No description available.

No description available.

Distributions of water transit times (TTDs), and related storage-selection (SAS) distributions, are spatially integrated metrics of hydrological transport within landscapes. Recent works confirm that the form of TTDs and SAS distributions should be considered time variant-possibly depending, in predictable ways, on the dynamic storage of water within the landscape. We report on a 28 day periodic-steady-state-tracer experiment performed on a model hillslope contained within a 1 m3 sloping lysimeter. Using experimental data, we calibrate physically based, spatially distributed flow and transport models, and use the calibrated models to generate time-variable SAS distributions, which are subsequently compared to those directly observed from the actual experiment. The objective is to use the spatially distributed estimates of storage and flux from the model to characterize how temporal variation in water storage influences temporal variation in flow path configurations, and resulting SAS distributions. The simulated SAS distributions mimicked well the shape of observed distributions, once the model domain reflected the spatial heterogeneity of the lysimeter soil. The spatially distributed flux vectors illustrate how the magnitude and directionality of water flux changes as the water table surface rises and falls, yielding greater contributions of younger water when the water table surface rises nearer to the soil surface. The illustrated mechanism is compliant with conclusions drawn from other recent studies and supports the notion of an inverse-storage effect, whereby the probability of younger water exiting the system increases with storage. This mechanism may be prevalent in hillslopes and headwater catchments where discharge dynamics are controlled by vertical fluctuations in the water table surface of an unconfined aquifer. Plain Language Summary Volumes of water reside within landscapes for varying amounts of time before they are discharged to a stream. That length of time determines how long the water has to interact chemically with soil and rock, and therefore influences the chemistry of water that ends up in stream channels. Quantifying the full range and variability of those travel times remains a challenge. We built an experimental hillslope, which allows us to keep track of all the water that enters and exits the soilsomething that is difficult to accomplish in open environmental systems. We introduced chemically distinct water into the hillslope at specific points in time and followed the movement of that water within, and upon exit from the soil. We discovered that the water being discharged from the hillslope tends to have resided in the landscape for shorter lengths of time when the hillslope is very wet (like a wetted sponge) than when it is very dry (like a dry sponge). This insight helps us understand how different rainfall regimes, and the associated wetness of the landscape, can potentially influence water transit times through the landscape, and their relationship with stream chemistry.

The increasing use of ethanol as a gasoline additive has raised concerns over the potential impacts ethanol might have on groundwater contamination. In North America, 10% ethanol is commonly being added to gasoline (termed E10). Ethanol is usually denaturated with gasoline compounds before being transported; consequently E95 (95% ethanol) mixtures are also common. Therefore, spills with compositions ranging from E10 to E95 can be anticipated. The compounds of main concern associated with gasoline spills are benzene, toluene, ethylbenzene and xylenes (BTEX), trimethylbenzenes (TMBs) and naphthalene, due to their higher mobility and potential risks to human health. Ethanol is thought to increase mobility of the NAPL, create higher hydrocarbon concentrations in groundwater due to cosolvency, and decrease the rate of gasoline hydrocarbon biodegradation, with consequent increase in the length of the dissolved plumes. The objective of this research was to improve the knowledge about ethanol fate in the subsurface and the impacts it might have on the fate of gasoline compounds. To investigate that, laboratory experiments and controlled field tests supported by numerical modeling were conducted. To evaluate the impact of ethanol on dissolved hydrocarbon plumes, data from a controlled field test were evaluated using a numerical model. The mass discharge of BTEX, TMB and naphthalene from three sources (E0, E10 and E95) emplaced below the water table was compared to simulation results obtained in the numerical model BIONAPL/3D. It was shown that if ethanol fuel mixtures get below the water table, ethanol is dissolved and travels downgradient fast, in a short slug. Mass discharge from the E0 and E10 sources had similar hydrocarbon decay rates, indicating that ethanol from E10 had no impact on hydrocarbon degradation. In contrast, the estimated hydrocarbon decay rates were significantly lower when the source was E95. The aquifer did not have enough oxygen to support the mass loss observed assuming complete mineralization. Assuming a heterogeneous distribution of hydraulic conductivity did little to overcome this discrepancy. A better match between the numerical model and the field data was obtained assuming partial degradation of hydrocarbons to intermediate compounds, with consequent less demand for oxygen. Besides depending on the concentration of ethanol in the groundwater, the impact of ethanol on hydrocarbon degradation appears to be highly dependent on the aquifer conditions, such as availability of electron acceptors and adaptation of the microbial community. Another concern related to ethanol biodegradation is formation of explosive levels of methane. In this study, methane δ13C from toluene and ethanol as substrates was evaluated in microcosm tests. It was shown that methane is enriched in δ13C when ethanol is the substrate. Ethanol derived methane δ13C is in the range of -20‰ to 30‰, while methane from gasoline is around -55‰. The different ranges of δ13C allow it to be used as a tool to identify methane’s origin. This tool was applied to seven ethanol-gasoline contaminated sites. Methane origin could be clearly distinguished in five of the seven sites, while in the other two sites methane appears to have been produced from both ethanol and gasoline. Both ethanol and gasoline were identified as the source of methane in hazardous concentrations. The behaviour of ethanol fuels in the unsaturated zone was evaluated in 2-dimensional (2-D) lab tests and in a controlled field test. In the 2-D lab tests, dyed gasoline and ethanol were injected in the unsaturated zone simulated in a transparent plexiglass box packed with glass beads. Tests were performed under both static conditions and with horizontal groundwater flow. It was confirmed that some ethanol can be retained in the unsaturated zone pore water. However, most of the ethanol went through the unsaturated zone and reached the pre-existing gasoline pool. Ethanol displaced the NAPL to deeper positions, and it was shown that for large ethanol releases much of the gasoline can be displaced to below the water table. The ethanol that reaches the capillary fringe was shown to travel downgradient rapidly at the top of the capillary fringe, while ethanol was also retained in the unsaturated zone. The behaviour of ethanol fuel spills was further evaluated in a controlled field test. 200L of E10 containing around 5% MTBE was released into the unsaturated zone. Groundwater concentrations of ethanol, MTBE, BTEX, TMB and naphthalene above and below the water table were monitored downgradient of the source in multilevel wells. Lab tests were performed to evaluate the applicability of these samplers for volatile organic compounds. It was shown that volatilization losses might be significant when bubbles formation in the sampling line could not be avoided. A method for losses estimation and correction of the concentrations was developed. Concentrations in the source zone were measured in soil samples. Despite the thin (35 cm) unsaturated zone at the site, most of the ethanol was retained in the unsaturated zone pore water, above the capillary fringe. Being in zones of low effective hydraulic conductivity, ethanol was not transported downgradient, and remained in the unsaturated zone for more than 100 days. Ethanol mass discharge was much lower than would be anticipated based solely on the ethanol fraction in the gasoline and on its solubility. Oscillations in the water table, particularly when a shallow position was maintained for prolonged periods, flushed some ethanol to zones with high water saturation, where horizontal transport occurred. The ethanol that reaches the saturated zone appears in the downgradient wells as a slug, with relatively low concentrations. No effect of ethanol on gasoline hydrocarbons was observed, a consequence of most of the ethanol being retained in the unsaturated zone. In summary, spills of ethanol fuels might have two different outcomes, depending on whether most of the ethanol is retained in the unsaturated zone or if most reaches the capillary fringe and the saturated zone. The relation between the ethanol volume spilled and the retention capacity of the unsaturated zone will control the spill behaviour. The volume of ethanol that can be retained in the unsaturated zone is a function of the volume of water that is contacted by the infiltrating NAPL. Therefore, the type of soil, heterogeneities, depth to the water table and area of the spill will be determinant factors. If a relatively large volume of ethanol reaches the capillary fringe, ethanol will travel rapidly in the groundwater possibly in high concentrations, potentially enhancing dissolved hydrocarbon plumes. However, when most of the ethanol is retained in the unsaturated zone, it will likely be detected downgradient only in low concentration, and in pulses spread in time. In this scenario, impact on hydrocarbon plumes will be minor.

The increasing use of ethanol as a gasoline additive has raised concerns over the potential impacts ethanol might have on groundwater contamination. In North America, 10% ethanol is commonly being added to gasoline (termed E10). Ethanol is usually denaturated with gasoline compounds before being transported; consequently E95 (95% ethanol) mixtures are also common. Therefore, spills with compositions ranging from E10 to E95 can be anticipated. The compounds of main concern associated with gasoline spills are benzene, toluene, ethylbenzene and xylenes (BTEX), trimethylbenzenes (TMBs) and naphthalene, due to their higher mobility and potential risks to human health. Ethanol is thought to increase mobility of the NAPL, create higher hydrocarbon concentrations in groundwater due to cosolvency, and decrease the rate of gasoline hydrocarbon biodegradation, with consequent increase in the length of the dissolved plumes. The objective of this research was to improve the knowledge about ethanol fate in the subsurface and the impacts it might have on the fate of gasoline compounds. To investigate that, laboratory experiments and controlled field tests supported by numerical modeling were conducted. To evaluate the impact of ethanol on dissolved hydrocarbon plumes, data from a controlled field test were evaluated using a numerical model. The mass discharge of BTEX, TMB and naphthalene from three sources (E0, E10 and E95) emplaced below the water table was compared to simulation results obtained in the numerical model BIONAPL/3D. It was shown that if ethanol fuel mixtures get below the water table, ethanol is dissolved and travels downgradient fast, in a short slug. Mass discharge from the E0 and E10 sources had similar hydrocarbon decay rates, indicating that ethanol from E10 had no impact on hydrocarbon degradation. In contrast, the estimated hydrocarbon decay rates were significantly lower when the source was E95. The aquifer did not have enough oxygen to support the mass loss observed assuming complete mineralization. Assuming a heterogeneous distribution of hydraulic conductivity did little to overcome this discrepancy. A better match between the numerical model and the field data was obtained assuming partial degradation of hydrocarbons to intermediate compounds, with consequent less demand for oxygen. Besides depending on the concentration of ethanol in the groundwater, the impact of ethanol on hydrocarbon degradation appears to be highly dependent on the aquifer conditions, such as availability of electron acceptors and adaptation of the microbial community. Another concern related to ethanol biodegradation is formation of explosive levels of methane. In this study, methane δ13C from toluene and ethanol as substrates was evaluated in microcosm tests. It was shown that methane is enriched in δ13C when ethanol is the substrate. Ethanol derived methane δ13C is in the range of -20‰ to 30‰, while methane from gasoline is around -55‰. The different ranges of δ13C allow it to be used as a tool to identify methane’s origin. This tool was applied to seven ethanol-gasoline contaminated sites. Methane origin could be clearly distinguished in five of the seven sites, while in the other two sites methane appears to have been produced from both ethanol and gasoline. Both ethanol and gasoline were identified as the source of methane in hazardous concentrations. The behaviour of ethanol fuels in the unsaturated zone was evaluated in 2-dimensional (2-D) lab tests and in a controlled field test. In the 2-D lab tests, dyed gasoline and ethanol were injected in the unsaturated zone simulated in a transparent plexiglass box packed with glass beads. Tests were performed under both static conditions and with horizontal groundwater flow. It was confirmed that some ethanol can be retained in the unsaturated zone pore water. However, most of the ethanol went through the unsaturated zone and reached the pre-existing gasoline pool. Ethanol displaced the NAPL to deeper positions, and it was shown that for large ethanol releases much of the gasoline can be displaced to below the water table. The ethanol that reaches the capillary fringe was shown to travel downgradient rapidly at the top of the capillary fringe, while ethanol was also retained in the unsaturated zone. The behaviour of ethanol fuel spills was further evaluated in a controlled field test. 200L of E10 containing around 5% MTBE was released into the unsaturated zone. Groundwater concentrations of ethanol, MTBE, BTEX, TMB and naphthalene above and below the water table were monitored downgradient of the source in multilevel wells. Lab tests were performed to evaluate the applicability of these samplers for volatile organic compounds. It was shown that volatilization losses might be significant when bubbles formation in the sampling line could not be avoided. A method for losses estimation and correction of the concentrations was developed. Concentrations in the source zone were measured in soil samples. Despite the thin (35 cm) unsaturated zone at the site, most of the ethanol was retained in the unsaturated zone pore water, above the capillary fringe. Being in zones of low effective hydraulic conductivity, ethanol was not transported downgradient, and remained in the unsaturated zone for more than 100 days. Ethanol mass discharge was much lower than would be anticipated based solely on the ethanol fraction in the gasoline and on its solubility. Oscillations in the water table, particularly when a shallow position was maintained for prolonged periods, flushed some ethanol to zones with high water saturation, where horizontal transport occurred. The ethanol that reaches the saturated zone appears in the downgradient wells as a slug, with relatively low concentrations. No effect of ethanol on gasoline hydrocarbons was observed, a consequence of most of the ethanol being retained in the unsaturated zone. In summary, spills of ethanol fuels might have two different outcomes, depending on whether most of the ethanol is retained in the unsaturated zone or if most reaches the capillary fringe and the saturated zone. The relation between the ethanol volume spilled and the retention capacity of the unsaturated zone will control the spill behaviour. The volume of ethanol that can be retained in the unsaturated zone is a function of the volume of water that is contacted by the infiltrating NAPL. Therefore, the type of soil, heterogeneities, depth to the water table and area of the spill will be determinant factors. If a relatively large volume of ethanol reaches the capillary fringe, ethanol will travel rapidly in the groundwater possibly in high concentrations, potentially enhancing dissolved hydrocarbon plumes. However, when most of the ethanol is retained in the unsaturated zone, it will likely be detected downgradient only in low concentration, and in pulses spread in time. In this scenario, impact on hydrocarbon plumes will be minor.

Physical and geophysical data collected at the Fractured Rock Research Site in Floyd County, Virginia indicate that recharge rates to the subsurface are controlled by a small scale thrust fault associated with regional thrust faulting within the Blue Ridge Province. Recharge rates appear to be correlated to spatial variation in the hydraulic conductivity of the regolith, which has been influenced by weathering rates and the metamorphic and structural history of the underlying parent material. Previous studies conducted at the Fractured Rock Research Site suggest that recharge potential can be separated into two regions: one over a vertically oriented shear zone associated with the small scale thrust fault, and the other overlying a thrust fault hanging wall. The angle of dip of the thrust fault shear zone and the fracturing within the crystalline rock adjacent to the fault plane appear to serve as geologic controls that preferentially direct infiltrated meteoric water to a deeper confined aquifer. The structural competence of the granulite gneiss thrust fault hanging wall appears to act as a barrier to deeper groundwater recharge, causing the formation of a shallow semi-confined aquifer within the overlying regolith. In-situ analysis of matric potential and moisture content shows two distinctly different recharge processes that are spatially correlated with the structure of the shallow subsurface (regolith overlying the vertically oriented shear zone and regolith overlying the thrust fault hanging wall), and have been shown to have strong temporal correlations with the dynamics of the underlying saturated conditions. Recharge flux estimates within the regolith overlying the thrust fault hanging wall are uncharacteristically high, and appear to be offset within the monitored region by the upward hydraulic gradient associated with the potentiometric surface of the underlying semi-confined aquifer. Because of the influence exerted by the upward hydraulic gradient on matric potential within the unsaturated regolith overlying the semi-confined aquifer, accurate recharge estimates could not be obtained from the matric potential data recorded by the tensiometers along this portion of the transect. Recharge flux within the regolith overlying the vertically oriented shear zone is strongly controlled by the orientation and aerial extent of the thrust fault shear zone, and highlights the importance of accurate delineation of recharge areas in crystalline rock aquifer systems. / Master of Science

Diffuse transfers of pesticides from agricultural land to ground and surface waters can lead to significant drinking water quality issues. This thesis describes the development and application of a parameter-efficient, numerical model to predict pesticide concentrations in raw water sources within an integrated hydrological framework. As such, it fills an unoccupied niche that exists in pesticide fate modelling for a computationally undemanding model that contains enough process complexity to be applicable in a wide range of catchments and hydrogeological settings in the UK and beyond. The model represents the key processes involved in pesticide fate (linear sorption and first-order degradation) and transport (surface runoff, lateral throughflow, drain flow, percolation to the unsaturated zone, calculated using a soil water balance) in the soil at a daily time step. Soil properties are derived from the national soil database for England and Wales and are used to define the boundary conditions at the interface between the subsoil and the unsaturated zone. This is the basis of the integrated hydrological framework which enables the application of the model to both surface water catchments and groundwater resources. The unsaturated zone model accounts for solute transport through two flow domains (accounting for fracture flow and intergranular matrix flow) in three hydrogeological settings (considering the presence and permeability of superficial deposits). The model was first applied to a small headwater sub-catchment in the upper Cherwell. Performance was good for drainflow predictions (Nash Sutcliffe Efficiency > 0.61) and performed better than the MACRO model and as well as the modified MACRO model. Surface water model performance was evaluated for eight pesticides in five different catchments. Performance was generally good for flow prediction (Nash Sutcliffe Efficiency > 0.59 and percentage bias below 10 %, in the validation period for all but two catchments). The 90th percentile measured concentration was captured by the model in 62 % of catchment-pesticide combinations. In theremaining cases predictions were within, at most, a factor of four of measured 90th percentile concentrations. The rank order of the frequency of pesticides detected over 0.1 μg L-1 was also predicted reasonably well (Spearman’s rank coefficient > 0.75; p < 0.05 in three catchments). Pesticide transport in the unsaturated zone model was explored at the point scale in three aquifers (chalk, limestone and sandstone). The results demonstrate that representing the unsaturated zone processes can have a major effect on the timing and magnitude of pesticide transfers to the water table. In comparison with the other catchment scale pesticide fate models that predict pesticide exposure at a daily time-step, the model developed stands out requiring only a small number of parameters for calibration and quick simulation times. The benefit of this is that the model can be used to predict pesticide exposure in multiple surface and groundwater resources relatively quickly which makes it a useful tool for water company risk assessment. The broad-scale approach to pesticide fate and transport modelling presented here can help to identify and prioritise pesticide monitoring strategies, to compare catchments in order to target catchment management and to highlight potential problems that could arise under different future scenarios.

Este trabalho tem como objetivos avaliar, para eventos de chuvas simuladas, o comportamento hidráulico de uma trincheira de infiltração experimental instalada na EESC/USP e propor uma nova metodologia de dimensionamento para trincheiras de infiltração. Para realizar a avaliação, foram conduzidos ensaios experimentais para determinar o volume infiltrado, sendo este calculado pela diferença entre o volume de entrada, medido através de vertedor triangular, e o volume armazenado na trincheira, medido a partir de medidor de nível. Em relação à nova metodologia de dimensionamento, esta foi proposta com base no modelo de Green-Ampt adaptado ao processo de infiltração horizontal. Os parâmetros de entrada do modelo: umidade, potencial matricial e condutividade hidráulica saturada, foram determinados, respectivamente, através de sondas que utilizam a Reflectometria no Domínio do Tempo (TDR), por curvas de retenção determinadas em laboratório pela técnica do papel filtro e câmara de Richards e por ensaio laboratorial com permeâmetro de carga constante. As curvas de retenção além de serem determinadas por dois métodos laboratoriais também foram avaliadas em função da distribuição uni e bimodal do tamanho médio dos poros. Para isto, utilizou-se a tomografia computadorizada para analisar o comportamento dos poros no solo estudado. O solo onde foi instalada a trincheira constitui-se de areia média a fina argilosa e apresenta a condutividade hidráulica saturada em torno de 9,06.\'10 POT.-6\' m/s. A câmara de Richards foi o método de determinação da curva de retenção que melhor caracterizou o solo quanto à sua porosidade. Os resultados demonstraram que a trincheira de infiltração é eficiente, controlando 100% do volume escoado, mesmo para chuvas com intensidades superiores aos projetos de microdrenagem. Verificou-se também que o modelo de Green-Ampt se ajusta bem ao processo de infiltração horizontal e que a metodologia de dimensionamento proposta contribui de forma eficiente e economicamente eficaz para o dimensionamento de dispositivos de infiltração. / This work assessed the hydraulic behavior of a experimental infiltration trench built at the EESC/USP and proposed a new sizing methodology for infiltration trench, for simulated events. In order to evaluate the hydraulic behavior experimental tests were taken to determine the infiltrated volume. The infiltrated volume was calculated through the difference between the entry volume, obtained by triangular weir, and the volume stored in the trench, determined by level meter. The sizing methodology proposed was based on Green-Ampt model adapted to horizontal infiltration process. The model parameters: water content, pressure head and saturated hydraulic conductivity were determined by Time Domain Reflectometer, retention curve and constant head permeameter, respectively. Retention curves were determined applying two laboratorial methods: Richards camara and filter-paper method, and were evaluate by the unimodal and bimodal distributions of the medium size of the pores. Tomography computerized was used to analyze the behavior of the soil pores. Infiltration trench was installed in a sand soil which presents the saturated hydraulic conductivity around 9,06.\'10 POT.-6\' m/s. The Richards\'s camera was the method that better represented the soil porosity. The results proved the efficiency of the infiltration trench, which controlled 100% of the runoff, even for events with intensities greater than drainage systems design storms. It was also verified that Green-Ampt model has good adjustments to horizontal infiltration process and the sizing methodology proposed is efficient for sizing of infiltration dispositive.

Groundwater use is of fundamental importance to meet rapidly expanding urban, industrial, and agricultural water requirements, particularly in semiarid zones. To quantify the current rate of groundwater recharge is thus a prerequisite for efficient and sustainable groundwater resource management in these dry areas, where such resources are often the key to economic development. Increased groundwater recharge has been documented where native vegetation or forest/shrub land was converted to grassland or pasture, or where the land was cleared for agricultural purposes. The basic argument for increased recharge is that evapotranspiration, primarily interception and transpiration, is higher in shrublands than grasslands. Chloride mass balance (CMB) has been used to estimate ancient recharge, but recharge from recent land-use change has also been documented, specifically where vegetation has been altered and deep-rooted species replaced with shallow-rooted grasses. Chloride concentrations are inversely related to recharge rates: low Clconcentrations indicate high recharge rates as Cl- is leached from the system; high Cl concentrations indicate low recharge rates since Cl- accumulates as a result of evapotranspiration. The objectives were (1) to assess the hypothesis that removal of woody-shrub vegetation and replacement with grasses increases deep drainage, (2) to quantify the amount of deep drainage after land-use change, and (3) to provide science-based data for a better understanding of changing land-use impacts on deep drainage. Eight soils from five locations in the Central Rolling Red Plains near Abilene and Sweetwater were sampled. Each location consisted of a pair of similar soils with contrasting vegetative cover: shrubland and grassland. At each site three to five soil cores were taken as deep as possible and samples were taken by horizon, but horizons were split when their thickness exceeded 0.25 m. Soil Cl- profiles under shrubland at three sites showed that virtually no water escapes beyond the root zone. High Cl- concentrations and inventories reflect soil moisture fluxes that approached 0 mm yr-1 with depth. Evapotranspiration may be largely responsible for Cl- enrichment in those profiles. Surprisingly, soil moisture flux past 200 cm under juniper woodlands was the highest with 2.6 mm yr-1. Evapotranspirative Cl- enrichment in the upper 300 cm was not observed and may suggest a different water uptake mechanism for this plant community. Soil Cl- profiles showed increased recharge rates under grassland vegetation ecosystem. Estimated deep drainage past 200 cm of 0.1 to 1.3 mm yr-1 was observed. Low Cl- concentrations and inventories suggest a leaching environment that may be in response to changes in land use/land cover.

Šiame darbe eksperimentiniais ir modeliavimo metodais buvo nagrinėjami skirtingose hidrogeologinėse sąlygose esančių branduolinių objektų aikštelių aeracijos zonos gruntuose vystantys drėgmės (ir priemaišų) pernašos procesai. Aeracijos zonos profiliai buvo palyginti atlikus eksperimentinius tyrimus ir apibendrinus duomenis apie gruntų hidraulines savybes, drėgnį ir izotopinę sudėtį. Krituliuose ir aeracijos zonos grunto profilių drėgmėje buvo nustatyti globaliai paplitusio radionuklido tričio bei vandens molekulės izotopinių traserių pasiskirstymo ypatumai. Nustatyta, kad krituliuose pasireiškiančios stabiliųjų izotopų sezoninės variacijos aeracijos zonoje ir ypač gruntiniame vandenyje beveik išnyksta, tačiau išlieka nežymūs sezoninių smailių pėdsakai, kurie Maišiagalos aikštelėje yra šiek tiek kontrastiškesni nei Stabatiškės aikštelėje, kadangi Maišiagalos aikštelėje drėgmės tranzito aeracijos zonoje laikas trumpesnis. Priėmus hipotetines žemės paviršiaus užtvindymo sąlygas esant žemam, vidutiniškam ir aukštam slėgiui, momentiniam ir pastoviam priemaišų įvedimui, palyginimui buvo išnagrinėti šeši scenarijai. Sudarytas tričio pernašos aeracijos zonoje modelis parodė, kad paros įvesties parametrų suvidurkinimas atskiram mėnesiui gali riboti tričio koncentracijos aeracijos zonos profilyje ir vandens balanso elementų pasiskirstymo atkūrimą. / Two nuclear waste disposal sites (Maišiagala radioactive waste storage facility site and Stabatiškė near surface repository site) were compared in terms of the contaminant movement through the unsaturated zone. Extensive data sets of the hydraulic properties, water content and isotope composition have been collected and summarized. Distribution features of globally widespread radionuclide tritium and the water molecule tracer isotopes in precipitation, unsaturated zone soil moisture profiles and groundwater were determined. Stable isotopes seasonal variation occurring in precipitation almost disappears in the unsaturated zone soil moisture and especially in shallow groundwater, but in both sites slight seasonal peak traces remain, which at the Maišiagala site are a bit more contrasting in comparison with the Stabatiškė site. Both sites were modelled by using unsaturated flow and transport models. Six scenarios have been developed to simulate the water and solute movement under low, moderate, and high water input, and instantaneous and continuous solute input conditions. In order to validate the representativeness of the hydraulic parameters, the model has been used to estimate the tritium distribution in the unsaturated zone, which properly represents the dynamics of the unsaturated zone. The uniformity of the daily input in each month should be kept in mind in the evaluation of the smooth changes of the tritium profile and water budget elements.