Evaluating the reliability of time series analysis to estimate variable riparian travel times by numerical groundwater modelling

Abstract

The transition zones between rivers and adjacent riparian aquifers are locations of high biogeochemical activities that contribute to a removal of potentially hazardous substances in the aquatic system. The potential of the removal processes depends highly on subsurface water travel times, which can be determined by using the propagation of electrical conductivity (EC) signal from the river into the riparian aquifer. Although this method has been applied and verified in many studies, we observe possible limitations for the usage of EC fluctuation analysis. Our findings are based on EC time series analyses during storm events and artificial hydropeaks induced by watermill operations. Travel times derived by cross‐correlation analysis were compared with travel times calculated based on backward particle tracking of a calibrated transient numerical groundwater flow model. The cross‐correlation method produced only reasonable travel times for the artificial hydropeaks. In contrast, cross‐correlation analysis of the EC data during natural storm events resulted in implausibly negative or unrealistically low travel times for the bulk of the data sets. We conclude that the reason for this behaviour is, first, the low EC contrast between river and groundwater in connection with a strong damping of the infiltrating river EC signal into the subsurface during storm events. Second, the existence of old and less‐mineralized riparian water between the river and the monitoring well resulted in bank‐storage‐driven EC breakthrough curves with earlier arrival times and the subsequent estimation of implausible riparian travel times.

Seasonal and short‐term controls of riparian oxygen dynamics and the implications for redox processes

“…deep DA DO were computed according to theoretical transit-times; (b) boxplot of high-frequency DA DO values presented in (a) following their same notation signals were strongly dampened in the observation wells, limiting the use of signal analysis methods for the EC data. Additionally, Nixdorf and Trauth (2018) had argued that for using EC timeseries to calculate riparian transit-times, EC variations and their propagation into the subsurface are a necessary but not sufficient condition to constrain travel times. Additionally, it is also required EC variations to be higher relative to stream discharge variations for successful application of EC-based transit-times analysis.…”

“…This study extends these previous studies and aims to address major features controlling spatio‐temporal variations of transit‐times and DO consumption rates in riparian aquifers with a specific focus on the interplay between seasonal and event‐scale variability. A fourth‐order reach of the Selke Stream, a well instrumented and studied site (Nixdorf & Trauth, 2018; Vieweg et al, 2016) within the Bode Observatory of the TERENO initiative (Wollschläger et al, 2017) is used as a test case. At the site, we combined GW tracer‐tests with high‐frequency data of water levels, EC, and DO in the stream and in groundwater.…”

Effects of Heterogeneous Stream‐Groundwater Exchange on the Source Composition of Stream Discharge and Solute Load

“…(2017), the Manning's roughness coefficient is set to 0.022 and the hydraulic conductivity of the streambed is set to 1.16 × 10 −5 –1.16 × 10 −4 m/s with a decreasing trend toward the outlet. According to the model setup in Nixdorf and Trauth (2018) and values measured in the field, the width of the Hauptseegraben channel is set to 3 m and the width of the Selke River is 1–10 m. The streambed thickness is assumed to be 1 m. Because the Hauptseegraben channel connects all the three lakes, for simplification, this stream is simulated starting from the discharge point connecting Concordia Lake with a specified inflow rate of 0.1 m 3 /s, which is the average abstraction rate from the Concordia Lake from 2014 to 2017. More details about the stream parameter could be found in Table .…”

How Important is Denitrification in Riparian Zones? Combining End‐Member Mixing and Isotope Modeling to Quantify Nitrate Removal from Riparian Groundwater

“…Water Resources Research proximity of the Selke River due to the meandering river channel depending on hydrologic conditions and channel morphology (Nixdorf & Trauth, 2018). Chloride concentrations in riparian groundwater and the river suggest that the A and B-N transects are less impacted by infiltrating river water compared to the other well clusters (Figure 1b; Trauth et al, 2018).…”

“…Fertilizer application on agricultural fields is the main N source in the catchment (Rode et al, 2016). Previous studies in the field site area have analyzed hyporheic exchange (Trauth et al, 2015; Trauth & Fleckenstein, 2017), hydrochemical gradients between the unsaturated zone and groundwater (Gassen et al, 2017), riparian travel times (Nixdorf & Trauth, 2018), and NO 3 − removal in the riparian zone (Trauth et al, 2018).…”