M.Sc Thesis

M.Sc StudentKhodorkovsky Miriam
SubjectInvestigation of Spatial and Seasonal Distribution of
Selected Pesticides (Chlorpyrifos, Methidathion
and Endosulfan) and their Degradation
Products in the Streams of
DepartmentDepartment of Civil and Environmental Engineering
Supervisors ASSOCIATE PROF. Yael Dubowski
PROF. Eran Friedler


Many pesticides end-up in surface- and ground-water, potentially risking aquatic life and drinking water. Their degradation products have different toxicity, mobility and persistency than the parent pesticides. Although they are often detected in water-bodies, not many studies focused on them. Their behavior and fate need better understanding.

Hula valley, largely exploited for agriculture, might serve as a major source of pesticides reaching Lake Kinneret. Endosulfan, Chloropyrifos and Methidathion were shown to be massively used in this area. Endosulfan degrades rapidly in water, but its high hydrophobicity enables longer persistence, by adsorption to particles. Environmental fate of its two stereoisomers, a- and b-endosulfan, varies: a-isomer is more volatile, while b- adsorbs more readily to particles. Endosulfan sulfate, the major derivative, is more persistent than parent isomers. Chlorpyrifos adsorbs strongly to soil particles, has low water solubility and is fairly stable to soil-degradation. Chlorpyrifos-oxon is its main photodegradation product. 3,5,6-trichloro-2-pyridinol, formed by abiotic and biotic degradation, is the more stable metabolite. These degradation products are more soluble and susceptible to leaching than parent compound. Methidathion is moderately water soluble, poorly bound by soils, and undergoes rapid degradation. Only parent compound was monitored, due to scarcity of data regarding its possible transformation products and their standards.

This study’s objective was to investigate spatial and temporal distribution of these pesticides and their derivatives in Hula watershed streams. The compounds were monitored in five different sites routinely, and more intensively during rain events. Observed pesticides levels were compared with rainfall data and information regarding pesticides application and degradation processes.

Compounds of interest were detected mainly during first major rain events. Their concentrations were usually <1μg/l to few μg/l. In some cases, degradation products concentrations were as or higher than their parent compounds. Chloropyrifos derivatives were detected more frequently and showed higher concentrations than chlorophyrifos. a-endosulfan was the dominant endosulfan compound detected. Methidathion was detected less frequently and reached lower maximal values than two other pesticides.

Obtained data indicate that Hula valley is a potential source of pesticides contamination to Jordan River, and further to Lake Kinneret.

Application time, rain intensity, differences in soil types, hydrology, distance from agricultural areas, etc. may influence levels and ratios of different residuals in streams.

Surface reactions seem to be significant among transformation processes that pesticides undergo after application. Monitoring campaigns should include degradation products, as well as sampling through rain events and in proximity to application episodes.