Investigation of lead contamination in Dhanmondi lake

Abstract

Heavy metal contamination of water bodies is a rising concern in recent years. Lead is a major toxic heavy metal which has wide industrial applications. Lead contamination is a common problem for lakes and reservoirs located in urban/industrial areas, especially for confined ones, which seriously threatens aquatic inhabitants as well as humans. Dhanmondi lake, located in the central part of Dhaka city, receives effiuents from a variety of sources and is likely to be contaminated with lead. The major objective of this study was to assess the level of lead contamination in Dhanmondi lake. Water and sediment samples were collected from seven sampling locations within the lake during three different seasons: dry season (from February to April), wet season (June and July) and interim season (April and May). In addition, different tissues of a Nilotika fish were tested in order to determine gross bioaccumualtion oflead in fish species of this lake. Average total aqueous lead concentration in Dhanmondi lake varies from 151 to 210 ~lg/I during dry season and 30-120 ~lg/I during wet season. Lead concentration in aqueous media was found to be much higher than that specified by Environmental Quality Standards of Bangladesh (EQS, 1991) for fishing and drinking (50 /.lg/I), industrial (10 /.lg/I) and irrigational (100 /.lg/l) purposes. Aqueous lead concentrations throughout the lake during any particular season were found to be more or less uniform. However, lead concentration decreased significantly (by a factor of about 2) from dry season to wet season primarily due to dilution effect resulting from increased rainfall and surface runoff during wet period. No seasonal variation oflead concentration in bOllom sediment was observed. Slightly higher concentrations of lead were found at the ends near Pilkhana and Dhanmondi road no. 15. This indicates that significant amount of lead may flow into the lake through inlets at these locations. Since toxicity oflead depends on its chemical form, equilibrium speciation calculations were performed to determine concentrations of various lead species. Equilibrium speciation calculations were performed using chemical equilibrium program M1NEQL' (Schecher, 1994). The major species present in lake water are aqueous PbeO", Pb2' and PbOH', of which, Pb2' is highly toxic. Although total aqueous lead concentration in Dhanmondi lake is high, concentration of toxic ionic lead species (Pb2') is relatively small. However, if in any case, lake water becomes acidic (e.g., due to discharge of acidic waste, acid rain etc.), concentration of Pb2+ may significantly increase and make the water highly toxic. Significant amount oflead was found in various tissues of fish of Dhanmondi lake. This result is alarming because this lake is a popular fishing spot and fishes of this lake are widely consumed by people. Thus lead contamination of Dhanmondi lake is endangering not only aquatic inhabitants but also humans.