Application des techniques physiques, géochimiques et isotopiques pour l'évaluation de l'infiltration sous climat tempéré

Depuis plusieurs années, la Zone Non Saturée d'une parcelle expérimentale au nord de l'Italie (plaine du Pô) a fait l'objet de nombreuses études sur les caractéristiques physiques et géologiques du milieu, ainsi que sur les modalités du mouvement des eaux d'infiltration. Le site a été équipé de 6 tensiomètres à mercure, 6 bougies poreuses pour l'échantillonnage de l'eau et d'un tube d'accès pour la sonde à neutrons.L'application des différentes méthodes d'évaluation a fourni, pour la période d'étude (Septembre 1994-Août 1995), des valeurs de l'infiltration sensiblement différentes. La méthode physique, basée sur le calcul du bilan hydrique annuel, et considérant le sol comme un unique réservoir monocouche, a indiqué que 19% du total des précipitations arrive à s'infiltrer. D'autre part, un modèle de simulation intégrant les caractéristiques hydrodynamiques du sol a permis d'évaluer la recharge à 29% des précipitations annuelles. Le calcul de la répartition de la teneur en eau sur le profil porte sur la description des fluctuations temporelles du plan de flux nul, en relation avec les épisodes pluvieux et l'intensité de l'évaporation.La méthode géochimique du "profil en ions conservés" entre l'eau de pluie et du sol a permis, quant à elle, d'évaluer la lame d'eau infiltrée à environ 12% des précipitations annuelles. La composition isotopique des eaux extraites du sol manifeste un enrichissement global en isotopes lourds (2H et 18O) par rapport aux eaux météoriques, conséquence d'une évaporation intense. L'infiltration semble être de type "piston flow" dispersif. Elle suit, seulement pendant l'hiver, un flux où la composante convective dans la microporosité est prédominante sur la composante diffusive qui a lieu, de préférence, dans la macroporosité.In groundwater management, it is necessary to define water movement, evaluate infiltration and evapotranspiration rates, and quantify the physico-chemical evolution of transported solutes throughout the unsaturated zone (UZ). Traditionally, in temperate regions, recharge rate is evaluated by the comparison between physical methods (based on direct measurements of hydrological parameters with lysimeters, tensiometers and neutron gauges) and geochemical approaches (conservative ions, stable isotope profiles and artificial tracers).In the Po Plain (Northern Italy) intensive agricultural irrigation and overexploitation of groundwater profoundly affect the quality and availability of shallow groundwater resources. The region has been intensively cultivated with corn and rice during the last 5 centuries. Irrigation canals have been constructed in order to distribute water from alpine rivers to areas more favourable to agriculture. In the past, only the water balance method has been applied to obtain recharge rates in these situations.An experimental field site has been operational since 1987 in the ENEA-EUREX Nuclear Centre of Saluggia. The experimental plot represents an unsaturated zone in fluvio-glacial deposits (Holocene) of the Dora Baltea River, formed by gravel and sand, interlayered with silt levels. The water table is usually at a depth of 200 cm but it varies as a function of the river level. The objectives of the present study were to describe water movement throughout the UZ and to evaluate infiltration and evapotranspiration rates using different physico-chemical methodologies. The Saluggia plot was equipped with six tensiometers, a neutron gauge hole and 6 porous cups, up to a depth of 160 cm.During 10 years of monitoring, the most favourable period for understanding infiltration processes and water movement through the UZ is September 1994 - August 1995. This period is characterised by an exceptional rain event (300 mm in 48 hours), followed by a river flood. Water samples collected with porous cups at various depths have been analysed for chemistry and stable isotope composition, together with rainfall and Dora Baltea River water.In October 1994 (beginning of the rainy period) and May 1995 (beginning of the dry period) soil samples were collected at different depths and water was extracted under vacuum conditions. Stable isotope analyses were performed on extracted waters. Soil samples were also analysed for granulometry, chemical and mineralogical composition.Reducing soil to an unique monolayer aquifer, the water balance method suggests that only 19% of the total precipitation infiltrates, whereas a simulation model taking into account soil hydrodynamic characteristics estimates an infiltration rate of 29% of the rainfall. The position of the Zero Flux Plane (ZFP) fluctuates seasonally between the surface and the maximum studied depth, as a consequence of precipitation events and evaporation fluxes. In some periods, the ZFP is level with the water table and direct discharge may occur.Measurements of the stable isotopic composition (δ2H and δ18O) of soil water allows a quantitative estimate of direct groundwater discharge. On a δ2H versusδ18O plot, pore waters from the UZ have an isotopic composition that differs from that of the majority of groundwater samples, plotting below the local rain water line and indicating some degree of evaporation during the recharge process. The isotopic enrichment is particularly significant at the evaporation front, suggesting that pore water in soil reflects a different recharge regime from those of the regional ground waters. Water isotope and solute composition were substantially modified from their original composition during the infiltration process within the soil, via mechanisms such as anion exchange with soil particles, salt precipitation/dissolution or isotopic fractionation.Arial recharge was also evaluated using the depth distribution of a conservative solute. Assuming that chloride is derived from precipitation alone, Cl- content in soil and in rain water suggests that 12% of total rainfall infiltrates. Therefore, the recharge rate is estimated to be less than 100 mm/a. These data are in agreement with other results obtained by the chloride concentration profile method, in areas of the Po Valley, but are considerably different from those evaluated by the traditional physical methods

Design of an air ambulance aircraft for the Australian environment

Australia is a large country with the majority of its population living in capital cities on the coast line. The inland part of the county, also referred to as the Outback, is sparsely populated and not easily accessible. The Royal Flying Doctor Service (RFDS) is a national organisation providing Australia an airborne medical transport and emergency paramedic service. Currently the RFDS covers an area of 7,150,000 km2, flying on average 65,544 km a day. Their fleet consists of about 60 aircraft, flying of 21 bases Australia wide. The performance of the aircraft that the RFDS uses are critical to the quality of their services they provide. To date, there has never been a custom build Air Ambulance, neither in Australia, or the rest of the world, a surprising fact, when it is considered that there or over 100 Aero-Medical companies operating in the USA. The aircraft currently in use are converted business or light regional aircraft that were not designed specifically for MediVac operations. These conversions are both time consuming and costly, with the last purchase and refit costing the RFDS over 8 million Australian dollars. This formed the basis of an undergraduate student design project to design an aircraft specifically for MediVac operations. The Request for Proposal was drawn up Air Ambulance Victoria whose staff also provided valuable support to the students. This paper gives a summary of the findings of that design project

Hg natural pollution of shallow aquifers induced by seawater intrusion at Monte Angentario (Central Italy): evidences from isotope geochemistry

Electronic Proceedings

Outlines of biogeochemical cycling in the San Vitale Pinewood

This paper is focused on the multicomponent exchange processes and elemental cycling in the subsurface and superficial environments which constitute the area of the Pineta di San Vitale and nearby farmlands. The coastal aquifers in the area of the Pineta underwent severe seawater intrusion due to subsidence and overpumping, yielding a decline in the water quality and driving a number of processes in aquifers and soils. These include cation-exchange reactions by clay minerals, even if the cation exchange capacity of the aquifer is probably low; iron oxyhydroxides extensive precipitation due to oxic seawater recirculation; adsorption processes of both essential micronutrient and potentially toxic elements on the surface of Fe and Al hydroxides. The development of local anoxic conditions due to respiration and decomposition of organic matter allows the reductive dissolution of the iron oxy-hydroxides, which liberate the sorbed elements to porewaters. This redox cycling appears to be important in the bioavailability and geochemical cycling of a number of trace metals in the subterranean environment of the Pineta. CO2 charged waters in soils account for the high alkalinity observed in most water samples, through the role of carbonates and the control of pH and the Ca2+ content. The active processes in the surface ecosystem of the plain bordering the Pineta di San Vitale, exploited for agricultural uses, include dry and wet atmospheric deposition from marine and anthropogenic sources, leaching processes during runoff and retention and transport of trace elements by Fe and Mn oxides colloidal particles, through the network of canals and ditches towards the coastland. These processes are sensitive to flow fluctuations, and are deeply influenced by the management of water resources during summer, when a high water supply in irrigated crops is required. Using geochemical simulation codes it is highlighted that the reverse phenomenon to salinization, leading to groundwater freshening, is a long process, also depending on the low cation exchange capacity of the site. Even if a detailed study concerning the recharge rate of the aquifers would be necessary, these preliminary observation indicate that attenuation planning for salinization is urgent

A Multi-Isotope Study of the Castrocaro Mineral Waters (Northeastern Apennines, Italy)

An applied multi-isotope approach (δ2H, δ18O, δ11B, δ13C, δ15N, δ34S), coupled with chemical evidence, allowed us to constrain a conceptual hydrogeological model for the Castrocaro sulfureous groundwater system that is composed of fresh, brackish, and saline components. In this model, the meteoric recharge enters the turbidite sequence of the Marnoso-Arenacea Fm., undergoing chemical and isotopic modifications by interaction with fine grained pelite rocks, mixing with fossil seawater, and biochemical reactions. The groundwater then rises into a biocalcarenite unit, which acts just as a reservoir from which the resource is finally exploited by wells feeding the Castrocaro Terme spa