Artificial Tracers Research Articles

Velocity and celerity dynamics at plot scale inferred from artificial tracing experiments and time-lapse ERT

The relationship between tracer velocities and wave or wetting front celerities is essential to understand water flowing from hillslopes to the stream. The connection between maximum velocity and celerities estimated by means of experimental techniques has not been explored. To assess the pattern of infiltrating water front and dominant flow direction, we performed sprinkling experiments at a trenched plot in the Weierbach catchment in Luxembourg. Maximum velocities and wetting front celerities were inferred at different depths using artificial tracers, soil moisture measurements (TDR), and geophysical techniques. The flow direction was predominantly vertical within the observed plot, with almost no lateral flow observed until depths of 2–3m; shallow trench flow was intermittent and associated with preferential flow. Average celerity estimates using TDR and geophysical techniques were equal to 707±234mmh−1 and 971±625mmh−1, respectively. Vertical maximum velocity estimates were tracer-dependent and had very variable ranges: 109.3±89.3mmh−1 (Cl−), 177.8±199.1mmh−1 (Br−), and 604.1±610.7mmh−1 (Li+). Preferential flow processes were inferred from maximum velocities apparently greater than celerities and scattered trench flow with highly variable tracer concentrations. The high variability between maximum velocities of different tracers indicated a complex pattern of tracer movement through the soil, not captured by celerity values alone. Our study demonstrated the importance to assess both velocities and celerities to understand flow dynamics in response to sprinkling while information on the wetting front alone would have missed important preferential flow processes.

Long-range transport pathways of tropospheric source gases originating in Asia into the northern lower stratosphere during the Asian monsoon season 2012

Abstract. Global simulations with the Chemical Lagrangian Model of the Stratosphere (CLaMS) using artificial tracers of air mass origin are used to analyze transport mechanisms from the Asian monsoon region into the lower stratosphere. In a case study, the transport of air masses from the Asian monsoon anticyclone originating in India/China by an eastward-migrating anticyclone which broke off from the main anticyclone on 20 September 2012 and filaments separated at the northeastern flank of the anticyclone are analyzed. Enhanced contributions of young air masses (younger than 5 months) are found within the separated anticyclone confined at the top by the thermal tropopause. Further, these air masses are confined by the anticyclonic circulation and, on the polar side, by the subtropical jet such that the vertical structure resembles a bubble within the upper troposphere. Subsequently, these air masses are transported eastwards along the subtropical jet and enter the lower stratosphere by quasi-horizontal transport in a region of double tropopauses most likely associated with Rossby wave breaking events. As a result, thin filaments with enhanced signatures of tropospheric trace gases were measured in the lower stratosphere over Europe during the TACTS/ESMVal campaign in September 2012 in very good agreement with CLaMS simulations. Our simulations demonstrate that source regions in Asia and in the Pacific Ocean have a significant impact on the chemical composition of the lower stratosphere of the Northern Hemisphere. Young, moist air masses, in particular at the end of the monsoon season in September/October 2012, flooded the extratropical lower stratosphere in the Northern Hemisphere with contributions of up to ≈ 30 % at 380 K (with the remaining fraction being aged air). In contrast, the contribution of young air masses to the Southern Hemisphere is much lower. At the end of October 2012, approximately 1.5 ppmv H2O is found in the lower Northern Hemisphere stratosphere (at 380 K) from source regions both in Asia and in the tropical Pacific compared to a mean water vapor content of ≈ 5 ppmv. In addition to this main transport pathway from the Asian monsoon anticyclone to the east along the subtropical jet and subsequent transport into the northern lower stratosphere, a second horizontal transport pathway out of the anticyclone to the west into the tropics (TTL) is found in agreement with MIPAS HCFC-22 measurements.

Radon as a natural tracer for underwater cave exploration

The Molnár János cave is one of the largest hypogenic caves of the Buda Thermal Karst (Budapest, Hungary) and mainly characterized by water-filled passages. The major outflow point of the waters of the cave system is the Boltív spring, which feeds the artificial Malom Lake. Previous radon measurements in the cave system and in the spring established the highest radon concentration (71 BqL−1) in the springwater. According to previous studies, the origin of radon was identified as iron-hydroxide containing biofilms, which form where there is mixing of cold and thermal waters, and these biofilms efficiently adsorb radium from the thermal water component. Since mixing of waters is responsible for the formation of the cave as well, these iron-hydroxide containing biofilms and the consequent high radon concentrations mark the active cave forming zones. Based on previous radon measurements, it is supposed that the active mixing and cave forming zone has to be close to the spring, since the highest radon concentration was measured there. Therefore radon mapping was carried out with the help of divers in order to get a spatial distribution of radon in the cave passages closest to the spring. Based on our measurements, the highest radon activity concentration (84 BqL−1) was found in the springwater. Based on the distribution of radon activity concentrations, direct connection was established between the spring and the István-room of the cave, which was verified by an artificial tracer. However, the distribution of radon in the cave passages shows lower concentrations (18–46 BqL−1) compared to the spring, therefore an additional deep inflow from hitherto unknown cave passages is assumed, from which waters with high radon content arrive to the spring. These passages are assumed to be in the active cave formation zone. This study proved that radon activity concentration distribution is a useful tool in underwater cave exploration.

Phase-speed Spectra of Eddy Tracer Fluxes Linked to Isentropic Stirring and Mixing in the Upper Troposphere and Lower Stratosphere

Abstract The regions around the subtropical jets in the upper troposphere and lower stratosphere (UTLS) are characterized by strong isentropic stirring and mixing. In this work, the wave spectrum of the associated eddy tracer fluxes is examined using an artificial passive tracer advected on isentropes by the two-dimensional flow. The eddy diffusivity computed from the flux–gradient relation captures the main features of the mixing structure. Eddy transport in the UTLS is strongest in the summer hemisphere, and weak eddy fluxes are found at the core and poleward of the subtropical jets, especially in the winter hemisphere. There is an important contribution of stationary planetary equatorial Rossby waves in the tropical upper troposphere. The transient eddy tracer transport is primarily linked to medium-scale waves (wavenumbers ~4–7) breaking in the regions of weak westerlies around the subtropical jets and to planetary-scale waves at high latitudes. Phase-speed spectra for transient eddy fluxes show a close relationship of waves to the background zonal wind. In the deep tropics, traveling equatorial and Rossby waves of extratropical origin lead to cross-equatorial tracer transport throughout the upper troposphere. Interannual changes show that eddy tracer fluxes closely follow the shifts in the zonal winds associated with El Niño–Southern Oscillation and the quasi-biennial oscillation.

Tracer Tests for Characterizing Malm Geothermal Reservoirs Within the German BMWi Project TRENDS: A Feasibility Study

The research project TRENDS (“Tracer-assisted evaluation of reservoir behavior under expansive deployment schemes for Malm geothermal resources in the Munich area”) aims at quantifying the hydrogeology of Malm geothermal resources with a focus on fluid transport parameters, and identifying possible restrictions to reservoir exploitation resulting from either hydraulic or geothermal heat supply ‘competition’ effects between ‘adjacent’ reservoirs. While it is beyond doubt that these aims cannot be achieved without the use of some fluid-based tracking procedure for quantifying fluid transport (viz.: tracer tests), the use of artificial tracers is challenged by the very large size of Malm geothermal reservoirs, implying very long residence times (RT) of circulating fluids and strong dilution of tracers therein. A series of M. Sc. thesis projects conducted at the University of Göttingen addresses tracer test design, evaluation and interpretation problems specifically associated with the ‘very long RT’ issue. Preliminary findings from these projects, and some recommendations for future tracer test design are presented.

Dynamics and anthropogenic impacts of multiple karst flow systems in a mountainous area of South China

The Xiangxi River basin, South China, is a steep terrane with well-developed karst features and an important Cambrian-Ordovician aquifer. Meteoric water in this mountainous area features a mean δ18O elevation gradient of –2.4 ‰/km. This gradient was used to estimate mean recharge elevations of 760 m for Shuimoxi (SMX) spring, 1,060 m for Xiangshuidong (XSD) spring, and 1,430 m for drill hole ZK03, indicating multiple flow paths in the Cambrian-Ordovician karst aquifer. Mean residence times of 230 and 320 days and ∼2 years were estimated for these features, respectively, using the damped running average model that predicts the isotopic variations in groundwater from those in precipitation. Groundwater in the regional karst flow system has the longest residence time, the highest recharge elevation, the longest flow paths, the lowest addition of anthropogenic components, and the greatest amount of water–rock interaction as indicated by its higher dissolved solids, Mg2+ concentrations and Mg/Ca ratios than the springs. In contrast, the local and shallow karst flow systems respond rapidly to recharge events. Artificial tracer tests prove that these shallow karst systems can also quickly transmit anthropogenic contaminants, indicating that they are highly vulnerable to human impacts, which include the enrichment of NO3 –. The intensity of water–rock interaction and groundwater vulnerability are mainly determined by the structure and dynamics of the multiple karst flow systems.

Artificial ultra-fine aerosol tracers for highway transect studies

The persistent evidence of health impacts of roadway aerosols requires extensive information for urban planning to avoid putting populations at risk, especially in-fill projects. The required information must cover both highway aerosol sources as well as transport into residential areas under a variety of roadway configurations, traffic conditions, downwind vegetation, and meteorology. Such studies are difficult and expensive to do, but were easier in the past when there was a robust fine aerosol tracer uniquely tied to traffic – lead. In this report we propose and test a modern alternative, highway safety flare aerosols. Roadway safety flares on vehicles in traffic can provide very fine and ultra-fine aerosols of unique composition that can be detected quantitatively far downwind of roadways due to a lack of upwind interferences. The collection method uses inexpensive portable aerosol collection hardware and x-ray analysis protocols. The time required for each transect is typically 1 h. Side by side tests showed precision at ± 4%. We have evaluated this technique both by aerosol removal in vegetation in a wind tunnel and by tracking aerosols downwind of freeways as a function of season, highway configuration and vegetation coverage. The results show that sound walls for at-grade freeways cause freeway pollution to extend much farther downwind than standard models predict. The elevated or fill section freeway on a berm projected essentially undiluted roadway aerosols at distances well beyond 325 m, deep into residential neighborhoods. Canopy vegetation with roughly 70% cover reduced very fine and ultra-fine aerosols by up to a factor of 2 at distances up to 200 m downwind.

Heat as a natural, low-cost tracer in mine water systems: The attenuation and retardation of thermal signals in a Reducing and Alkalinity Producing Treatment System (RAPS)

A combined chemical and thermal tracer test has been attempted, using iced water and sodium chloride, in the compost and limestone Tan-y-Garn Reducing and Alkalinity Producing System (RAPS), designed to treat iron-rich net acidic mine water in South Wales, UK. The applied thermal signal was, however, inadequate to produce a temperature anomaly in the output water, being overwhelmed by the summer solar/atmospheric daytime heat input to the lagoon. Nevertheless, natural diurnal temperature signals could be discerned in the outlet water, being retarded relative to the input signal. Thermal retardation could be calculated as approximately 2.3 to 2.9, and various estimates of the RAPS bulk volumetric heat capacity tend to fall between 2 and 3MJm−3K−1. The natural diurnal thermal signal potentially provides a low cost substitute for artificial tracers in estimating RAPS retention times.

Petrothermal and aquifer-based EGS in the Northern-German Sedimentary Basin, investigated by conservative tracers during single-well injection-flowback and production tests

The use of artificial tracers in single-well tests in conjunction with EGS-related fracturing or stimulation of sedimentary and crystalline formations in the N-German Basin is evaluated. Conservative-tracer signal analysis is suggested as a tool to quantify individual fracture contributions to multiple-fracture discharge, and illustrated with preliminary data from the Groß Schönebeck site. The proposed approach helps to avoid investing in unnecessary wellbore completion-integrated devices used for inflow profiling, especially when the productivity of the target formation is uncertain. Conservative-tracer signal analysis is further applied to estimate the thermal-lifetime contributions from the petrothermal and the aquifer components of a hybrid EGS developed in sedimentary formations at the Horstberg site. Long-term observation of tracer signals is recommended for both sites.

(奥地利)阿尔卑斯地区东部Niedere Tauern山脉残留的Schöneben岩石冰川地下水流和储存库的确定和评价

More than 2,600 relict rock glaciers are known in the Austrian Alps but the knowledge of their hydraulic properties is severely limited. The relict Schoneben Rock Glacier (Niedere Tauern Range, Austria), with an extension of 0.17 km2, was investigated based on spring data (2006–2014) and seismic refraction survey. Spring-discharge hydrographs and natural and artificial tracer data suggest a heterogeneous aquifer with a layered internal structure for the relict rock glacier. The discharge behavior exhibits a fast and a delayed flow component. The spring discharge responds to recharge events within a few hours but a mean residence time of several months can also be observed. The internal structure of the rock glacier (up to several tens of meters thick) consists of: an upper blocky layer with a few meters of thickness, which lacks fine-grained sediments; a main middle layer with coarse and finer-grained sediments, allowing for fast flow; and an approximately 10-m-thick basal till layer as the main aquifer body responsible for the base flow. The base-flow component is controlled by (fine) sandy to silty sediments with low hydraulic conductivity and high storage capacity, exhibiting a difference in hydraulic conductivity to the upper layer of about three orders of magnitude. The high storage capacity of relict rock glaciers has an impact on water resources management in alpine catchments and potentially regulates the risk of natural hazards such as floods and related debris flows. Thus, the results highlight the importance of such aquifer systems in alpine catchments.

A deuterium-based labeling technique for the investigation of rooting depths, water uptake dynamics and unsaturated zone water transport in semiarid environments

Summary Non- or minimum-invasive methods for the quantification of rooting depths of plants are rare, in particular in (semi-)arid regions; yet, this information is crucial for the parameterization of SVAT (Soil–Vegetation–Atmosphere Transfer) models and understanding of processes within the hydrological cycle. We present a technique utilizing the stable isotope deuterium (2H) applied as artificial tracer to investigate the vertical extent of the root zone, characterize water uptake dynamics of trees and shrubs at different depths and monitor transport of water through the unsaturated zone of dry environments. One liter of 35% deuterated water (2H2O) was punctually applied at several depths (0.5 m, 1 m, 2 m, 2.5 m and 4 m) at six different plots at a natural forested site in the Cuvelai–Etosha Basin (CEB), Namibia/Angola. Subsequently, uptake of the tracer was monitored by collecting plant samples (xylem and transpired water) up to seven days after tracer injection. Soil profiles at the plots were taken after the campaign and again after six months in order to evaluate the transport and distribution of 2H within the unsaturated zone. Of 162 plant samples taken, 31 samples showed clear signals of artificially introduced 2H, of which all originate from the plots labeled up to 2 m depth. No artificially injected 2H was found in plants when tracer application occurred deeper than 2 m. Results further indicate a sharing of water resources between the investigated shrubs and trees in the upper 1 m whilst tree roots seem to have better access to deeper layers of the unsaturated zone. The soil profiles taken after six months reveal elevated 2H-concentrations from depths as great as 4 m up to 1 m below surface indicating upward transport of water vapor. Purely diffuse transport towards the soil surface yielded an estimated 0.4 mm over the dry season. Results are of particular significance for a more precise parameterization of SVAT models and the formulation of water balances in semiarid areas. The developed methodology is beneficial for site-specific investigations in complex and data scarce environments, where the root zone plays a major role for the overall water balance. For arid and semiarid environments experiencing low recharge rates, water transported in its vapor phase is found to play an important role for the overall soil water balance. The use of 2H2O is cost-effective and provides the opportunity to investigate multiple effects along the soil–vegetation interface that have been difficult to deal with previously.

Impact of different Asian source regions on the composition of the Asian monsoon anticyclone and of the extratropical lowermost stratosphere

Abstract. The impact of different boundary layer source regions in Asia on the chemical composition of the Asian monsoon anticyclone, considering its intraseasonal variability in 2012, is analysed by simulations of the Chemical Lagrangian Model of the Stratosphere (CLaMS) using artificial emission tracers. The horizontal distribution of simulated CO, O3, and artificial emission tracers for India/China are in good agreement with patterns found in satellite measurements of O3 and CO by the Aura Microwave Limb Sounder (MLS). Using in addition, correlations of artificial emission tracers with potential vorticity demonstrates that the emission tracer for India/China is a very good proxy for spatial distribution of trace gases within the Asian monsoon anticyclone. The Asian monsoon anticyclone constitutes a horizontal transport barrier for emission tracers and is highly variable in location and shape. From the end of June to early August, a northward movement of the anticyclone and, during September, a strong broadening of the spatial distribution of the emission tracer for India/China towards the tropics are found. In addition to the change of the location of the anticyclone, the contribution of different boundary source regions to the composition of the Asian monsoon anticyclone in the upper troposphere strongly depends on its intraseasonal variability and is therefore more complex than hitherto believed. The largest contributions to the composition of the air mass in the anticyclone are found from northern India and Southeast Asia at a potential temperature of 380 K. In the early (mid-June to mid-July) and late (September) period of the 2012 monsoon season, contributions of emissions from Southeast Asia are highest; in the intervening period (early August), emissions from northern India have the largest impact. Our findings show that the temporal variation of the contribution of different convective regions is imprinted in the chemical composition of the Asian monsoon anticyclone. Air masses originating in Southeast Asia are found both within and outside of the Asian monsoon anticyclone because these air masses experience, in addition to transport within the anticyclone, upward transport at the southeastern flank of the anticyclone and in the tropics. Subsequently, isentropic poleward transport of these air masses occurs at around 380 K with the result that the extratropical lowermost stratosphere in the Northern Hemisphere is flooded by the end of September with air masses originating in Southeast Asia. Even after the breakup of the anticyclonic circulation (around the end of September), significant contributions of air masses originating in India/China are still found in the upper troposphere over Asia. Our results demonstrate that emissions from India, China, and Southeast Asia have a significant impact on the chemical composition of the lowermost stratosphere of the Northern Hemisphere, in particular at the end of the monsoon season in September/October 2012.

Estimation of groundwater recharge via deuterium labelling in the semi-arid Cuvelai-Etosha Basin, Namibia

The stable water isotope deuterium (2H) was applied as an artificial tracer (2H2O) in order to estimate groundwater recharge through the unsaturated zone and describe soil water movement in a semi-arid region of northern central Namibia. A particular focus of this study was to assess the spatiotemporal persistence of the tracer when applied in the field on a small scale under extreme climatic conditions and to propose a method to obtain estimates of recharge in data-scarce regions. At two natural sites that differ in vegetation cover, soil and geology, 500 ml of a 70 % 2H2O solution was irrigated onto water saturated plots. The displacement of the 2H peak was analyzed 1 and 10 days after an artificial rain event of 20 mm as well as after the rainy season. Results show that it is possible to apply the peak displacement method for the estimation of groundwater recharge rates in semi-arid environments via deuterium labelling. Potential recharge for the rainy season 2013/2014 was calculated as 45 mm a−1 at 5.6 m depth and 40 mm a−1 at 0.9 m depth at the two studied sites, respectively. Under saturated conditions, the artificial rain events moved 2.1 and 0.5 m downwards, respectively. The tracer at the deep sand site (site 1) was found after the rainy season at 5.6 m depth, corresponding to a displacement of 3.2 m. This equals in an average travel velocity of 2.8 cm d−1 during the rainy season at the first site. At the second location, the tracer peak was discovered at 0.9 m depth; displacement was found to be only 0.4 m equalling an average movement of 0.2 cm d−1 through the unsaturated zone due to an underlying calcrete formation. Tracer recovery after one rainy season was found to be as low as 3.6 % at site 1 and 1.9 % at site 2. With an in situ measuring technique, a three-dimensional distribution of 2H after the rainy season could be measured and visualized. This study comprises the first application of the peak displacement method using a deuterium labelling technique for the estimation of groundwater recharge in semi-arid regions. Deuterium proved to be a suitable tracer for studies within the soil–vegetation–atmosphere interface. The results of this study are relevant for the design of labelling experiments in the unsaturated zone of dry areas using 2H2O as a tracer and obtaining estimations of groundwater recharge on a local scale. The presented methodology is particularly beneficial in data-scarce environments, where recharge pathways and mechanisms are poorly understood.

Transfer function approach for artificial tracer test interpretation in karstic systems

Summary A karstic formation consists in a three-dimensional hydrological system which involves horizontal and vertical, diphasic or saturated water transfers characterised by a large range of velocity. These subsurface flow processes correspond to various water pathways through fractured, fissured, and underground streams or conduits leading to a nonlinear global behaviour of the system. An efficient way of investigating of a karstic system behaviour consists in the injection of artificial tracer tests at loss points and in careful analysis of the recovery tracer fluxes at one or several outlets of the systems. These injections are also an efficient way of providing hypotheses on characteristic time of contaminant transfer in these type of aquifers. Here, we propose a Laplace-transform transfer function of the Residence Time Distribution function that allows to discriminate between a quick-flow advection-dominated component and a slow-flow advection–dispersion/dominated component in the artificial tracer transfer in the system. We apply this transfer function on five high resolution sampling rate artificial tracer tests operated on the Baget system in the Pyrenees (France) in order to illustrate the advantages and limitations of this approach. We provide then an interpretation of the relationship between tracer test recovery shape and karstic system organisation between inlet and outlet site.

Characterizing Aquifer Heterogeneity Using Bacterial and Bacteriophage Tracers

Gravel aquifers act as important potable water sources in central western Europe, yet they are subject to numerous contamination pressures. Compositional and textural heterogeneity makes protection zone delineation around groundwater supplies in these units challenging; artificial tracer testing aids characterization. This paper reappraises previous tracer test results in light of new geological and microbiological data. Comparative passive gradient testing, using a fluorescent solute (Uranine), virus (H40/1 bacteriophage), and comparably sized bacterial tracers and , was used to investigate a calcareous gravel aquifer's ability to remove microbiological contaminants at a test site near Munich, Germany. Test results revealed relative recoveries could exceed those of H40/1 at monitoring wells, 10 m and 20 m from an injection well, by almost four times; recoveries varied by a factor of up to three between wells. Application of filtration theory suggested greater attenuation of H40/1 relative to similarly charged occurred due to differences in microorganism size, while estimated collision efficiencies appeared comparable. By contrast, more positively charged experienced greater attenuation at one monitoring point, while lower attenuation rates at the second location indicated the influence of geochemical heterogeneity. Test findings proved consistent with observations from nearby fresh outcrops that suggested thin open framework gravel beds dominated mass transport in the aquifer, while discrete intervals containing stained clasts reflect localized geochemical heterogeneity. Study results highlight the utility of reconciling outcrop observations with artificial tracer test responses, using microbiological tracers with well-defined properties, to characterize aquifer heterogeneity.

Tracer use for the Protection of Water Resources in Nuclear Sites

Natural and artificial tracers can be used for nuclear sites management. Tracer techniques coupled with modelling and environmental monitoring activities are an effective tool to characterize and to foresee the radionuclide dynamic in the environment. In particular, the safeguard of water resources for human purposes must be guaranteed. In this work, the transport of tracers H-3 and I-129 in groundwater and subsoil was evaluated by means of Hydrus 1D and AMBER codes. Information on how radionuclides migrate in the environment were obtained and preliminary hypothesis on how to design the environmental monitoring network of the investigated site were deduced.

236U and 239,240Pu ratios from soils around an Australian nuclear weapons test site

The isotopes 236U, 239Pu and 240Pu are present in surface soils as a result of global fallout from nuclear weapons tests carried out in the 1950's and 1960's. These isotopes potentially constitute artificial tracers of recent soil erosion and sediment movement. Only Accelerator Mass Spectrometry has the requisite sensitivity to measure all three isotopes at these environmental levels. Coupled with its relatively high throughput capabilities, this makes it feasible to conduct studies of erosion across the geographical extent of the Australian continent.In the Australian context, however, global fallout is not the only source of these isotopes. As part of its weapons development program the United Kingdom carried out a series of atmospheric and surface nuclear weapons tests at Maralinga, South Australia in 1956 and 1957. The tests have made a significant contribution to the Pu isotopic abundances present in the region around Maralinga and out to distances ∼1000 km, and impact on the assessment techniques used in the soil and sediment tracer studies. Quantification of the relative fallout contribution derived from detonations at Maralinga is complicated owing to significant contamination around the test site from numerous nuclear weapons safety trials that were also carried out around the site. We show that 236U can provide new information on the component of the fallout that is derived from the local nuclear weapons tests, and highlight the potential of 236U as a new fallout tracer.

Stalactite drip-water monitoring and tracer tests approach to assess hydrogeologic behavior of karst vadose zone: case study of Han-sur-Lesse (Belgium)

This paper focuses on the unsaturated zone and epikarst of karst aquifers in order to identify the hydrogeological behavior of these important but poorly understood parts of karst systems. A high-resolution monitoring of stalactite drips and artificial tracer test experiment in the vadose zone allows to draw a conceptual model for the local recharge processes through the unsaturated zone of karst aquifers. Those aquifers represent major groundwater resources in Belgium. Seven years of high-resolution stalactite dripping were recorded on 3 sites of the Han-sur-Lesse cave system (South of Belgium). The three sites show very different discharge variations but a similar behavior from one year to another. Analysis and comparison of the dripping cycles over 7 years highlight the vadose zone behavior and its relationship with surface infiltration (water excess). For each site, a water excess threshold at the beginning of the winter has been identified. This threshold is needed to refill the vadose zone/epikarst capacity before the dripping. The value of the threshold is relatively constant for each site during the 7 years, indicating the regularity of the capacitive system of the vadose zone. Unknown features have also been observed with discharge decrease during high water excess periods, linked to the drainage processes of epikarst. Finally, artificial tracer test reveals the duality of the unsaturated zone behavior, with fast transmissive pathways (velocity of >6 m/h) related to a highly capacitive system (>150 days of tracer restitution).

Backward-in-time methods to simulate large-scale transport and mixing in the ocean

In oceanography and meteorology, it is important to know not only where water or air masses are headed for, but also where they came from as well. For example, it is important to find unknown sources of oil spills in the ocean and of dangerous substance plumes in the atmosphere. It is impossible with the help of conventional ocean and atmospheric numerical circulation models to extrapolate backward from the observed plumes to find the source because those models cannot be reversed in time. We review here recently elaborated backward-in-time numerical methods to identify and study mesoscale eddies in the ocean and to compute where those waters came from to a given area. The area under study is populated with a large number of artificial tracers that are advected backward in time in a given velocity field that is supposed to be known analytically or numerically, or from satellite and radar measurements. After integrating advection equations, one gets positions of each tracer on a fixed day in the past and can identify from known destinations a particle positions at earlier times. The results provided show that the method is efficient, for example, in estimating probabilities to find increased concentrations of radionuclides and other pollutants in oceanic mesoscale eddies. The backward-in-time methods are illustrated in this paper with a few examples. Backward-in-time Lagrangian maps are applied to identify eddies in satellite-derived and numerically generated velocity fields and to document the pathways by which they exchange water with their surroundings. Backward-in-time trapping maps are used to identify mesoscale eddies in the altimetric velocity field with a risk to be contaminated by Fukushima-derived radionuclides. The results of simulations are compared with in situ mesurement of caesium concentration in sea water samples collected in a recent research vessel cruise in the area to the east of Japan. Backward-in-time latitudinal maps and the corresponding material-line techniques are applied to document transport of water masses across strong currents. Backward-in-time drift maps are shown to be useful in identifying the Lagrangian fronts favorable for fishery.

Assessing the Nonlinearity of Karst Response Function under Variable Boundary Conditions.

The hydraulic and transfer response of karst aquifers is complex and often highly nonlinear: due to their high transmissivity and connection with the surface, such systems are very sensitive to modifications of their boundary conditions. The aim of this study was to assess the variation of the response depending on both upstream and downstream parameters, and propose a methodology to simulate the response of the karst system depending on those parameters. The impact of the variations of multiple environmental parameters on the response of a karstic system submitted to tidal variations (Normandy, France) was investigated after a campaign of artificial tracer tests acquired in very different hydrologic conditions (rainfall events, low tide, high tide, low/high piezometric level, and low/high waters). Principal components analysis and hierarchical clustering were applied on both environmental variables and karstic system response variables (parameters of the residence time distribution [RTD] curves). Equations between the RTD parameters and the most relevant variables were established using a symbolic regression algorithm. It appeared that the variations of the RTD parameters depend mainly on the cumulated rainfall preceding the injection, the piezometric level of the aquifer, and on the tide parameters. The hydraulic conditions downstream of the aquifer have a strong influence on the hydraulic and transfer response of the aquifer. The response of the aquifer in various and extreme conditions has been simulated using the equations resulting from the symbolic regression algorithm. Such relationships can be useful for management of water resources in karst media, and support decision making.