Under the weather: the influence of land-use and climate on surface water fecal contamination.




St Laurent, Jacques

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The risk of waterborne infections acquired from the consumption of contaminated water is related to changes in source water fecal contamination, which is often influenced by land-use and hydro-meteorological conditions in the surrounding watershed. The impact of land-use composition on surface water contamination was explored in order to determine the risk of surface water contamination associated with land-use change. Highest contamination was observed in watersheds characterized by more than 12.5% agricultural and more than 1.6% urban land (mean fecal coliform (FC) concentration of these 5 sites = 135 CFU 100ml-1 while the British Columbia (BC) raw water quality guideline = 100 CFU 100ml-1). Contamination increased exponentially, and violated BC raw water quality guidelines with greater frequency, in relation to greater agricultural land in the upstream watershed. Additional factors, such as sewage treatment plants, low dilution in smaller streams, and higher temperatures were also associated with greater contamination. These results indicate the high level of risk posed by agricultural and urban development and the need for source water protection. Fecal contamination levels in source water are also influenced by rainfall and snowmelt-induced surface runoff that transport diffuse fecal contaminants into surface water. Seasonal levels of fecal contamination in surface water was related to the watershed hydro-climatic regime for around half of the watersheds examined. Watersheds with snowmelt-dominant (SD) runoff regimes showed stronger evidence of hydro-meteorological variability driving seasonal contamination levels than those with rainfall and snowmelt-influenced (RSI) and rainfall-dominant (RD) runoff regimes, and thus are more prone to experiencing changes to seasonal variability resulting from climate change. Projected increases in mean annual temperatures of between 1.70C and 4.00C towards the end of the 21st century will alter existing runoff regimes within watersheds. For SD watersheds that remain below freezing and continue to accumulate snowpack during the cold season, transport of fecal contamination will likely occur earlier in the year with greater intensity. Fecal coliform transport in summer is likely to decrease, especially in SD watersheds in which fecal contamination is driven by summer rainfall events. Snowmelt-dominant watersheds transitioning toward a RD runoff regime will experience less contamination during spring but increased contamination during late fall and winter. The extent to which these changes in runoff regime will influence surface water fecal contamination will vary among watersheds. Further investigation is required to identify factors that enhance or mitigate the association of surface water fecal contamination with rainfall and snowmelt-induced runoff in order to identify specific site vulnerability to changing seasonal contamination levels. Total precipitation within BC is projected to increase by 20-30% towards the end of the 21st century. The association of annual FC variability with snowmelt and rainfall variability was examined in order to assess the capacity of such increases to raise the level of surface water fecal contamination. Greater total annual and seasonal rainfall and/or river discharge increased surface water fecal contamination for 58% (11/19) of the sites examined. Hydro-meteorological variability influenced FC concentration during winter, the season of greatest precipitation, and spring, the season of greatest snowmelt, but not during summer or fall. Reduced contamination levels during the El Niño event in 2002/03 were associated with a mean reduction in river discharge during spring and summer. These associations suggest that the risk of increased surface water fecal contamination in response to higher precipitation is likely to be greatest in winter for RD watersheds and spring for SD watersheds, although the magnitude of impact will vary among sites. Climate change and land-use activities within watersheds have the capacity to alter the timing and amount of surface water fecal contamination. These factors are likely to act synergistically by increasing the presence and transport of fecal contaminants within watersheds. Such relationships should be carefully considered to aid the assessment and mitigation of the risk of source water contamination associated with land-use and climate change.



waterborne disease, climate change, drinking water, land-use, fecal contamination