Faecal indicator bacteria and microbial source tracking as tools for testing water quality and sources of contamination of Rawan-Oya tributary of the Mahaweli River, in Sri Lanka

Abstract

Faecal indicator bacteria (FIB) are used to assess water quality, indicating potential contamination from faeces, while microbial source tracking (MST) identifies the specific source of that contamination. These are essential tools for assessing the level and source of faecal contamination in water, and this study evaluated faecal contamination in the Rawan-Oya tributary of the Mahaweli River, a crucial drinking water source in Sri Lanka, using these tools. Water samples were collected from nine sites representing forested (n = 19), agricultural (n = 38), rural (n = 38), semi-urban (n = 38), and urban (n = 38) areas. These samples were analysed for FIBs including total coliforms (TC), faecal coliforms (FC), Escherichia coli, and faecal streptococci (FS) using membrane filtration method. DNA markers were used in order to identify human and animal faecal contaminant sources. Results showed an increase in FIB concentrations from forested to urban areas, indicating increased numbers in urban areas compared to those in forested areas. TC concentration in water ranged from 2.7 × 10² to 1.6 × 10⁵ CFU/100 mL. FC counts increased from 16 to 3.2×10⁴ CFU/100 mL, and FS counts fluctuated between 5.4×10² and 6.2×10³ CFU/100 mL. All samples tested positive for the universal FIB DNA marker. Microbial source tracking analysis using human- and cattle-specific Bacteroides markers indicated the highest levels of human faecal contamination in urban sites, while significant cattle faeces contamination was observed in both agricultural and urban areas. The microbial quality of water did not meet the drinking water standards (FC and TC, 0 CFU/100 mL) in any of the sites tested. However, the forested area met the recreational water standards (FC ≤ 235 CFU/100 mL). This study identifies the potential of using FIB and MST as a means of confirming the specific sources and locations of faecal contamination in water bodies, which is crucial for developing targeted water management strategies and for alerting local communities and authorities about the status of drinking water quality and factors that contribute towards the deterioration of the quality of water bodies.