Profiling of pesticide residues in Mobego-Kabianga wetland ecosystem and its retention efficiency

Abstract

Wetland ecosystems play a critical role in the environment. It provides global significance environmentally, economically, and socially. Wetlands have several important functions which include storage of carbon, storage of nutrients, retention of sediments and pollutants. Wetlands also serve as sinks and transformators in water and nutrient cycle. The unrestricted use of natural wetlands has posed a great concern as these wetlands acts as sinks for point and non-point sources of pollution in surface water runoff from municipal and agricultural sectors. These pollutants can have adverse effects on the working of the wetland in terms of retention and removal of such pollutants. Therefore, this study was undertaken to profile pesticide residues in Mobego-Kabianga wetland ecosystem. In addition, the retention efficiency of pesticide residues was also investigated. Values of physical-chemical parameters including pH, Total Dissolved Solids (TDS), Total Suspended Solids (TSS), temperature, conductivity, dissolved oxygen (DO) and flow rate were measured in situ at the inflows, midpoints and at outflows to the wetland ecosystems using Lab Quest vernier caliper instrument. Water, soil/sediments and selected plant materials samples were collected and analyzed for residual pesticides using coupled Gas Chromatography - Mass Spectrometric/Mass Spectrometric (GC-MS/MS) and Liquid Chromatography - Mass Spectrometric/Mass Spectrometric (LC-MS/MS) methods. All results were recorded and analyzed for measure of central tendencies using SAS statistical system version 9.4. From the study, levels of physicochemical parameters ranged from 0.259-1.079 m/s (flow rate), 0.233- 10.03 mg/l (dissolved oxygen), 592-764 µs/cm (conductivity), 17-20 oC (temperature), 6- 8 (pH), 384-1646 mg/l (TDS) and 700-910 mg/l (TSS). Variation in the levels of physico chemical parameters was noted with p≤0.05. However, TSS was found to have no significant difference with p=0.06. In analysis of residual pesticides, 24 prominent residual pesticides were detected and quantified by GC-MS/MS and LC-MS/MS and they include: diazinon, dimethoate, chlorpyriphos, malathion, methidathion, aldrin, BHC, chlordane, DDT, DDD, DDE, metachlor, alachlor, dieldrin, metolachlor, aldicarb, carbaryl, carbofuran, methiocarb, methomyl, deltamethrin, tetramethrin, pyrethrin and cypermethrin. Their levels ranged from 0.001 to 0.861 ppb in water; 0.001 to 0.501 ppb in sediments and 0.001 to 0.466 ppb in plant materials. The calculated retention efficiency was 85 %. However, cypermethrin and dieldrin recorded negative retention efficiency. Significant variation was observed in levels of physicochemical parameters, residual pesticides and retention efficiencies of the wetland ecosystem. It is recommended that a study be carried out to develop environmentally friendly molecules which can be used for pest control and management practices with an aim of protecting wetland ecosystems from effects of chemical pesticides and hence improve retention efficiencies of natural tropical wetland ecosystems.