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

Abstract

Riparian zones are highly-dynamic transition zones between surface water (SW) and groundwater (GW) and function as key biogeochemical-reactors for solutes transitioning between both compartments. Infiltration of SW rich in dissolved oxygen (DO) into the riparian aquifer can supress removal processes of redox sensitive compounds like NO 3 − , a nutrient harmful for the aquatic ecosystem at high concentrations. Seasonal and short-term variations of temperature and hydrologic conditions can influence biogeochemical reaction rates and thus the prevailing redox conditions in the riparian zone. We combined GW tracer-tests and a 1-year high-frequency dataset of DO with data-driven simulations of DO consumption to assess the effects of seasonal and event-scale variations in temperature and transit-times on the reactive transport of DO. Damköhler numbers for DO consumption (DA DO) were used to characterize the system in terms of DO turnover potential. Our results suggest that seasonal and short-term variations in temperature are major controls for DO turnover and the resulting concentrations at our field site, while transit-times are of minor importance. Seasonal variations of temperature in GW lead to shifts from transport-limited (DA DO > 1) to reaction-limited conditions (DA DO < 1), while shortterm events were found to have minor impacts on the state of the system, only resulting in slightly less transport-limited conditions due to decreasing temperature and transit-times. The data-driven analyses show that assuming constant water temperature along a flowpath can lead to an over-or underestimation of reaction rates by a factor of 2-3 due to different infiltrating water temperature at the SW-GW interface, whereas the assumption of constant transit-times results in incorrect estimates of NO 3 − removal potential based on DA DO approach (40%-50% difference).

Transit‐Time and Temperature Control the Spatial Patterns of Aerobic Respiration and Denitrification in the Riparian Zone

“…Moreover, infiltrating SW with different temperature from GW can affect subsurface reaction rate coefficients, which can be especially important within the first few meters around the streambed (Engelhardt et al., 2013; Munz et al., 2017; Nogueira et al., 2021; Song et al., 2018). But we claim that, due to strongly damped higher frequency heat signals from the stream, the reaction rate coefficients and the overall DO z around the stream would not change to a great extent as they are mainly driven by the seasonal temperature variations (Figure S5 in Supporting Information ).…”

“…Detailed descriptions of these experiments can be found in Nogueira et al. (2021). DO concentrations were monitored by self‐contained loggers (HOBO Dissolved Oxygen Data Logger) in both stream and in groundwater with a 10‐min interval.…”

“…A rising stream stage will increase the hydraulic gradients between the stream and local groundwater (GW), increasing stream water (SW) infiltration fluxes and reducing the transit‐times of infiltrating water parcels. Additionally, short‐term and seasonal temperature variations affect biogeochemical reaction rates, leading to different chemical patterns depending on the combination of transport and reaction processes at a given time and location (Nogueira et al., 2021; Song et al., 2018; Vieweg et al., 2016), which are often not explicitly considered in studies on SW‐GW interactions.…”

Aerobic respiration in riparian exchange zones of regulated river corridors

“…found that incorporating seasonal temperature changes was key for modelled oxygen and nitrogen concentrations in the riparian zone to match field observations. The importance of seasonal temperature fluctuations on riparian zone oxygen and nitrogen dynamics is also supported byNogueira et al, 2021, who found that seasonal temperature variations exert a strong control on riparian DO conditions and nitrogen reaction rates. Future work could incorporate temperature dynamics into reactive transport modelling to assess the influence of seasonal or shortterm temperature conditions on riparian zone respiration.…”

Advancing measurements and representations of subsurface heterogeneity and dynamic processes: towards 4D hydrogeology

“…Based on such data, the strong temperature dependence of aerobic respiration rates has been demonstrated (Diem et al 2014), which may dominate turnover rates compared to the effects of variable transit times (Nogueira et al,2021a). Similar effects were found to affect complex spatio-temporal patterns of riparian denitrification, which seem to be jointly controlled by hydraulically driven variability in exchange fluxes and transit times, supply of organic carbon as an electron donor from stream water and riparian sediments and seasonal temperature variations (Trauth et al 2018, Lutz et al 2020, Nogueira et al 2021b. Besides high-resolution data sets of key variables like oxygen concentration, it is often the combination of these rich data sets with innovative methods for analysis and modeling (e.g.…”