Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo
Autor(a) principal: | |
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Data de Publicação: | 2020 |
Outros Autores: | , , |
Tipo de documento: | Artigo |
Idioma: | eng |
Título da fonte: | Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos) |
Texto Completo: | http://hdl.handle.net/10773/37613 |
Resumo: | The groundwater chemistry of the semi-arid volcanic island of Porto Santo, part of the Madeira archipelago, Atlantic Ocean, was investigated. Generally, the groundwater was brackish, containing 2–10 mol % seawater. Groundwater with up to 20 mM alkalinity and a Na enrichment of up to 30 mM, as compared to the Na concentration predicted by the seawater Na/Cl ratio, was found in the main aquifer. Also notable are the high concentrations of F (up to 0.3 mM), B (up to 0.55 mM), As (up to 0.35 μM), all in excess of WHO recommendations, as well as up to 6 μM V. Geochemical modeling, using the PHREEQC code, was used to explore different scenarios that could explain the genesis of the observed bulk groundwater chemistry. First, a model for aquifer freshening with the displacement of resident seawater from the aquifer by infiltrating freshwater, was tested. This scenario leads to the development of NaHCO3 waters as observed in many coastal aquifers. However, the measured alkalinity concentration in the groundwater was far higher than the concentration predicted by the freshening model. In addition, the behavior of modelled pH and PCO2 were at variance with their distributions in the field data. The second model explored the possible effect of volcanic glass leaching on the groundwater chemistry. Using insight derived from studies of volcanic glass surface alteration as well as experimental work on water-volcanic glass interactions, a geochemical model was developed in which the exchange of H+ for Na+ on the volcanic glass surface is the main mechanism but the exchange of other cations on the volcanic glass surface is also included. The uptake of H+ by the glass surface causes the dissociation of carbonic acid, generating bicarbonate. This model is consistent with the local geology and the field data. It requires, however, volcanic glass leaching to occur in the unsaturated zone where there is an unlimited supply of CO2. The exchange reaction of H+ for Na+ is confined to the surface layer of volcanic glass as otherwise the process becomes limited by slow solid state diffusion of H+ into the glass and Na+ out of the glass. Therefore, volcanic ash deposits, with their high volcanic glass surface areas and matrix flow, are the aquifers where this type of high NaHCO3 waters can be expected, rather than in basalts, which predominantly feature fracture flow. The trace components F, B, As and V are believed to originate from hyaloclastites, consisting of predominantly (90%) of trachy-rhyolite volcanic glass. Although stratigraphically older than the main calcarenite aquifer, topographically they are often located at higher altitudes, above the phreatic level and located along the main recharge flow path. In addition, the semi-arid climate conditions provide a long groundwater residence time for the reactions as well as limited aquifer flushing. |
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Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto SantoGroundwaterGeochemistryVolcanicsGlassWeatheringAridSodiumBicarbonateFluorideArsenicBoronVanadiumModelThe groundwater chemistry of the semi-arid volcanic island of Porto Santo, part of the Madeira archipelago, Atlantic Ocean, was investigated. Generally, the groundwater was brackish, containing 2–10 mol % seawater. Groundwater with up to 20 mM alkalinity and a Na enrichment of up to 30 mM, as compared to the Na concentration predicted by the seawater Na/Cl ratio, was found in the main aquifer. Also notable are the high concentrations of F (up to 0.3 mM), B (up to 0.55 mM), As (up to 0.35 μM), all in excess of WHO recommendations, as well as up to 6 μM V. Geochemical modeling, using the PHREEQC code, was used to explore different scenarios that could explain the genesis of the observed bulk groundwater chemistry. First, a model for aquifer freshening with the displacement of resident seawater from the aquifer by infiltrating freshwater, was tested. This scenario leads to the development of NaHCO3 waters as observed in many coastal aquifers. However, the measured alkalinity concentration in the groundwater was far higher than the concentration predicted by the freshening model. In addition, the behavior of modelled pH and PCO2 were at variance with their distributions in the field data. The second model explored the possible effect of volcanic glass leaching on the groundwater chemistry. Using insight derived from studies of volcanic glass surface alteration as well as experimental work on water-volcanic glass interactions, a geochemical model was developed in which the exchange of H+ for Na+ on the volcanic glass surface is the main mechanism but the exchange of other cations on the volcanic glass surface is also included. The uptake of H+ by the glass surface causes the dissociation of carbonic acid, generating bicarbonate. This model is consistent with the local geology and the field data. It requires, however, volcanic glass leaching to occur in the unsaturated zone where there is an unlimited supply of CO2. The exchange reaction of H+ for Na+ is confined to the surface layer of volcanic glass as otherwise the process becomes limited by slow solid state diffusion of H+ into the glass and Na+ out of the glass. Therefore, volcanic ash deposits, with their high volcanic glass surface areas and matrix flow, are the aquifers where this type of high NaHCO3 waters can be expected, rather than in basalts, which predominantly feature fracture flow. The trace components F, B, As and V are believed to originate from hyaloclastites, consisting of predominantly (90%) of trachy-rhyolite volcanic glass. Although stratigraphically older than the main calcarenite aquifer, topographically they are often located at higher altitudes, above the phreatic level and located along the main recharge flow path. In addition, the semi-arid climate conditions provide a long groundwater residence time for the reactions as well as limited aquifer flushing.Elsevier2023-05-09T14:54:34Z2020-03-01T00:00:00Z2020-03info:eu-repo/semantics/publishedVersioninfo:eu-repo/semantics/articleapplication/pdfhttp://hdl.handle.net/10773/37613eng0883-292710.1016/j.apgeochem.2019.104470Condesso de Melo, Maria TeresaShandilya, Raghwendra NarayanSilva, João Baptista PereiraPostma, Diekeinfo:eu-repo/semantics/openAccessreponame:Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)instname:Agência para a Sociedade do Conhecimento (UMIC) - FCT - Sociedade da Informaçãoinstacron:RCAAP2024-02-22T12:12:47Zoai:ria.ua.pt:10773/37613Portal AgregadorONGhttps://www.rcaap.pt/oai/openaireopendoar:71602024-03-20T03:08:13.627776Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos) - Agência para a Sociedade do Conhecimento (UMIC) - FCT - Sociedade da Informaçãofalse |
dc.title.none.fl_str_mv |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
title |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
spellingShingle |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo Condesso de Melo, Maria Teresa Groundwater Geochemistry Volcanics Glass Weathering Arid Sodium Bicarbonate Fluoride Arsenic Boron Vanadium Model |
title_short |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
title_full |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
title_fullStr |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
title_full_unstemmed |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
title_sort |
Volcanic glass leaching and the groundwater geochemistry on the semi-arid Atlantic island of Porto Santo |
author |
Condesso de Melo, Maria Teresa |
author_facet |
Condesso de Melo, Maria Teresa Shandilya, Raghwendra Narayan Silva, João Baptista Pereira Postma, Dieke |
author_role |
author |
author2 |
Shandilya, Raghwendra Narayan Silva, João Baptista Pereira Postma, Dieke |
author2_role |
author author author |
dc.contributor.author.fl_str_mv |
Condesso de Melo, Maria Teresa Shandilya, Raghwendra Narayan Silva, João Baptista Pereira Postma, Dieke |
dc.subject.por.fl_str_mv |
Groundwater Geochemistry Volcanics Glass Weathering Arid Sodium Bicarbonate Fluoride Arsenic Boron Vanadium Model |
topic |
Groundwater Geochemistry Volcanics Glass Weathering Arid Sodium Bicarbonate Fluoride Arsenic Boron Vanadium Model |
description |
The groundwater chemistry of the semi-arid volcanic island of Porto Santo, part of the Madeira archipelago, Atlantic Ocean, was investigated. Generally, the groundwater was brackish, containing 2–10 mol % seawater. Groundwater with up to 20 mM alkalinity and a Na enrichment of up to 30 mM, as compared to the Na concentration predicted by the seawater Na/Cl ratio, was found in the main aquifer. Also notable are the high concentrations of F (up to 0.3 mM), B (up to 0.55 mM), As (up to 0.35 μM), all in excess of WHO recommendations, as well as up to 6 μM V. Geochemical modeling, using the PHREEQC code, was used to explore different scenarios that could explain the genesis of the observed bulk groundwater chemistry. First, a model for aquifer freshening with the displacement of resident seawater from the aquifer by infiltrating freshwater, was tested. This scenario leads to the development of NaHCO3 waters as observed in many coastal aquifers. However, the measured alkalinity concentration in the groundwater was far higher than the concentration predicted by the freshening model. In addition, the behavior of modelled pH and PCO2 were at variance with their distributions in the field data. The second model explored the possible effect of volcanic glass leaching on the groundwater chemistry. Using insight derived from studies of volcanic glass surface alteration as well as experimental work on water-volcanic glass interactions, a geochemical model was developed in which the exchange of H+ for Na+ on the volcanic glass surface is the main mechanism but the exchange of other cations on the volcanic glass surface is also included. The uptake of H+ by the glass surface causes the dissociation of carbonic acid, generating bicarbonate. This model is consistent with the local geology and the field data. It requires, however, volcanic glass leaching to occur in the unsaturated zone where there is an unlimited supply of CO2. The exchange reaction of H+ for Na+ is confined to the surface layer of volcanic glass as otherwise the process becomes limited by slow solid state diffusion of H+ into the glass and Na+ out of the glass. Therefore, volcanic ash deposits, with their high volcanic glass surface areas and matrix flow, are the aquifers where this type of high NaHCO3 waters can be expected, rather than in basalts, which predominantly feature fracture flow. The trace components F, B, As and V are believed to originate from hyaloclastites, consisting of predominantly (90%) of trachy-rhyolite volcanic glass. Although stratigraphically older than the main calcarenite aquifer, topographically they are often located at higher altitudes, above the phreatic level and located along the main recharge flow path. In addition, the semi-arid climate conditions provide a long groundwater residence time for the reactions as well as limited aquifer flushing. |
publishDate |
2020 |
dc.date.none.fl_str_mv |
2020-03-01T00:00:00Z 2020-03 2023-05-09T14:54:34Z |
dc.type.status.fl_str_mv |
info:eu-repo/semantics/publishedVersion |
dc.type.driver.fl_str_mv |
info:eu-repo/semantics/article |
format |
article |
status_str |
publishedVersion |
dc.identifier.uri.fl_str_mv |
http://hdl.handle.net/10773/37613 |
url |
http://hdl.handle.net/10773/37613 |
dc.language.iso.fl_str_mv |
eng |
language |
eng |
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0883-2927 10.1016/j.apgeochem.2019.104470 |
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info:eu-repo/semantics/openAccess |
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openAccess |
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application/pdf |
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Elsevier |
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Elsevier |
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Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos) |
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