Submarine groundwater discharge in a subsiding coastal lowland: A 226Ra and 222Rn investigation in the Southern Venice lagoon

Gattacceca, Julie C. and Mayer, Adriano and Cucco, Andrea and Claude, Christelle and Radakovitch, Olivier and Vallet-Coulomb, Christine and Hamelin, Bruno (2011) Submarine groundwater discharge in a subsiding coastal lowland: A 226Ra and 222Rn investigation in the Southern Venice lagoon. Applied Geochemistry, 26 (5). pp. 907-920. DOI 10.1016/j.apgeochem.2011.03.001

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Abstract

Several recent studies have suggested that submarine groundwater discharge (SGD) occurs in the Venice lagoon with discharge rates on the same order or larger than the surface runoff, as demonstrated previously in several other coastal zones around the world. Here, the first set of 222Rn data, along with new 226Ra data are reported, in order to investigate the occurrence and magnitude of SGD specifically in the southern basin of the lagoon. The independent connection with the Adriatic Sea (at the Chioggia inlet), in addition to the relative isolation of the water body from the main lagoon, make this area an interesting case study. There is probably only minimal fresh groundwater flux to the lagoon because the surrounding aquifer is subsiding and mainly has a lower hydraulic head than seawater.<br/>The data show that the Ra and Rn activities are in slight excess in the lagoon compared to the open sea, with values on the same order as those observed in the northern and central basins. Taking into account the water exchange rate between the lagoon and adjacent seawater provided by previous hydrodynamic numerical modelling, it is shown that this excess cannot be supported at steady state by only riverine input and by diffusive release from the sediment interstitial water. High activities observed in groundwater samples collected from 16 piezometers tapping into the shallow aquifer over the coastal lowland substantiate that the excess radioactivity in the lagoon may indeed be due to the advection of groundwater directly into the lagoon bottom water through the sediment interface. However, the data show that the groundwater composition is extremely heterogeneous, with high Ra activities concentrated within a narrow coastal strip where the contact between fresh and saline water takes place, while Rn strongly decreases when approaching the lagoon shore across the 20 km coastal plain. Assuming that the average groundwater activities measured in the coastal strip are representative of the SGD composition, a SGD flux of 7.7 ± 3.5 × 105 and 2.5 ± 2 × 106 m3/d is calculated using a 226Ra and 222Rn budget, respectively, (i.e. about 1-3 times the surface runoff), substantially lower than in previous studies. The influence of all assumptions on SGD estimates (groundwater heterogeneity, diffusive sediment flux, one-box versus multi-boxes model calculations) is discussed, and a sensitivity analysis of the influence of imperfect exchange and mixing at the lagoon outlets that affects the lagoon composition is provided. Finally, the results confirm that the SGD flux, calculated with these assumptions, is largely (~80%) composed of saline lagoon water circulating through the sediment under the lagoon margin, and that the fresh water discharge associated with SGD is at most a minor term in the lagoon hydrologic balance.

Item Type: Article
Uncontrolled Keywords: NIL AREP;
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Journal or Publication Title: Applied Geochemistry
Volume: 26
Page Range: pp. 907-920
Identification Number: 10.1016/j.apgeochem.2011.03.001
Depositing User: Sarah Humbert
Date Deposited: 04 Jun 2011 13:23
Last Modified: 23 Jul 2013 10:02
URI: http://eprints.esc.cam.ac.uk/id/eprint/2075

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