Jean-Baptiste Charlier

Transport of a nematicide in surface and groundwaters in a tropical volcanic catchment

The aim of this article is to determine how the nematicide cadusafos [S,S-di-sec-butyl O-ethyl phosphorodithioate] contaminates water and soils at two scales, subcatchment and catchment. The study site was a small banana (Musa spp.)-growing catchment on the tropical volcanic island of Guadeloupe in the Caribbean. Two application campaigns were conducted, one in 2003 on 40% of the catchment and one in 2006 on 12%. The study involved monitoring for 100 d the surface water and groundwater flows and the cadusafos concentrations in the soil and in surface and groundwaters in a 2400 m(2) subcatchment and a 17.8 ha catchment. The results show that at the subcatchment scale the high retention in the A horizon of the soil limits the transport of cadusafos by runoff, whereas the lower retention of the molecule in the B horizon favors percolation toward the shallow groundwater. Comparing the losses of cadusafos at the subcatchment and at the catchment scales revealed that the nematicide re-infiltrated in the hydrographic network. Two successive phases of stream water contamination were observed, corresponding to two distinct contamination mechanisms: an event-dominated contamination phase (of <30 d) when transport was linked to overland flow during precipitation shortly after application, and a stabilized contamination phase when transport originated mainly from the drainage of the shallow aquifer. Lastly, comparing the losses of the two phases during 2003 and 2006 showed that shallow groundwater, which is promoted in such permeable soils under abundant tropical rainfalls, seems to be the main contributor to stream contamination.

Use of Dissolved Organic Carbon to Characterize Infiltration in a Small Karst System in the French Jura Mountains (Fertans, France)

Due to the high input of organic soil substances in infiltrated water, dissolved organic carbon (DOC) is an interesting tracer to determine water origin in hydrosystems. The aim of this study was to use continuous measurements of DOC natural fluorescence in spring water to characterize infiltration in a karst system at the event time scale. Hydrological data were recorded at the outlet of a small perennial spring within a cliff at Fertans in the French Jura mountains since July 2009. Results showed that the response of the system was fast (response time of 3 hours) and was produced by two combined flow types. First, discharge and turbidity peaks, and electrical conductivity troughs were produced by a piston-type flow enabling an unclogging of the fractures. Second, this fast infiltration process was followed by a slower one enriched in DOC, produced by a more diffuse-type flow through the matrix compartment.

Use of a flood-routing model to assess lateral flows in a karstic stream: implications to the hydrogeological functioning of the Grands Causses area (Tarn River, Southern France)

The aim of this study is to assess the spatio-temporal variability of lateral flows in the streams of a large karstic basin to construct a conceptual model of karst contributions to flood generation. The lateral flows of the Tarn River, which crosses the Grands Causses karst zone in Southern France, were investigated between several gauging stations along the river. First, through analysing the lateral flows on an event time scale of 30 floods, it was possible to identify the losing and gaining reaches, highlighting a highly variable attenuation/amplification role of karsts on flood generation. Second, the diffusive wave model was used to quantify the lateral flows on an hourly time step within a flood event. The simulations show a high variability of lateral outflows and inflows within a same reach according to the hydrometeorological conditions, with in some cases an inversion of the lateral flow direction during the flood. The results highlight complex surface/groundwater exchanges during a single flood event, with high river losses despite the concurrent flow of large springs feeding the stream. This spatio-temporal variability of the karst influence on flood generation was linked to the aquifer’s structure, which has improved the understanding of the hydro(geo)logical functioning of the Grands Causses massif. Finally, the new methodology proposed here opens challenging perspectives towards a framework for the analysis of surface–groundwater exchanges in karstic rivers.

How Karst Areas Amplify or Attenuate River Flood Peaks? A Response Using a Diffusive Wave Model with Lateral Flows

This paper investigates the role of karst aquifers on flood generation and propagation using the Hayami Diffusive Wave (DW) model accounting for uniformly distributed lateral flows. The inverse model was applied on the main channel reaches of the Tarn basin at Millau (2,400 km2) in southern France to assess lateral inflows from karstic springs as well as lateral outflows from river losses. Results show that the DW model, which is simple, parsimonious, and easy-to-use, is able to quantify lateral flows avoiding difficult parameterisation. Surface/groundwater exchanges were characterised on several reaches along the stream, showing a highly variable attenuation/amplification influence of flood peak by karst units during a single flood event. We showed that the upstream part of the karst area have a dominant attenuation role by re-infiltrating part of runoff from the head-water basin in hard-rock areas, while the downstream part have a dominant amplification role due to high contributions of karst groundwater. These results improved the conceptual hydrogeological model of the Grands Causses region.

Assessing Freshwater Resources in Coastal Karstic Aquifer Using a Lumped Model: The Port-Miou Brackish Spring (SE France)

Freshwater resources in coastal aquifers are restricted by seawater intrusion. Studying brackish spring can be an appropriate approach to assess this saline intrusion and to elaborate a conceptual model of the karst aquifer. The aim of this study is to model the hydrogeological flows and the salinity of a brackish spring using a lumped numerical model (Rainfall-Discharge-Salinity), and to quantify the freshwater discharge available. The model is based on a classical karst model composed of connected reservoirs representing the main storage elements of the karst aquifer, which can be deduced from the analysis of discharge and salinity recorded time series. The model was successfully applied on the Port-Miou spring (400 km2), in SE France, which is one of the main submarine springs around the Mediterranean Sea. Four reservoirs were used to model spring discharge and salinity: a SOIL reservoir feeding a DEEP brackish reservoir impacted by seawater intrusion, and two FAST and SLOW reservoirs representing the shallower freshwater resource. We showed that the spring water is always brackish more or less diluted by freshwater during flood events. These results improved the conceptual hydrogeological model of the Port-Miou spring and showed the effectiveness of lumped models to simulate discharge and salinity in coastal karst aquifers.

Comparative Study of the Physicochemical Response of Two Karst Systems During Contrasting Flood Events in the French Jura Mountains

This paper presents preliminary results from two karst systems belonging to the “Jurassic Karst” observatory in the French Jura Mountains. The sites are characterized by localized and diffuse recharge. Physicochemical monitoring was performed at the karst outlet (springs), as well as in the unsaturated zone (cave and epikarstic spring). During two contrasting flood events, water level, temperature, electrical conductivity, dissolved organic carbon, turbidity, and dissolved oxygen were recorded at high frequency and compared. These preliminary results allow to propose a conceptual model for both sites. It was possible to distinguish specific autogenic and allogenic recharge mechanisms and to characterize the respective contribution of the saturated and unsaturated zones.