Impact of rainfall on microbial contamination of surface water

Purpose - Increased annual precipitation and more frequent episodes with heavy precipitation are expected in Norway due to climate change. The purpose of this paper is to use two case studies to investigate effects of precipitation on the amounts of faecal indicator bacteria and parasitic protozoa (Cryptosporidium and Giardia) loaded to surface waters from catchment areas exposed to different faecal sources. Design/methodology/approach - In the first case study, the loads of faecal indicator bacteria and Cryptosporidium and Giardia, were investigated in relation to precipitation in a stream from a small valley where cattle and sheep are grazed. In the second case study, historical data (monthly values from 2004-2009) regarding faecal coliforms and water flow in five tributaries (urban and rural) of a lake used as a drinking water source, were used for calculating loads of faecal indicator bacteria. These loads were evaluated in relation to historical data on precipitation. Additional sampling during/after rainfall, including analysis of samples for Cryptosporidium and Giardia, was performed. Findings - The study visualises how heavy rainfall may increase the load of faecal microorganisms and potential pathogens in Norwegian water sources. Originality/value - This study provides supplementary information about microbial contamination (including by parasites) of Norwegian surface waters during or after rainfall. This is useful input for decision making regarding protection of vulnerable water sources and for risk assessments. Data are also provided for evaluation of water treatment needs for Norwegian municipalities and waterworks responsible for providing safe drinking water under future climatic conditions.