Evaluation of the short-term fate and transport of chemicals of emerging concern during soil-aquifer treatment using select transformation products as intrinsic redox-sensitive tracers

In this study, known products from oxic transformation of the X-ray contrast medium iopromide were introduced for the first time as intrinsic tracer for in situ characterization of the transition zone between oxic and suboxic conditions during the initial phase of soil-aquifer treatment (SAT). Two wet-dry cycles of a full-scale infiltration basin were monitored to characterize hydraulic retention times, redox conditions, removal of bulk organic parameters and the fate of chemicals of emerging concern (CECs). Tracer tests at the site showed an average hydraulic retention time of < 20 h before collection in drainage pipes located approximately 1.5 m below surface. Dissolved oxygen at different depth rapidly depleted and only increased towards the end of the flooding event. Transformation of iopromide and all known intermediates to persistent transformation products (TPs) usually occurring during oxic biodegradation was very limited in samples from suction cups immediately underneath the basin. But transformation was complete in samples collected from the drainage outlet indicating that dissolved oxygen had been introduced to the system before sample collection in the combined drainage outlet. Similar to iopromide and its TPs, removal of several CECs (diclofenac, bezafibrate, mecoprop, TCEP) was inefficient after 90 cm infiltration (< 35%) but significantly enhanced in the combined drainage outlet (> 80%). These results highlight that the analysis of iopromide along with its intermediates and persistent TPs can serve as a promising probing tool to determine overall efficiency of CEC biodegradation and to identify potential in situ oxygen limitations.