Thick Vadose Zone Research Articles

Spatial and temporal variability of groundwater recharge in a sandstone aquifer in a semiarid region

Abstract. With the aim to understand the spatial and temporal variability of groundwater recharge, a high-resolution, spatially distributed numerical model (MIKE SHE) representing surface water and groundwater was used to simulate responses to precipitation in a 2.16 km2 upland catchment on fractured sandstone near Los Angeles, California. Exceptionally high temporal and spatial resolution was used for this catchment modeling: hourly climate data, a 20 m×20 m grid in the horizontal plane, and 240 numerical layers distributed vertically within the thick vadose zone and in the upper part of the groundwater zone. The finest practical spatial and temporal resolutions were selected to accommodate the large degree of surface and subsurface variability of catchment features. Physical property values for the different lithologies were assigned based on previous on-site investigations, whereas the parameters controlling streamflow and evapotranspiration were derived from calibration to continuous streamflow at the outfall and to average hydraulic heads from 17 wells. Confidence in the calibrated model was enhanced by validation through (i) comparison of simulated average recharge to estimates based on the applications of the chloride mass-balance method to data from the groundwater and vadose zones within and beyond the catchment, (ii) comparison of the water isotope signature (18O and 2H) in shallow groundwater to the variability of isotope signatures for precipitation events over an annual cycle, and (iii) comparison of simulated recharge time series and observed fluctuation of water levels. The average simulated recharge across the catchment for the period 1995–2014 is 16 mm yr−1 (4 % of the average annual precipitation), which is consistent with previous estimates obtained by using the chloride mass balance method (4.2 % of the average precipitation). However, one of the most unexpected results was that local recharge was simulated to vary from 0 to >1000 mm yr−1 due to episodic precipitation and overland runoff effects. This recharge occurs episodically with the major flux events at the bottom of the evapotranspiration zone, as simulated by MIKE SHE and confirmed by the isotope signatures, occurring only at the end of the rainy season. This is the first study that combines MIKE SHE simulations with the analysis of water isotopes in groundwater and rainfall to determine the timing of recharge in a sedimentary bedrock aquifer in a semiarid region. The study advances the understanding of recharge and unsaturated flow processes and enhances our ability to predict the effects of surface and subsurface features on recharge rates. This is crucial in highly heterogeneous contaminated sites because different contaminant source areas have widely varying recharge and, hence, groundwater fluxes impacting their mobility.

Depth-Dependent Seasonal Variation of Soil Water in a Thick Vadose Zone in the Badain Jaran Desert, China

In a vadose zone the soil water content can change seasonally, driven by seasonal variations of meteorological factors. This dynamic behavior is depth-dependent, which controls the groundwater recharge from infiltration, and plays an essential role in the environments in arid and semi-arid regions. In particular, the depth-dependent seasonal variations of soil water were investigated in the Badain Jaran Desert (BJD), China, where the vadose zone is thick. The monitoring results showed that the amplitudes of temperature and soil moisture content in the shallow vadose zone (depth < 3 m) significantly decrease with depth. For the deep vadose zone (depth >3 m), the depth-dependent dynamic was synthetically estimated with both numerical and analytical models. Results show that the penetration depth of seasonal fluctuation is about 47 m, below which the infiltration flux stabilizes at a level of 30.7 ± 4 mm/yr. The depth to water table in the BJD is generally larger than 50 m, up to 480 m. As a consequence, groundwater recharge from infiltration in this area almost does not change in different seasons.

Estimation of natural groundwater recharge in the Aguascalientes semiarid valley, Mexico

Groundwater recharge (GWR) is analyzed and evaluated within the Aguascalientes Valley by means of a modified linearized Boussinesq equation and the Water Table Fluctuations (WTF) technique. These techniques use the specific yield, water table variations and the subsurface drainage of groundwater. The methodology is applied to the semiarid Aguascalientes valley, which contains a thick vadose zone. The combination of the analytical solution based on the Boussinesq equation and the WTF technique, allows the method to be applied in areas with deep groundwater levels, such as the Aguascalientes valley, without the need for high temporal resolution data. The data for the application of the method were provided by various government agencies and includes information on 145 wells positioned within the valley during the period 1985-2015. The specific yield and the transmissivity were integrated from the results of two previous investigations. Results indicate that the variation of recharge ranges from 0.86 to 525.69 mm/year in the analyzed period, with the highest recharge rates occurring in the north and center parts of the valley and is likely attributed to high specific yield and vertical hydraulic conductivity due to the presence of coarse permeable soils present in the area. Conversely, the lowest recharge rates were found to occur near the north and south borders of the valley.

In Situ and GRACE‐Based Groundwater Observations: Similarities, Discrepancies, and Evaluation in the High Plains Aquifer in Kansas

AbstractRelating Gravity Recovery and Climate Experiment (GRACE)‐derived terrestrial water storage anomaly (aTWS) variations to in situ groundwater data is important to understanding GRACE data utility. Here GRACE‐derived annual changes in aTWS (ΔTWS) from National Aeronautics and Space Administration's Jet Propulsion Laboratory and Goddard Space Flight Center are compared to annual changes in saturated groundwater volumes (ΔGW) in the High Plains aquifer (HPA) and overlying alluvial aquifers in Kansas. Regression analysis suggests that trends in ΔTWS are strongly related to trends in ΔGWalluvial and weakly related to trends in ΔGWHPA, although the magnitude of ΔTWS is much larger than ΔGWalluvial and is similar to ΔGWHPA. Unlike alluvial aquifers, a thick vadose zone overlies the HPA. Estimates of changes in vadose zone water content (ΔVZ) using GRACE products are similar in magnitude to annual variations in North American Land Data Assimilation soil moisture data, suggesting an unexpected dominance of ΔVZ in ΔTWS despite significant groundwater depletion. Results demonstrate that water storage changes in shallow alluvial aquifers, the root zone, the deep vadose zone, and the HPA itself are all significant in the HPA region of Kansas. Consequently, GRACE observations cannot, alone, be taken to reflect variations in saturated zone groundwater storage in the HPA, or other large, heavily used aquifers subject to substantial groundwater depletion, when deep unsaturated zones exist. GRACE can best provide guidance on regional aquifer storage changes where water storage changes in the vadose zone and perched or adjacent aquifers are well constrained.

Relationship between soil water content and soil particle size on typical slopes of the Loess Plateau during a drought year

In the context of global climate change as well as local climate warming and drying on the Loess Plateau of China, understanding the relationship between soil particle size and soil water distribution during years of atypical precipitation is important. In this study, fractal geometry theory is used to describe the mechanical composition and texture of soils to improve our understanding of hydropedology and ecohydrology in the critical zone on the Loess Plateau. One grassland slope and two shrubland slopes were selected in the hilly and gully region of the Loess Plateau, and soils were sampled along hillslope transects at depths of 0–500 cm. Fractal theory and redundancy analysis (RDA) were used to identify relationships between the fractal dimension of soil particle-size distributions and the corresponding van Genuchten parameters for the soil-water-characteristic curves. The oven-drying method was used to measure soil water content, and the high-speed centrifugation method was used to generate soil-water-characteristic curves. The results show that (1) the soil water that can be used by Caragana korshinskii during a drought year is distributed below 2 m from the surface, whereas the soil water that can be used by grass is below 1.2 m; (2) Caragana korshinskii promotes the conservation of fine soil particles more than does natural restored grass, and the soil particle-size distribution fractal dimension changes with depth and position; and (3) soil hydraulic properties correlate strongly with soil pedological properties such as bulk density and the soil particle-size distribution fractal dimension. These results provide a case study of the relationships among soil distributions, hydrologic and geomorphic processes for vegetation restoration in drylands with a thick vadose zone. More studies on soil property changes are needed to provide case studies and empirical support for ecological restoration in the Loess Plateau of China.

Aquifer recharge with stormwater runoff in urban areas: Influence of vadose zone thickness on nutrient and bacterial transfers from the surface of infiltration basins to groundwater

Stormwater infiltration systems (SIS) have been built in urban areas to reduce the environmental impacts of stormwater runoff. Infiltration basins allow the transfer of stormwater runoff to aquifers but their abilities to retain contaminants depend on vadose zone properties. This study assessed the influence of vadose zone thickness (VZT) on the transfer of inorganic nutrients (PO43−, NO3−, NH4+), dissolved organic carbon (total -DOC- and biodegradable -BDOC-) and bacteria. A field experiment was conducted on three SIS with a thin vadose zone (<3 m) and three SIS with a thick vadose zone (>10 m). Water samples were collected at three times during a rainy period of 10 days in each infiltration basin (stormwater runoff), in the aquifer impacted by infiltration (impacted groundwater) and in the same aquifer but upstream of the infiltration area (non-impacted groundwater). Inorganic nutrients, organic matter, and dissolved oxygen (DO) were measured on all water samples. Bacterial community structures were investigated on water samples through a next-generation sequencing (NGS) scheme of 16S rRNA gene amplicons (V5-V6). The concentrations of DO and phosphate measured in SIS-impacted groundwaters were significantly influenced by VZT due to distinct biogeochemical processes occurring in the vadose zone. DOC and BDOC were efficiently retained in the vadose zone, regardless of its thickness. Bacterial transfers to the aquifer were overall low, but data obtained on day 10 indicated a significant bacterial transfer in SIS with a thin vadose zone. Water transit time and water saturation of the vadose zone were found important parameters for bacterial transfers. Most bacterial taxa (>60%) from impacted groundwaters were not detected in stormwater runoff and in non-impacted groundwaters, indicating that groundwater bacterial communities were significantly modified by processes associated with infiltration (remobilization of bacteria from vadose zone and/or species sorting).

Spatial and temporal dynamics of deep percolation, lag time and recharge in an irrigated semi-arid region

The time required for deep percolating water to reach the water table can be considerable in areas with a thick vadose zone. Sustainable groundwater management, therefore, has to consider the spatial and temporal dynamics of groundwater recharge. The key parameters that control the lag time have been widely examined in soil physics using small-scale lysimeters and modeling studies. However, only a small number of studies have analyzed how deep-percolation rates affect groundwater recharge dynamics over large spatial scales. This study examined how the parameters influencing lag time affect groundwater recharge in a semi-arid catchment under irrigation (in northeastern Iran) using a numerical modeling approach. Flow simulations were performed by the MODFLOW-NWT code with the Vadose-Zone Flow (UZF) Package. Calibration of the groundwater model was based on data from 48 observation wells. Flow simulations showed that lag times vary from 1 to more than 100 months. A sensitivity analysis demonstrated that during drought conditions, the lag time was highly sensitive to the rate of deep percolation. The study illustrated two critical points: (1) the importance of providing estimates of the lag time as a basis for sustainable groundwater management, and (2) lag time not only depends on factors such as soil hydraulic conductivity or vadose zone depth but also depends on the deep-percolation rates and the antecedent soil-moisture condition. Therefore, estimates of the lag time have to be associated with specific percolation rates, in addition to depth to groundwater and soil properties.

Mineral N stock and nitrate accumulation in the 50 to 200 m profile on the Loess Plateau

Nitrogen (N) stored in deep profiles is important in assessing regional and/or global N stocks and nitrate leaching risk to groundwater. The Chinese Loess Plateau, which is characterized by significantly thick loess deposits, potentially stores immense stocks of mineral N, posing future threats to groundwater quality. In order to determine the vertical distributions of nitrate and ammonium content in the region, as well as to characterize the potential accumulation of nitrate in the deep loess profile, we study loess samples collected at five sites (Yangling, Changwu, Fuxian, An'sai and Shenmu) through a 50 to 200m loess profile. The estimated storage of mineral N varied significantly among the five sites, ranging from 0.46 to 2.43×104kgNha−1. Ammonium exhibited fluctuations and dominated mineral N stocks within the whole profile at the sites, except for the upper 20–30m at Yangling and Changwu. Measured nitrate content in the entire profile at Fuxian, An'sai and Shenmu is low, but significant accumulations were observed to 30–50m depth at the other two sites. Analysis of δ15N and δ18O of nitrate indicates different causes for accumulated nitrate at these two sites. Mineralization and nitrification of manure and organic N respectively contribute nitrate to the 0–12 and 12–30m profile at Changwu; while nitrification of NH4+ fertilizer, NO3− fertilizer and nitrification of organic N control the nitrate distribution in the 0–3, 3–7 and 7–10m layer at Yangling, respectively. Furthermore, our analysis illustrates the low denitrification potential in the lower part of the vadose zone. The accumulated nitrate introduced by human activities is thus mainly distributed in the upper vadose zone (above 30m), indicating, currently, a low nitrate leaching risk to groundwater due to a high storage capacity of the thick vadose zone in the region.

Advantages and challenges of using soil water isotopes to assess groundwater recharge dominated by snowmelt at a field study located in Canada

ABSTRACTSeasonal signals of stable isotopes in precipitation, combined with measurements of isotope ratios in soil water, can be used for quantitative estimation of groundwater recharge rates. This study investigates the applicability of using the piston flow principle and the peak shift displacement method to estimate actual groundwater recharge rates in a humid Nordic region located in the province of Quebec, Canada. Two different sites with and without vegetation (C1 and C2) in an unconfined aquifer were tested by measuring soil water isotope ratios (18O/16O and 2H/1H) and volumetric pore water content. Core samples were obtained along the vadose zone down to the groundwater table at the two sites (2.45 m for Site C1 and 4.15 m for Site C2). The peak shift method to estimate groundwater recharge rates was shown to be accurate only in certain specific conditions inherent to the soil properties and the topographical situation of the investigated sites. Indeed, at Site C2, recharge from the snowmelt could not be estimated because of heterogeneity in the lower part of the vadose zone. At this same site the later recharge after the snowmelt (in the period from late spring to early autumn) could be estimated accurately because the upper part of the vadose zone was homogeneous. Furthermore, at site C1, runoff/runon phenomena hampered calculations of actual infiltration and thus produced inaccurate results for recharge. These two different site effects (heterogeneity in the first site and runoff/runon in the other site) were identified as being limiting factors in the accurate assessment of actual recharge. This study therefore recommends the use of the peak shift method for (1) humid Nordic regions, (2) homogeneous and thick vadose zones, and (3) areas with few or limited site effects (runoff/runon).

Modelling Vadose Zone Processes for Assessing Groundwater Recharge in Semi-Arid Region

Normally groundwater recharge is estimated using methods based on water balance, water table fluctuations, fixed factor of annual rainfall and tracer movement. In many of these methods water stored in the vadose zone and evapotranspiration are not accounted properly. These factors control groundwater recharge to a large extent, particularly in arid and semi-arid regions which are normally characterized by a deep water table, thick vadose zone and high evapotranspiration. In this study, HYDRUS-1D and MODFLOW models were used to assess the recharge flux and groundwater recharge in an area under a semi-arid region giving due consideration to important vadose zone processes. Cumulative recharge flux at water table in various sub-areas varied from 20.01 cm to 23.43 cm (29.26% to 34.26% of the monsoon rainfall). The average groundwater recharge was 22.2%. Total surface runoff in various sub-areas varied from 3.39 cm to 14.36 cm (5% to 21% of the monsoon rainfall). Evapotranspiration was found to be a major recharge controlling factor. Reference evapotranspiration varied from 37.19 cm to 45 cm (54% to 66% of the monsoon rainfall). Natural recharge under the prevailing pumping rate and pumping schedule was 23.3% of the monsoon rainfall. Simulation results revealed that if all the surface runoff is retained in the area, water table will rise by 1.46 m.

Global patterns of nitrate storage in the vadose zone

Global-scale nitrogen budgets developed to quantify anthropogenic impacts on the nitrogen cycle do not explicitly consider nitrate stored in the vadose zone. Here we show that the vadose zone is an important store of nitrate that should be considered in future budgets for effective policymaking. Using estimates of groundwater depth and nitrate leaching for 1900–2000, we quantify the peak global storage of nitrate in the vadose zone as 605–1814 Teragrams (Tg). Estimates of nitrate storage are validated using basin-scale and national-scale estimates and observed groundwater nitrate data. Nitrate storage per unit area is greatest in North America, China and Europe where there are thick vadose zones and extensive historical agriculture. In these areas, long travel times in the vadose zone may delay the impact of changes in agricultural practices on groundwater quality. We argue that in these areas use of conventional nitrogen budget approaches is inappropriate.

Deep Vadose Zone Contaminant Flux Evaluation at the Hanford BY-Cribs Site Using Forward and Imposed Concentration Modeling Approaches

For sites with vadose zone contamination, the nature and extent of groundwater contaminant plumes are related to the flux into groundwater. In thick vadose zones at arid sites, transport is likely relative slow, complicating direct measurement of contaminant flux. A predictive numerical analysis was applied to estimate contaminant flux to groundwater at the Hanford BY Cribs site. The site received large amounts of waste water in the 1950’s, containing contaminants such as technetium-99 and nitrate. Recently, large increases in groundwater concentrations have been observed in site groundwater wells and, for potential remediation purposes, vadose zone fluxes into groundwater needed to be evaluated. The analysis approach uses forward simulations, tracking contaminants during and after disposal, and imposed concentration (IC) simulations, where concentrations, derived from electrical conductivity data obtained in 2007, are imposed on the flow field. A total of three IC methods, relating electrical conductivity to nitrate concentrations, are developed and tested. The numerical analysis predicts high fluxes during and directly after disposal, with decreasing trends beginning in 1970. The IC simulations also show declining nitrate fluxes over time. The predicted fluxes of the forward and IC simulations after 2007 are in reasonable agreement. The observed concentration increases in the groundwater below the BY Cribs site cannot be explained by an increase in contaminant flux from the vadose zone. An alternative explanation, based on the observed decrease in aquifer thickness below the site, is presented.

Depth of the vadose zone controls aquifer biogeochemical conditions and extent of anthropogenic nitrogen removal

We investigated biogeochemical conditions and watershed features controlling the extent of nitrate removal through microbial dinitrogen (N2) production within the surficial glacial aquifer located on the north and south shores of Long Island, NY, USA. The extent of N2 production differs within portions of the aquifer, with greatest N2 production observed at the south shore of Long Island where the vadose zone is thinnest, while limited N2 production occurred under the thick vadose zones on the north shore. In areas with a shallow water table and thin vadose zone, low oxygen concentrations and sufficient DOC concentrations are conducive to N2 production. Results support the hypothesis that in aquifers without a significant supply of sediment-bound reducing potential, vadose zone thickness exerts an important control of the extent of N2 production. Since quantification of excess N2 relies on knowledge of equilibrium N2 concentration at recharge, calculated based on temperature at recharge, we further identify several features, such as land use and cover, seasonality of recharge, and climate change that should be considered to refine estimation of recharge temperature, its deviation from mean annual air temperature, and resulting deviation from expected equilibrium gas concentrations.

Flow dynamics in vadose zones with and without vegetation in an arid region

Flow dynamics in a thick vadose zone in an arid region, China was investigated using a field experiment at plots with bare soils and vegetated soils. Detailed pressure head profile along a depth of 8m, groundwater level, soil moisture content at surface, air temperature, and precipitation were observed over one year's time span. The temporal and spatial variations of pressure heads and hydraulic gradients over the time span elucidate the role of air temperature, precipitation, and soil stratification, the growth of vegetation, on the flow dynamics in the vadose zone. The dynamics includes freezing and thawing of surface soils, infiltration, evapotranspiration, distribution of moisture, and groundwater recharge. Estimated hydraulic gradients based on the observed pressure heads suggest that vegetation affected flow dynamics even at 3m below land surface during its growth seasons. The pressure head distributions at the vadose zone over the time span were found correlated well with soil stratification or heterogeneity. Afterward, we estimated the land-atmosphere interface flux, water uptake rate by the plants, and we then discussed the relationship between seasonal variation of temperature, precipitation, evaporation, plant growth, soil stratification (heterogeneity) and the flow dynamics in the vadose zone of the region.

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

AbstractUnderstanding 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.

Controls on solute concentration‐discharge relationships revealed by simultaneous hydrochemistry observations of hillslope runoff and stream flow: The importance of critical zone structure

AbstractWe investigated controls on concentration‐discharge relationships of a catchment underlain by argillite by monitoring both groundwater along a hillslope transect and stream chemistry. Samples were collected at 1–3 day intervals over 4 years (2009–2013) in Elder Creek in the Eel River Critical Zone Observatory in California. Runoff at our study hillslope is driven by vadose zone flux through deeply weathered argillite (5–25 m thick) to a perched, seasonally dynamic groundwater that then drains to Elder Creek. Low flow derives from the slowly draining deepest perched groundwater that reaches equilibrium between primary and secondary minerals and saturation with calcite under high subsurface pCO2. Arriving winter rains pass through the thick vadose zone, where they rapidly acquire solutes via cation exchange reactions (driven by high pCO2), and then recharge the groundwater that delivers runoff to the stream. These new waters displayed lower solute concentrations than the deep groundwater by less than a factor of 5 (except for Ca). Up to 74% of the total annual solute flux is derived from the vadose zone. The deep groundwater's Ca concentration decreased as it exfiltrates to the stream due to CO2 degassing and this Ca loss is equivalent of 30% of the total chemical weathering flux of Elder Creek. The thick vadose zone in weathered bedrock and the perched groundwater on underlying fresh bedrock result in two distinct processes that lead to the relatively invariant (chemostatic) concentration‐discharge behavior. The processes controlling solute chemistry are not evident from stream chemistry and runoff analysis alone.

Experimental and numerical investigations of soil water balance at the hinterland of the Badain Jaran Desert for groundwater recharge estimation

Quantification of groundwater recharge from precipitation in the huge sand dunes is an issue in accounting for regional water balance in the Badain Jaran Desert (BJD) where about 100 lakes exist between dunes. In this study, field observations were conducted on a sand dune near a large saline lake in the BJD to investigate soil water movement through a thick vadose zone for groundwater estimation. The hydraulic properties of the soils at the site were determined using in situ experiments and laboratory measurements. A HYDRUS-1D model was built up for simulating the coupling processes of vertical water–vapor movement and heat transport in the desert soil. The model was well calibrated and validated using the site measurements of the soil water and temperature at various depths. Then, the model was applied to simulate the vertical flow across a 3-m-depth soil during a 53-year period under variable climate conditions. The simulated flow rate at the depth is an approximate estimation of groundwater recharge from the precipitation in the desert. It was found that the annual groundwater recharge would be 11–30mm during 1983–2012, while the annual precipitation varied from 68 to 172mm in the same period. The recharge rates are significantly higher than those estimated from the previous studies using chemical information. The modeling results highlight the role of the local precipitation as an essential source of groundwater in the BJD.

An experimental study on isotope fractionation in a mesoporous silica-water system with implications for vadose-zone hydrology

Soil water dynamics within a thick vadose (unsaturated) zone is a key component in the hydrologic cycle in arid regions. In isotopic studies of soil water, the isotopic composition of adsorbed/pore-condensed water within soils has been assumed to be identical to that of bulk liquid water. To test this critical assumption, we have conducted laboratory experiments on equilibrium isotope fractionation between adsorbed/condensed water in mesoporous silica (average pore diameter 15nm) and the vapor at relative pressures p/po=0.3–1.0 along the adsorption–desorption isotherm at 30°C. The isotope fractionation factors between condensed water in the silica pores and the vapor, α(2H) and α(18O), are smaller than those between liquid and vapor of bulk water (1.074 and 1.0088, respectively, at 30°C). The α(2H) and α(18O) values progressively decrease from 1.064 and 1.0083 at p/po=1 to 1.024 and 1.0044 at p/po=0.27 for hydrogen and oxygen isotopes, respectively, establishing trends very similar to the isotherm curves. Empirical formulas relating α(2H) and α(18O) to the proportions of filled pores (f) are developed. Our experimental results challenge the long-held assumption that the equilibrium isotope fractionation factors for the soil water–vapor are identical to those of liquid water–vapor system with potential implications for arid-zone and global water cycles, including paleoclimate proxies in arid regions.

Evaluation of deep vadose zone contaminant flux into groundwater: Approach and case study

For sites with a contaminant source located in the vadose zone, the nature and extent of groundwater contaminant plumes are a function of the contaminant flux from the vadose zone to groundwater. Especially for thick vadose zones, transport may be relatively slow making it difficult to directly measure contaminant flux. An integrated assessment approach, supported by site characterization and monitoring data, is presented to explain current vadose zone contaminant distributions and to estimate future contaminant flux to groundwater in support of remediation decisions. The U.S. Department of Energy Hanford Site (WA, USA) SX Tank Farm was used as a case study because of a large existing contaminant inventory in its deep vadose zone, the presence of a limited-extent groundwater plume, and the relatively large amount of available data for the site. A predictive quantitative analysis was applied to refine a baseline conceptual model through the completion of a series of targeted simulations. The analysis revealed that site recharge is the most important flux-controlling process for future contaminant flux. Tank leak characteristics and subsurface heterogeneities appear to have a limited effect on long-term contaminant flux into groundwater. The occurrence of the current technetium-99 groundwater plume was explained by taking into account a considerable historical water-line leak adjacent to one of the tanks. The analysis further indicates that the vast majority of technetium-99 is expected to migrate into the groundwater during the next century. The approach provides a template for use in evaluating contaminant flux to groundwater using existing site data and has elements that are relevant to other disposal sites with a thick vadose zone.

Dynamic, structured heterogeneity of water isotopes inside hillslopes

AbstractUse of the stable isotopes of water (δD, δ18O) to determine vegetative water sources, runoff paths, and residence times generally assumes that, other than shallow evaporative enrichment, the isotopic composition of precipitation is conserved as it travels through the subsurface to the stream channel. Here we follow rainfall through a thick (up to 25m) vadose zone of soil, saprolite, and weathered bedrock mostly composed of argillite, and underlying a steep (32°) forested hillslope. We discover a persistent structured heterogeneity of water isotopes inside the hillslope. Summer dry season causes evaporative enrichment of the soil, but not in the saprolite and weathered bedrock. In winter, the mobile water, generated by successive rainstorms with widely varying isotopic composition, mixes in the vadose zone, elevating soil and rock moisture content, and eventually recharging the groundwater with isotopically invariant water similar to the seasonally averaged rainfall. Yet throughout the winter the less mobile winter soil and rock moisture remains relatively light, and water extracted from the interior of argillite lies well to the left of the local meteoric water line. This persistently light composition of soil and rock moisture and the deviation from average meteoric values suggest that subsurface fractionation, or the inheritance of paleo‐meteoric rock moisture associated with rock uplift may lead to large enduring isotopic differences between high and low mobility water. These differences suggest that the use of water isotopes as tracers must consider the possibility of subsurface isotopic evolution and the influence of exchange with more tightly held water.