Vadose Zone Groundwater Research Articles

Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3−) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3− pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3− are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3− biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3− in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60–80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3− consumption, leading to increased NO3− leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3− concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3− concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3− to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3− biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.

Extending the Overlay and Index: A Simple Method for Assessing Aquifer Vulnerability in a Combined Vadose Zone—Groundwater Flow System

This work extends the overlay and index methods for intrinsic groundwater vulnerability, that typically involve the soil surface and the vadose zone, to groundwater (saturated) transport. The method is “hybrid” as it combines the standard overlay and index methods with a simplified process-based approach for the groundwater component. For the latter, we make use of concept and methodologies based on geomorphological analysis, employing tools that are generally implemented in Geographic Information Systems (GIS). The proposed method is based on a simple probabilistic analysis, in which the overlay and index method (that can be any) provides the probability that the contamination reaches the groundwater through the vadose zone, while the probability that the contaminant reaches a generic location in the groundwater system is determined by analyzing the groundwater streamlines. The analysis leads to the definition of the combined vulnerability index υ, that takes care of both transport in the vadose zone and the aquifer. The method is applied to a groundwater catchment in the Campania region, Southern Italy. The method is simple and effective in assessing aquifer vulnerability in a combined vadose zone—groundwater flow system, useful for preliminary, screening analysis of the intrinsic aquifer vulnerability.

A case study on the migration of nuclides in the coupled vadose zone-groundwater system at a proposed spent fuel reprocessing site in Gansu, China

Nuclear energy represents a relatively novel clean energy source, and potential nuclear contamination must be considered while developing the nuclear industry. In the context of a hypothetical nuclear accident, a nuclide migration model was established based on regional geological and hydrological data to investigate the migration characteristics of the radionuclides 3H, 99Tc(VII), 137Cs, and 79Se(IV) in a proposed spent fuel reprocessing site (SFRS) in Gansu, China. The temporal and spatial evolution of these four nuclides as well as their effects on the environment and local population were discussed in detail. The model of nuclide migration in the vadose zone was coupled to a large-scale regional nuclide migration model of groundwater in the form of boundary conditions. The results under the least favorable conditions showed that nuclides with high adsorption rates (137Cs and 79Se(IV)) remained in the vadose zone and decayed out completely. In contrast, nuclides with low adsorption rates (3H and 99Tc(VII)) penetrated the vadose zone and polluted the groundwater. The vadose zone delayed the arrival of 3H and 99Tc(VII) to the phreatic surface and reduced their concentrations. It was predicted that the concentration of 3H in the groundwater would fall below the threshold after 40 years, and the distance from the leakage area to the leading edge of the contamination plume would be 374 m. 99Tc(VII), which has a slightly higher adsorption rate and a lower decay rate than 3H, spread 833 m after 3000 years when its concentration was below the threshold. 3H and 99Tc(VII) did not migrate to downstream farms to threaten the safety of residential drinking water. This research presents a safety assessment of a nuclear area in Gansu, China, and provides a significant reference for the future construction of nuclear facilities and nuclear contamination control.

Quantify the effects of groundwater level recovery on groundwater nitrate dynamics through a quasi-3D integrated model for the vadose zone-groundwater coupled system

Groundwater level (GWL) recovery in some semiarid regions, attributed to mitigation countermeasures for groundwater depletion, potentially causes nitrate accumulated in the vadose zone to be introduced into the aquifer. However, the extent to which GWL recovery affects interactions between the vadose zone and saturated aquifers, migration pathways of soil nitrogen and groundwater nitrate dynamics have not been explicitly determined. This study established a quasi-3D feedback model for the vadose zone-groundwater coupled system in a typical GWL recovery area and quantitatively evaluated the effects of GWL recovery on nitrate-N leaching fluxes via the vadose zone and groundwater nitrate-N dynamics. Within the framework of the integrated model, both the water/contaminant leaching fluxes and the depth to groundwater were exchanged at each flow time step. The obtained results reveal that the temporal changes in nitrate-N leaching fluxes depended on the behaviors of precipitation, farmland irrigation and lithology of the vadose zone, while its spatial patterns were determined by both the GWL undulation and the vertical profiles of nitrate-N content. Furthermore, the GWL recovery caused the magnitude of the nitrate-N leaching fluxes into the aquifer to increase by 44.4%. Along with the GWL recovery, the phreatic aquifer volume increased by 7.47%, and the nitrate-N mass herein increased by 40.06%, which was largely driven by the nitrate-N leaching flux. Consequently, the average groundwater nitrate-N concentration in the GWL recovery region increased by approximately 2.4 mg/L, apart from the artificial recharge route. This finding suggests that the intensified leaching of soil contaminants, given the circumstances of GWL recovery, has a negative effect on groundwater quality. An appropriate groundwater management scheme is therefore urgently required to achieve an optimal balance between GWL recovery and groundwater environment.

Tracing the Sources and Fate of NO3- in the Vadose Zone-Groundwater System of a Thousand-Year-Cultivated Region.

Excess nitrate (NO3-) loading in terrestrial and aquatic ecosystems can result in critical environmental and health issues. NO3--rich groundwater has been recorded in the Guanzhong Plain in the Yellow River Basin of China for over 1000 years. To assess the sources and fate of NO3- in the vadose zone and groundwater, numerous samples were collected via borehole drilling and field surveys, followed by analysis and stable NO3- isotope quantification. The results demonstrated that the NO3- concentration in 38% of the groundwater samples exceeded the limit set by the World Health Organization. The total NO3- stock in the 0-10 m soil profile of the orchards was 3.7 times higher than that of the croplands, suggesting that the cropland-to-orchard transition aggravated NO3- accumulation in the deep vadose zone. Based on a Bayesian mixing model applied to stable NO3- isotopes (δ15N and δ18O), NO3- accumulation in the vadose zone was predominantly from manure and sewage N (MN, 27-54%), soil N (SN, 0-64%), and chemical N fertilizer (FN, 4-46%). MN was, by far, the greatest contributor to groundwater NO3- (58-82%). The results also indicated that groundwater NO3- was mainly associated with the soil and hydrogeochemical characteristics, whereas no relationship with modern agricultural activities was observed, likely due to the time delay in the thick vadose zone. The estimated residence time of NO3- in the vadose zone varied from decades to centuries; however, NO3- might reach the aquifer in the near future in areas with recent FN loading, especially those under cropland-to-orchard transition or where the vadose zone is relatively thin. This study suggests that future agricultural land-use transitions from croplands to orchards should be promoted with caution in areas with shallow vadose zones and coarse soil texture.

Interaction Simulation of Vadose Zone Water and Groundwater in Cele Oasis: Assessment of the Impact of Agricultural Intensification, Northwestern China

Agricultural intensification has boosted land productivity, but it has also created new sustainability issues. As one of the most important human habitations and agricultural farming areas in arid areas, the Cele Oasis has a very developed agricultural system. This paper studies the long-term effects of different types of agricultural intensification strategies on groundwater level fluctuations in the Cele Oasis. A soil water flow (HYDRUS-1D) and aquifer simulation (MODFLOW) coupling model were used to construct the geometric structures of the vadose zone and saturated zone in the Cele Oasis and to analyze the recharge and discharge mechanism of the oasis. The results showed that HYDRUS-1D accurately simulated soil moisture transport in the Cele Oasis, providing reliable data for calibration of the MODFLOW model. The groundwater level simulated by MODFLOW was in good agreement with the observed value. The results of the R2, RMSE, and NSE were ranges of 0.77–0.90, 0.45–0.74 m, and 0.76–0.87, respectively. The errors were acceptable limits. The coupling model predicted the responses of different agricultural types and cropping scenarios to groundwater. Predictions showed that the groundwater level in the Cele Oasis remained stable under the current cropping scenario (100% cropping intensity), and that the groundwater level decreased slightly under the cropping scenario (110% cropping intensity and 130% cropping intensity). When the cropping scenario was at 170% cropping intensity, the groundwater level decreased rapidly, and the maximum drawdown value was 7 m. Therefore, the maximum cropping intensity of the Cele Oasis in the future should be 130% of the current cropping intensity.

Fate and transport modelling framework for assessing risks to soil and groundwater from chemicals accidentally released during surface operations: An Australian example application from shale gas developments

Shale and tight gas developments in the Beetaloo (28,000 km2) and Cooper (139,000 km2) basins of Australia are subject to stringent State and Federal Government controls and assessments. Several scientific investigations are ongoing to improve the scientific basis of the risks from unconventional gas developments to water and the environment. In this study a framework was developed to derive estimates of chemical dilution associated with leakage to groundwater from accidental release of chemicals used for shale and tight gas extraction in Australia. The quantitative assessment accounted for key landscape parameters that determine natural attenuation: soil type, depth to groundwater and groundwater velocity. Both basins were discretised into 1000 × 1000 m2 grids for which the unsaturated zone and groundwater dilution factors were derived. Migration of chemicals through deep unsaturated zones was calculated with the HYDRUS-1D simulator, taking account of best-available hydraulic properties from a digital soil database. A three-dimensional analytical solution of the advection–dispersion equation provided estimates of dilution in groundwater after solutes travelled 500 m from the centre (source location) to the edge of every grid cell. The combined vadose zone-groundwater dilution factors were used to determine under which conditions concentrations of hydraulic fracturing chemicals or flowback water accidentally released into the environment would decrease to levels that are no longer considered harmful to the environment. When the method was applied to 39 hydraulic fracturing chemicals scheduled for stimulation of a shale gas well, ecotoxicological risk quotients (RQ) were calculated to indicate which chemicals were of no environmental concern. This work contributes to increasing the efficiency of quantitative impact assessments and provides a framework to develop dedicated monitoring and management practices to support regulation and management of the gas industry in Australia.

Modelling vadose zone flows and groundwater dynamics of alluvial aquifers in Eastern Gangetic Plains of India: evaluating the effects of agricultural intensification

Modelling vadose zone flows and groundwater dynamics of alluvial aquifers in Eastern Gangetic Plains of India: evaluating the effects of agricultural intensification

Analytical Model for Volatile Organic Compound Transport in the Coupled Vadose Zone-Groundwater System.

A 3D mathematical model that describes transport of volatile organic compounds in a coupled vadose-saturated zone system is proposed. The subsurface processes incorporated in the model include advection, dispersion, interphase mass transfer, and diffusive mass exchange between two horizontal porous media formations, as well as the time-dependent mass loading from a source zone. The analytical solutions are derived subject to the specific initial and boundary conditions. The solutions are evaluated by numerical Laplace inverse transform. The model solutions can be used to study the fate and transport in subsurface formations composed of a vadose zone and a water table aquifer, where the volatile organic compound is released from entrapped nonaqueous phase liquid in the vadose zone, or the dissolved volatile organic compound transports with groundwater accompanied by diffusive mass transfer into the overlying soil formations. Mass transfer between two layers is demonstrated to have back-diffusion characteristics, which results in secondary contamination and retains low levels of contaminant concentrations over a prolonged period of time. The model solutions are specifically useful in assessing the vapor intrusion process in a contaminated site where a vadose zone is underlain by a water table aquifer contaminated with volatile organic compounds.

Long-term nitrogen retention and transit time distribution in agricultural catchments in western France

Elevated nitrogen (N) concentrations have detrimental effects on aquatic ecosystems worldwide, calling for effective management practices. However, catchment-scale annual mass-balance estimates often exhibit N deficits and time lags between the trajectory of net N inputs and that of N riverine export. Here, we analyzed 40-year time series of N surplus and nitrate-N loads in 16 mesoscale catchments (104–10 135 km2) of a temperate agricultural region, with the aim to (1) investigate the fate of the ‘missing N’, either still in transit through the soil—vadose zone—groundwater continuum or removed via denitrification, and (2) estimate the transit time distribution of N by convoluting the input signal with a lognormal model. We found that apparent N retention, the ‘missing N’, ranged from 45%–88% of then N net input, and that topsoil N accumulation alone accounted for ca. two-thirds of this retention. The mode of the nitrate-N transit time distribution ranged from 2–14 years and was negatively correlated with the estimated retention. Apparent retention was controlled primarily by average runoff, while the transit time mode was controlled in part by lithology. We conclude that the fate of the soil ‘biogeochemical legacy’, which represents much of the catchment-scale ‘missing N’, is in our hands, since the N accumulated in soils can still be recycled in agroecosystems.

Distribution and molecular chemodiversity of dissolved organic nitrogen in the vadose zone-groundwater system of a fluvial plain, northern China: Implications for understanding its loss pathway to groundwater

Nitrogen (N) pollution in groundwater has become a worldwide environmental geological issue due to the excessive N application into the vadose zone and furthered N leaching. Dissolved organic nitrogen (DON) are proposed as an overlooked pathway of N loss from agricultural systems to groundwater recently. Here, we collected soil (0–320 cm) and groundwater samples in a historic agricultural area to characterize the distribution and chemodiversity of DON in the vadose zone-groundwater system, and identified specific linkages between DON traits and the bacterial community. The results showed that DON and NO3−-N were the main forms of dissolved N in the vadose zone-groundwater system. The deep vadose zone (> 100 cm) was an important storage area for DON (44.9%), having implications for long-term groundwater quality degradation. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) revealed that the DON was dominated by condensed aromatics and lignins (57.2%) in the vadose zone, whereas amino sugars, proteins, peptides and lignins (72.5%) were dominant in groundwater. By analyzing shared and ubiquitous DON molecular formulas detected among different layers, it was found that < 2.52% of DONs could be leached from surface soil to groundwater directly, and most DONs went through biological conversion during the whole leaching path. It was identified that bacterial community played an important role in DONs transformation. The most active bacteria in the transformation were Nitrospira, Bacillus, and Sphingomonas and they tended to interact with DON of high N/C and H/C ratios, causing molecules with high unsaturation, high aromaticity and high oxidation to accumulate. The results would be helpful to elucidate DON occurrence in groundwater and track the key processes governing DON transport from the surface soil to groundwater.

Implementation of biocementation for a partially saturated problematic soil of the UK railway network

This paper refers to biocementation of a problematic soil of the UK railway network as a potential stabilisation technique of this soil using indigenous ureolytic bacteria. The soil is peat, a soft foundation soil also subject to oxidation wastage. As the peat is under existing embankments, electrokinetics (EK) is proposed as a promising technique to implement treatments. In the context of unsaturated soils the paper thus focuses on two particular aspects relevant for the implementation of treatments and the stability of this soil, namely: a) the effect of degree of saturation of the peat on the bio-electrokinetic treatment ; b) the soil water retention curve of the soil affecting flow and transport; these are relevant as we focus on understanding and modelling the implementation of treatments through electrokinetics; moreover for the peat it is of importance to understand moisture exchange in the vadose zone and control groundwater table levels (e.g. during electrokinetics) in order to prevent further oxidation. After isolation and screening of indigenous microorgansisms Bacillus licheniformis was selected for further testing. The results in terms of unconfined compressive strength, CaCO3 content, swelling and compression behaviour and water retention proved the feasibility of biocementation using this indigenous microorganism. Ongoing work is assessing the required treated soil characteristics and related required biocementation degree to solve UK rail's peat foundation problems. Upscaling of the techniques towards in situ implementation is also planned in the next stage of the research.

Infiltration Rate of Groundwater in Vadose Zone of Uplifted Carbonate Island

Solute contents in the vadose zone of limestone strata on Miyako Island of the Ryukyu Archipelago in Japan were measured by means of electrical conductivities (EC) of the extracted solutions of limestone core samples. A vertically nonuniform distribution of solute content was observed, which should lead to temporal variations of solute concentration in the groundwater under the water table. On the other hand, the data of chloride concentration in groundwater from 13 wells on the island was analyzed to see their long time variations. Both of these two approaches resulted in recharge rates of around 10^-7 m s^-1 for the vadose zone in the limestone strata of this typical uplifted carbonate island of the western Pacific Ocean.

Salinity control in a clay soil beneath an orchard irrigated with treated waste water in the presence of a high water table: A numerical study

Summary A citrus orchard planted on a structured, clay soil associated with a high water table, irrigated by drip irrigation system using treated waste water (TWW) and local well water (LWW) was considered here. The scope of the present study was to analyze transport of mixed-ion, interacting salts in a combined vadose zone–groundwater flow system focusing on the following issues: (i) long-term effects of irrigation with TWW on the response of the flow system, identifying the main factors ( e.g. , soil salinity, soil sodicity) that control these effects, and (ii) salinity control aiming at improving both crop productivity and groundwater quality. To pursue this two-fold goal, 3-D numerical simulations of field-scale flow and transport were performed for an extended period of time, considering realistic features of the soil, water table, crop, weather and irrigation, and the coupling between the flow and the transport through the dependence of the soil hydraulic functions, K ( ψ ) and θ ( ψ ), on soil solution concentration C, and sodium adsorption ratio, SAR. Results of the analyses suggest that in the case studied, the long-term effect of irrigation with TWW on the response of the flow system is attributed to the enhanced salinity of the TWW, and not to the increase in soil sodicity. The latter findings are attributed to: (i) the negative effect of soil salinity on water uptake, and the tradeoff between water uptake and drainage flux, and, concurrently, solute discharge below the root zone; and, (ii) the tradeoff between the effects of C and SAR on K ( ψ ) and θ ( ψ ). Furthermore, it was demonstrated that a data-driven protocol for soil salinity control, based on alternating irrigation water quality between TWW and desalinized water, guided by the soil solution salinity at the centroid of the soil volume active in water uptake, may lead to a substantial increase in crop yield, and to a substantial decrease in the salinity load in the groundwater.

Stochastic analysis of transport in a combined heterogeneous vadose zone–groundwater flow system

First‐order analysis was used to investigate the effect of a few characteristics of the vadose zone and the solute source at the soil surface on the expected breakthrough curves (BTCs) of tracer solutes in a combined unsaturated‐saturated spatially heterogeneous formation with a three‐dimensional, statistically anisotropic structure. The analysis is based on a stochastic continuum representation of the stationary and nonstationary Eulerian velocity vectors in the unsaturated and saturated domains, respectively, and a general Lagrangian description of the solute mass flux in steady state flow. Results of the present analysis suggest that the length scales and the flow characteristics of the spatially heterogeneous, unsaturated zone that affect the travel time probability density function between the soil surface and the water table also affect the expected solute BTC at a control plane located downstream in the groundwater. The flow in the unsaturated zone might enhance the early arrival time and the spreading of the expected solute BTC in the groundwater, especially when the vertical extent of the unsaturated zone is relatively large; when the unsaturated zone consists of relatively dry, coarse‐textured, stratified soil material; and when the horizontal extent of the solute source in the direction of the mean groundwater flow is relatively large. The effect of the characteristics of the vadose zone and the solute source at the soil surface on the expected solute BTC in the groundwater decreases as the distance to the control plane increases and as the process of the solute breakthrough is more advanced. The relatively good agreement between the results of the present first‐order, stochastic analysis with the results of detailed, numerical analyses of flow and transport in a three‐dimensional heterogeneous, combined vadose zone–groundwater flow domain suggests that the first‐order stochastic analyses may be sufficiently reliable to show appropriate trends which might occur in realistic combined vadose zone–groundwater flow systems.

Cementitious Wasteforms for Immobilization of Low-Activity Radioactive Wastes

AbstractSolidification of low-activity wastes with cementitious materials is a widely accepted technique that contains and isolates waste from the hydrologic environment. The radionuclides I-129, Se-75, Tc-99, and U-238 are identified as long-term dose contributors. The anionic nature of these radionuclides in aqueous solutions allows them to readily leach into the subsurface environment. Any failure of concrete encasement may result in water intrusion and consequent mobilization of radionuclides from the waste packages via mass flow and/or diffusion into the surrounding subsurface environment. Assessing the long-term performance of waste grouts for encasement of radionuclides requires understanding the: 1) speciation and interaction of the radionuclides within the concrete wasteform, 2) diffusion of radionuclide species when contacted with vadose zone porewater or groundwater under environmentally relevant conditions, and 3) long-term durability and weathering of concrete waste forms. An improved understanding of the interactions of long-lived radionuclides in cementitious matrices will improve predictions of the long-term fate of these sequestered contaminants. An integrated laboratory investigation has been conducted including a: 1) multifaceted spectroscopic investigation to interrogate the speciation and interaction of radionuclides within concrete wasteforms, 2) solubility tests to quantify the stability of solid phases identified as radionuclide-controlling phases, 3) quantify the diffusion of radionuclides from concrete wasteforms into surrounding subsurface sediment under realistic moisture contents (4%, 7%, and 15% by weight moisture content), 4) quantify the long-term durability of concrete waste forms as a function environmental parameters relevant to depository conditions, and 5) identify the formation of secondary phases or processes (microcracking) that influence radionuclide retention. Data obtained from this investigation provides valuable information for understanding the speciation, behavior, and fate of radionuclides immobilized within concrete wasteforms under vadose zone conditions and underscores the necessity for robust, multi-disciplinary performance assessments for concrete waste forms.

Equivalent vadose zone steady state flow: An assessment of its capability to predict transport in a realistic combined vadose zone–groundwater flow system

The applicability of an equivalent steady state vadose zone flow model to a realistic transient three‐dimensional, heterogeneous, combined vadose zone–groundwater flow system was tested here. Two cases pertinent to semiarid regions and the presence (or absence) of an irrigated crop were analyzed and are presented in detail using the hydraulic properties from the Bet Dagan trench. In addition, the case of a more humid region characterized by additional summer precipitation and soils of different textures was analyzed and is discussed briefly. Results of the analyses suggested that when the water table is located at sufficiently large distance from the soil surface, the vadose zone can be divided conceptually into two distinct zones: a highly transient, near‐surface zone and a deeper, quasi steady state zone. Under these conditions, an equivalent steady state influx, obtained by averaging the cumulative flux of the net applied water over the relevant time period, may provide a relatively good approximation of the cumulative water flux that crossed the water table. As for solute transport, in all cases examined, the trajectories of the centroid of the solute plume and the mean and the standard deviation of the solute breakthrough curves at a given horizontal control plane, CP (expressed as functions of the cumulative flux crossing the pertinent horizontal CP), associated with the transient flows were in relatively good agreement with their counterparts associated with the equivalent steady state flows, particularly under noncropped conditions and when the soil texture is coarser. The main finding of this study is that for the case in which the water table is located at sufficiently large distance from the soil surface, equivalent steady state vadose zone flow may reconstruct the “true” solute BTC (or the solute travel time PDF) at a horizontal CP located in the vicinity of the water table depth. The latter finding applies to heterogeneous soils of different textures and to different atmospheric forcing conditions at the soil surface and in the presence (or absence) of vegetation and despite the fact that features of the transport unique to vadose zone transient flow (induced by the temporal fluctuations in the water table depth) cannot be reproduced by the equivalent steady state vadose zone flow.

Evaluating Interactions between Groundwater and Vadose Zone Using the HYDRUS‐Based Flow Package for MODFLOW

In the past, vadose zone processes in groundwater flow models were dramatically simplified (or even neglected) due to constraints on computational resources. The one‐dimensional unsaturated flow package HYDRUS, recently developed for the groundwater model MODFLOW, was evaluated and compared with other contemporary modeling approaches used to characterize vadose zone effects in groundwater models. Being fully incorporated into the MODFLOW program, the HYDRUS package provides MODFLOW with recharge fluxes at the water table, while MODFLOW provides HYDRUS with the position of the groundwater table that is used as the bottom boundary condition in the package. The performance of the HYDRUS package was analyzed for three case studies of increasing complexity: (i) a one‐dimensional infiltration experiment; (ii) a two‐dimensional water table recharge experiment; and (iii) a hypothetical regional‐scale groundwater flow problem. The computational need and modeling efficiency of the HYDRUS package was compared with other relevant MODFLOW packages (VSF, UZF1, and REC‐ET). For smaller scale problems (up to two dimensions), the VSF process and the HYDRUS and UZF1 packages performed comparably well in terms of modeling efficiency and simulation times. Because of the high computational demand, it was not feasible to use the VSF process on a typical personal computer for the hypothetical large‐scale groundwater problem. The HYDRUS package provided a much more efficient alternative to VSF for this large‐scale groundwater problem and could better account for vadose zone processes than the UZF1 and REC‐ET packages. For large‐scale groundwater problems, the HYDRUS package provides an optimal tradeoff between computational effort and accuracy of model simulations for coupled vadose zone–groundwater problems.

Dissolution rates and vadose zone drainage from strontium isotope measurements of groundwater in the Pasco Basin, WA unconfined aquifer

Strontium isotope ratios ( 87Sr/ 86Sr) measured in groundwater samples from 273 wells in the Pasco Basin unconfined aquifer below the US Department Of Energy Hanford Site show large and systematic variations that provide constraints on groundwater recharge, weathering rates (for saturated zone dissolution) of the aquifer host rocks, communication between unconfined and deeper confined aquifers, and vadose zone–groundwater interaction. The impact of millions of cubic meters of wastewater discharged to the vadose zone (10 3–10 5 times higher than ambient drainage) shows up strikingly on maps of groundwater 87Sr/ 86Sr. Extensive access through the many groundwater monitoring wells at the site allows for an unprecedented opportunity to evaluate the strontium geochemistry of a major aquifer, hosted primarily in unconsolidated sediments, and relate it to both long term properties and recent disturbances. Groundwater 87Sr/ 86Sr increases systematically from 0.707 to 0.712 from west to east across the Hanford Site, in the general direction of groundwater flow, as a result of addition of Sr from the weathering of aquifer sediments and from diffuse drainage through the Pasco Basin vadose zone. The lower 87Sr/ 86Sr groundwater reflects recharge waters that have acquired Sr from Columbia River Basalts. Based on a steady-state model of Sr reactive transport and drainage, there is an average natural drainage flux of 0–1.4 mm/yr near the western margin of the Hanford Site, and ambient drainage may be up to 30 mm/yr in the center of the site assuming an average bulk rock weathering rate of 10 −7.5 g/(g/yr). The weathering rates cannot be higher than those assumed by more than a factor of 2–3, in order to match Sr isotope data.

Catchment hydrologic response with a fully distributed triangulated irregular network model

This study explores various aspects of catchment hydrology based on a mechanistic modeling of distributed watershed processes. A new physics‐based, distributed‐parameter hydrological model that uses an irregular spatial discretization is introduced. The model accounts, on a continuous basis, for the processes of rainfall interception, evapotranspiration, moisture dynamics in the unsaturated and saturated zones, and runoff routing. Simulations of several mid‐ to large‐sized watersheds (∼103 km2) highlight various dynamic relationships between the vadose zone–groundwater processes and their dependence on the land surface characteristics. It is argued that the model inferences can be used for interpretation of distributed relationships in a catchment. By exploiting a multiple‐resolution representation, the hydrologic features of the watershed terrain are captured with only 5–10% of the original grid nodes. This computational efficiency suggests the feasibility of the operational use of fully distributed, physics‐based models for large watersheds.