Noble Gases to Analyse the Physical Mechanisms Influencing Arsenic Dynamics in Groundwater
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Date
2023Type
- Doctoral Thesis
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Abstract
This work focuses on the application of noble gases as tracers in an arsenic contaminated aquifer situated along the Red River delta in Vietnam. Noble gases are utilised to evaluate the gas and groundwater dynamics in the contaminated aquifer, with the aim to improve knowledge on the physical mechanism(s) controlling As transport and distribution. Since the inert property of noble gases ensures they are solely affected by physical processes, separation from any biogeochemical processes, which additionally affect As mobilisation, can thus be achieved.
The main source of noble gas concentrations in groundwater of the contaminated aquifer, are derived from the atmosphere and are therefore mainly dependant on the local conditions (i.e., temperature, salinity and atmospheric pressure) from the last moment the sampled groundwater parcel was in contact with the atmosphere. Deviation from these expected, atmospherically derived concentrations allows identification of the specific physical processes affecting gas concentrations in groundwater. The major physical processes identified via noble gas analysis in the groundwater of this study are: excess air formation, radiogenic decay and degassing.
The most important of these identified processes relevant to this work is degassing, which refers to the loss of dissolved atmospheric gases in groundwater, as a result of a newly formed free gas phase. Such gas phases can be formed through over-saturation of biogeochemical gases produced within an aquifer. Noble gases in the groundwater partition into the newly produced gas phase or gas bubbles, which is subsequently removed from the aquifer, leaving behind groundwater depleted in atmospheric gases.
Importantly, the observation of a free gas phase present within the studied aquifer, results in a hydraulically slowed zone where the gas phase (or gas bubbles) are present. The area locally producing the gas bubbles behaves as an interruption of the dominating groundwater flow path, affecting As transport and distribution. Further, the degassed groundwater has been shown to correlate alongside high CH₄ production, which in-turn has been reported (also in other study sites) to occur concomitantly with As mobilisation, although often with an incomplete understanding of the CH₄-As relationship. A leading hypothesis offered in this thesis, is that As mobilised within high CH₄ producing zones, will correlate positively with CH₄ since As (and solute) transport out of such zones is buffered due to the reduced hydraulic conductivity.
However, it is important to note that specific solutes (i.e., organic carbon) are additionally required to sustain both As mobilisation and CH₄ production. Noble gases acquired in the pore space of the overlying sediments of the contaminated aquifer, thus additionally provided some insights into the potential input of solutes and the impact on bubble formation in the CH₄ producing zones of the studied aquifer. These overlying sediments show a stratification of alternating saturated and unsaturated layers, effectively reducing the hydrostatic load on the underlying aquifer. A reduced hydrostatic pressure further promotes free gas formation within the aquifer as the specific saturation concentration of dissolved gases is lowered.
Finally, obtaining groundwater residence times to better constrain groundwater flow and thus, As transport, proved to be highly challenging, which is a common issue where groundwater is degassed. Attempting to correct for degassing provided mixed results, with about only 50% of the total number of samples taken able to yield robust information on groundwater residence times. However, in light of the highly dynamic nature of the groundwater at the studied field site, groundwater flow can be qualitatively understood additionally in terms of the spatial distribution of excess air, degassing and the accumulation of radiogenic ⁴He in the specific (different) parts of the studied aquifer. Collectively, such observations have allowed for an improved understanding on groundwater flow at the study site, even with a restricted understanding on the groundwater residence times.
he groundwater residence times. In conclusion, this work provides insight into how the physical processes identified, can impact As mobilisation, transport and spatial heterogeneity in the studied aquifer. Further, a physical framework is conceptualised onto which the biogeochemistry of the studied aquifer can be built, and which aims to aid understanding of the physical processes affecting As distribution, in other highly biogeochemically active aquifers. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000602341Publication status
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Contributors
Examiner: Kipfer, Rolf
Examiner: Casacuberta, Núria
Examiner: Berg, Michael
Examiner: Brennwald, Matthias
Publisher
ETH ZurichOrganisational unit
02350 - Dep. Umweltsystemwissenschaften / Dep. of Environmental Systems Science
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