Ph.D Thesis

Ph.D StudentDinerman Efrat
SubjectFate of Chlorpyrifos under Semi-Arid Conditions and its
Impact on Chlorpyrifos Loss from Orchard
Soil Surface
DepartmentDepartment of Civil and Environmental Engineering
Supervisors PROF. Eran Friedler
Full Thesis textFull thesis text - English Version


Pesticides are widely used for both agricultural and nonagricultural purposes. Once released to the environment, pesticides may degrade and form various degradation products (DPs) which may vary significantly from their parent compounds.

In semi-arid climates, as in Israel, applied pesticide may remain at the surface-air interface for a long period of time due to completely dry summer and limited irrigation. Information on surface reactions of pesticides applied to fields is still limited since most previous studies focused on pesticide degradation in bulk soil or in water.  Surface loss of pesticides may be due to photo-degradation, volatilization, and degradation as well as sorption-desorption to/from soil. Hence, proper understanding of their environmental fate requires information on kinetics and products of each of these processes.

The main objective of the present study was to better understand and quantify the fate, and transformation of Chlorpyrifos (CP) and its DPs - Chlorpyrifos-oxon (CPO) and 3,5,6,Trichloro-2-pyridinol (TCP), in a single field scale. The research combined laboratory studies under controlled conditions and development of a conceptual model for predicting the CP and its main DPs concentrations in soil as a function of time from application. Preliminary evaluation of the model was obtained by comparing its predicted concentrations with those measured in orchards following CP application.

Sorption experiments showed that CP has higher sorption capacity to the soil than its oxon and pyridinol derivatives. Increasing contact time with soil (aging) resulted in increased difficulty to release CP but did not affect TCP desorption properties. Aging experiments were not conducted with CPO due to its very fast degradation in soil. In general, soil degradation rates followed the order CPO>CP >TCP.  CP was found to degrade faster under altering dry-wet soil conditions than under continuous dry conditions.

Photolysis of CP, CPO, and TCP as thin film on thin soil layer was studied under solar radiation. CPO was identified as a photo-product of CP, and photolysis rates were relatively fast (t1/2: 2-9 h) for all three compounds. Absorption spectra of the three compounds as thin films were measured (for the first time), and together with actinic flux measurements enabled calculating their direct photolysis quantum yields.

Leaves and soil were sampled during field campaigns with time following CP application. CP was mainly detected in soil surface. In both compartments, concentrations decreased with time. Calculated soil concentrations of CP and TCP from the conceptual model were in good agreement with our field observations.