Using the radium quartet to quantify submarine groundwater discharge and porewater exchange

Rodellas, Valentí, Garcia-Orellana, Jordi, Trezzi, Giada, Masqué, Pere, Stieglitz, Thomas C., Bokuniewicz, Henry, Cochran, J. Kirk, and Berdalet, Elisa (2017) Using the radium quartet to quantify submarine groundwater discharge and porewater exchange. Geochimica et Cosmochimica Acta, 196. pp. 58-73.

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Abstract

The specific ingrowth rates of different radium isotopes make them valuable tracers to distinguish processes occurring at different temporal scales. Here we demonstrate the use of the radium quartet (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) to differentiate flows of submarine groundwater discharge and porewater exchange to a coastal embayment (Alfacs Bay, NW Mediterranean Sea), based on the assumption that these processes occur on different time scales. In order to evaluate the seasonal dynamics of groundwater and porewater inputs to the bay, we conducted three seasonal samplings at Alfacs Bay, during which samples for Ra isotopes were collected from bay waters, groundwater springs, porewaters and irrigation channels. Activities of short-lived Ra isotopes in the bay showed a strong seasonality, (e.g. average ²²⁴Ra activities in summer (∼32 dpm 100 L⁻¹) up to 4 times higher than in winter (∼8 dpm 100 L⁻¹)). In contrast, the activities of the long-lived Ra isotopes were fairly constant throughout the year (e.g. activities of ²²⁶Ra were ∼16 and ∼14 dpm 100 L⁻¹ in summer and winter, respectively). The relatively short exposure to sediments of recirculation fluxes resulted in porewaters significantly enriched in short-lived Ra isotopes relative to the long-lived ones (e.g. ²²⁴Ra = 1100–1300 dpm 100 L⁻¹; ²²⁶Ra = 17–99 dpm 100 L⁻¹), whereas coastal groundwaters were enriched in all the Ra isotopes (e.g. ²²⁴Ra = 120–150 dpm 100 L⁻¹; ²²⁶Ra = 200–400 dpm 100 L⁻¹). The distinct signatures of different sources allowed us to construct seasonal Ra mass balances to estimate both groundwater discharge, which ranges from (40 ± 60)·10³ m³·d⁻¹ in summer to (310 ± 200)·10³ m³·d⁻¹ in winter, and porewater exchange fluxes, ranging from (1200 ± 120)·10³ m³·d⁻¹ in summer to (270 ± 40)·10³ m³·d⁻¹ in winter. Whereas the seasonal variability of groundwater inputs is likely governed by the terrestrial hydraulic gradient, a qualitative evaluation of the drivers of porewater exchange suggested that the strong seasonality of the seawater recirculation inputs is likely mediated by seasonal cycles on the activity of benthic infauna. Ra isotopes are thus valuable tracers to differentiate fluxes of both submarine groundwater discharge and porewater exchange, allowing a more accurate evaluation of the fluxes of freshwater and solutes to coastal ecosystems, as well as their implications for coastal biogeochemical cycles.

Item ID: 46133
Item Type: Article (Research - C1)
ISSN: 1872-9533
Keywords: radium isotopes; submarine groundwater discharge; porewater exchange; bioirrigation; Mediterranean; Alfacs Bay
Funders: Ministry of Economy and Competetiveness, Spain (MINECO), Spanish Government (SG), European Union (EU), Campus France (CF), Generalitat de Catalunya to MERS (GC-MERS), Agence Nationale de la Recherche (ANR)
Projects and Grants: MINECO ECOALFACS (CTM2009-09581), SG PhD fellowship AP2008-03044, EU FP7 Marie Curie Actions PCOFUND-GA-2013-609102, CF PRESTIGE program, EU FP7 Marie Curie Actions METTRANS-ITN-290336, GC-MERS 2014 SGR-1356, ANR @RAction chair (ANR-14-ACHN-0007-01), ANR Labex OT-Med (ANR-11-LABEX-0061), ANR Investissements d'Avenir program through the A*MIDEX project (ANR-11-IDEX-0001-02)
Date Deposited: 21 Oct 2016 01:33
FoR Codes: 04 EARTH SCIENCES > 0406 Physical Geography and Environmental Geoscience > 040603 Hydrogeology @ 30%
04 EARTH SCIENCES > 0406 Physical Geography and Environmental Geoscience > 040608 Surfacewater Hydrology @ 30%
04 EARTH SCIENCES > 0405 Oceanography > 040503 Physical Oceanography @ 40%
SEO Codes: 96 ENVIRONMENT > 9609 Land and Water Management > 960903 Coastal and Estuarine Water Management @ 50%
96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961102 Physical and Chemical Conditions of Water in Coastal and Estuarine Environments @ 50%
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