Managed aquifer recharge with marginal water for irrigation and contaminant attenuation

Abstract

Managed aquifer recharge with marginal water allows for contaminants of emerging concern (CECs) to be attenuated in-situ, and for water to be more efficiently used. In phreatic aquifers underlying agricultural fields, this also increases water availability for crops. Existing drainage systems may be adapted for both injection (subirrigation) and extraction (drainage). We address subirrigation with treated domestic wastewater by considering numerical simulations and an experimental field site implemented as a proofof-concept. The transport, adsorption, and biodegradation of tracers, and relatively mobile and persistent CECs (using carbamazepine as an example), are analyzed. For this scenario, almost all non-biodegraded solutes irrigated into the soil are advected laterally towards surface water channels by regional groundwater flow. During the crop season, tracers may rise to the root zone, but only directly above irrigation drains. Accordingly, although up to 10% of tracers irrigated over four years are passively taken up by crops, this is concentrated in the plants situated directly above irrigation drains. No significant spreading of carbamazepine to the root zone occurs, and <1% of the total irrigated amount is taken up by crops after four years. Due to the annual precipitation surplus, solutes that are not very mobile are unlikely to enter the root zone even after decades of irrigation, because the length of the short-term precipitation shortage (crop season) is insufficient for them to rise to the root zone. Furthermore, carbamazepine biodegrades almost completely before being advected any significant distance from the drains. As carbamazepine is a relatively mobile and persistent CEC, the risks of crop, soil and surface water contamination by most other CECs is even smaller. The fraction of the irrigated CEC mass that seeps into surface water channels from the phreatic aquifer decreases exponentially with the distance between the agricultural plot and the channel, and increases exponentially with the agricultural plot width. Thus, the studied subsurface wastewater irrigation system reduces the environmental impacts of marginal water discharge and agricultural water use, with minimal risk of crop and environmental contamination.