Groundwater dynamics in a restored tidal marsh are limited by historical soil compaction

In places where tidal marshes were formerly embanked for agricultural land use, these marshes are nowadays increasingly restored with the aim to regain important ecosystem services. However, there is growing evidence that restored tidal marshes and their services develop slowly and differ from natural tidal marshes in many aspects. Here we focus on groundwater dynamics, because these affect several key ecosystem functions and services, such as nutrient cycling and vegetation development. We hypothesize that groundwater dynamics in restored tidal marshes are reduced as compared to natural marshes because of the difference in soil structure. In the macro-tidal Schelde estuary (Belgium), in both a natural and a restored (since 2006) freshwater tidal marsh, we measured depth profiles of soil properties (grain size distribution, LOI (loss on ignition), moisture content and bulk density) and temporal dynamics of groundwater levels along a transect with increasing distance from a tidal creek. X-ray micro CT-scanning was used to quantify soil macroporosity. The restored marsh has a two layered soil stratigraphy with a topsoil of freshly accreted sediment (ranging in depth between 10 and 60 cm, deposited since 2006) and a subsoil of compact relict agricultural soil. We found that both the soil in the natural marsh and the topsoil of the restored marsh consist of loosely packed sediment rich in macropores and organic matter, whereas the relict agricultural soil in the restored marsh is densely packed and has few macropores. Our results show that groundwater level fluctuations in the restored marsh are restricted to the top layer of newly deposited sediment (i.e. on average 0.08 m depth). Groundwater level fluctuations in the natural marsh occur over a larger depth of the soil profile (i.e. on average 0.28 m depth). As a consequence, the reduced groundwater dynamics in restored tidal marshes are expected to alter the subsurface fluxes of water and nutrients, the source sink function and the development of marsh vegetation

Contaminant mass flux measurement in groundwater with passive samplers - A field application

In France and more generally speaking in Europe, passive sampling is an emerging method to measure groundwater quality and as a consequence to monitor contaminated sites. Contaminant mass flux measurement in groundwater can be of great interest for the management of contaminated sites because it can better define pollution sources and plumes and therefore improve practically all aspects of site characterization and remediation. For example, it can allow a better definition of contaminant transfer pathways, a better calculation of natural attenuation rates or a more efficient design of remediation techniques. In some countries like in Switzerland, the remediation urgency is already evaluated thanks to contaminant mass flux. So far, contaminant mass flux evaluation in groundwater is not as precise as it could be, because they are calculated from concentrations measured in snapshot groundwater samples and generic Darcy velocities and therefore do not account for possible concentration and groundwater flux variations over time and space. Passive samplers for contaminant mass flux such as passive flux meters (PFMs) can therefore be an interesting alternative to improve contaminant mass flux determination in groundwater. To date, this kind of passive sampler is not used in Europe and feedback is needed in order to promote their use and encourage consultants to use them. In this context, this work aimed at evaluating PFMs to measure contaminant mass flux in groundwater at a site contaminated with chlorinated solvents. On site, 6 monitoring wells were equipped with PFMs at different depths. 2 phases of exposure were carried out and PFMs were exposed during 3 months in each phase. The tests on site consisted in comparing the concentrations in groundwater given by the PFMs (average concentrations over the exposure time) to the average concentrations calculated from snapshot samples obtained with the conventional sampling method before the installation and after the retrieval of the PFMs, that is to say well purging prior to groundwater sampling with a pump. Concentrations coming from the PFMs were as well compared with an integrative sampler (ceramic dosimeter) having a constant sampling rate and giving average concentrations over the exposure time. Contaminant mass flux benefit was as well evaluated. Chlorinated solvent concentrations in groundwater snapshot samples taken before and after passive sampler exposure showed little variation. Therefore, concentrations given by the ceramic dosimeters were consistent with the ones obtained from the conventional sampling method. Nevertheless, PFMs showed a vertical distribution of the contamination and significantly higher concentrations were measured at some depths. These results showed that concentrations could be underestimated with the conventional sampling technique, leading to an underestimation of the risk for Human and Environment. PFMs offered complementary information because they allowed multi-level sampling in the wells and took into account variations in time and space, which could be of great value and seems very promising for contaminated sites characterization and remediation improvement

Mesure de flux massiques de contaminants dans les eaux souterraines. Tests sur sites atelier

Les flux massiques de contaminants peuvent permettre une meilleure gestion des sites et sols pollués. Cependant, à l’heure actuelle, ils sont estimés à partir d’échantillons moyens d’eaux souterraines et du flux d’eaux souterraines obtenu par la méthode de Darcy. Dans cette estimation, les variations spatiales et temporelles de ces données ne sont pas prises en compte, ce qui induit une forte incertitude sur le flux massique de contaminants. Les échantillonneurs passifs de flux (EPF) permettent de les mesurer directement. Dans le cadre du projet Passiflux, deux d’entre eux (les PFMs d’Enviroflux et les échantillonneurs iFLUX) ont été exposés au cours de 4 campagnes sur 2 sites. Les concentrations moyennes déduites des EPF sont généralement comparables aux concentrations obtenues à partir d’échantillons d’eau prélevés selon la technique conventionnelle, à la lumière des horizons productifs en pompage. L’intérêt de ces dispositifs pour une mesure multi-niveaux et une meilleure caractérisation de la contamination a également pu été démontré dans le cadre de ce projet. Ces dispositifs demandent désormais à être testés pour mesurer la charge massique traversant un plan de référence sur site réel, au moyen de forages positionnés dans le plan

Recent advances for monitoring groundwater and pollutant fluxes using single-well applied tracer techniques

In many different hydrogeological investigations, quantifying groundwater fluxes is essential but often challenging due to the variability of hydraulic conditions in space and time. Traditional approaches used to estimate groundwater fluxes are based on hydraulic conductivity obtained from field pumping or slug tests that provide only order-of-magnitude estimates and hydraulic gradients that can also vary, especially in areas of active groundwater discharge or pumping. The Finite Volume Point Dilution Method (FVPDM) is a recently developed applied tracer technology able to measure accurately groundwater fluxes and to monitor continuously their changes with time. We report 10 years of application of the FVPDM in contrasted hydrogeological contexts, from porous alluvial to fractured-rock aquifers, including strong interactions with surface water and contrasting groundwater flow dynamics. The obtained results prove that the FVPDM is able to measure a wide range of groundwater fluxes from a few centimetres per day to hundreds of metres per day. These results also emphases the variability in groundwater fluxes, (1) with time in aquifers influenced by variable hydraulic conditions such as tidal effects and (2) in space where orders of magnitude difference in groundwater fluxes are observed between nearby monitoring wells at a given site. Preliminary results of continuing work have also shown the potential for the FVPDM approach to be coupled with contaminant specific sensors and with passive sampling technologies to quantify contaminant mass fluxes in the subsurface. Recent developments have also investigated the ability to assess groundwater flow directions at the well scale