Assessment of groundwater contamination in Dhaka city from tannery waste

Abstract

The Tannery industries located at Hazaribagh are one of the major polluting industries in Dhaka. The tanning process requires a huge amount of chromium especially in blue green section of the industry. At present wastewater from the tannery industries is discharged through open drains into a wastewater pond/lagoon, which finally flows into the Buriganga River though three outfalls. The stagnant pond area serves to retain the tannery wastewater for prolonged periods of time, allowing dissolved constituents including chromium to percolate into the subsurface and possibly contaminate the aquifer below. The present study aims at assessing the groundwater contamination, particularly chromium contamination, from tannery waste in Dhaka. Groundwater samples were collected from 21 DTW stations from the six Zones of DWASA, including all seven stations located in Zone 2 covering Hazaribagh area, and analyzed for a wide range of parameters including chromium. Chromium concentrations in the groundwater samples from Zone 2 were found to be significantly higher than those from the other DWASA zones. Speciation data suggest that only a small fraction of chromium is present in Cr(VI) form and majority of dissolved chromium in groundwater is present in less toxic Cr(III) form. Out of the 14 groundwater samples from Zone 2, two exceeded the Bangladesh drinking water standard of 0.05 mg/l. Average chromium concentration in groundwater from the seven tubewells of Zone 2 is about 0.036 mg/I, about ten times higher than the average chromium concentration of 0.004 mg/! in water samples from 14 tubewells from 5 other zones. In the absence of any natural source of chromium and the presence of a large number of tanneries in the Hazaribagh area, it appears that chromium from tannery wastewater is already contaminating the groundwater in and around Hazaribagh area. Concentration of sulfide was also found to be higher in the groundwater samples from Zone 2, which also appears to suggest that the groundwater in the Hazaribagh area is being contaminated by the tannery wastewater. Apart from chromium, alarmingly high lead concentrations, varying from 0.001 to 0.14 mg/I, were also detected in many of the DWASA groundwater samples. Among the 21 samples analyzed, 17 exceeded the WHO guideline value (for drinking water) of 0.01 mg/I and two exceeded the Bangladesh drinking water standard of 0.05 mg/l. Sediment samples were collected from two boreholes at the effiuent discharge area and were analyzed for pH, lead and chromium. Very high accumulation of chromium (reaching close to 15,000 mg/kg of sediment) was detected in the top layer of sediment. The accumulation of chromium appears to be restricted primarily to the first 10 feet of soil. This is probably due to the fact that most of the chromium in the tannery wastewater is present in Cr(III) form and Cr(III) generally is not transported to great distance by groundwater because of its low solubility. However, groundwater with low levels of Cr(III), leached from the heavily contaminated sediments, can still move downward and slowly contaminate the aquifer below. Results from this research work suggest that a huge quantity of chromium has accumulated in the sediment at the effiuent discharge area at Hazaribagh, which can potentially contaminate the groundwater aquifer and thus endanger the water supply of the Dhaka city dwellers.