Study on the Effects of Hydraulic Dredging and Disposal Operations on Water Quality of Nworie River, South-Eastern Nigeria

A study was done to ascertain the effect of hydraulic dredging and spoils disposal on the physicochemical parameters of Nworie River. The study was carried out from July 2010 to January, 2012. Water samples were collected prior to, during and monitored several months after dredging. The samples were collected from 5 stations: station 1 was the dredging point, stations 2 and 3 were 250m and 500m upstream of the point, while and stations 4 and 5 were 250m and 500m downstream of it respectively. The physicochemical parameters of the water were analysed prior to dredging and most of the values obtained were within acceptable limit of established water standards. The pH was within the range of 6.5-7.8, Temperature 27.6-29.9°C, Conductivity 34-196μs/cm, Turbidity 0-76NTU, TDS 22.1-127.4 mg/l, TSS 9-51 mg/l, Nitrate 1.2- 33.6mg/l, Phosphate 0.2-3.7mg/l, Sulphate 0- 33 mg/l, Dissolved Oxygen 4.3-9.4 mg/L and BOD 5 2.5 - 7.9 mg/l. During the dredging, the water physicochemistry changed: pH values decreased drastically to 4.6, DO to 1.3mg/l, Nitrate to 1.2 mg/l. Temperature increased to 30. 2°C, Turbidity 620NTU, TSS 349mg/L, Conductivity 498μs/cm, TDS 249 mg/l, BOD 5 19.1mg/l, Sulphate 33mg/l and Phosphate was 0.92mg/l. Three months after the dredging, the values for the parameters returned to their pre-dredging concentrations, however during rainy season, as the leachates and silts from the spoils were washed into the river, this prolonged the recovery of the river system to 9 months. This implies that the effect of dredging on the river is of short term but the disposal of the excavated materials at the river bank compounded and prolonged these impacts.