Quantitative-qualitative hydrodynamic simulation and routing of transmission of pollution in domestic rivers using a combination of WASP and HEC-RAS models

Hydrodynamic prediction and simulation of rivers’ qualitative parameters is very important for determining the rate of loading of pollutants and their manner of transmission in aquatic ecosystems, in relation to rivers’ self-purification capacity. From the beginning of human life, rivers and surface waters have always been considered essential because of the need for water for living purposes. Cities and industrial and agricultural centers, in fact entire civilizations, arose near the rivers in order to use sources of water. However, by developing industry and technology, human beings started destroying nature. Knowledge of the quality of water resources is one of the most important requirements in the planning and development of water resources and their conservation and control. Hence, in order to ensure the monitoring and management of the quality of this natural resource, some methods can be used that entail the least cost and time to attain these objectives. To develop these studies, sampling and environmental tests for water qualitative parameters were initially conducted. For performing this step, six stations were determined for monitoring in the basin of the Balikhlouchai. The parameters considered were then measured and the results were compared with existing standards. The purpose of conducting this study is to route the pollution of Balikhlochai River, establishing a relationship between the results of two quantitative and qualitative simulated hydrodynamic models (HEC-RAS and WASP) and comparing the results with statistical indices (mean absolute error, root mean square error, Relative volume error, and Nash–Sutcliffe model efficiency coefficient). Given their self-purification and self-regulation capabilities, rivers in normal situations can undertake the natural load of pollution imposed by the environment and solve them. In situations in which pollution has human origins and the load is more than the river’s carrying capacity, this will be associated with destruction, and the death of river ecosystems. The reasons for the increase in pollution load are the entry of surface runoff from the rainfall into the river, washing of various pesticides and organic fertilizers, phosphate and nitrate due to drainage of agricultural land, and the organic load entering the urban and rural. Wastewater into the river along the route is the source of food for algae and, as a result, increases the pollution burden and reduces the health of the river. Calibration and validation of the model were performed based on samples taken from six monitoring stations during 2015-2016. According to the validation results, MAE, RMSE, RE and NSE statistical indices for DO were 0.04, 7.2, 4.7 and 0.84, respectively, and 0.07, 8.2, 5.2 and 0.81 for the river flow rate, respectively, representing the optimal and reliable results of the output of simulation models. The results obtained from statistical indices showed that the simulation of the research variables was well performed, the simulation results were reliable and both models have good performance in predicting water quality. Due to the increasing population and growth of various kinds of pollution, the qualitative conditions of the rivers and aquifers become worse in the field of surface water. According to the results of the models’ output, the river discharge along the route ranged from 1.2 to 0.3 m³/s and DO ranged from 7.5 to 3.5 mg/l, indicating a severe reduction in the river flow rate and DO. Comparing these results gives a more accurate analysis of the pollution process in the Balkhlohai River; as the river's downstream is lowered, the loading rate of the pollutants is increased. So, the river is naturally unable to reduce pollutants and return the river to its normal state. It happens because both velocity of the water flow and the depth of the river are reduced. Therefore the amount of water mixing in the river decreases, as well as the amount of dissolved oxygen in the river due to reduced mixing. So, the pollution load and pollution of the river increases. Studying the pollution process in Balikhlochai River indicated that river pollution was due to the entry of urban wastewater and runoff from agricultural drainage-water into the basin, as well as the severe reduction in river self-purification capacity and Eutrophication phenomenon. The decade's prediction shows that if the measures needed to manage the current status of river water quality are not met, the river's health is compromised and should be stepped up to restore the river. The results indicate that the river downstream from the point of view of quality management should be prioritized and the study results can be used in determining the strategies for coping with pollution and promoting management in the Basin of Balikhlochai River.