Flood-Controlled Excess-Air Formation Favors Aerobic Respiration and Limits Denitrification Activity in Riparian Groundwater

Abstract

The saturated riparian zones of rivers act as spatially and temporally variable biogeochemical reactors. This complicates the assessment of biogeochemical transport and transformation processes. During a flood event, excess-air formation, i.e., the inclusion and dissolution of air bubbles into groundwater, can introduce high amounts of dissolved O 2 and thereby affect biogeochemical processes in groundwater. With the help of a field-installed membrane-inlet mass-spectrometer we resolved the effects of flood induced excess-air formation on organic carbon (OC) and nitrogen transformations in groundwater of different riparian zones of a restored section of the River Thur, Switzerland. The results show that the flood event triggered high aerobic respiration activity in the groundwater below a zone densely populated with willow plants. The flood introduced high concentrations of O 2 (230 µmol L −1 ) to the groundwater through the formation of excess air and transported up to ∼400 µmol L −1 OC from the soil/root layer into groundwater during the movement of the water table. A rapid respiration process, quantified via the measurements of O 2 , CO 2 , and noble-gas concentrations, led to fast depletion of the introduced O 2 and OC and to high CO 2 concentration (590 µmol L −1 ) in the groundwater shortly after the flood. The synchronous analysis of different nitrogen species allowed studying the importance of denitrification activity. The results indicate that in the willow zone excess-air formation inhibited denitrification through high O 2 concentration input. Instead, the observed decrease in nitrate concentration (∼50 µmol N L −1 ) may be related to fostered nitrate uptake by plants. In the other riparian zones closer to the river, no significant excess-air formation and corresponding respiration activity was observed. Overall, analyzing the dissolved gases in the groundwater significantly contributed to deciphering biogeochemical processes in the riparian aquifer characterized by pronounced changes in the flow regime and by spatial heterogeneity of the vegetation. Measuring excess-air formation helped identifying and explaining the low denitrification activity in a zone with high OC turnover and to quantify OC sources.

A Portable and Autonomous Mass Spectrometric System for On-Site Environmental Gas Analysis

“…Gases are therefore highly useful proxies to study the dynamics of environmental processes and can be of great utility to study the interplay of these processes. In particular, chemically inert gases (noble gases; He, Ne, Ar, Kr, Xe) were successfully used to study and quantify physical transport and mixing of fluids as well as the exchange between gas and water − or biofluids. , Such quantitative information on the physical processes controlling the dynamics of gases in environmental systems is required to quantify and interpret the turnover and the fate of biogeochemically active species (e.g., O 2 , CO 2 , CH 4 , N x O x ) in terms of chemical and biological processes ,,− …”

“…However, this gas depletion biases the partial pressures relative to the assumed gas/water equilibrium in the membrane contactor. While both approaches have illustrated the great potential of continuous analysis of selected gas species, ,,,,, the remaining analytical limitations of these instruments (limited sets of gas species, lack of absolute partial pressure analysis) and their bulkiness and high power consumption (several hundred watts) prevented widespread use in diverse environmental research applications in the field.…”

“…The currently applied methods for environmental gas analyses are usually targeted at a single 35 gas species or limited set of gases, and are often based on expensive and highly specialised instru-36 ments and techniques for sampling and subsequent laboratory based quantification. [36][37][38][39][40][41][42] Acquiring 37 data sets comprising a sufficiently complete suite of both inert and active gas species at the re-38 quired spatial and temporal resolutions therefore poses numerous practical limitations related to 39 field-work logistics, access to specialised laboratories and data turnaround time that often prevent 40 acquisition of suitable data sets.…”

Field Continuous Measurement of Dissolved Gases with a CF-MIMS: Applications to the Physics and Biogeochemistry of Groundwater Flow

“…However, their concentrations in natural waters can be significantly altered by biogeochemical reactions that typically occur in aquatic environments . Therefore, the measurement of these reactive dissolved gases is critical to the understanding of the biogeochemical reactivity of aquatic environments. − …”