Evaluation of water quality and pollution sources of gulshan lake

Abstract

Gulshan Lake is the northernmost lake in a chain of water bodies in Dhaka (Gulshan Lake, Hatirjheel, Begunbari Khal, Balu River and Shitalakhya River), all suffering from serious pollution. Gulshan Lake is about 3800m long having an average depth of about 2.5m. Encroachment, disposal of untreated domestic and commercial wastewater and dumping of solid wastes have degraded the overall quality of the Lake. Gulshan Lake is one of the last remaining water bodies of Dhaka city; not only is its presence important for the sustenance of the eco-system, it is also considered as major source of groundwater recharge. This study focuses on the assessment of water and sediment qualities of Gulshan Lake, identification of sources of pollution and characterization of selected major outfalls of the Lake. The major types of wastewater/ storm water outfalls contributing to the pollution of Gulshan Lake include (i) Storm sewer pipes, (ii) Open channels, (iii) Box culverts, and (iv) Small private outfalls. A detailed inventory of outfalls of Gulshan Lake has been prepared. A total of 102 outfalls have been identified, which included 24 major (diameter/ size 24 inch and above) outfalls. Measured dry weather flow rate through 4 major outfalls varied from about 2 to 2.8 L/sec. Outfall discharges are characterized by high concentrations of BOD5 (56-160 mg/l), COD (129-188 mg/l), Ammonia (16-28 mg/l), Phosphate (3.55-13.4 mg/l), TDS (277-413 mg/l) and Color (164-321 Pt-Co unit). These are comparable to the characteristics of raw sewage. Water quality of Gulshan Lake has been monitored during November 2009 to August 2010, covering both dry season and wet season through six sampling campaigns (five in dry season and one in wet season). Water samples were collected from sixteen locations along eight cross sections along Gulshan Lake during the study. The water samples were tested for a range of parameters including pH, Color, EC, TDS, TSS, DO, BOD5, COD, Fecal Coliform, NH3-N, NO3-N, NO2-N, PO4, SO4, and S2-. Results showed fluctuations in chemical composition of Lake water, both spatially and temporally. The Lake water has been characterized by very low DO (mostly below 2 mg/l) and very high fecal coliform (FC) throughout the year, indicating severe organic/fecal pollution of the water body. The high BOD5 (up to 46.0 mg/l) and COD (up to 130 mg/l) values also indicated significant organic pollution. Many dead fishes have been found afloat in the Lake during the study. A trend of decreasing DO and increasing BOD with the advancement of dry season was observed; the situation improved during the wet season, but only marginally. The Lake water has been found to contain high concentration of nutrients [mainly ammonia (10 to 20 mg/l), and phosphate (up to 8.55 mg/l)], especially during the dry season. Among the parameters tested, Color, TDS and Ammonia showed the most significant seasonal variation due to the influence of rain and storm runoff; decreasing concentration of Color, TDS and Ammonia and increasing concentration of TSS was observed during the wet season (i.e., August). EC and phosphate showed significant spatial variation, with increasing concentration towards downstream. Along a particular cross-section, quality of Lake water, collected from both banks, did not vary significantly (except for few locations), irrespective of the location of outfalls; possibly indicating that the Lake is reasonably well-mixed. Relatively high concentrations of Cadmium and Lead have been found in the Lake sediments. The Lake sediments also have very high oxygen demand (pSOD), which may be responsible for persistent low DO of Lake water throughout the year. The poor water quality of Gulshan Lake is also contributing to the pollution of Hatirjheel, where a restoration project is currently being implemented, and the water bodies downstream. It is very important to take immediate steps to restore the water and sediment qualities of Gulshan Lake. Development of domestic sewer network in the Gulshan Lake watershed areas for diverting domestic sewage (and also industrial effluent, if any) flows away from the Lake is the most important and urgent task in this regard. It is also important to develop a comprehensive management plan for revival of all lakes, khals, and wetlands within Dhaka city.