Combined Surface-Subsurface Stream Restoration Structures Can Optimize Hyporheic Attenuation of Stream Water Contaminants.

There is a design-to-function knowledge gap regarding how engineered stream restoration structures can maximize hyporheic contaminant attenuation. Surface and subsurface structures have each been studied in isolation as techniques to restore hyporheic exchange, but surface-subsurface structures have not been investigated or optimized in an integrated manner. Here, we used a numerical model to systematically evaluate key design variables for combined surface (i.e., weir height and length) and subsurface (i.e., upstream and downstream baffle plate spacing) structures. We also compared performance metrics that place differing emphasis on hyporheic flux versus transit times. We found that surface structures tended to create higher flux, shorter transit time flowpaths, whereas subsurface structures promoted moderate-flux, longer transit time flowpaths. Optimal combined surface-subsurface structures could increase fluxes and transit times simultaneously, thus providing conditions for contaminant attenuation that were many times more effective than surface or subsurface structures alone. All performance metrics were improved by the presence of an upstream plate and the absence of a downstream plate. Increasing the weir length tended to improve all metrics, whereas the optimal weir height varied based on metrics. These findings may improve stream restoration by better aligning specific restoration goals with appropriate performance metrics and hyporheic structure designs.

Transformation of organic micropollutants along hyporheic flow in bedforms of river-simulating flumes.

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.

Using recirculating flumes and a response surface model to investigate the role of hyporheic exchange and bacterial diversity on micropollutant half-lives.

Enhancing the understanding of the fate of wastewater-derived organic micropollutants in rivers is crucial to improve risk assessment, regulatory decision making and river management. Hyporheic exchange and sediment bacterial diversity are two factors gaining increasing importance as drivers for micropollutant degradation, but are complex to study in field experiments and usually ignored in laboratory tests aimed to estimate environmental half-lives. Flume mesocosms are useful to investigate micropollutant degradation processes, bridging the gap between the field and batch experiments. However, few studies have used flumes in this context. We present a novel experimental setup using 20 recirculating flumes and a response surface model to study the influence of hyporheic exchange and sediment bacterial diversity on half-lives of the anti-epileptic drug carbamazepine (CBZ) and the artificial sweetener acesulfame (ACS). The effect of bedform-induced hyporheic exchange was tested by three treatment levels differing in number of bedforms (0, 3 and 6). Three levels of sediment bacterial diversity were obtained by diluting sediment from the River Erpe in Berlin, Germany, with sand (1 : 10, 1 : 1000 and 1 : 100 000). Our results show that ACS half-lives were significantly influenced by sediment dilution and number of bedforms. Half-lives of CBZ were higher than ACS, and were significantly affected only by the sediment dilution variable, and thus by bacterial diversity. Our results show that (1) the flume-setup is a useful tool to study the fate of micropollutants in rivers, and that (2) higher hyporheic exchange and bacterial diversity in the sediment can increase the degradation of micropollutants in rivers.

Plant Assimilation Kinetics and Metabolism of 2-Mercaptobenzothiazole Tire Rubber Vulcanizers by Arabidopsis.

2-Mercaptobenzothiazole (MBT) is a tire rubber vulcanizer found in potential sources of reclaimed water where it may come in contact with vegetation. In this work, we quantified the plant assimilation kinetics of MBT using Arabidopsis under hydroponic conditions. MBT depletion kinetics in the hydroponic medium with plants were second order (t1/2 = 0.52 to 2.4 h) and significantly greater than any abiotic losses (>18 times faster; p = 0.0056). MBT depletion rate was related to the initial exposure concentration with higher rates at greater concentrations from 1.6 µg/L to 147 µg/L until a potentially inhibitory level (1973 µg/L) lowered the assimilation rate. 9.8% of the initial MBT mass spike was present in the plants after 3 h and decreased through time. In-source LC-MS/MS fragmentation revealed that MBT was converted by Arabidopsis seedlings to multiple conjugated-MBT metabolites of differential polarity that accumulate in both the plant tissue and hydroponic medium; metabolite representation evolved temporally. Multiple novel MBT-derived plant metabolites were detected via LC-QTOF-MS analysis; proposed transformation products include glucose and amino acid conjugated MBT metabolites. Elucidating plant transformation products of trace organic contaminants has broad implications for water reuse because plant assimilation could be employed advantageously in engineered natural treatment systems, and plant metabolites in food crops could present an unintended exposure route to consumers.