Determination of assimilative capacities of stagnant water bodies of Yesilirmak basin

The runoff from urban and industrial wastewater, agriculture, forest areas and livestock activities change the existing water quality by directly or indirectly effect to body of water physically, chemically and biologically. Point and nonpoint pollutant loads are transported to the downstream and depending on width of the basin, there may be more than one water body in the drainage area of the stagnant water body. For this reason, the pollutant loads of water body within its own drainage area are transferred from upstream to downstream. Thus, the loads that come to the stagnant water bodies are divided into three, such as point loads, nonpoint loads and upstream. The transported nutrient loads cause eutrophication, depending on the capacity of the water body to exceed the assimilation capacity. Excessive increase in total nitrogen and total phosphorus pollution parameters in water bodies causes water quality deterioration in connection with eutrophication. Eutrophication causes problems such as algae bloom, deterioration of aquatic life, decreasing dissolved oxygen and decreasing light transmission. The natural process of eutrophication is slow in geological time but this process can be accelerated through anthropogenic activities. Assimilation capacity; in the Regulation on Surface Water Quality Management, it is expressed as "the capacity of the water body to take all pollutans without damaging the aquatic organisms or the people consuming water to return to the unpolluted state. Determining the assimilation capacity of total nitrogen and total phosphorus in stagnant water bodies is important for preventing eutrophication and protecting aqutaic life. With the calculated assimilation capacity, identified the water body has a tolerance to total nitrogen and total phosphorus pollutant. For this reason, models are needed to predict the effects of eutrophication on river, lake, pond and dam ecosystems. In addition to this, scenario analysis related to assimilation capacity and current situation and measures to be taken against pressures are also made. Yesilirmak Basin is one of the 26 basins in Turkey, which discharges into the Black Sea with the Yesilirmak River. The basin covers a portion of 5% of Turkey with approximately 40,000 km2. Yesilirmak, the main river of the basin; the branch river with Kelkit River, Cekerek Stream and Corum Stream has 5.80 billion m3 of water potential. According to 2017 census data, the basin population is 2,728,147. According to CORINE 2012 data, 44% of Yesilirmak Basin is composed of agricultural land, 37% forest area and approximately 19% of pasture and meadow area. In order to determine the assimilation capacity of stagnant water bodies in Yesilirmak Basin, it was ensured to reach the mesotrophic state corresponding to the water quality criteria in Table 9 of the Regulation on Surface Water Quality. It is assumed that the water bodies in the basin are completely mixed and steady state conditions. It has been assumed that the pollutants are reduced with first degree degradation kinetics in the model of assimilation capacity. It is calculated how much deviation of these water bodies from the assimilation capacity in the case of the current nutrient loading or what should be the nutrient loading to reach assimilation capacity of these water bodies. To measure the accuracy of the model, comparisons were made with the measured data in the field. The assimilation capacities of the modeled water bodies were calibrated with the total nitrogen and total phosphorus concentrations measured in the field. Hydrological analysis was determined by using WEAP model in order to calculatethe water budget of Yesilirmak Basin. In the basin, between the years 2000-2017, the data of the water budget components were obtained by taking the average of the hydrological model data of the water bodies. The WEAP model was set up by taking into account water bodies in the basin, dam characteristics (elevation – volume curves, dam operating data, evaporation data), water transfers, drinking water requirements, agricultural irrigation and point discharges. The pollutant loads from the water body/bodies at the upstream of the water body, point and nonpoint constitute the total load. Point sources are urban direct, industrial and wastewater treatment plants discharges. Nonpoint sources are fertilizer, livestock activities, forest land, pasture land, urban settlement drainage, rural settlement drainage, septic tanks, solid waste areas and seafood. Topographic and geological conditions, environmental impacts and the size of the drainage area are the main factors of the nonpoint pollution level. To calculate the assimilation capacity, a spreadsheet format has been created in Excel, which includes water budget of rivers and stagnant water bodies in Yesilirmak Basin and total nitrogen and total phosphorus components as state variables. In addition, the unit loads of point and nonpoint pollutants in Yesilirmak Basin were calculated and entered into the spreadsheet. With this spreadsheet model, the assimilation capacity of the stagnant water bodies' are calculated by using the matrices, including convection–transformation processes and water quality kinetics. One of these matrices is response matrix and the other is the matrix of upstream loads. The response matrix is the algebraic state of the internal interactions within the system and describes the response of the water body to external influences. In the Yesilirmak Basin, 134 water bodies were determined, of which 53 were stagnant and 81 were river water bodies. As a result of the study, it was observed that 25 stagnant water bodies exceeded their assimilation capacities. In terms of total nitrogen, the extraction capacity of 5 lakes, 7 ponds and 4 dams was exceeded. On the other hand, in terms of total phosphorus, the extraction capacity of 6 lakes, 8 ponds and 6 dams was exceeded. It has been determined that the dominantly pollutant load transported to these stagnant water bodies whose capacities are exceeded is nonpoint, point or upstream loading. With these determined loads, the source of total nitrogen or total phosphorus pollutant load and the load reduction required to reach assimilation capacity was detected. By determining the source of the total nitrogen or total phosphorus pollutant load and determining the load reduction required to exceed the assimilation capacity, the water bodies are expected to recover from the eutrophic situation to the mesotrophic state. Thus, it will be possible to determine the measures to be taken in order to reduce the total incoming loads and to improve the water quality of stagnant water which is not exceeded but at risk. In the future, the model can be developed to take account of the stratification of water bodies. In this study, it has been studied in a single basis and it can be developed in the future in order to take into account the stratification in water bodies.