Deep Tree Roots at Risk of Accelerating Ground Water Pollution Beneath Glacial Aquitards

Glacial aquitards deposited by Pleistocene glaciers stretch over millions of square kilometers in the Northern Hemisphere. These aquitards are supposed to protect underlying aquifers from surface pollution, depending on the presence or absence of deep open fractures. We show that previously unaddressed deep abandoned root macropores within the fractures from modern and past tree vegetation, the latter identified by ancient DNA, are major vertical flow paths in glacial aquitards across Denmark and may sustain very rapid water flow and contaminant transport to 10 – 20 m below ground surface (mbgs). In comparison, the contribution to dissolved contaminant transport by fractures without relic root macropores is marginal below 1–2 mbgs and negligible below water tables in the aquitards. The consistent absence of deep open fractures indicate that embedded sedimentary heterogeneities (interconnecting with the root macropores) represent principal leaks and contaminant pathways into aquifers beneath rooting depths in the aquitards. Model simulations show that targeting groundwater protection in areas with rooting depths exceeding aquitard thickness and leaky aquitards is pivotal for restoring and maintaining groundwater quality within drinking-water standards. Formation of deep root macropores by afforestation and reforestation is at risk of accelerating contamination of aquifers threatened by surface pollution.