Abstract
AbstractRiparian 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 NO3−, 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 (DADO) 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 (DADO > 1) to reaction‐limited conditions (DADO < 1), while short‐term 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 NO3− removal potential based on DADO approach (40%–50% difference).
Highlights
The notion that surface water (SW) and groundwater (GW) should be perceived as one entity rather than two separate components has been established over the last 20 years (Fleckenstein et al, 2010; Winter et al, 1998)
In order to assess effects of short-term fluctuations of temperature and stream discharge on riparian dissolved oxygen (DO) dynamics, which are likely not captured by tracer-tests, we extended the analyses of GW transittimes and DO consumption rates to a high-frequency dataset
As denitrification is known to occur at the site (Trauth et al, 2018), we can estimate the NO3− fractions remaining after a water parcel has travelled through the riparian zone directly from Damköhler numbers for DO consumption (DADO) values taking into account that sufficient dissolved organic carbon (DOC) is available for the process after DO depletion
Summary
The notion that surface water (SW) and groundwater (GW) should be perceived as one entity rather than two separate components has been established over the last 20 years (Fleckenstein et al, 2010; Winter et al, 1998). Nixdorf and Trauth (2018) reported that the observed EC-signals in some riparian wells at losing stream sections could be attributed to direct surface water infiltration, but were instead affected by stream water that had previously been stored in the banks They found the estimated transit-times to be as much as two orders of magnitude lower compared to ‘true’ transit-times derived from introduced (e.g., natural gradient) tracer-tests. These complex interactions are difficult to characterize in the field and it is not surprising that to date only a few studies have attempted to explore them in field studies of GW– SW systems (Vieweg et al, 2016; Zarnetske et al, 2012) 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. The relationship between DO consumption rates and transit-times was evaluated using the concept of Damköhler numbers in order to characterize the reactive state of the system
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