Fate of organophospate pesticides in wetland sediments in Meric-Ergene basin

Technological advances in modern agriculture and the application of synthetic fertilizers and manure to agricultural crops increased crop yields and food production for the world's growing population. Together with the fertilizers, agricultural pesticides are also frequently used in modern farming to maintain high levels of production. Pesticides are organic and inorganic chemicals that threaten humans and other living things due to their harmful effects against non-target organisms. An important pathway for unintentional introduction of these compounds into aquatic ecosystems is surface runoff from agricultural fields. Particulate-associated pesticides can be mobilized and transported by runoff into adjacent water bodies, where they may impact non-target aquatic organisms. In our world where a large part consists of water, water resources have a great importance to all living things. Among the 25 basins in Turkey, Meriç-Ergene Basin has recently been the focus of environmental scrutiny due to water quality problems. The Meriç-Ergene basin, one of the major river systems of the eastern Balkans, is shared by Bulgaria, Greece and Turkey. The Meriç transboundary river system rises in Bulgaria and flows along the Turkish Greek border into the Aegean Sea. The Meriç Delta area is a significant ecological site, listed in Class A of International Wetlands and protected under the RAMSAR Convention. The Meriç-Ergene basin is a sub-basin of Meriç transboundary river system located at the north-western part of Turkey. Water quality in the basin suffers from agricultural activities. Overall, 37.9% of the total area in the basin is agricultural land dominated by paddy fields providing 54% of the rice production in Turkey (Republic of Turkey Ministry of Forestry and Water Affairs, 2012). The wetlands in the Meriç-Ergene basin receive agricultural drainage during the irrigation season. Thus, nonpoint source pollutants such as nutrients and pesticides from adjacent farmland that enter into these wetlands pose a big threat to the aquatic life and species in the wetland system. Furthermore, these pollutants have potential to reach downstream water bodies including Meriç River, negatively impacting water quality as well as the overall biodiversity and productivity of the transboundary Meriç Delta. In a previous study conducted in Gala Lake wetland (Öterler 2009), organophosphate pesticides such as azinphos-methyl, diazinon, dichlorvos, terbufos and malathion have been detected at μg/l levels in the wetland water. Given the hydrophobic nature of some of these insecticides, much higher levels of them are expected to be present in the wetland sediment. Pesticides carry out their persistence in nature by sorption on the surface of particles, sediments and soil surfaces and can be transported through different environmental compartments. Bioaccumulation occurs when pesticides entering non-target organisms are transported upwards in the food chain. The stability of the pesticide in the environment depends on the structural and chemical properties of the compound. Pesticides applied to an environment can be degraded biologically, chemically or by photolysis under the influence of sunlight depending on their chemical and physical structures. In recent years, organophosphate pesticides have been commonly used replacing organochlorine pesticides, because of their greater biodegradation potential and a greater effect on target organisms. Nevertheless, it is necessary to determine the fate of these compounds that persist in different environmental compartments depending on their structural characteristics. Natural treatments systems such as filter strips, vegetated drainages and treatment wetlands have been offered as best management practices (BMPs) to reduce the impacts of nonpoint source pesticide pollution. There have been several studies focusing on the removal of organophosphate pesticides by sorption to wetland sediments and assessing the removal efficiencies by measuring inlet and outlet concentrations in water samples. However, it is also important to know the fate of these compounds within the wetland system. Particle-associated organophosphate pesticides adsorbed to wetland sediments can be transported via runoff and reach downstream surface water systems in the long term. Therefore, it is important to quantify the ultimate fate and transformation rate of pesticides within the system before reaching conclusions about how effective a natural treatment system is with respect to pesticide removal. In 2013, The Turkish Ministry of Forestry and Water Affairs established a research group to study the impact of pesticides and herbicides on water quality and to set environmental standards on the usage of these compounds. Within the scope of these studies, a priority list of compounds has been prepared based on the past and current usage in Turkey. When selecting the substances identified in the priority list, their toxicity and permanence / bioaccumulation characteristics have also been taken into account. The organophosphate pesticides to be considered within the scope of the thesis (Chlorpyrifos, Fenthion, Diazinon, Dichlorvos) have been selected based on this list. Studies for the selected organophosphate pesticides have been carried out on the water, sediment and plant samples from the Gala Lake in the Ergene-Meriç Basin and the surrounding wetlands. Sterilized killed controls and a pesticide blank control without sediments were also included to account for hydrolysis and other possible sources of pesticide dissipation. The initial concentration for all pesticides was 100 µg/g. At days 0, 3, 7, 10, 14 and 21, triplicate 10 ml aliquots of the sediment slurry were taken from the flasks and extracted with 2 ml hexane before HPLC analysis. The data were fitted to first order decay curves and first order degradation rates and half-lives were calculated from these curves. In addition to the biotransformation experiments, the adsorption of the pesticides to sediment samples was measured and the linear partitioning coefficients (Kd) were determined using a batch equilibrium method. The Koc coefficient, which changes depending on the organic content of the soil, was calculated by performing the Kd / Koc conversion with the organic carbon fraction in the soil. Biodegradation experiments were carried out using three types of reactors, one in which biological activity is inhibited, the other in biological activity, and the other in control experiments. The results provided information on the half-life and biodegradation rate of the pesticides. Adsorption percentages of pesticides on plant surfaces have also been examined for collected plant samples. A mesocosmos scale wetland reactor was constructed using water, sediment and plant samples taken from Gala Lake and surrounding wetlands. The established mesocosm scale reactor was operated as a batch reactor and the biodegradation of pesticides in these mesocosm scale open reactors were investigated. The results provided a possibility to make comparisons for laboratory scale and mesocosm scale reactors and provided insight into the effectiveness of wetlands as BMPs for pesticide removal. Operation and application evaluations were carried out for the design of artificial wetlands according to the findings obtained from the experiments. The results of this study are part of an effort to understand factors controlling the biodegradation of organophosphate pesticides in riparian wetlands receiving agricultural drainage.