether groundwater quality variables: (i) are normally distributed, (ii) exhibit seasonal patterns and (iii) are correlated in time. Groundwater quality data from about 40 well locations in north-western zone of Bangladesh were used in this study. Results of investigation on nine parameters, such as chloride, nitrate, TDS, pH, SAR, sodium, iron, calcium and magnesium suggest that parameter concentrations are well within acceptable limits. Results of statistical analyses further showed that most groundwater quality variables are not normally distributed. but have skewed right distribution. As such, most of the variables appeared to be long normally distributed. Seasonality and serial dependence of water quality data were also investigated. Chloride, nitrate and TDS showed ,seasonal patterns and pH, SAR, sodium, calcium and magnesium showed significant serial correlation. Spatial and temporal variability of these parameters were also investigated. Spatial variability was found to be much higher than the temporal variability. Finaliy, leaching potential hazards of various pesticides used were.aiso studied for'one location, Gangachara thana in Rangpur district. A methodology to calculate Leaching Potential Index (LPI) was described and this index was used to rank the relative vulnerability of groundwater areas to contamination by agriculturally applied pesticides. The method uses the iv advection-dispersion equation for chemical transport in soils to develop leaching potential index which is an indicator of the relative susceptibility of each area. This index is physically based and uses chemical and environmental properties in the susceptibility evaluation. The leaching potential indices were calculated for 60 sites in Gangachara thana, Rimgpur district to rank the relative susceptibility of sites to pesticide leaching. Based on these computed leaching potential indices, eight most vulnerable groundwater sites were identified and groundwater sample was collected from each site for analysis in laboratory for pesticide residues. The collected samples were tested for pesticide residues in the Institute of Food and Radiation Biology under Atomic Energy Commission at Savar. Tests were carried out for residues of most commonly used pesticides, such as heptachlor, aldrin, DOE, dieldrin, endriri, DOD, and DDT of organochlorine group arid methyl parathion, diazinon, parathion. methyl trithion, methoxychlor, malathion, disulfoton, phorate and ethion of organo-phosphorous group and none of them were detected in the tests. However, the method of LPI can be a useful planning tool for early identification of agricultural areas susceptible to grou'ndwater contamination and efficient management of sampling resources.