Groundwater‐surface water exchange as key control for instream and groundwater nitrate concentrations along a first‐order agricultural stream

Abstract

Lower‐order streams (first‐ and second‐order) define the initial, landscape‐related, chemical signature of stream water in catchments. To date, first‐order streams have been perceived as predominantly draining systems, which collect water and solutes from the surrounding groundwater and surface runoff and simply mirror the chemical composition of the inputs. In this study, the impact of stream‐groundwater exchange fluxes on water chemistry of a first order agricultural stream (Schönbrunnen) and its connected groundwater in south‐western Germany was assessed combining 222Rn, dissolved ions (chloride, sulphate, nitrate), and salt tracer tests with investigations of stream discharge and groundwater hydraulic gradients. The findings suggest that stream‐water chemistry in lower‐order streams is governed by an intricate interplay between dynamic, bidirectional water and solute exchange between groundwater and the stream leading to a pronounced hydrologic turnover along the studied reaches. High nitrate concentrations (up to 79 mg/L as NO3−) in stream water were attenuated in downstream direction (a mean value of 39 mg/L) without an increase in discharge, suggesting that redox processes occurring during sediment passage in sequential infiltration and exfiltration zones affect stream water chemistry. Nitrate in stream water infiltrating into the aquifer at distinct losing spots was subject to denitrification within the first few decimetres of the streambed, while concurrent exfiltration of low‐nitrate groundwater into the stream at gaining spots compensated for flow losses and in turn diluted instream nitrate concentrations. In summary the findings imply that (1) instream mixing resulting from the bidirectional exchange of water between groundwater and the stream (hydrologic turnover) affects instream nitrate concentrations, (2) denitrification in the streambed of losing reaches and the near‐stream aquifer significantly contributes to reactive nitrate turnover and elimination, and (3) oxidation of ammonium could be a secondary source of nitrate inputs into the stream.

Assessing impacts of cemeteries on water quality in an urban headwater watershed with mixed human‐built infrastructure

“…However, the high‐concentration cemetery groundwater carries enough nitrate relative to the stream water to offset the nitrate losses that are associated with a net loss of discharge and result in increased nitrate loads during these time periods. Jimenez‐Fernandez et al (2022) describes a headwater stream undergoing various hyporheic exchange patterns including both net gaining from groundwater input and net losing to groundwater input throughout the 500‐meter reach studied. Meadowbrook likely undergoes a similar pattern throughout the cemetery and future research should identify more specific areas of the gaining and losing stream.…”

“…This reach likely experiences dynamic hyporheic exchange with a small net loss of stream water to groundwater during this time. However, the highconcentration cemetery groundwater carries enough nitrate relative to the stream water to offset the nitrate losses that are associated with a net loss of discharge and result in increased nitrate loads during these time periods Jimenez-Fernandez et al (2022). describes a headwater stream undergoing various hyporheic exchange patterns including both net gaining from groundwater input and net losing to groundwater input throughout the 500-meter reach studied.…”

Catchment properties shape seasonal variation in groundwater- surface water interaction - geogenic silicate as a proxy for hydrological turnover induced mixing

“…These reaches display distinct geomorphological settings, evident in their valley cross-section profiles (Figure 4), leading to contrasting hydrological conditions. HT magnitudes can vary across short distances (Jimenez-Fernandez et al, 2022). To address this, we designed the experimental setup for both total reach and sub-reach HT estimations (Figure 2).…”

“…In other studies, HT measurements were carried out in one catchment during summer baseflow recession (Payn et al, 2009), in four sets under baseflow conditions (Ward et al, 2013), in two short campaigns at baseflow conditions in summer and winter (Jimenez-Fernandez et al, 2022), or in seven campaigns at different streams over one hydrological year (Jäkhel et al, 2022). We present a dataset where all these periods are covered by measurements during two hydrological years.…”

Catchment Properties Shape Seasonal Variation in Groundwater‐Surface Water Interaction—Geogenic Silica as a Proxy for Hydrological Turnover Induced Mixing

“…Altogether, these processes induce the spatial and temporal variability of HT processes. However, reach specific features on, that is, hillslope topography, geology, vegetation, and valley bottom structure influence streamflow dynamics and GW‐SW interaction (Bergstrom et al., 2016; Jähkel et al., 2022; Jimenez‐Fernadez et al., 2021) studies measuring HT at the reach and catchment scale show that discharge can explain the variance in HT quite well (Covino et al., 2011; Mallard et al., 2014).…”