Physicochemical and isotopic similarity between well water and intruding surface water is not synonymous with similarity in prokaryotic diversity and community composition

Highlights • Multiple methods were applied to study surface water intrusion in shallow wells. • One well showed large-scale intrusion from a nearby gravel pit pond. • There, physicochemistry and stable water isotopes were similar in well and pond. • Yet well water had higher prokaryotic diversity and a distinct community composition. • Thus, understanding intrusion in shallow wells requires a diverse array of methods. ABSTRACT: Intruding surface water can impact the physicochemical and microbiological quality of groundwater. Understanding these impacts is important because groundwater provides much of the world's potable water, and reduced quality is a potential public health risk. In this study, we monitored six shallow groundwater wells and three surface water bodies in the North Ostrobothnia region of Finland twice monthly for 12 months (October 2021–October 2022) via (i) on-site and off-site measurements of physicochemical water quality parameters, (ii) determination of stable water isotope compositions, and (iii) analysis of microbial communities (via amplicon sequencing of the V3–V4 16S rRNA gene sub-regions). Water from one well showed clear overall physicochemical and isotopic similarity with a nearby pond, as well as temporal fluctuations in water temperature and isotopes that mirrored those of the pond. Isotope mixing analyses suggested that about 80–95 % of the well water comes from the pond. Such large-scale intrusion might be expected to reduce prokaryotic diversity and composition in the aquifer, either by strong influx of surface water taxa or changes to aquifer physicochemistry. Compared to the pond, however, prokaryotic communities from the well showed significantly higher alpha diversity and a composition more similar to a nearby well unaffected by intrusion. The finding that physicochemical and isotopic similarity between well water and intruding surface water is not synonymous with similarity in prokaryotic diversity and community composition makes clear the need for a multi-method approach when studying the impact of surface water intrusion on shallow wells.