Gwénaëlle Chaillou

Dose-dependent response of a benthic system to biodeposition from suspended blue mussel (Mytilus edulis) culture.

This study reports the results of a field experiment using benthic mesocosms that examined dose-dependent effects of mussel biodeposition on the benthic environment. Mesocosms were placed in the natural sea bottom and subjected to one of eight levels of biodeposition (from 0 to 1400 mussels m⁻²). Most analyses indicated non-linear (i.e., threshold) effects. Sediment characteristics changed significantly between 200 and 400 mussels m⁻² as did multivariate community structure. Community structure effects were characterised by changes in abundances of species that are very sensitive or tolerant to organic loading. The multivariate AZTI Marine Biotic Index (M-AMBI) indicated that the benthic status changed from High to Good in all mesocosms receiving biodeposits. Sediments acted as a sink for oxygen (O₂), but results suggest O₂ sediment demand was not sensitive enough to evaluate organic loading impacts. Results from this and improved experiments can be used to determine the environmental carrying capacity of sites for bivalve culture.

Transport and transformations of groundwater-borne carbon discharging through a sandy beach to a coastal ocean

Submarine groundwater discharge (SGD) is now recognized as an important coastal process affecting local and regional sources of solutes to coastal oceans. The objective of this study was to compare SGD estimates through Martinique Beach (Îles-de-la-Madeleine, QC, Canada) using different sampling techniques in order to quantify SGD fluxes of dissolved organic and inorganic carbon (DOC, DIC) exported to a coastal embayment. These fluxes were estimated using hydrogeological and geochemical methods, including direct measurement via seepage meters and hydrogeological estimation using Darcy’s law. Darcy estimates led to a fresh SGD flow ranging from 0.020 m3/s at the shoreline to 0.030 m3/s at the beach face. Direct measurements by seepage meters revealed higher SGD flows, with a mean value of 0.090 m3/s. These SGD flows were mainly composed of recirculating seawater flushing by the falling tide. The unconfined aquifer of Martinique Beach is a biogeochemically reactive zone, where DOC and DIC are nonconservative. The groundwater residence time (~32 days), low oxygen conditions (~20%) and high DOC concentrations (~ 2 mmol/L) represent a suitable environment for biogeochemical reactions to occur, and subsequently alter groundwater-borne carbon concentrations. We calculated a fresh-groundwater carbon load of 147 kg/d of DIC and 27 kg/d of DOC at the beach face. Even though recirculating seawater dominated the volume of total SGD, fresh SGD was an important carbon pathway, accounting for 12 and 20% of total DIC and DOC, respectively. This site-specific study is the first attempt to estimate volumetric and chemical groundwater fluxes to a coastal Canadian ocean, and demonstrates the strong interaction between fresh groundwater and coastal systems.

Dynamics of groundwater floodwaves and groundwater flood events in an alluvial aquifer

In gravelly floodplains, streamflood events induce groundwater floodwaves that propagate through the alluvial aquifer. Understanding groundwater floodwave dynamics can contribute to groundwater flood risk management. This study documents groundwater floodwaves on a flood event basis to fully assess environmental factors that control their propagation velocity, their amplitude and their extension in the floodplain, and examines the expression of groundwater flooding in the Matane River floodplain (Québec, Canada). An array of 15 piezometers equipped with automated level sensors and a river stage gauge monitoring at 15-minute intervals from September 2011 to September 2014 were installed within a 0.04-km2 area of the floodplain. Cross-correlation analyses were performed between piezometric and river-level time series for 54 flood events. The results reveal that groundwater floodwave propagation occurs at all flood magnitudes. The smaller floods produced a clear groundwater floodwave through the floodplain, while the largest floods affected local groundwater flow orientation by generating an inversion of the hydraulic gradient. Propagation velocities ranging from 8 to 13 m/h, which are two to three orders of magnitude higher than groundwater velocity, were documented while the induced pulse propagated across the floodplain to more than 230 m from the channel. Propagation velocity and amplitude attenuation of the groundwater floodwaves depend both on flood event characteristics and the aquifer characteristics. Groundwater flooding events are documented at discharge below bankfull (< 0.5 Qbf). This study highlights the role of flood event hydrographs and environmental variables on groundwater floodwave properties and the complex relationship between flood event discharge and groundwater flooding. The role that groundwater floodwaves play in flood mapping and the ability of analytical solutions to reproduce them are also discussed.

Hydrogeochemical evolution and groundwater mineralization of shallow aquifers in the Bas-Saint-Laurent region, Québec, Canada

This study presents the first regional groundwater hydrogeochemical portrait of the Bas-Saint-Laurent region (BSL), a region shaped by the Appalachians, a strong Quaternary glacial heritage, and coastal dynamics from the St. Lawrence Estuary. The proximity of BSL’s aquifers to St. Lawrence Estuary and its geological history with the past Goltwait Sea transgression create unique issues with respect to groundwater mineralization and sustainability in the region. The study is based on the distribution of major and trace elements and stable isotope signatures of water and inorganic carbon (δ18O, δ2H, δ13CDIC) in 145 groundwater samples collected in private and municipal wells distributed evenly over the study area. Groundwater shows a wide range of composition as indicated by the seven facies revealed by their composition of major elements. Ca-HCO3 and Na-HCO3 facies mainly dominate the regional groundwater composition, representing respectively 66 and 20% of the samples. Nevertheless, no significant relation between the geology, the aquifer confinement, and the geochemical facies emerged. This suggests that factors other than the hydrological settings may control the chemical composition of the groundwater in the study area. A hierarchical cluster analysis (HCA), including major, minor and trace elements, was performed, allowing the water samples to be distributed into four distinct geochemical groups that reveal a gradient from less mineralized (C4 and C2 groups with a dominant Ca-HCO3 facies) in the recharge areas to more mineralized (C1 with a Ca-HCO3 facies, to C3 with a Na-HCO3 facies) in the coastal discharge areas. Based on geochemical graphs and isotopic signatures, a conceptual model is proposed to explain this hydrogeochemical evolution at the regional scale. The most remarkable finding is that groundwater mineralization does not originate from modern seawater mixing despite the proximity of St. Lawrence seawater. Most of the hydrochemical evolution and groundwater mineralization is induced by the mixing with evaporated or remnant seawater originated from past transgressions, cation exchanges and mineral dissolution.