Impact of the 2018 European drought on microbial groundwater quality in private domestic wells: A case study from a temperate maritime climate

A significant volume of research over the past two decades has highlighted both direct and indirect links between climate change and groundwater quality. However, to date, few studies have sought to explore the relationship (s) between drought conditions and groundwater quality in i) private (unregulated) groundwater sources, or ii) temperate maritime climates not commonly prone to drought events. The Republic of Ireland (ROI) represents an appropriate case-study due to its' high reliance on private groundwater supplies, and while the region is largely unaffected by climatological extremes, modelling studies indicate that drier summers and drought conditions will increase in frequency. Accordingly, the current study sought to quantify the effects of the Summer 2018 drought experienced throughout Europe on private groundwater quality in the southwest of Ireland via an opportunistic field study. A repeated measures sampling campaign comprised of "drought" (June/July) and "post-drought" (October/November) analyses of 74 wells was undertaken, with complementary mapping and statistical analyses. Both Total Coliforms (TCs) and Escherichia coli (E. coli) were present during both drought (TCs: 42/74; 56.8%, E. coli: 7/74; 9.5%) and post-drought (TCs: 42/74; 56.8%, E. coli: 18/74; 24.3%) sampling periods. E. coli contamination during drought conditions was unexpected due to an absence of recharge or infiltration for microbial transport. Bivariate analyses suggest a hydrodynamic change, with the significance of E. coli sources and pathways shown to switch between sampling periods i.e. a shift from a combination of regional and local (site specific) contamination mechanisms, to solely site-specific mechanisms. More specifically, during drought conditions, septic tank density (p = 0.001) and local subsoil type (p = 0.009) were both associated with the presence of E. coli, while neither variable was significant during post-drought conditions. The current study is the first to provide a quantitative comparison of private groundwater quality during and after a large-scale drought event in a temperate maritime climate and may be used to improve our understanding of the effects of extreme events, and thus necessary preventative and monitoring strategies, going forward.