Hydrobiogeochemical Function of Soil Based Onsite Wastewater Treatment Systems: Insights from High-Resolution O2 Imaging

Abstract

Innovative/alternative onsite wastewater treatment systems (I/A OWTS) are increasingly installed to protect groundwater and surface water from excess nitrogen. Improved understanding of the relationships between hydraulic and biogeochemical processes can help to make more informed decisions on OWTS design to maximize their performance and resilience. In this study, for the first time, oxygen (O2) imaging was used to investigate redox dynamics within a soil-based I/A OWTS during the dosed application of artificial wastewater. Mesocosms consisted of an unsaturated sand matrix (nitrifying layer) above a water-saturated sand-sawdust matrix (denitrifying layer). Mesocosms consistently removed >85% of total inorganic nitrogen, with estimated nitrification and denitrification rates of 8.9–14.3 and 13.8–14.3 nmol N h−1 cm−3, respectively. Dosed application of artificial wastewater resulted in intermittent oxygenation of the upper horizon of the sand-sawdust layer due to the pulsed release of oxic water from the overlying unsaturated sand layer. Hydraulic disturbance, simulated by effluent port closure, inhibited this pulsed water release and resulted in the formation of water-saturated pockets in the sand layer, which rapidly turned anoxic. Water ponding at the surface and the loss of interconnected air-filled pore spaces inhibited diffusive O2 supply from the atmosphere, which caused O2 partial pressure to decline at rates between 4.2 and 6.7 matm h−1 throughout the sand layer, most likely as a result of nitrification. When the hydraulic function was restored, significant volumes of the sand layer remained water-saturated, and anoxic regions intensified, indicative of capillary hysteresis associated with wetting and drainage at low suction pressures during the hydraulic disturbance and sustained increased water content following the disturbance. Consequently, the nitrifying performance of the system declined, with effluent NH4+ concentrations linearly increasing with decreasing areas of the oxic unsaturated sand. Overall, this study highlights the critical role of unsaturated matrix composition and compaction for the system performance and resilience, and illustrates the potential of chemical imaging to coregister hydrological and biogeochemical processes in soil-based I/A OWTS.