Advances in Biological Research 2 (5-6): 97-103, 2008 ISSN 1992-0067 © IDOSI Publications, 2008 Coressponding Author: Elijah I. Ohimain, Biological Sciences Department, Niger Delta University, Wilberforce Island, Amassoma, Bayelsa State, Nigeria. 97 Variations in Heavy Metal Concentrations Following the Dredging of an Oil Well Access Canal in the Niger Delta Elijah I. Ohimain, Gbolagade Jonathan and Steve O. Abah 1 1,2 3 Biological Sciences Department, Niger Delta University, 1 Wilberforce Island, Amassoma, Bayelsa State, Nigeria Department of Botany and Microbiology, University of Ibadan, Nigeria 2 College of Medicine, Ambrose Alli University, Ekpoma, Edo State, Nigeria 3 Abstract: In a study to assess the impact of dredging on heavy metal contamination, surface water was monitored for over one year, from December 1997 to December 1998. Samples were collected twice before dredging, in December 1997 and in June 1998, corresponding to dry and raining seasons respectively. Samples were also collected immediately after dredging in July 1998 and were monitored in August, September and December 1998. Samples were collected analysed from five stations within the study area, station 1-5. Station 1 was in the dredged canal, which was originally a side branch of the Warri River tributary. In the Warri River tributary, Station 2 was 500m upstream and Station 3 was 1000m upstream of the mouth of the dredged canal, whilst Stations 4 and 5 were respectively 500m and 1000m downstream of it. Stations 3 and 5 represented the reference situation to which possible dredging effects could be compared. Prior to dredging, the concentration of heavy metals in the surface water samples of Warri River occurred in traces; lead (0.01-0.28 mg l ), 1 zinc (0.04-1.02 mg l ), copper (0.00-0.17 mg l ), iron (0.22-0.88 mg l ), chromium (0.00-0.03 mg l ) and cadmium 1 1 1 1 (0.03-0.14 mg l ). These values are comparable to values obtained elsewhere in the Niger Delta. But following 1 the dredging of an oil well access canal, the concentration of heavy metals increased in several folds depending on the metal; lead (2.57 mg l ) zinc (9.56 mg l ), copper (15.36 mg l ), iron (229.50 mg l ), 1 1 1 1 manganese (14.30 mg l ), chromium (0.27 mg l ) and cadmium (0.39). The results indicated a significant 1 1 elevation of heavy metal concentrations over the background values. The percentage increase are as follows; 2333, 2130, 20,750, 51,113, 1960, 1266 and 703% for lead, zinc, copper, iron, chromium, manganese and cadmium respectively. However, 6 months after the dredging, the concentration of heavy metal reduced drastically to 533, 499, 4973, 6013, 480, 328 and 188% for lead, zinc, copper, iron, chromium, manganese and cadmium respectively over the background values. Notwithstanding the effect of tidal dilutions, the impact of dredging on heavy metals is medium to long term. Key words: Dredging Heavy metals Niger Delta Mangrove Oil exploration Warri River Water quality INTRODUCTION contamination [4-6], which has resulted in vegetation Hydrocarbon exploration in the Niger Delta is a result of dredging have been reported to cause often constrained by access difficulties due to the alteration in the population and diversity of meandering wetland of the region. Dredging is often phytoplankton [8] and zooplankton [9, 10]. Benthic required to make oil exploration sites accessible for drilling invertebrates are also impacted by dredging [6]. In a study and development. During dredging, waterway sediment, carried out by Lewis et al. [11], dredging caused a soil, creek banks and vegetation along the right of way are reduction in numbers of benthic species, increased typically removed and deposited as dredge spoils at the turbidity, reduction of primary productivity and bank of the newly dredged canal, resulting in a number mobilization and increased bioavailability of sediment of environmental impacts including alteration of trace metals. Dredging has been reported to cause the topography and hydrology [1-3], acidification and water re-suspension of sediments, which is linked to the damage [7] and fish kills [6]. Turbidity plumes created as