Rapid Cleanup of a Perchlorate Plume from Fireworks

Perchlorate was detected in a municipal wellfield in Evart, Michigan in April 2015. Perchlorate concentrations were detected initially in six of the City's wells at concentrations ranging up to 20 μg/L. An investigation to identify the source determined that the perchlorate was from fireworks launched during the annual 4th of July show held at the fairgrounds located upgradient from the wellfield. The use of approximately 600 kg of fireworks during the annual display resulted in an annual loading of approximately 4 kg of perchlorate to groundwater. An aggressive groundwater extraction system began operation in June 2016 to restore water quality in the affected aquifer, and the 2016 fireworks display was relocated to a location outside the capture zone of the water supply wells. Within 18 months average perchlorate concentrations in the water supply wells had been reduced to about 0.6 μg/L. The extraction system continued to operate through the end of 2019, by which time the average perchlorate concentrations in the water supply wells were reduced to 0.2 μg/L. In 2019, approximately 0.4 kg of perchlorate were removed from the aquifer, about one-half of the amount removed in 2018, reflecting the slow leaching of perchlorate of fireworks residuals from vadose zone soils.     

1,4‐Dioxane Soil Remediation Using Enhanced Soil Vapor Extraction: I. Field Demonstration

1,4‐Dioxane is totally miscible in water, sequestering in vadose pore water that can serve as a source of long‐term groundwater contamination. Although some 1,4‐dioxane is removed by conventional soil vapor extraction (SVE), remediation is typically inefficient. SVE efficiency is hindered by low Henry’s Law constants at ambient temperature and redistribution to vadose pore water if SVE wells pull 1,4‐dioxane vapors across previously clean soil. It was hypothesized that heated air injection and more focused SVE extraction (“Enhanced SVE” or XSVE) could increase the efficiency of 1,4‐dioxane vadose treatment, and this new process was tested at former McClellan Air Force Base, CA. The XSVE system had four peripheral heated air injection wells surrounding a 6.1 m × 6.1 m × 9.1 m deep treatment zone with a central vapor extraction well. After 14 months of operation, soil temperatures reached as high as ~90 °C near the injection wells and the treatment zone was flushed with ~20,000 pore volumes of injected air. Post‐treatment sampling results showed reductions of ~94% in 1,4‐dioxane and ~45% in soil moisture. Given the simplicity of the remediation system components and the promising demonstration test results, XSVE has the potential to be a cost‐effective remediation option for vadose zone soil containing 1,4‐dioxane.     

Perchlorate accumulation and migration in the deep vadose zone in a semiarid region

For 25 years, a plant in Israel manufacturing ammonium perchlorate disposed of untreated wastewater in four unlined ponds. This study explores the transport mechanisms of perchlorate infiltrated from 1965 to 1990 from one of these active storage ponds into a deep (40 m) layered vadose zone and the underlying Israeli coastal aquifer. Perchlorate migration from 1990, when wastewater disposal ceased, until today, with infiltration due only to natural rain (500 mm y⁻¹), was also studied. Several indirect methods were used, including: mass balance in the unsaturated zone profile, δ¹⁸O and δ²H profiles below the pond, and a comparison of the same sediment profiles in 2005 and 2007. The isotopic composition of the pore water could be divided into two separate groups: lighter (depleted) and heavier (enriched) samples. All samples in the lighter group were from the shallow vadose zone, above two clayey layers, and represent natural infiltration of rainwater. The enriched samples were from the deeper section of the unsaturated zone (20–40 m) and represent water used for perchlorate manufacturing 14 years prior to drilling. Consequently, the overall maximum infiltration rate was estimated to be 1.4 m y⁻¹. Below the clayey layer almost identical perchlorate concentrations were found along the sediment profile in 2005 and 2007 (two boreholes, 3 m apart). Very different perchlorate profiles were observed above the clayey layers. This suggests that perchlorate below the clay layers (20–40 m) is practically stagnant under the current natural conditions. The reduction in perchlorate concentration in groundwater below the ponds vs. its increased concentration further downgradient supports the contention that the current migration of perchlorate from the vadose zone to the groundwater is very small. We estimate that perchlorate concentration in the groundwater under the infiltration pond, which was 187 mg l⁻¹ in 2004, will reach 10 μg l⁻¹ within about 14 years. The existence of a clayey layer crossing the thick vadose zone was thus found to significantly change the infiltration rate when ponded conditions were replaced with natural precipitation.     

Seepage response along an alluvial valley in a semi‐arid catchment in north‐central California

Over a period of 12 months, soil moisture content and potential was monitored in an annual‐grass‐dominated 20 ha catchment in order to determine flow paths leading to exfiltration at the catchment outlet. Water was found to enter the catchment valley either through flow originating in the slopes or through surface infiltration during rainfall events. Although subsurface flow from the slopes to the catchment outlet occurred throughout the year, surface recharge was restricted to a few events during the wet season. In the deeper saturated profile of the valley, flow was directed upwards along the valley edges and gradually became horizontal towards the central axis of the valley. During the peak of the rainfall season, horizontal flow close to the catchment outlet intercepted the gradually sloping surface, resulting in exfiltration. Plants influenced the hydrology of the catchment by removing moisture from the root zone during spring and early summer, resulting in evapotranspiration losses from the vadose zone. Heterogeneities within the valley soil were evident as variable‐permeability layers that resulted in a seasonally confined water table within the valley. This investigation shows that the vadose zone plays an important role in redistributing surface recharge and emphasizes the importance of accounting for effective moisture in low‐yielding catchments with ephemeral surface runoff. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantification of nitrate storage in the vadose (unsaturated) zone: a missing component of terrestrial N budgets

National terrestrial nitrogen budgets for many developed countries have been calculated as part of the management of impacts of N on the environment, but these rarely represent the subsurface explicitly. Using estimates of vadose zone travel time and agricultural nitrate loading, we quantify, for the first time, the total mass of nitrate contained in the vadose zone of aquifers in England and Wales. This mass peaked in 2008 at 1400 kt N (800 to >1700 kt N from sensitivity analyses), which is approximately 2.5 to 6 times greater than saturated zone estimates for this period and indicates that the subsurface is an important store of reactive nitrogen. About 70% of the nitrate mass is estimated to be in the Chalk, with the remainder split between the Permo‐Triassic sandstones, the Jurassic Oolitic limestones and minor aquifers. Current controls on fertilizer application mean that the vadose zone is now a nitrate source, and in 2015 we estimate the net flux from the unsaturated zone to groundwater to be 72 kt N/a. The mass of nitrate in the vadose zone should be included in future terrestrial nitrogen budgets at national and global scales to improve ecosystem management. British Geological Survey © NERC 2015. Hydrological Processes © 2015 John Wiley & Sons, Ltd.     

Design of Lysimeter Leak Detector Networks for Surface Impoundments and Landfills

Sampling of soil pore moisture in the vadose zone underneath land disposal facilities (landfills and surface impoundments) for hazardous waste has been suggested as an "early warning system" to detect leakage from these facilities. Some states require vadose zone moisture sampling at such sites. Given a leak of a particular size, mathematical models can estimate the necessary moisture sample volume collection times and lysimeter spacings to guarantee detection of the leak in a homogeneous medium. Examination of 47 hazardous waste sites existing in 1984 indicated the most were located in areas with water tables too shallow to permit vadose zone detection monitoring. Several of the 47 sites had soils that could be described as loamy sand, silt loam or silty clay. Using these three soils as examples, the process of lysimeter leak-detector network design has been illustrated. For a particular loamy sand with a saturates hydraulic conductivity of 10^-6 cm/ sec, the maximum ceramic lysimeter spacing is 15.5 feet at a depth of 30 feet to collec a moisture sample of 10 mL in one week from a 1 ft2 leak. For a silt loam, maximum lysimeter spacing would be 17 feet at depth of 15 feet. For silty clays, the maximum lysimeter spacing is 7 feet at a depth of 2 feet; maximum emplacement depth is about 9 feet. Calculations show that in some soils, suction lysimeters will not be able to collect usable moisture samples. Since soil properties vary widely and lysimeter spacing is strongly dependent on soil-moisture characteristics appropriate soil measurements and modeling must be performed at each disposal facility to estimate lysimete performance and to select locations for emplacement.     

1,4‐Dioxane Soil Remediation Using Enhanced Soil Vapor Extraction (XSVE): II. Modeling

1,4‐Dioxane is a volatile organic compound that is fully miscible in water, allowing it to sequester in vadose zone pore water and serve as a long‐term source of groundwater contamination. Conventional soil vapor extraction (SVE) removes 1,4‐dioxane; however, substantial 1,4‐dioxane can remain even after other colocated chlorinated solvents have been remediated. A field demonstration of “enhanced SVE” (XSVE) with focused extraction and heated injection was conducted at former McClellan AFB, CA, achieving 94% reduction in soil concentrations. A screening‐level tool, HypeVent XSVE, was created to assist in system design and data reduction and to anticipate how operating factors affect XSVE performance (e.g., cleanup level, remediation time, etc.). It assumes well‐mixed conditions, and combines an energy balance, mass balances for water and contaminant, and a temperature‐dependent 1,4‐dioxane Henry's Law constant. User inputs include the target treatment zone size, initial 1,4‐dioxane and soil moisture concentrations, and ambient site and injection/extraction conditions (temperature, humidity). Projections based on inputs representative of demonstration site conditions adequately anticipated the observed macroscopic field results. Sensitivity analyses show that removal increases with increasing heated air injection temperature and relative humidity and decreasing initial soil moisture content.     

Vadose Zone Monitoring: An Early Warning System

Currently, vadose zone monitoring is required under the Resource Conservation and Recovery Act (RCRA) only at land treatment facilities. Contaminant leak detection through ground water monitoring is very important, but it is considered to be after the fact. Remedial action costs can be reduced considerably by monitoring the vadose zone for compounds that exhibit high rates of movement. Volatile organic compounds (VOCs) exhibit this property and are present at many municipal landfills, recycling facilities, and treatment storage and disposal facilities (TSDFs). Through the authors'personal experience, it has been noted that gaseous phase transport of VOCs through the vadose zone is at least an order of magnitude greater than advective transport of VOCs in ground water. Therefore, VOCs in soil gas are an effective early warning leak detection parameter. Downward movement of leachate can be intercepted by porous cup lysimeters. Attenuation in the vadose zone slows the apparent movement of contaminants; however, it is only a matter of time before leachate reaches the water table. The authors believe that soil-gas and pore-water monitoring should and eventually will be required at all RCRA sites. If vadose zone monitoring becomes an additional requirement under RCRA, both the facility owner and the taxpayer will benefit. During the interim, facility owners can benefit by employing vadose zone monitoring techniques coupled with either qualitative or quantitative chemical analyses.     

Characterising deep vadose zone water movement and solute transport under typical irrigated cropland in the North China Plain

Understanding the dynamics and mechanisms of soil water movement and solute transport is essential for accurately estimating recharge rates and evaluating the impacts of agricultural activities on groundwater resources. In a thick vadose zone (0–15 m) under irrigated cropland in the piedmont region of the North China Plain, soil water content, matric potential, and solute concentrations were measured. Based on these data, the dynamics of soil water and solutes were analysed to investigate the mechanisms of soil water and solute transport. The study showed that the 0–15‐m vadose zone can be divided into three layers: an infiltration and evaporation layer (0–2 m), an unsteady infiltration layer (2–6 m), and a quasi‐steady infiltration layer (6–15 m). The chloride, nitrate, and sulphate concentrations all showed greater variations in the upper soil layer (0–1 m) compared to values in the deep vadose zone (below 2 m). The average concentrations of these three anions in the deep vadose zone varied insignificantly with depth and approached values of 125, 242, and 116 mg/L. The accumulated chloride, sulphate, and nitrate were 2,179 ± 113, 1,760 ± 383, and 4,074 ± 421 kg/ha, respectively. The soil water potential and solute concentrations indicated that uniform flow and preferential flow both occurred in the deep vadose zone, and uniform flow was the dominant mechanism of soil water movement in this study. The piston‐like flow velocity of solute transport was 1.14 m per year, and the average value of calculated leached nitrate nitrogen was 107 kg/ha?year below the root zone. The results can be used to better understand recharge processes and improve groundwater resources management.     

Nitrate in the Intermediate Vadose Zone Beneath Irrigated Cropland

More than 1000 feet of fine-textured, unsaturated zone core beneath nitrogen-fertilized and irrigated farmland was collected, leached and analyzed for nitrate-nitrogen. Fertility plots treated with 200, 300, and 400 Ibs N/acre/yr accumulated significant quantities of nitrate-nitrogen in the vadose zone below the crop rooting zone. The average nitrate-nitrogen concentration approximately doubled with each 100 lbs N/acre/yr increment above the 100 lbs N/ acre/ yr treatment. Nitrate loading estimates for the plots treated with 400 lbs N/ acre/ yr indicate that over 1200 lbs N/ acre was in the vadose zone beneath the crop rooting zone. In 15 years, the nitrate moved vertically at least 60 feet through these fine-textured, unsaturated sediments. As much as 600 lbs N/acre have accumulated in the vadose zone under independent corn producers'fields.
Vadose zone sampling is effective in predicting future non-point nitrate-contaminated areas.     

Physical controls on septic leachate movement in the vadose zone at the hillslope scale,Putnam County,New York,USA

The fate and transport of contaminants in the vicinity of septic fields remains poorly understood in many hydrogeomorphological environments. We report hydrometric data from an intensive hillslope‐scale experiment conducted between 29 August and 11 November 1998 at a residential leach field in New York State. The objective of our study was to characterize water flux within the vadose zone, understand the physical controls on the flux, and predict how this ultimately will affect subsurface water quality. Soil‐water flux was calculated using matric potential measurements from a network of 25 tensiometer nests, each nest consisting of three tensiometers installed to depths of 10, 50 and 130 cm. Unsaturated hydraulic conductivity curves were derived at each depth from field‐determined time‐domain reflectometry–tensiometry moisture‐release curves and borehole permeametry measurements. Flownets indicated that a strong upward flux of soil water occurred between rainstorms. Following the onset of (typically convective) rainfall, low near‐surface matric potentials were rapidly converted to near‐saturated and saturated conditions, promoting steep vertical gradients through the near‐surface horizons of the hillslope. Lateral hydraulic gradients were typically 10 times smaller than the vertical gradients. Resultant flow vectors showed that the flux was predominantly vertical through the vadose zone, and that the flux response to precipitation was short‐lived. The flux response was controlled primarily by the shape of the unsaturated hydraulic conductivity curves, which indicated a rapid loss of conductivity below saturation. Thus, soil water had a very high residence time in the vadose zone. The absence of rapid wetting at 130 cm and the delayed and small phreatic zone response to rainfall indicated that water movement through macropores did not occur on this hillslope. These results are consistent with a Cl tracing experiment, which demonstrated that the tracer was retained in the vadose zone for several months after injection to the system. Copyright © 2002 John Wiley & Sons, Ltd.     

Comparing Natural Source Zone Depletion Pathways at a Fuel Release Site

Natural source zone depletion (NSZD) refers to processes within chemically impacted vadose and saturated zones that reduce the mass of contaminants remaining in a defined source control volume. Studies of large petroleum hydrocarbon release sites have shown that the depletion rate by vapor phase migration of degradation products from the source control volume through the vadose zone (V‐NSZD) is often considerably higher than the rate of depletion from the source control volume by groundwater flow carrying dissolved petroleum hydrocarbons arising from dissolution, desorption, or back diffusion, and degradation products arising from biodegradation (GW‐NSZD). In this study, we quantified vadose zone and GW‐NSZD at a small unpaved fuel release site in California typical of those in settings with predominantly low permeability media. We estimated vadose zone using a dense network of efflux monitoring locations at four sampling events over 2 years, and GW‐NSZD using groundwater monitoring data downgradient of the source control volume in three depth intervals spanning up to 9 years. On average, vadose zone was 17 times greater than GW‐NSZD during the time interval of comparison, and vadose zone was in the range of rates quantified at other sites with petroleum hydrocarbon releases. Estimating vadose zone and GW‐NSZD rates is challenging but the vadose zone rate is the best indicator of overall source mass depletion, whereas GW‐NSZD rates may be useful as baselines to quantify progress of natural or engineered remediation in portions of the saturated zone in which there are impediments to loss of methane and other gases to the vadose zone.     

Prediction of regional-scale groundwater recharge and nitrate storage in the vadose zone: A comparison between a global model and a regional model

Extensive nitrogen loads at the soil surface exceed plant uptake and soil biochemical capacity, and therefore lead to nitrogen accumulation in the deep vadose zone. Studies have shown that stored nitrogen in the vadose zone can eventually reach the water table and affect the quality of groundwater resources. Recently, global scale models have been implemented to quantify nitrate storage and nitrate travel time in the vadose zone. These global models are simplistic and relatively easy to implement and therefore facilitate analysis of the considered transport processes at a regional scale with no further requirements. However, the suitability of applying these models at a regional scale has not been tested. Here, we evaluate, for the first time, the performance and utility of global scale models at the regional scale. Applied to the Loess Plateau of China, we compare estimates of groundwater recharge and nitrate storage derived from global scale models with results from a regional scale approach utilizing the Richards and advection-dispersion equations. The estimated nitrate storage was compared to nitrate observations collected in the deep vadose zone (>50 m) at five sites across the Loess Plateau. Although both models predict similar spatial patterns of nitrate storage, the recharge fluxes were three times smaller and the nitrate storage was two times higher compared with the regional model. The results suggest that global scale models are a potentially useful screening tool, but require refinement for local scale applications.     

Characterization of recharge through complex vadose zone of a granitic aquifer by time-lapse electrical resistivity tomography

The vadose zone is the main region controlling water movement from the land surface to the aquifer and has a very complex structure. The use of non-invasive or minimally invasive geophysical methods especially electrical resistivity imaging is a cost-effective approach adapted for long-term monitoring of the vadose zone. The main aim of this work is to know the fractures in the vadose zone, of granitic terrene, through which the recharge or preferred path recharge to the aquifer takes place and thus to relate moisture and electrical resistivity. Time lapse electrical resistivity tomography (TLERT) experiment was carried out in the vadose zone of granitic terrene at the Indian Geophysical Research Institute, Hyderabad along two profiles to a depth of 18 m and 13 m each. The profiles are 300 m apart. Piezometric, rainfall and soil moisture data were recorded to correlate with changes in the rainfall recharge. These TLERT difference images showed that the conductivity distribution was consistent with the recharge occurring along the minor fractures. We mapped the fractures in hard rock or granites to see the effect of the recharge on resistivity variation and estimation of moisture content. These fractures act as the preferred pathways for the recharge to take place. A good correlation between the soil moisture and resistivity is established in the vadose zone of granitic aquifer. Since the vadose zone exhibits extremely high variability, both in space and time, the surface geophysical investigations such as TLERT have been a simple and useful method to characterize the vadose zone, which would not have been possible with the point measurements alone. The analyses of the pseudosection with time indicate clearly that the assumption of the piston flow of the moisture front is not valid in hard rocks. The outcome of this study may provide some indirect parameters to the well known Richard's equation in studying the unsaturated zone.     

Evidence for climatic and hillslope‐aspect controls on vadose zone hydrology and implications for saprolite weathering

Through the delivery of water in snowmelt, climate should govern the rate and extent of saprolite formation in snow‐dominated mountain watersheds, yet the mechanisms by which water flows deeply into regolith are largely unexplored. In this study we link rainfall, snow depth, and water content data from both soil and shallow saprolite to document vadose zone dynamics in two montane catchments over 2 years. Measurements of snow pack thickness and soil moisture reveal strong contrasts between north‐ and south‐facing slopes in both the timing of meltwater delivery and the duration of significant soil wetting in the shallow vadose zone. Despite similar magnitudes of snowmelt recharge, north‐facing slopes have higher sustained soil moisture compared to south‐facing slopes. To help interpret these observations, we use a 2D numerical model of vadose zone dynamics to calculate the expected space–time moisture patterns on an idealized hillslope under two wetting scenarios: a single sustained recharge pulse versus a set of short pulses. The model predicts that the duration of the recharge event exerts a stronger control on the depth and residence time of water in the upper unsaturated zone than the magnitude of the recharge event. Model calculations also imply that water should move more slowly through the subsurface and downward water flux should be substantially reduced when water is applied in several pulses rather than in one sustained event. The results suggest that thicker soil and more deeply weathered rock on north‐facing slopes may reflect greater water supply to the deep subsurface. Copyright © 2015 John Wiley & Sons, Ltd.     

Major Procedural Discrepancies in Soil Extracted Nitrate Levels and Nitrogen Isotopic Values

Preliminary vadose zone nitrate extraction experiments have revealed the extractant (2N KCl) volume affects the determination of nitrate and its nitrogen isotope ratio. In five cores, extractable NO₃-N concentrations increased an average of 1.7 times after the soil-to-extractant ratio was increased from 1:1 to 1:10. An increased extractant volume resulted in a large positive shift of stable nitrogen values (δ¹⁵N), which averaged +6.2%. An underestimation of available NO₃-N for leaching and transport through the vadose zone and a biased source interpretation from the δ¹⁵N values probably would occur if a procedure which leads to incomplete extraction of nitrate is used.     

An approach to catchment‐scale groundwater nitrate risk assessment from diffuse agricultural sources: a case study in the Upper Bann,Northern Ireland

DRASTIC has drawbacks in groundwater risk assessment that are important in guiding activities to prevention agricultural diffuse groundwater pollution. This paper presents an improved and GIS‐based D‐DRASTIC approach for groundwater nitrate risk assessment from diffuse agricultural sources based on DRASTIC. D‐DRASTIC considers the risk concept, nitrate loading, pollutant transport with runoff, depth to water, net recharge, aquifer media, soil media, topography, impact of the vadose zone media, and the hydraulic conductivity of the aquifer. D‐DRASTIC was developed within an ArcGIS environment and applied to the Upper Bann Catchment, Northern Ireland as a case study. D‐DRASTIC shows that ‘very high’ and ‘high’ zones of groundwater nitrate risk occupy 5% and 11% of the case study area, respectively. When considering groundwater pollution sources and pathways, the results using D‐DRASTIC are helpful in guiding the activities of groundwater pollution prevention at the catchment scale in the context of better implementation of the EU Water Framework Directive. Copyright © 2008 John Wiley & Sons, Ltd.     

Improved Recharge Estimation from Portable,Low‐Cost Weather Stations

Groundwater recharge estimation is a critical quantity for sustainable groundwater management. The feasibility and robustness of recharge estimation was evaluated using physical‐based modeling procedures, and data from a low‐cost weather station with remote sensor techniques in Southern Abbotsford, British Columbia, Canada. Recharge was determined using the Richards‐based vadose zone hydrological model, HYDRUS‐1D. The required meteorological data were recorded with a HOBO^TM weather station for a short observation period (about 1 year) and an existing weather station (Abbotsford A) for long‐term study purpose (27 years). Undisturbed soil cores were taken at two locations in the vicinity of the HOBO^TM weather station. The derived soil hydraulic parameters were used to characterize the soil in the numerical model. Model performance was evaluated using observed soil moisture and soil temperature data obtained from subsurface remote sensors. A rigorous sensitivity analysis was used to test the robustness of the model. Recharge during the short observation period was estimated at 863 and 816 mm. The mean annual recharge was estimated at 848 and 859 mm/year based on a time series of 27 years. The relative ratio of annual recharge‐precipitation varied from 43% to 69%. From a monthly recharge perspective, the majority (80%) of recharge due to precipitation occurred during the hydrologic winter period. The comparison of the recharge estimates with other studies indicates a good agreement. Furthermore, this method is able to predict transient recharge estimates, and can provide a reasonable tool for estimates on nutrient leaching that is often controlled by strong precipitation events and rapid infiltration of water and nitrate into the soil.     

Effects of Access Tube Material and Grout on Neutron Probe Measurements in the Vadose Zone

A test-stand experiment was conducted on neutron probe measurements using steel and Schedule 40 PVC access tubes to determine the effect of backfill grout in boreholes. The experiment used a moveable simulated vadose zone. Access tubes and grout do have masking effects on vadose zone measurements, but vadose zone moisture was detected through all configurations tested. Steel tubing has a smaller masking effect than PVC tubing. The masking effect of grout increased with borehole diameter. Although this experiment produced numerical results, conclusions drawn are qualitative in nature, rather than a quantitative calibration of the technique.     

Assessment of groundwater recharge processes through karst vadose zone by cave percolation monitoring

Recharge processes of karst aquifers are difficult to assess given their strong heterogeneity and the poorly known effect of vadose zone on infiltration. However, recharge assessment is crucial for the evaluation of groundwater resources. Moreover, the vulnerability of karst aquifers depends on vadose zone behaviour because it is the place where most contamination takes place. In this work, an in situ experimental approach was performed to identify and quantify flow and storage processes occurring in karst vadose zone. Cave percolation monitoring and dye tracing were used to investigate unsaturated zone hydrological processes. Two flow components (diffuse and quick) were identified and, respectively, account for 66% and 34% of the recharge. Quickflow was found to be the result of bypass phenomenon in vadose zone related to water saturation. We identify the role of epikarst as a shunting area, most of the storage in the vadose zone occurring via the diffuse flow component in low permeability zones. Relationship between rainfall intensity and transit velocity was demonstrated, with 5 times higher velocities for the quick recharge mode than the diffuse mode. Modelling approach with KarstMod software allowed to simulate the hybrid recharge through vadose zone and shows promising chances to properly assess the recharge processes in karst aquifer based on simple physical models.