Water Quality Monitoring and Eutrophication Simulation of Dez River

Document Type : Original Research

Authors
S. Jahanimehr 1
M. Zakermoshfegh 1
B. Lashkar-Ara 2
F. Gholinezhad 3
1 Jundi-Shapur university of technology, Dezful, Iran.
2 Jundi-shapur university of technology
3 General Department of environmental protection of Khuzestan province.
10.22034/24.6.39
Abstract
The biological response to high concentrations of nutrients such as phosphorus and nitrogen in a river is called eutrophication in water-resources engineering. Due to increasing withdrawals of water, successive dry years, and the discharge of urban, industrial, and agricultural wastewater into the Dez River, the quality of the river is being threatened. In this research, for the purpose of field monitoring and then simulating the eutrophication of Dez river, eight monitoring stations were selected considering the way pollutants enter the river. A number of water quality variables, including dissolved oxygen and water temperature, were measured directly by the portable device, while phosphate and nitrate variables were measured through sampling and laboratory tests. Then, the water quality simulation of the river with 18 Km of length was conducted by the HEC-RAS model. A comparison was made between the simulation results and the observed data, in which parameters including dissolved oxygen, temperature, nitrate, and phosphate were in good agreement with the observed data. To measure the accuracy of the model, the root mean square error (RMSE) statistical function was used. The results of the sensitivity analysis showed that the amount of dissolved oxygen is more sensitive to the parameters of oxygen demand and wind speed function coefficients, and also the model is less sensitive to the parameters of diffusion coefficient and dust coefficient, and the change of these two parameters has no effect on the dissolved oxygen graph. This was used in the model calibration process so that only parameters affecting the model were used in the calibration process. The results showed that the variables of nitrate, water temperature, dissolved oxygen and phosphate were modeled with appropriate accuracy. In model calibration, RMSE values for nitrate, water temperature, dissolved oxygen and phosphate parameters were calculated as 0.25, 0.29, 0.67 and 0.67 mg/L, respectively. Also, the results of the model confirmation show the acceptable compatibility of the simulated and observed values. After the model Sensitivity analysis, calibration and validation, it was used to simulate the river's response to the nutrition reduction scenario, which involved the impact of the discharge increasing on the concentration reduction of nutrition in the river.

The results of nitrate and phosphate concentration changes during the simulation period in the river course showed that the amount of phosphate and nitrate concentration increases from the upstream to the downstream of the river. Also, the simulation results for the dissolved oxygen variable show that in most of the sampling months, the concentration of dissolved oxygen is higher than 4 mg/l.

 The simulation results showed that in the scenario of discharge increasing, the nutrition concentration reduced significantly. On average, in all the stations, with the increase of the flow by 70 cubic meters per second, there is an increase in DO by 0.5-0.9 mg/l, a decrease in nitrate between 0.4 and 6 mg/l, and a decrease in phosphate by 3 0.0 mg/l and a decrease of 0.5 to 1 degree in water temperature. This shows the impact of this scenario on the amount of nitrate and phosphate variables, which are the main factors in creating the phenomenon of eutrophication and algal growth.

 

Keywords

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Modares Civil Engineering journal
Volume 24, Issue 6
November and December 2024
Pages 39-53