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Receiving Water Quality Models for Estimation of Total Maximum Daily Pollutant Loads: Case Study of the Küçük Menderes River Basin (Turkey)

IWRA 2021 Online Conference One Water, One Health
Theme 1: How can we better manage water for food and public health in a changing world?
Author(s): Nilsu Gevrekçioğlu, Alper Elçi, Mehmet Dilaver, Selma Ayaz

Nilsu Gevrekçioğlu1, Alper Elçi2, Mehmet Dilaver3, Selma Ayaz3

1 Dokuz Eylul University, Graduate School of Natural and Applied Sciences, Department of Environmental Engineering, Izmir, Turkey
2 Dokuz Eylul University, Faculty of Engineering, Department of Environmental Engineering, Buca, Izmir, Turkey
3 Tubitak Marmara Research Center, Environment and Cleaner Production Institute, Gebze, Kocaeli, Turkey

Keyword(s): Numerical modeling, AQUATOX model, river water quality, wastewater discharge, non-point source pollution


(a) Purpose or objectives and status of study or research hypothesis:
Mathematical water quality models that simulate transport and reaction processes in the receiving water body in response to contaminant loads are important tools to estimate the capacity of a watershed to sustain environmental quality standards (EQS). This study focuses on the application of a water quality model to determine the total maximum loads of nutrients, organic compounds, PCB-138, and three heavy metals. The application is demonstrated for the Küçük Menderes River Basin, which is under significant environmental pollution pressures due to localized industrial wastewater discharges and widespread agricultural activities.

(b) Key issue(s) or problem(s) addressed:
The study area is a watershed of an intermittent-flowing river that becomes dominated by wastewater effluents during dry periods. The modeling approach provides quantitative guidance on achieving EQSs and controlling point and distributed sources in an environmentally stressed watershed.

(c) Methodology or approach used
AQUATOX is used to simulate concentration time series for the main river reach of the study watershed. The large size of the watershed (3225 km2) and the high number of wastewater discharges complicate the use of a single model, therefore a modular approach is taken by considering five sub-watersheds and setting up sub-models for each of them. A total of 30 wastewater discharges (industrial and domestic), and diffuse source loads of nutrients and organic carbon are included in the model. Input data on river discharge and water quality of the river and the wastewater discharges were collected bi-monthly for one year. The maximum allowable pollutant loads were calculated by varying the point source loads in the calibrated model and running it multiple times to obtain receiving water concentrations that comply with EQSs.

(d) Results and conclusions derived from the project
Current total pollutant loads and corresponding concentrations in the river water are determined with the calibrated model for each sub-watershed. Modeling results suggest that even drastic reductions in total nitrogen and total phosphorus loads are not sufficient to achieve water quality goals for downstream reaches of the river. Concentrations in the receiving river are influenced by the influx of nutrients from diffuse sources during wet periods
and by point sources during intermittent periods. It is evident that strict measures are required to control discharges and prevent further impairment.

(e) Implications of the project relevant to selected conference theme, theory and/or practice
Water quality modeling is required to assess and mitigate the effects of human activities on the water cycle. The study demonstrates also how to predict the effectiveness of controlling these activities and identifying solutions specific to the watershed.

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