Pesticides play an important role in the economy of an agricultural country like Pakistan. These are used to combat the infestation of pests and diseases that not only affect the crop yields and crop value but also damage the country’s export reputation resulting in economic losses. However, the repeated and intensive applications of pesticides may result in contamination of agricultural lands and water bodies. Therefore, it is important to understand the fate of pesticides in the soil. Factors which can affect pesticide persistence include soil properties (pH, OM, CEC), volatilization, leaching and run-off, and co-contamination with heavy metals. Many soils in Pakistan contain high levels of metals arising from the application of various agrochemicals, sewage sludge and industrial wastewater. These metals may either cause certain pesticides to persist or may enhance degradation of some other pesticides. Thus it is important to study the effects of co-contamination of heavy metals on the fate of commonly used pesticides in the agricultural soils of Pakistan. With these considerations in mind, the study was carried out to investigate the effect of co-existing metals on the persistence/degradation of pesticide residues in cotton/wheat cropping pattern soil. To accomplish the task, the study was carried out in various phases. The first phase of study focused on the determination of trace metals status in cotton/wheat field’s soil. Representative soil samples from six cotton/wheat growing areas of Okara, Faisalabad, Jaranwala, Rahim yar khan, Multan and Khanewal were collected and the concentrations of trace metals i.e., Cu, Fe, Ni, Zn, Cd, Mn, As, Hg and Pb were estimated by AAS. Soil physico-chemical parameters such as pH, E.C, CEC, O.M (organic matter), M.C (moisture content), Cl-, NO3-, SO42- and PO43- were also determined by using the standard methods. Trace metal analysis in different soil depth and speciation studies of metals from selected intensive cotton/wheat growing areas i.e. Multan and Khanewal were also included in the study. The soil of cotton/wheat cropping pattern exhibited enhanced levels of Fe and Cd in Okara, Ni, Cu and Cd in Faisalabad, Fe, Ni, Cu and Cd in Jaranwala, Cu, Ni, As and Cd in Rahim Yar Khan, Fe, As and Cd in Multan and As and Cd in Khanewal soils exceeding the NEQS, Pakistan. Jaranwala cotton/wheat field’s soil was deficient of Zn and phosphorous, while remaining five cotton/wheat growing areas were only deficient of phosphorus. The high content of chloride in all of the studied areas also affected the soil quality. The analysis of trace metals in different depths of soil indicated that appreciable concentrations of Fe and As were present in soil up to 60 cm down the soil surface. Soil speciation analysis indicated that Fe, As and Cu were mostly associated with Fe-Mn oxide bond fraction, organic fraction and residual fractions. A number of significant metal-to-metal correlations were studied in the soil samples from different areas. The Multivariate PCA and CA pointed toward the multiple sources of these enhanced metal levels in soil including excessive use of phosphate fertilizers, use of sewerage water for irrigation purposes and past use of Cu and As based pesticides. The study thus suggested that there is need to monitor the sources of these trace metals in cotton/wheat soils as quality of wheat gains may be affected due to excessive metal contents. The second phase of study was based on the analysis of residues of commonly used pesticides (chlorpyrifos, α-cypermethrin, λ-cyhalothrin, bifenthrin, deltamethrin, MCPA, triazophos, profenofos, imidacloprid, fenitrothion and 3-PBA) for both crops (cotton and wheat) and their stable metabolite in cotton/wheat cropping pattern soil of selected areas of Punjab, Pakistan. Chlorpyrifos, α-cypermethrin, λ-cyhalothrin, bifenthrin, deltamethrin, MCPA, triazophos profenofos and fenitrothion, stable metabolite of pyrethroids 3-PBA were determined by using modified EPA multiresidue pesticide analysis method by GC-MS while imidacloprid residues were determined by HPLC-UV method. Most of the soil samples from the selected study areas were found to be highly contaminated with imidacloprid, chlorpyrifos, and α-cypermethrin residues. Significant concentration of 3-PBA was also present in these areas. Soil properties such as pH, CEC, E.C, M.C. and O.M significantly influenced the pesticides persistence and their mobility to other environmental media. Multivariate analysis helped in understanding the sources of these selected pesticide residue, and the influence of various soil components on their persistence in the soil. Moreover, the univariate and multivariate statistics were carried out to explore the mutual pesticides interactions. Soil pH was positively associated with profenofos, triazophos and α- cypermethrin residues while soil organic matter content was concomitant with bifenthrin, λ-cyhalothrin and deltamethrin residues. The positive association of imidacloprid with soil moisture indicated that imidacloprid leaching down the soil profile was enhanced under moist and flooded conditions while its substantial relationship with soil chloride and sulphate content indicated their role in its leaching. xvii Co-occurrence of trace metals and pesticide residues in soil and their interaction and potential synergistic toxicity has not been well studied. Metals can enhance the degradation/persistence of pesticides in the environment. Therefore, in the third phase, pesticides residues and trace metal data obtained in previous two phases was analyzed by using univariate and multivariate statistical tools to understand their mutual/synergistic relationships and their common sources in these soils. Various significant positively and negatively correlated metal-pesticides pairs were observed at p < 0.05 in the present study. α-cypermethrin and λ-cyhalothrin showed positive correlations with Cu and Pb in Okara and Jaranwala. Fe was found to be significantly positively correlated with imidacloprid in the soil of Okara, Jaranwala and Rahim yar khan, that indicated that these metals affected the fate of pesticide in these areas. Similarly, MCPA and fenitrothion were negatively correlated with Cd metal in Rahim yar khan, Multan and Khanewal. Cluster analysis indicated that Fe, Cd and Pb were grouped with imidaclopid in most of the selected areas and Mn and Cu were grouped with chlorpyrifos in the loamy soil of Okara which pointed toward the fact that these trace metals might affect the transformation of pesticides in these areas. The influences of trace metals on the photodegradation of soil incorporated imidacloprid; α-cypermethrin and chlorpyrifos were studied in the last phase of study. The soil samples spiked with pesticides and trace metal ions were irradiated with UV light for different time intervals and analyzed by HPLC. The control sterile and unsterile soil samples spiked with pesticides and selected trace metals were incubated in dark at 25oC for the same interval of time. The results obtained showed that photodegradation of imidaclopid and chlorpyrifos followed the first order kinetics while α-cypermethrin followed biphasic reaction kinetics. The dissipation t1/2 was observed to be decreased from 144 days to 21 days for imidaclopid, 100 days to 1 day for α-cypermethrin and 41 days to 20 days for chlorpyrifos under UV irradiation. Trace metals (Cu2+, Zn2+, Cd2+ and Fe2+) were found to affect the degradation of all three selected pesticides. The presence of Fe2+, Zn2+ and Cu2+ in the soil significantly enhanced the rate of imidacloprid photodegradation and resulted in reduced t1/2 from 21 days to 7, 7.9 and 9.4 days respectively while Cd2+ increased the persistence of imidacloprid in the soil. Fe2+, Cu2+ and Zn2+ catalyzed the degradation of imidacloprid to some extent in the absence of UV light while Cd2+ had enhanced its dissipation in the dark. The photodegradation half-life of α-cypermethrin was hindered in the presence of Cu2+ and Zn2+ in the soil. Cu2+ exhibited slightly greater inhibition effect than Zn2+ and thus led to an increase in α-cypermethrin t1/2 from 0.64 hours to 4.7 and 0.71 hours respectively. The t1/2 values for the metal assisted biodegradation of α-cypermethrin in soil followed the order; Zn2+ < Cu2+ < Fe2+ days. The proliferated soil Fe2+ and Cd2+ levels however enhanced the photodegradation of α-cypermethrin while Cd2+ also decreased the microbial degradation of α- cypermethrin. Photodegradation rate of chlorpyrifos was enhanced in the presence of all the four studied trace metals i.e. Cu2+, Zn2+, Fe2+ and Cd2+ resulting in the reduction in t1/2 from 19.8 to 4.39, 4.39, 12.6 and 19.25 days respectively. Cu2+ and Zn2+ led to a 4.5 fold increase in the rate of photodegradation while 2 and 2.5 folds increase in microbial degradation of chlorpyrifos. Fe2+ and Cd2+ had negligible effect on the microbial degradation of chlorpyrifos. The studied trace metals also affected the abiotic dissipation in the following order Cu2+ > Zn2+ >Fe2+ > Cd2+. On overall basis, it was concluded that all the three selected pesticides were photodegradable and metals play a significant role in their dissipation. Fe2+ accelerated the photodegradation of all three studied pesticides that belonged to different classes, while Cu2+, Zn2+ and Cd2+ played dual role in photodegradation of studied pesticides. Cu2+ and Zn2+ enhanced the photodegradation of imidacloprid and chlorpyrifos while they extended the dissipation rate of α-cypermethrin in soil. The photodegradation rate of chlorpyrifos and α-cypermethrin were enhanced in Cd2+ polluted soil while imidacloprid became persistent in the presence of excess of soil Cd2+ content." xml:lang="en_US