Spatial Variability in Trace Organic Compound Reactivity During Urban River Infiltration Into an Alluvial Aquifer

Abstract Trace organic compounds (TrOCs) in lotic systems pose risks for ecosystem and human health. TrOCs can enter groundwater along losing river sections, yet little is known about the spatial extent of infiltration zones and the biogeochemical factors controlling the spatial variability of TrOC reactivity and removal, particularly along river‐to‐groundwater flow paths originating in rivers dominated by treated wastewater. We measured 33 TrOCs, redox‐sensitive ions and fluorescence‐derived properties of dissolved organic carbon (DOC) in river water and an adjacent alluvial aquifer. Using previously derived travel times and a convolution‐based model approach combined with a Monte Carlo analysis, we estimate distributions of apparent first‐order removal‐rate constants (λ) for different sections along a set of river‐to‐groundwater flow paths. Trace organic compound reactivity was generally highest near the river, where changes of redox‐sensitive ions and DOC composition were also most pronounced. Median values (±the interquartile range) of λ range from −0.7 ± 0.1 d −1 (valsartan acid) to 2.2 ± 0.4 d −1 (iopromid). For many TrOCs, λ correlated with zero‐order DOC turnover rates, suggesting that under eutrophic and similar redox conditions reactivity of these TrOCs is controlled by DOC turnover. Some TrOCs (e.g., primidone, valsartan acid) were found to be transported over long (>200 m) distances from the river. Although hydraulic gradients permitted surface water to enter the alluvial aquifer throughout the site, river‐to‐groundwater fluxes were substantial only along a short (<20 m) section of the investigated river reach. Unintended riverbank filtration, even when highly localized, may thus contribute to the development of partially closed urban water cycles.