Suspected faecal contamination of Phewa Lake: Spatial patterns and effects of rainfall

Abstract

Phewa Lake was once popular for swimming, but most local people now perceive it to be unsafe due to urban development and beliefs about sewage contamination. Nevertheless some people, especially children, still swim or bathe in Phewa’s waters. Our objectives were to determine whether swimmers are at risk of waterborne disease, how the risk varies around the lake, and how it varies with rainfall. We measured concentrations of faecal indicator bacteria (thermotolerant coliforms) in lake water samples from five points along the urbanized eastern shore, from the mainly agricultural northern shore, the mainly forested southern shore, and the middle of the lake. We also sampled two urban streams. We sampled on ten occasions from October 2021 to January 2023, including monsoon, post-monsoon and winter. During dry periods concentrations of thermotolerant coliforms were low (median 3-60 cfu/100 mL) in all lake samples and very high (median 3000-45,000 cfu/100 mL) in the urban streams. After rain, concentrations in lake samples near the urban area rose to median 400-2000 cfu/100 mL, exceeding USA guideline values. Concentrations were generally higher on the urbanized side of the lake than on the mainly agricultural and forested sides, and generally increased along the presumed flow path towards the lake outlet. We recommend further investigations to determine the sources of contamination. We also recommend developing guideline faecal indicator bacteria concentrations for recreational waters in Nepal, with regular monitoring and public signage to protect people from illness at popular swimming sites.