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Abstract Interpreting the fate of wastewater contaminants in streams is difficult because their inputs vary in time and several processes synchronously affect reactive transport. We present a method to disentangle the various influences by performing a conservative-tracer test while sampling a stream section at various locations for chemical analysis of micropollutants. By comparing the outflow concentrations of contaminants with the tracer signal convoluted by the inflow time series, we estimated reaction rate coefficients and calculated the contaminant removal along a river section. The method was tested at River Steinlach, Germany, where 38 contaminants were monitored. Comparing day-time and night-time experiments allowed distinguishing photo-dependent degradation from other elimination processes. While photo-dependent degradation showed to be highly efficient for the removal of metroprolol, bisoprolol, and venlafaxine, its impact on contaminant removal was on a similar scale to the photo-independent processes when averaged over 24 h. For a selection of compounds analyzed in the present study, bio- and photodegradation were higher than in previous field studies. In the Steinlach study, we observed extraordinarily effective removal processes that may be due to the higher proportion of treated wastewater, temperature, DOC and nitrate concentrations, but also a higher surface to volume ratio from low flow conditions that favorizes photodegradation through the shallow water column and a larger transient storage than observed in comparable studies. Graphical abstract Unlabelled Image Highlights • Studying reactive processes in river channels is inherently difficult. • Tracer tests allowed separating conservative transport from the measured signal. • Comparison of day and night experiments highlighted photodegradation. • Photodegradation is weaker than photo-independent elimination on the long term. [ABSTRACT FROM AUTHOR]

Abstract The estimation of gas-exchange rates between streams and the atmosphere is of great importance for the fate of volatile compounds in rivers. For dissolved oxygen, this exchange process is called reaeration, and its accurate and precise estimation is essential for the quantification of metabolic rates. A common method for the determination of gas-exchange rates is through artificial gas-tracer tests with a proxy gas. We present the implementation of a portable gas-equilibrium membrane inlet mass spectrometer (GE-MIMS) to record concentrations of krypton and propane injected as tracer compound in the context of a gas-tracer test. The field-compatible GE-MIMS uses signals of atmospheric measurements for concentration standardization, and allows recording the dissolved-gas concentrations at a high temporal resolution, leading to overall low measurement uncertainty. Furthermore, the in-situ approach avoids loss of gas during the steps of sampling, transport, storage, and analysis required for ex-situ gas measurements. We compare obtained gas-exchange rate coefficients, reaeration and derived metabolic rates from the in-situ measurements to results obtained from head-space sampling of propane followed by laboratory analysis, and find much lower uncertainties with the in-situ method. Graphical Abstract Highlights • Determination of gas-exchange rates in streams from gas-tracer tests • Quantification of gas tracers using a portable gas-equilibrium mass spectrometer • Reliable estimation of reaeration rates from krypton and propane injections • In-situ analysis avoids gas loss and improves the determination of reaeration rates. [ABSTRACT FROM AUTHOR]

Improved understanding of stream solute transport requires meaningful comparison of processes across a wide range of discharge conditions and spatial scales. At reach scales where solute tracer tests are commonly used to assess transport behavior, such comparison is still confounded due to the challenge of separating dispersive and transient storage processes from the influence of the advective timescale that varies with discharge and reach length. To better resolve interpretation of these processes from field-based tracer observations, we conducted recurrent conservative solute tracer tests along a 1 km study reach during a storm discharge period and further discretized the study reach into six segments of similar length but different channel morphologies. The resulting suite of data, spanning an order of magnitude in advective timescales, enabled us to (1) characterize relationships between tracer response and discharge in individual segments and (2) determine how combining the segments into longer reaches influences interpretation of dispersion and transient storage from tracer tests. We found that the advective timescale was the primary control on the shape of the observed tracer response. Most segments responded similarly to discharge, implying that the influence of morphologic heterogeneity was muted relative to advection. Comparison of tracer data across combined segments demonstrated that increased advective timescales could be misinterpreted as a change in dispersion or transient storage. Taken together, our results stress the importance of characterizing the influence of changing advective timescales on solute tracer responses before such reach-scale observations can be used to infer solute transport at larger network scales. [ABSTRACT FROM AUTHOR]

Time-variable discharge is known to control both transport and transformation of solutes in the river corridor. Still, few studies consider the interactions of transport and transformation together. Here, we consider how diurnal discharge fluctuations in an intermittent, headwater stream control reach-scale solute transport and transformation as measured with conservative and reactive tracers during a period of no precipitation. One common conceptual model is that extended contact times with hyporheic zones during low discharge conditions allows for increased transformation of reactive solutes. Instead, we found tracer timescales within the reach were related to discharge, described by a single discharge-variable StorAge Selection function. We found that Resazurin to Resorufin (Raz-to-Rru) transformation is static in time, and apparent differences in reactive tracer were due to interactions with different ages of storage, not with time-variable reactivity. Overall we found reactivity was highest in youngest storage locations, with minimal Raz-to-Rru conversion in waters older than about 20 h of storage in our study reach. Therefore, not all storage in the study reach has the same potential biogeochemical function and increasing residence time of solute storage does not necessarily increase reaction potential of that solute, contrary to prevailing expectations. [ABSTRACT FROM AUTHOR]