Intensive agriculture, a pesticide pathway to >100 m deep groundwater below dryland agriculture, Cordoba Pampas, Argentina

[Display omitted] •Agrochemicals reaching deep artesian groundwater below dryland agriculture.•‘Young’ groundwater tracers indicate rapid flow to deep wells.•Phreatic level rise acts as ‘doorway’ for surface pollutants into deep groundwater.•Land modification and flooding appears critical to deep pollutant transport.•Bypass flow mechanisms such as leaky wells require further investigation. Groundwater pesticide pollution in shallow groundwater is a well-established global phenomenon. However, deep aquifers are widely thought to be naturally protected from such modern contaminants, by confining geological barriers and upwards hydraulic gradients. Here we document pervasive pesticide pollution in >100 m deep artesian wells in a sedimentary aquifer below dryland agriculture. The vertical distribution of key groundwater markers, including numbers and concentrations of pesticides, stable (δ18O & δ2H) and radioactive (3H &14C) isotopes and ion concentrations were used to develop a conceptual model of pollutant transport to deep groundwater. Tritium, stable isotope and pesticide distributions in unconfined groundwater indicate that water table rise to