Numerical Analysis Of Water Quality Impact Of Water Intake On Dongjiang Caused By Dongguan Canal Drainage

Dongjiang River is a dominating drinking water source of Dongguan, in whichthere’re many water intakes of drinking water plants. Dongguan Canal is the city’sdrainage channel, and water quality is worse than the class Ⅴ due to the wastewaterdischarging. In rainy season, the water quality of Dongjiang River is impacted by thefrequent drainage from Dongguan Canal that threats to drinking water safety in urban.The water quality of the Dongguan reach of Dongjiang River and DongguanCanal was consecutively monitored weekly during March-July in2010. The observedresults show that the average of DO, CODMn, ammonia nitrogen and TP in dry seasonare3.9,2.5,1.1and0.10mg/L respectively, and the average in rainy season are6.2,2.4,1.30and0.14mg/L, respectively, in Dongjiang River. The average of DO,CODMn, ammonia nitrogen and TP are2.5,10.7,18.8and1.5mg/L in the dry season,and in the rainy season they are3.4、7.2、9.4、0.69mg/L, respectively, in DongguanCanal. Water quality is worse than the class Ⅴ. CODMn, ammonia nitrogen and TP ofDongguan Canal are much higher than that of Dongjiang. So the water quality inDongjiang River will be impacted obviously by the canal drainage in the rainy season,especially ammonia nitrogen, which is the key pollution indicator. The maximumobserved is4.8mg/L, and it is the main limit factor for water plant.CE-QUAL-W2model is applied to simulate the hydrodynamic and water qualityin Dongjiang River network. The simulated results of water level and ammonianitrogen agree well with the field data. The average errors of high and low waterlevels are0.16m and0.09m, respectively, and the average error of ammonia nitrogenis about0.13mg/L. The model studies the different scenarios of water quality impactin the river network caused by the Canal drainage under different hydrologicalconditions. The numerical results show that when the Canal drainage at the medianwater level, the pollutant zone is distributed uniformly. Under this situation theconcentration in the pollutant zone is as low as2.6mg/L, the length of the pollutantzone is2940m, and the discharge flow is relatively large, an average of302.4m3/s. It is a suitable way for the Canal drainage.EFDC model is applied to simulate the hydrodynamic and water quality inDongguan reach of Dongjiang River. The numerical results of water level, velocityand ammonia nitrogen agree well with the field data and the result of1D Dongjiangriver network model. The model studies the different scenarios of water qualityimpact near the water intake of the drinking water plant of Dongguan caused by theCanal drainage under different hydrological conditions and the role of the baffle. Thenumerical results show that, when the Canal drainage at the median water levelwithout the baffle, the ammonia nitrogen is relatively small, about4.0mg/L near thewater intake; it is a suitable way. If a150m long baffle is set up at the upstream of theintake. The shelter function of baffle is obvious when drainage at the low water level.Compared with the situation without the baffle, the pollutant concentration dropsfrom8.5~10.5mg/L to2.6-4.5mg/L, greatly improve the water quality near the intake.The flow velocity is relative large when canal drainage at median water level, and thepollutant mix quickly behind the baffle whose effect is not obvious.