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Hyporheic Zone Processes – A training network for enhancing the understanding of complex physical, chemical and biological process interactions

Periodic Reporting for period 2 - HypoTRAIN (Hyporheic Zone Processes – A training network for enhancing the understanding of complex physical, chemical and biological process interactions)

Reporting period: 2017-01-01 to 2018-12-31

Rivers are important ecosystems under various anthropogenic pressures. One of them is contaminants that enter streams on different routes, e.g. by (treated) wastewater. Especially organic micropollutants such as pharmaceuticals and their daughter products are often neither degraded by the human metabolism nor the treatment processes in the wastewater treatment plant. Being discharged into rivers they might even be transported to the groundwater. Rivers and groundwater are important drinking water resources which is why attenuation or degradation of pollutants is relevant not only for ecosystems but also for human health. Rivers have some self-purification capacity which is assumed to result from the degradation of pollutants by (micro-) organisms and retention by sorption in the hyporheic zone (HZ). HZs, i.e. the sediments in the river bed, are highly dynamic and productive river compartments. Surface water enters the river bed and travels through the sediment matrix. On its way it might mix with groundwater since HZs are often interconnected with adjacent aquifers (Fig. 1 and 2). Physical, biological and chemical processes occurring simultaneously in HZs are assumed to stimulate retention and/or degradation of pollutants. HypoTRAIN gathered a multi-disciplinary team from hydrology, ecology, microbiology, engineering, environmental physics, contaminant science, and modelling to generate new mechanistic insights into the functioning of HZs and enable a more holistic design of river management and restoration.
Knowledge on hyporheic fluxes is a prerequisite for quantifying micropollutant degradation/attenuation. For this, temperature and electric conductivity (EC) were used as tracers in innovative approaches: Vertical and horizontal temperatures in HZs were measured with temperature lances and heat-pulse sensors. The results were used to model advective heat transport from which fluxes can be deduced. Similarly, salt tracer tests were conducted at reach scales and EC breakthrough was measured in surface water and HZs. Diurnal fluctuations of EC in river water due to varying wastewater discharge rates and their propagation in the HZ were used to quantify degradation, attenuation and dilution of pollutants. Dissolved noble gases were used as tracers to identify surface water infiltration into the HZ and groundwater exfiltration to rivers. Numeric models identified when and where hyporheic exchange is most intense during dynamic discharge events. Results show that both regional groundwater conditions and local channel geomorphology determine spatiotemporal hyporheic exchange patterns. Dunes, ripples and alternating bars increase hyporheic exchange volumes.
For many parent compounds the transformation products are not identified so far. We contributed to this by developing and validating a method for the analysis of 26 parent compounds and 35 transformation products.
Sorption of molecules to the river sediments plays an important role in the attenuation of pollutants. However, for a lot of these components sorption characteristics are not fully known yet. We contributed to that by batch and column experiments to determine sorption isotherms and sorption rate constants for different micropollutants.
The degradation mechanisms of compounds with different physicochemical properties were investigated. Biodegradation was determined to be a major attenuation mechanism of most compounds while sorption and other physicochemical processes only played a minor role.
The multiple expertise of HypoTRAIN was fully exploited in two joint experiments where biogeochemical, hydrological and ecological approaches were coupled to quantify attenuation/degradation on both reach and local scales. An interactive map gives an overview on the datasets generated during the field experiments at a lowland river fed by treated wastewater in Germany:

A flume experiment which provided controlled external conditions aimed at comparing the effects of hyporheic microbial diversity and varying bedforms on reduction of pollutants (Fig. 3). Preliminary results for one micropollutant showed increased degradation in flumes with most hyporheic exchange and highest bacterial diversity.
A video describes the setup of the flume study:

Our results will be published in more than 50 peer-reviewed articles of which 14 are already published:

DOI: 10.1002/2016WR019195
DOI: 10.1002/ece3.3031
DOI: 10.1002/2017wr021144
DOI: 10.1016/j.scitotenv.2017.08.036
DOI: 10.1039/c8em00390d
DOI: 10.1038/s41598-018-34206-z
DOI: 10.1021/acs.est.8b03117
DOI: 10.5194/hess-22-6163-2018
DOI: 10.1029/2018WR023185
DOI: 10.1002/hyp.13350
DOI: 10.1029/2018WR022993
DOI: 10.1016/j.envsoft.2018.09.006
DOI: 10.1029/2018WR024609
DOI: 10.1021/acs.est.8b05488
Summarizing, we can eventually confirm that the HZ is a hotspot of biogeochemical processes which can significantly contribute to the removal or attenuation of pollutants. This effect strongly depends on hydrological fluxes transporting surface and groundwater into the river bed. Those fluxes enable the exposition of pollutants to the biogeochemical mechanisms driving their decrease in the HZ. It can be concluded that river restoration measures with a focus on reduction of pollutants should include features to increase hyporheic flow by creating physical barriers, e.g. woody deposits or rocks. These features will promote physico-chemical conditions favoring sorption and biodegradation in the HZ.
Furthermore, key microbial taxa associated with degradation of several organic micropollutants of interest have been identified using biological molecular techniques (qPCR, Next Generation Illumina sequencing). Their interaction with hyporheic geochemical parameters revealed that oxygen distribution directly influences the microbial guilds occupying particular HZ compartments. This in turn influences the degradation pathway of micropollutants reaching these microzones. Due to relatively low concentrations of micropollutants compared to other growth substrates, cometabolism is predictably an important biodegradation process in the HZ, compounded by an inadequate enzyme catalogue to degrade the ever-dynamic generation of new anthropogenic compounds. The data on the microbial community structure and response to micropollutants form a basis for optimization of conventional wastewater treatment and/or manipulations of receiving rivers.
We improved approaches to sample hyporheic water on extremely small vertical scales (e.g. mini-point samplers and an innovative passive sampling method). Analytical methods were advanced to quantify very low micropollutant concentrations. The development of “enantiomeric fractionation” enabled the differentiation of biodegradation processes from sorption and dilution. Furthermore, the performance of models describing and predicting hyporheic processes and the fate of pollutants was improved. At best, our results will facilitate the generation of better tools for assessing the effectiveness of restoration measures, river regulation, and the impact of climate change on hyporheic processes.