Investigation of pesticide transport in soil and groundwater

Abstract

Agricultural chemicals, such as pesticides are indispensable in modem agriculture. Pesticides are highly beneficial to the crops being grown, but only as long as they remain in the root zone of the soil. If a portion of the pesticides applied to ahTficultural fields leaves the target soil compartment, they can no longer complete their proposed purpose; they even can pose hannflll effects to the environment. The presence of pesticides in surface and hTfOlmdwater has led to public concem about the intensive use of ahTfochemicals. The ability to predict the fate of such chemicals released into the environment is an essential prerequisite to their efficient and safe management as well as to the establishment of realistic regulatory controls. In this research Rangpur sadar thana has been selected as the study area. For evaluating the susceptibility of hTfOundwater contamination, Leaching Potential Index (LPI) of sixty nine sites of RanhTur sadar thana has been calculated. This index is used to rank the relative vulnerability of hTfOundwater areas to contamination by agricultural applied pesticides. The method of LPI is based on one-dimensional advection dispersion equation and the index is physically based and uses chemical and environmental properties of pesticides in the susceptibility calculation. The numerical ranks of computed LPI are grouped by quantiles into very high, high, moderate, low and very low categories and based on these ranking, one most vulnerable site from the very high categories is selected for MT3D application. MT3D, one of the four modules of the groundwater flow and transport modelling system MODFLOW, is a solute transport model and is used to obtain the fate and transport of agricultural pesticides. Most commonly used pesticides; diazinon, malathion, chlorpyrifos, cypennethrin and fenitrothion of organophosphorous hTfOUP, carbofuran, carbalyl of carbamate group and heptachlor and endosulfan of organochlorine hTfOliPare used in MT3D model to study their fate. Model results indicate that pesticides with high sorptivity and moderate to high persistence (cypennethrin, endosulfan, heptachlor) are found to have low potential impact on hTfOundwater. Pesticides with moderate sorption and low persistence (malathion, fenitrothion) may be generally considered nonthreatening to health. Again pesticides with low to moderate persistence and low XVIII sorptivity (carbofuran, carbaryl) should be taken as a serious concem because they pose a si!:,'11ificantimpact on groundwater. And pesticides with moderate sorption and persistence (diazinon, chlorpyrifos) have moderate potential impact on groundwater. Decreasing organic matter content of soil and increasing half-life of pesticides in deeper depth would not make Si!:,lllificantchange. MT3D results also justified that dispersion phenomenon has less effect in mass transport in porous media. Six soil samples are collected from the same site considered as the most vulnerable. The samples are collected from the !:,'found surface and 5/, 10/, 15/, 20/, and 25/ below the ground surface for the pesticide residue analysis. The collected samples are tested at the Agrochemical and Environmental Research Division (AERO), Institute of Food and Radiation Biology (IFRB), Atomic Energy Commission (AEC), Savar, Dhaka. Tests are carried out for pesticides, such as diazinon, malathion, chlorpyrifos, cypennethrin, fenitrothion of organophosphorous group, carbofuran, carbaryl of carbamate group using High perfonnance Liquid Chromato!:,'faph (HPLC) and DDT, DOE, dieldrin, aldrin, heptachlor, lindane, BHC, endrin, TOE, DOD, endosulfan of organochlorine group using Gas Chromato!:,'faph (GC). None of the pesticides were detected in the test. In the field survey, it was found that Sumithion (50 EC), chemical group of fenitrothion was applied on that site two months before the soil sample collection. As fenitrothion is a low persistent organophosphorous insectic,ide, it may be degraded or may be washed out within two months period. But some unknown peaks found in the organochlorines chromato!:,'fam indicate the presence of some unknown chemicals. These may be a new fonn of chemical fonned by the combination of different pesticides applied or an unknown brand of pesticide used in the area.