Groundwater vulnerability to pesticides in Northwest Bangladesh

The transport and leaching potential hazards of various pesticides were studied in a shallow unconfined aquifer located in Northwest Bangladesh. Pesticide leaching potential was quantified using a one-dimensional advective–dispersive transport equation for a non-conservative chemical that follows first-order decay and linear adsorption in soils. Leaching potential index (LPI) was calculated for 69 sites in the study area to evaluate the relative vulnerability to pesticide leaching and to prioritize sites for model study and soil sampling. The numerical ranks of computed LPI were grouped by quantiles into very high, high, moderate, low and very low categories; and based on these rankings, the most vulnerable site was selected. The fate and transport of pesticides in this most vulnerable site was modeled using MT3D. The model results indicate that pesticides with high sorptivity and moderate to high persistence have low potential impact on groundwater. Top soils are found to be particularly vulnerable to the accumulation of organochlorine pesticides. Results also revealed that decreasing the soil organic matter and increasing the half-life of the pesticides at deeper depths did not make any significant change. Finally, six soil samples were collected from the same site at depths of 0.0, 1.5, 3.0, 4.5, 6.0, and 7.5 m for the analysis of pesticide residues. The soil–water was extracted from the samples following standard extraction technique and tested using gas chromatography (GC) and high-performance liquid chromatography (HPLC) for pesticide residues. Results showed no trace of pesticide residues in the soil–water; however, a few unknown peaks were detected indicating the use of some unknown brand of chemicals in the study area.