Reactive transport modeling of biogeochemical dynamics in subterranean estuaries: Implications for submarine groundwater discharge of nutrients
Notice bibliographique
Résumé
The quality of groundwater, in particular in coastal areas, is increasingly deteriorating due to the input of nutrients (NO3-, NH4+ and PO4) from septic systems and agricultural leaching. The discharge of groundwater to coastal waters, termed submarine groundwater discharge (SGD), is now recognized as an important transport pathway of nutrients from the land to the sea. The chemical composition of SGD is not only determined by the landward sources, but also by the biogeochemical reactions that occur when groundwater travels through the subterranean estuary, the zone in a coastal aquifer where freshwater and seawater interact. The aims of this thesis are: (1) To identify and quantify the biogeochemical processes that remove or transform nutrients in subterranean estuaries. (2) To assess the impact of landward source concentrations, aquifer characteristics, groundwater flow rates and dispersive transport on nutrient transformations in coastal aquifers and on the SGD of nutrients. The methodology used throughout this study is reactive transport modeling. First, a 1D finite difference reactive transport model (RTM) is used to describe the biogeochemistry of of phosphorus (P) in two oxic, electron donor-poor, Canadian aquifers, characterized by different aquifer material (calcareous and iron oxide-rich). The aquifer material, in particular iron oxide, is found to play a major role in the downgradient attenuation of P through sorption, whereas N, mainly in the form of NO3-, travels conservatively without major removal through denitrification. The investigation of the role of iron oxides in controlling phosphate concentrations is then extended to a subterranean estuary subject to saltwater intrusion. The behaviour of P along physico-chemical gradients of ionic strength and pH which develop during estuarine mixing, is simulated using an electrostatic surface complexation model (SCM) coupled to a 2D variable-density finite-element RTM. The transient evolution of phosphate sorption upon saltwater intrusion in a coastal aquifer and the subsequent mobilization of phosphate related to the sharp increase in groundwater pH are investigated. In many coastal aquifers, the sorption capacity of phosphate is related to the occurrence of zones of iron oxide accumulation, often referred to as “Iron Curtains”. The mechanism which drives the formation of such an “Iron Curtain” in the mixing zone of Waquoit Bay, MA, USA and the subsequent attenuation of P are studied. A 2D density-dependent flow coupled to a biogeochemical model is then applied to the extensive field data along a beach transect in the same study site, to help unravel the nutrient biogeochemical dynamics in this subterranean estuary. Results show that the conditions required for denitrification are not met and thus, SGD in Waquoit Bay is mainly a source of NO3- to the coastal zone. As illustrated by this field study, redox conditions strongly determine the extent of nutrient transformation/removal in subterranean estuaries. Therefore, four generic model subterranean estuaries, representing the end members of oxic/anoxic aquifer and seawater redox conditions are defined, to investigate further the role of organic matter reactivity, alternative denitrification pathways, as well as groundwater flow velocity and transverse dispersivity on the nutrient distributions and fluxes to the coastal waters.
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Comment cette classification a été obtenuedéplier
Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,001 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,001 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,000 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».