A MATHEMATICAL MODEL TO INVESTIGATE THE WATER POLLUTION OF DIYALA RIVER REACH WITHIN BAGHDAD CITY

Abstract

Diyala river is one of the most important tributaries of Tigris river. Diyala river suffers from the effect of pollutants due to the effluents from different point sources of pollution such as, Army canal outfall and Rustimiyah wastewater treatment plant that discharged to the river without adequate treatment. Five outfalls are located at different positions along the lower reach of the river and taken them as a point sources of pollutants. This study involves application of a one- dimensional, steady flow river and stream water quality model (QUAL2K) which is developed by United States Environmental Protection Agency (USEPA) , to predict some water quality parameters of lower reach of the Diyala river for 16.90km distance starting from 2km upstream of Rustimiyah third expansion and extending to the confluence of the Tigris-Diyala rivers within Baghdad city. The model was calibrated and validated for DO, CBODu , Temp , TSS , pH and Ec using hydraulic and water quality data collected from Ministry of Water Resources for the period(January-April 2014). Google Earth and Arc-GIS technique were used in this study as supporting tools to provide some QUAL2K input hydro-geometric data. The model parameters were calibrated by trial and error until the simulated results agreed well with the observed data. The performance of the model was evaluated using statistics criteria based on MAE , RMS and RE . The results show that the predicted values based on QUAL2K model are in close agreement with measured values and model was found to be an acceptable tool for the modeling of Diyala river water quality parameters. The model was calibrated, for the dry flow data and the results showed that TSS concentration and pH value in the entire river were within the allowable limits according to Iraqi and WHO standards for drinking and irrigation purposes, were with range values (102 - 123.83 mg/L) and (7.25 - 7.50) respectively, but EC concentration was larger than allowable limits with range values (2773 - 2829µS/cm). DO and CBODu concentrations were not within the allowable limits for fish and aquatic life, with range (2.51 - 4.80mg/L), (27.94 - 37.45mg/L) respectively and water temperature value was with range of (20.99 - 21.78). The most polluted zone in the study region which located upstream of old Diyala bridge, that begins at (8.45km) downstream of headwater and extends to (12.90km). The model was applied to simulate various water quality management scenarios during critical period (low flow) to maintain the targeted water quality criteria (minimum DO at or above 4 mg/L) ,considering: (i) pollution loads modification; (ii) flow augmentation; (iii) local oxygenation. The scenarios result indicated that the pollution loads (BOD5) for wastewater modification and local oxygenation is effective in raising DO levels. While flow augmentation does not give a significant results in which the level of DO decrease even with reduction in the BOD5for point sources. The combination of wastewater modification and local oxygenation are necessary to ensure minimum DO concentrations. Thus the BOD5 of the discharged effluent from point sources should not exceed 15 mg/L with weir construction at critical positions 6.67km from the beginning of the study region with 1m height, or the BOD5 of the discharged effluent from point sources should not exceed 10 mg/L with weir construction at critical positions 6.67km with 0.6m height