Deep tree roots at risk of accelerating groundwater pollution beneath clay-rich aquitards.

Glacial aquitards from the last Pleistocene glaciation cover vast areas of the Northern Hemisphere. Due to their clay-rich composition, these aquitards are considered to protect underlying aquifers from surface contaminations, provided they are not penetrated by widespread deep fractures. In this study we combine ancient DNA (aDNA) with detailed hydrological techniques to investigate contaminant transport in the aquitards - an interdisciplinary approach that was not possible when the current understanding of the dominant role of fractures was established. We show that deep fractures (> 2 m below surface) in the glacial aquitards across Denmark are closed or marginal as pathways of dissolved contaminant transport unless they contain flow channels from deep modern and ancient tree roots. Our study reveals that deep tree roots can compromise natural groundwater protection and accelerate groundwater pollution by opening fractures and generating deep macropores. The results highlight that thorough attention needs to be paid to land-use history and widespread legacy and future soil pollution prior to reforestation and afforestation of aquitards above drinking water resources. We furthermore demonstrate that the protection of aquitard areas with less thickness than historical rooting depths and high degrees of lithological heterogeneity (related to aquitard glacial till types) is crucial to preserve drinking water resources and to restore groundwater quality beneath glacial aquitards.