Abstract
Urban streams receive increasing loads of organic micropollutants from treated wastewaters. A comprehensive understanding of the in-stream fate of micropollutants is thus of high interest for water quality management. Bedforms induce pumping effects considerably contributing to whole stream hyporheic exchange and are hotspots of biogeochemical turnover processes. However, little is known about the transformation of micropollutants in such structures. In the present study, we set up recirculating flumes to examine the transformation of a set of micropollutants along single flowpaths in two triangular bedforms. We sampled porewater from four locations in the bedforms over 78 days and analysed the resulting concentration curves using the results of a hydrodynamic model in combination with a reactive transport model accounting for advection, dispersion, first-order removal and retardation. The four porewater sampling locations were positioned on individual flowpaths with median solute travel times ranging from 11.5 to 43.3 h as shown in a hydrodynamic model previously. Highest stability was estimated for hydrochlorothiazide on all flowpaths. Lowest detectable half-lives were estimated for sotalol (0.7 h) and sitagliptin (0.2 h) along the shortest flowpath. Also, venlafaxine, acesulfame, bezafibrate, irbesartan, valsartan, ibuprofen and naproxen displayed lower half-lives at shorter flowpaths in the first bedform. However, the behavior of many compounds in the second bedform deviated from expectations, where particularly transformation products, e.g. valsartan acid, showed high concentrations. Flowpath-specific behavior as observed for metformin or flume-specific behavior as observed for metoprolol acid, for instance, was attributed to potential small-scale or flume-scale heterogeneity of microbial community compositions, respectively. The results of the study indicate that the shallow hyporheic flow field and the small-scale heterogeneity of the microbial community are major controlling factors for the transformation of relevant micropollutants in river sediments.
Highlights
Urban streams receive increasing loads of organic micropollutants from treated wastewaters
In previous studies we described the experiment, in which in total 20 river-simulating flumes were set up to investigate the influence of bacterial diversity and hyporheic exchange on the fate of organic micropollutants in the surface water (SW)
Concentration time trends of parent compounds and transformation products (TPs) in the bedforms. 40 compounds were analysed in SW and PW samples of Flumes 1 and 2, with 19 of the compounds representing TPs (Supplementary Table S2)
Summary
Urban streams receive increasing loads of organic micropollutants from treated wastewaters. Li et al.[23] investigated attenuation of micropollutants in bedforms built in recirculating flumes, similar to the setup of the present study They found formation of four transformation products (TPs) exclusively in the sediment (carbamazepine-10,11-epoxide, saluamine, metoprolol acid, and 1-naphthol). The identification of micropollutant transformation processes caused by shallow hyporheic exchange in bedforms is important to understand and eventually predict reach-scale attenuation of micropollutants and formation of TPs. The present study was conducted as part of an interdisciplinary joint experiment comprised of several subprojects. Supported by hydrodynamic and reactive transport models, the study aims to investigate the behavior and fate of a set of organic micropollutants and related TPs along specific shallow hyporheic flowpaths in two of the bedforms of each flume. The objectives were to identify (1) the effect of specific flowpath characteristics, such as residence times, on degradation of parent compounds and (2) the conditions favoring formation of TPs
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