Human health implications of extreme precipitation events and water quality in California, USA: a canonical correlation analysis
Authors: Alexander Gershunov, Tarik Benmarhnia, Rosana Aguilera
Journal: The Lancet Planetary Health (2018)

BackgroundPathogens and pollutants collect on the land surface or in infrastructure between strong rainfall episodes and are delivered via storm runoff to areas of human exposure, such as coastal recreational waters. In California, USA, precipitation events are projected to become more extreme and simultaneously decrease in frequency as storm tracks move poleward due to polar-amplified global warming. Precipitation extremes in California are dominated by atmospheric rivers, which carry more moisture in warmer climates. Thus, the physical driver of extreme precipitation events is expected to grow stronger with climate change, and pollutant accumulation and runoff-generated exposure to those pollutants are expected to increase, particularly after prolonged dry spells. Microbiological contamination of coastal waters during winter storms exposes human populations to elevated concentrations of microorganisms such as faecal bacteria, which could cause gastrointestinal and ear infections, and lead to exposure to pathogens causing life-threatening conditions, such as hepatitis A. The aim of this study was to quantitatively assess the effect of precipitation on coastal water quality in California.MethodsWe used a recently published catalogue of atmospheric rivers, in combination with historical daily precipitation data and levels of three indicators of faecal bacteria (total and faecal coliforms, and Escherichia coli) detected at roughly 500 monitoring locations in coastal waters along California's 840-mile coastline, to explore weekly associations between extreme precipitation events, particularly those related to atmospheric rivers, and the variability in water quality during 2003–09. We identified ten principal components (together explaining >90% of the variability) in precipitation and faecal bacteria time-series to reduce the dimensionality of the datasets. We then performed canonical correlation analysis of the principal components to identify and quantify associations between precipitation and faecal bacteria.FindingsWe found that the temporal evolutions of the leading canonical correlation patterns for precipitation and the faecal bacteria indicators were highly correlated. By looking at the Pearson correlation between precipitation and faecal bacteria canonical correlation patterns and the corresponding original time-series, we observed that the spatial pattern for all variables was strongest in southern California, and that it was mainly related to atmospheric river extreme precipitation events. Specific cases, such as peaks in precipitation and faecal pollution associated with an intense atmospheric river during January 7–12, 2005, were examined in detail. Overall, we found that precipitation over southern California explained over half of the weekly variability in faecal pollution in regional coastal waters.InterpretationThe associations between precipitation and water quality will be used to identify mechanisms of faecal pollutant delivery to coastal recreational waters via storm runoff and to track sources of pollution common in the region, such as sewage outfalls and homeless encampments near rivers and streams. This work should help to reduce vulnerability to extreme weather and delineate measures, such as an early warning system, that improve the response and resilience of human populations and ecosystems to a varying and changing climate. These results will also be used in the context of down-scaled climate change projections to estimate effects of future precipitation regime changes on coastal water quality and related health impacts.FundingUniversity of California Office of the President via Multicampus Research Programs and Initiatives.