Date of Award

2004

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Science

First Advisor

Dr. Munjed Maraqa

Second Advisor

Abdel Mohsen Mohamed

Third Advisor

Warren W, Wood

Abstract

Proper management of pesticides application in the UAE is greatly lacking, causing waste of resources and environmental concerns due to the excessive use of these chemicals. It has been reported that the rate of pesticides use in UAE reaches about 10 kg/hectare/yr, posing a potential threat to the quality of water in underlying aquifers. Little, however, is known about the mobility of applied pesticides in the UAE subsurface environment and the impact of these pesticides on soil and groundwater quality.

The aim of this study was to gain an understanding of the fate and mobility of pesticides in the UAE subsurface environment. This study focused on the role of mechanisms like advection, dispersion, degradation, and sorption that affect pesticides transport in the soil. The study was conducted in the laboratory using soil packed columns. Batch experiments were also conducted to assess the impact of some of the above mechanisms.

In this study, three pesticides that are commonly used in UAE were employed (i.e. dimethoate, metalaxyl and cymoxanil). Two sandy soils from Al Foah and Abu Samra areas in AI Ain district, Abu Dhabi Emirate were collected and utilized in the study. Both soils are alkaline with similar texture but have different organic matter content that ranges between 0.2% to 0.8%. Results showed that the pesticides are subject to various extent of degradation with cymoxanil undergoes the highest rate especially at high pH values. The study revealed that there was no influence of light and indigenous microorganisms on the degradation of the used pesticides within an experimental time of 3 days.

Pesticides sorption to soil is increased by the increased soil organic matter content with dimethoate and metalaxyl are being more readily sorbed on soil with high organic matter compared to sorption on soil with lower organic matter. Pesticides sorption is also affected by the compound hydrophobicity with metalaxyl being more sorbed than dimethoate on a given soil. Both dimethoate and metalaxyl sorb linearly on soil with low organic matter, while metalaxyl undergoes nonlinear sorption on the soil with high organic matter. Cymoxanil sorption to both soils was not evaluated due to the loss of the chemical from solution by hydrolysis.

Generated breakthrough curves (BTCs) for the ideal tracer (bromide) and the employed pesticides were analysed using the nonlinear least-squares model (CXTFIT2.0). BTCs obtained for bromide were used to describe the dispersive behaviour of the two soils. It was found that the dispersivity of Abu Samra soil is 0.12 cm while that of Al Foah 0.34 cm. Generated BTCs for the used pesticides were analyzed by moment analysis to determine the retardation coefficient (R) and the first-order degradation rate constant (λ). Simulation of BTCs using the determined coefficients showed that the equilibrium model adequately described transport of dimethoate in Abu Samra soil, but failed to describe breakthrough data for metalaxyl. A good description of the BTC for metalaxyl we obtained when the two-site sorption nonequilibrium model was used. The mass-transfer rate coefficient for metalaxyl can be predicted using empirical relationships reported in the literature.

Comparison between transport parameters determined by moment analysis and those determined by curve fitting showed that the use of moment analysis is sufficient to obtain R and λ. It was also observed that the values of the sorption distribution coefficient (Kd) determined from the column studies were 2-10 times higher than their counterparts obtained from batch experiments. This study also revealed that there appears to be an enhanced degradation of the pesticides in a soil environment as compared to their hydrolysis in the aqueous solution.

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