Meltem Kaçıkoç Mehmet Beyhan Faculty of Engineering, Department of Environmental Engineering, Suleyman Demirel University, Isparta, Turkey Research Article Hydrodynamic and Water Quality Modeling of Lake E  girdir The main objective of this study was to determine the impact of current point and nonpoint pollution loads on the water quality of Lake E girdir. For this purpose, hydrodynamics and water quality status of Lake E girdir were investigated by using the Delft3D model. The monthly monitoring of pH, dissolved oxygen (DO), chlorophyll-a (Chl-a), and forms of nitrogen and phosphorus was carried out, as was the seasonal monitoring of total organic carbon. In all, seven different sampling points were chosen from the lake and influent streams in order to monitor the lake during the period from December 2010 to November 2011, with the aim of calibrating the model. The performance of the dynamic water quality model was evaluated using the relative root mean square error (RRMSE) values for both the calibration and the validation periods. The results of the model showed that the DO had the lowest, and Chl-a had highest associated RRMSE value. The simulation results showed that Segments 1 and 7 of the system had relatively high concentrations of almost all the pollutants. It was also found that increasing the nonpoint nutrient loads from agricultural sources has a greater effect than increasing the nonpoint nutrient loads from the inhabited areas around the lake. Keywords: Delft3D model; Eutrophication; Monitoring; Pollution; Wastewater treatment Received: June 6, 2013; revised: December 2, 2013; accepted: January 17, 2014 DOI: 10.1002/clen.201300455 1 Introduction A reliable supply of clean, fresh water is essential for meeting the growing demands of the population, for household use, as well as for recreation, irrigation, aquaculture, and the protection of habitats [1]. Natural and artificial lakes constitute a large portion of the total fresh water resources that are sufficiently easily accessible to meet human needs [2]. The threat of contamination is ever-present, however, and pollutants are highly variable in quantity and quality, requiring an ever more detailed understanding of their behavior in surface waters [3]. In particular, lakes are very sensitive to pollutants, yet it is here that they have tendency to accumulate over time [4]. Sources of pollutants may be categorized as point and nonpoint; the latter are problematic in terms of their control, in comparison with point sources. Many healthy lake ecosystems contain small quantities of naturally occurring nitrogen and phosphorus [5]. Inappropriate agricultural activities are the most important source of nonpoint pollution, with runoff from inhabited areas also being an important source, though not as much as agriculture [6, 7]. Excessive nutrient inputs upset the natural balance of the ecosystem of a lake [8]. Legislation and regulatory frameworks have been used to improve ecosystems polluted by point and nonpoint pollution. The most widely applied current legislation is enshrined in the European Union (EU) Water Framework Directive (WFD) 2000/60/EC [9]. As a candidate country in the process of adapting new EU legislation, Turkey has been adapting its own existing laws on water-related issues. Fulfilling the requirements of the WFD has been of the utmost importance, and to this end water quality models can be useful in that they allow the simulation of water quality in lakes and other water bodies, which then provides opportunities for managers to develop policies relating to point and nonpoint pollution loading, using meteorological data and the characteristics of the receiving environment [10–12]. The first models of surface water were proposed by Lotka–Volterra and Streeter–Phelps in the 1920s. In the 1970s, these approaches and their variations were used more extensively in environmental management [13, 14], and in recent years, hydrodynamic and water quality models have been developed from the original one- dimensional steady-state models, resulting in transient, unsteady three-dimensional models. According to international criteria, Lake E girdir is an “A class wetland”, which has considerable significance in terms of its role in protecting natural biodiversity [15]. To date, in terms of its compliance with standards related to water quality for the supply of drinking and agricultural irrigation water, for fisheries, tourism, and energy, the lake has generally been in a satisfactory state. In recent years, however, signs of pollution and other adverse Correspondence: M. Beyhan, Faculty of Engineering, Department of Environmental Engineering, Suleyman Demirel University, 32260 Isparta, Turkey E-mail: mehmetbeyhan@sdu.edu.tr Abbreviations: APHA, American Public Health Association; Chl-a, chlorophyll-a; Delft3D-FLOW, hydrodynamic module; Delft3D-WAQ, water quality module; DO, dissolved oxygen; DSI, State Hydraulic Works; EU, European Union; RRMSE, relative root mean square error; SWQMR, Surface Water Quality Management Regulation; TIP, total inorganic phosphorus; TKN, total Kjeldahl nitrogen; TN, total nitrogen; TOC, total organic carbon; TON, total organic nitrogen; TOP, total organic phosphorus; TP, total phosphorus; WFD, Water Framework Directive 1573 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com Clean – Soil, Air, Water 2014, 42 (11), 1573–1582