Towards the Implementation of Permeable Reactive Interceptors On Agricultural Landscapes in Ireland.

There is consensus that denitrifying bioreactors are capable of high rates of nitrate (NO₃^-) remediation but further scope still exists to improve their efficiency and expand their remit in diverse environmental and agricultural contexts. In Ireland, they are developed to achieve sustainable intensification in grassland, where there is a need to remediate highly dynamic mixed-contaminant sources (including pollution swapping, i.e. bioreactor generated contaminants). To tackle this, our group developed novel laboratory and field permeable reactive interceptors (PRIs), through modification of denitrifying bioreactors concepts. First, NO₃^- spiked water (>19.5 mg NO₃-N L^-1) was circulated in 0.8-m long laboratory columns through a mixture of soil and carbon media. High water residence times (>8.5 days) favoured full denitrification, but enhanced pollution swapping, through media leaching or microbial activity (e.g. dissimilatory NO₃^- reduction to ammonium (NH₄⁺) or methanogenesis). Subsequently, similar concentrations of NO₃^- spiked water were circulated at 0.2 m³ day^-1 into a field-scale semi-controlled bioreactor (7 x 3 x 1.5 m engineered plastic tank installed at the Teagasc Environment Research Centre, Ireland), made of interconnected cells alternatively filled with soil and woodchip. Pollution swapping occurred in areas of long water residence times (i.e. at depth and towards the tank outlet), with full denitrification occurring in areas of short residence times (i.e. first two cells) linked to elevated nitrous oxide (N₂O) emissions. This suggests that PRI design can 1) adapt adequate timing of water/media interactions to variations in contaminant fluxes and 2) remediate dissolved N₂O, providing an additional remediation sequence (e.g. soil or biochar cap) prevents losses to air. In Ireland, PRIs are being implemented at spring discharge points draining farms with historically high NO₃^- loadings and at end of pipe discharge points in shallow and deep drainage outlets. Remediation sequences favouring contaminant recycling, such as NH₄⁺ adsorption on zeolite, are now being tested.