Today, there is a shortage of food due to the increase in the world population. For this reason, pesticides are used to remove plant pests in order not to cause product loss in agriculture. Especially in agriculture, synthetic pesticides created to achieve high efficiency are increasing day by day. The pesticides used get into the soil and then into the groundwater. In this study, ways of decomposition of the pesticides pyraclostrobin from the fungicide variety, pinoxaden from the herbicide variety, and gammacyhalothrin from the insecticide variety in groundwater were theoretically investigated. Optimized geometries were drawn with Gauss View 5.0, and then geometric optimization was carried out by the Gaussian 09W program using functional density theory (DFT) and Hartree-Fock methods. The geometrical structure (bond angles and bond lengths) of all three molecules and their possible degradation products were calculated at the level of B3LYP theory within the Hartre-Fock (HF) method and DFT in the basic set of 6-31G(d). Thus, the possible degradation mechanisms of these three pesticide molecules in water were determined. These results will guide experimental studies. © 2023 Bulgarian Academy of Sciences, Union of Chemists in Bulgaria.