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Atmospheric nitrous oxide contributes directly to global warming, yet models of the nitrogen cycle do not account for bedrock, the largest pool of terrestrial nitrogen, as a source of nitrous oxide. Although it is known that release rates of nitrogen from bedrock are large, there is an incomplete understanding of the connection between bedrock-hosted nitrogen and atmospheric nitrous oxide. Here, we quantify nitrogen fluxes and mass balances at a hillslope underlain by marine shale. We found that, at this site, bedrock weathering contributes 78% of the subsurface reactive nitrogen, while atmospheric sources (commonly regarded as the sole sources of reactive nitrogen in pristine environments) account for only the remaining 22%. About 56% of the total subsurface reactive nitrogen denitrifies, including 14% emitted as nitrous oxide. The remaining reactive nitrogen discharges in porewaters to a floodplain where additional denitrification probably occurs. We also found that the release of bedrock nitrogen occurs primarily within the zone of the seasonally fluctuating water table and suggest that the accumulation of nitrate in the vadose zone, often attributed to fertilization and soil leaching, may also include contributions from weathered nitrogen-rich bedrock. Our hillslope study suggests that, under oxygenated and moisture-rich conditions, weathering of deep, nitrogen-rich bedrock makes an important contribution to the nitrogen cycle. Weathering of deep bedrock releases reactive nitrogen into the subsurface, which contributes to the flux of nitrous oxide to the atmosphere, according to a field study that combines soil, rock and groundwater data within a river catchment.

Author(s): Wan, J; Tokunaga, TK; Dong, W; Williams, KH; Kim, Y; Conrad, ME; Bill, M; Riley, WJ; Hubbard, SS | Abstract: Understanding terrestrial carbon cycling has relied primarily on studies of topsoils that are typically characterized to depths shallower than 0.5nm. At a semiarid site instrumented down to 7nm, we measured seasonal- and depth-resolved carbon inventories and fluxes and groundwater and unsaturated zone flow rates. Measurements showed that ~30% of the CO2 efflux to the atmosphere (60% in winter) originates from below 1nm, contrary to predictions of less than 1% by Earth System Model land modules. Respiration from deeper roots and deeper microbial communities is supported by favorable subsurface temperatures, moisture, and oxygen availability. Below 1nm, dissolved organic carbon fluxes from the overlying soil and C from deep roots and exudates are expected to be important in sustaining microbial respiration. Because these conditions are characteristic of semiarid climate regions, we contend that Earth System Model land modules should incorporate such deeper soil processes to improve CO2 flux predictions.

Recharge of alluvial aquifers is a key component in understanding the interaction between floodplain vadose zone biogeochemistry and groundwater quality. The Rifle Site (a former U-mill tailings site) adjacent to the Colorado River is a well-established field laboratory that has been used for over a decade for the study of biogeochemical processes in the vadose zone and aquifer. This site is considered an exemplar of both a riparian floodplain in a semiarid region and a post-remediation U-tailings site. In this paper we present Sr isotopic data for groundwater and vadose zone porewater samples collected in May and July 2013 to build a mixing model for the fractional contribution of vadose zone porewater (i.e. recharge) to the aquifer and its variation across the site. The vadose zone porewater contribution to the aquifer ranged systematically from 0% to 38% and appears to be controlled largely by the microtopography of the site. The area-weighted average contribution across the site was 8% corresponding to a net recharge of 7.5 cm. Given a groundwater transport time across the site of ~1.5 to 3 years, this translates to a recharge rate between 5 and 2.5 cm/yr, and with the average precipitation to the site implies a loss from the vadose zone due to evapotranspiration of 83% to 92%, both ranges are in good agreement with previously published results by independent methods. A uranium isotopic (234U/238U activity ratios) mixing model for groundwater and surface water samples indicates that a ditch across the site is hydraulically connected to the aquifer and locally significantly affects groundwater. Groundwater samples with high U concentrations attributed to natural bio-reduced zones have 234U/238U activity ratios near 1, suggesting that the U currently being released to the aquifer originated from the former U-mill tailings.

Capillary pressure (Pc) – saturation (Sw) relations are essential for predicting equilibrium and flow of immiscible fluid pairs in soils and deeper geologic formations. In systems that are difficult to measure, behavior is often estimated based on capillary scaling of easily measured Pc–Sw relations (e.g., air-water, and oil-water), yet the reliability of such approximations needs to be examined. In this study, seventeen sets of brine drainage and imbibition curves were measured with air-brine, decane-brine, and supercritical (sc) CO2-brine in homogeneous quartz and carbonate sands, using porous plate systems under ambient (0.1 MPa, 23 ˚C) and reservoir (12.0 MPa, 45 ˚C) conditions. Comparisons between these measurements showed significant differences in residual nonwetting phase saturation, Snw,r. Through applying capillary scaling, changes in interfacial properties were indicated, particularly wettability. With respect to the residual trapping of the nonwetting phases, Snwr, CO2 > Snwr, decane > Snwr, air. Decane-brine and scCO2-brine Pc–Sw curves deviated significantly from predictions assuming hydrophilic interactions. Moreover, neither the scaled capillary behavior nor Snw,r for scCO2-brine were well represented by decane-brine, apparently because of differences in wettability and viscosities, indicating limitations for using decane (and other organic liquids) as a surrogate fluid in studies intended to apply to geological carbon sequestration. Thus, challenges remain in applying scaling for predicting capillary trapping and multiphase displacement processes across such diverse fields as vadose zone hydrology, enhanced oil recovery, and geologic carbon sequestration. This article is protected by copyright. All rights reserved.

Author(s): Tokunaga, TK; Kim, Y; Wan, J; Bill, M; Conrad, M; Dong, W | Abstract: Transport from the soil surface to groundwater is commonly mediated through deeper portions of the vadose zone and capillary fringe, where variations in temperature and water saturation strongly influence biogeochemical processes. This technical note describes a sediment column design that allows laboratory simulation of thermal and hydrologic conditions found in many field settings. Temperature control is particularly important because room temperature is not representative of most subsurface environments. A 2.0-m-tall column was capable of simulating profiles with temperatures ranging from 3 to 22°C, encompassing the full range of seasonal temperature variation observed in the deep vadose zone and capillary fringe of a semiarid floodplain in western Colorado. The water table was varied within the lower 0.8-m section of the column, and profiles of water content and matric potential were measured. Vadose zone CO2 collected from depth-distributed gas samplers under representative seasonal conditions reflected the influences of temperature and water table depth on microbial respiration. Thus, realistic subsurface biogeochemical dynamics can be simulated in the laboratory through establishing column profiles that represent seasonal thermal and hydrologic conditions.

© 2016 In order to understand the transport and humification processes of dissolved organic matter (DOM) within sediments of a semi-arid floodplain at Rifle, Colorado, fluorescence excitation–emission matrix (EEM) spectroscopy, humification index (HIX) and specific UV absorbance (SUVA) at 254 nm were applied for characterizing depth and seasonal variations of DOM composition. Results revealed that late spring snowmelt leached relatively fresh DOM from plant residue and soil organic matter down into the deeper vadose zone (VZ). More humified DOM is preferentially adsorbed by upper VZ sediments, while non- or less-humified DOM was transported into the deeper VZ. Interestingly, DOM at all depths undergoes rapid biological humification process evidenced by the products of microbial by-product-like (i.e., tyrosine-like and tryptophan-like) matter in late spring and early summer, particularly in the deeper VZ, resulting in more humified DOM (e.g., fulvic-acid-like and humic-acid-like substances) at the end of year. This indicates that DOM transport is dominated by spring snowmelt, and DOM humification is controlled by microbial degradation, with seasonal variations. It is expected that these relatively simple spectroscopic measurements (e.g., EEM spectroscopy, HIX and SUVA) applied to depth- and temporally-distributed pore-water samples can provide useful insights into transport and humification of DOM in other subsurface environments as well.

© Soil Science Society of America. Although CO2fluxes from soils are often assumed to originate within shallow soil horizons (

© 2016, US Government. Flood plains play a potentially important role in the global carbon cycle. The accumulation of organic matter in flood plains often induces the formation of chemically reduced groundwater and sediments along riverbanks. In this study, our objective is to evaluate the cumulative impact of such reduced zones, water table fluctuations, and temperature gradients on subsurface carbon fluxes in a flood plain at Rifle, Colorado located along the Colorado River. 2-D coupled variably-saturated, non-isothermal flow and biogeochemical reactive transport modeling was applied to improve our understanding of the abiotic and microbially mediated reactions controlling carbon dynamics at the Rifle site. Model simulations considering only abiotic reactions (thus ignoring microbial reactions) underestimated CO2partial pressures observed in the unsaturated zone and severely underestimated inorganic (and overestimated organic) carbon fluxes to the river compared to simulations with biotic pathways. Both model simulations and field observations highlighted the need to include microbial contributions from chemolithoautotrophic processes (e.g., Fe+2and S−2oxidation) to match locally-observed high CO2concentrations above reduced zones. Observed seasonal variations in CO2concentrations in the unsaturated zone could not be reproduced without incorporating temperature gradients in the simulations. Incorporating temperature fluctuations resulted in an increase in the annual groundwater carbon fluxes to the river by 170 % to 3.3 g m−2d−1, while including water table variations resulted in an overall decrease in the simulated fluxes. We conclude that spatial microbial and redox zonation as well as temporal fluctuations of temperature and water table depth contribute significantly to subsurface carbon fluxes in flood plains and need to be represented appropriately in model simulations.

Leakage of saline-alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature ( approximately 21 degrees C). Relatively good agreement was also obtained at high temperature ( approximately 70 degrees C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.

Long-term historic spills of uranium at the 300 Area fuel fabrication site (58,000 kg of disposed uranium over 32 yr) and at the 200 East Area BX tank farm (7000 kg of spilled uranium in one event), both within the Hanford formation in the Hanford Site, Washington State, were investigated by subsurface sampling and subsequent microscale investigations of excavated samples. The 200 Area sediments contained uranyl silicate mineralization (sodium boltwoodite) in restrictive microfractures in granitic clasts, in the vadose zone over a narrow range in depth. Well logging and column experiments indicated that tank wastes migrated deeper than observed in core samples. The 300 Area sediments included metatorbernite and uranium at low concentrations associated with detrital aluminosilicates, along with other mineral phases that could accommodate uranyl, such as uranophane and calcium carbonate. The association of contaminant uranyl with Hanford formation sediments provided a persistent source of uranium to groundwater. The results of both studies suggest that the formation of secondary solid uranyl-bearing phases infl uences the subsequent release of uranium to the environment and that our understanding of these processes and individual waste sites is incomplete.

Leakage of highly saline and alkaline radioactive waste from storage tanks into underlying sediments is a serious environmental problem at the Hanford Site in Washington State. This study focuses on geochemical evolution of tank waste plumes resulting from interactions between the waste solution and sediment. A synthetic tank waste solution was infused into unsaturated Hanford sediment columns (0.2, 0.6, and 2 m) maintained at 70°C to simulate the field contamination process. Spatially and temporally resolved geochemical profiles of the waste plume were obtained. Thorough OH − neutralization (from an initial pH 14 down to 6.3) was observed. Three broad zones of pore solutions were identified to categorize the dominant geochemical reactions: the silicate dissolution zone (pH > 10), pH-neutralized zone (pH 10 to 6.5), and displaced native sediment pore water (pH 6.5 to 8). Elevated concentrations of Si, Fe, and K in plume fluids and their depleted concentrations in plume sediments reflected dissolution of primary minerals within the silicate dissolution zone. The very high Na concentrations in the waste solution resulted in rapid and complete cation exchange, reflected in high concentrations of Ca and Mg at the plume front. The plume-sediment profiles also showed deposition of hydrated solids and carbonates. Fair correspondence was obtained between these results and analyses of field borehole samples from a waste plume at the Hanford Site. Results of this study provide a well-defined framework for understanding waste plumes in the more complex field setting and for understanding geochemical factors controlling transport of contaminant species carried in waste solutions that leaked from single-shell storage tanks in the past.

Waste disposal into seepage basins has generated groundwater contaminant plumes at many locations. At the F Area within the Savannah River Site, Pu was extracted from depleted U from 1955 to 1988, with wastewater discharged into seepage basins. Basin 3 was the largest F-Area seepage basin, receiving acidic wastewater containing radionuclides (including 3 H, 129 I, and multiple isotopes of U, Pu, Sr, and Cs), elevated NO 3 , and some metals. Contaminants transported into the groundwater migrate toward Fourmile Branch, a tributary to the Savannah River. We developed a two-compartment model and used 20 yr of groundwater quality data to estimate the post-closure drainage of waste solutions through its vadose zone into the aquifer. Tritium, NO 3 − , and specific conductance were used as tracers in the model to estimate drainage rates. Our calculations indicate that early stages of post-closure waste drainage occurred with high water fluxes (~0.5 m yr −1 ) and quickly declined. Even 20 yr after basin closure, however, drainage continues at several centimeters per year. While the magnitude of this late-stage drainage rate is low, its impact is large because of the high concentrations of contaminants it continues to supply to the groundwater. These estimated drainage fluxes constrain predictions on the waste plume behavior, especially with respect to its trailing gradient and time scales suitable for monitored natural attenuation. Our methodology requires only groundwater monitoring data and a small number of well-constrained input quantities. This approach can be useful for understanding contaminant dissipation at other locations as well, especially where the hydrogeological setting is relatively simple.

This paper explores the significance of the gas-water interface on colloid sorption and transport. Three types of common saturation conditions were simulated in packed sand columns: (1) a completely water-saturated condition, (2) gas bubbles trapped by capillary forces as a nonwetting residual phase (15% gas), and (3) gas present as a continuous phase (46% gas), or in other words, a vadose zone situation. Different saturations provided different interfacial conditions. Two types of polystyrene latex particles (0.2 μm), hydrophilic and hydrophobic, were used in each of the three saturations. Relative surface hydrophobicity of latex particles was characterized by contact angle measurements. Each experiment was repeated five times. A total of 30 columns with good reproducible packing and gas content gave reproducible particle breakthrough curves. The retention of both hydrophilic and hydrophobic colloids increased with gas content of the porous medium. Colloids preferentially sorbed onto the gas-water interface relative to the matrix surface. The degree of sorption increased with the increase of colloid surface hydrophobicity. The results validate and quantify direct microscopic scale visualization observations made in two-dimensional pore network glass micromodels (Wan and Wilson, 1994). These findings suggest an additional mechanism for filtration and for particulate transport in the subsurface environment, whenever more than one fluid phase is present. The results also give clear evidence that the presence of inadvertently trapped residual gas could help explain the discrepancy between water-saturated laboratory column experiments and related theory.

A saline-alkaline brine containing high concentration of U(VI) was accidentally spilled at the Hanford Site in 1951, introducing 10 tons of U into sediments under storage tank BX-102. U concentrations in the deep vadose zone and groundwater plumes increase with time, yet how the U has been migrating is not fully understood. We simulated the spill event in laboratory soil columns, followed by aging, and obtained spatially resolved U partitioning and speciation along simulated plumes. We found after aging, at apparent steady state, that the pore aqueous phase U concentrations remained surprisingly high (up to 0.022 M), in close agreement with the recently reported high U concentrations (up to 0.027 M) in the vadose zone plume (1). The pH values of aged pore liquids varying from 10 to 7, consistent with the measured pH of the field borehole sediments varying from 9.5 to 7.4 (2), from near the plume source to the plume front. The direct measurements of aged pore liquids together with thermodynamic calculations using a Pitzer approach revealed that UO2(CO3)3(4-) is the dominant aqueous U species within the plume body (pH 8-10), whereas Ca2UO2(CO3)3 and CaUO2(CO3)32- are also significant in the plume frontvicinity (pH 7-8), consistent with that measured from field borehole pore-waters (3). U solid phase speciation varies at different locations along the plume flow path and even within single sediment grains, because of location dependent pore and micropore solution chemistry. Our results suggest that continuous gravity-driven migration of the highly stable U02(CO3)34 in the residual carbonate and sodium rich tank waste solution is likely responsible for the detected growing U concentrations in the vadose zone and groundwater.

Effect of Saline Waste Solution Infiltration Rates on Uranium Retention and Spatial Distribution in Hanford Sediments Jiamin Wan 1* , Tetsu K. Tokunaga 1 , Yongman Kim 1 , Zheming Wang 2 , Antonio Lanzirotti 3 , Eduardo Saiz 4 , and R. Jeffrey Serne 2 1. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 2. Pacific Northwest National Laboratory, Richland, WA 99352 3. University of Chicago, Chicago, IL 60637 4. Material Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 ABSTRACT The accidental overfilling of waste liquid from tank BX-102 at the Hanford Site in 1951 put about 10 metric tons of U(VI) into the vadose zone. In order to understand the dominant geochemical reactions and transport processes occurred during the initial infiltration and help understand current spatial distribution, we simulated the waste liquid spilling event in laboratory sediment columns using synthesized metal waste solution. We found that, as the plume propagating through sediments, pH decreased greatly (as much as 4 units) at the moving plume front. Infiltration flow rates strongly affect U behavior. Slower flow rates resulted in higher sediment-associated U concentrations, and higher flow rates ( 5 cm/day) permitted practically unretarded U transport. Therefore, given the very high K sat of most of Hanford formation, the low permeability zones within the sediment could have been most important in retaining high concentrations of U during initial release into the vadose zone. Massive amount of colloids, including U-colloids, formed at the plume fronts. Total U concentrations (aqueous and colloid) within plume fronts exceeded the source concentration by up to 5-fold. Uranium colloid

The Hanford Site is the DOE's largest legacy waste site, with uranium (U) from plutonium processing being a major contaminant in its subsurface. Accidental release of highly concentrated high-level wastes left large quantities of U in the vadose zone under tank farms. The U contamination has been found in groundwater beneath the tank farms, indicating U is mobile.

The overall objective of this renewal proposal is to construct an experimentally supported conceptual model of U fate and transport, during and after the tank leakage, at heavily Uncontaminated areas within the Hanford Site. Through simulating the tank leakage event in laboratory columns, we will better understand the spatial and temporal distribution of the spilled U, and its mobility. These laboratory results will specifically be used to compare with and help explain borehole data from underneath tank BX-102. Our research focuses on the following basic issues: (1) Determining the spatial distribution of U within the plume during the early stage of waste plume development. (2) Determining the forms and amount of U partitioned in the pore water relative to the matrix associated U. Identify dissolved U and suspended mobile U-rich colloids in the aqueous phase, and characterize matrix-associated forms (deposited U colloids, sorbed and precipitated U species). (3) Determine how all the above change with time. (4) Determining the mobility of U (release and transport) as function of aging. (5) Developing a mechanistic reactive transport model capable of accounting for both dissolved and particulate U(VI) in the Hanford vadose zone. An overall understanding of U-precipitation vs. adsorption, stability of nanoparticles and their transport, will improve our ability to predict current and future development of U-waste plumes.

Although gravels comprise large portions of some vadose zones, their unsaturated hydraulic properties have received relatively little attention. This study examines moisture retention relations in the 2- and 6-mm size fractions of gravels from the Hanford formation vadose zone (Washington State). Understanding flow and transport within this formation is important because parts of it have become contaminated by leakage of radioactive wastes at the Hanford Site. Moisture retention relations were obtained for a very wide energy range, with attention to water retained in intragranular pores and along grain surfaces. External surfaces of these gravels have root mean-squared roughnesses (rmsr) in the micrometer range, with sparsely distributed deep (hundreds of micrometers) pits. Water films on these external surfaces are volumetrically insignificant at matric potentials less than about −2 kPa. Residual water in these gravels occurs in intragranular pores, accounts for about 10% of the total porosity, and is effectively hydraulically immobile. The intragranular domain in Hanford gravels also has a large specific surface area of about 11 m 2 g −1 . Thus, exchanges of solutes (including contaminants) between the intragranular domain of Hanford gravels and their immediate surrounding are significant and diffusion limited.

Unsaturated flow in very coarse sediments is a critical area for improving our basic understanding of vadose zone flow and transport because it contains important aspects that are beyond the realm predictable by classical Miller-Miller scaling analyses. In particular, very coarse-textured (>1 mm grain-size) media can sustain high flow rates at relatively low saturations, doing so via film flow rather than by flow through an interconnected network of saturated pores. Thus, the physics of fast flow processes in unsaturated very coarse media is fundamentally different from that traditionally recognized in finer textured sediments. Our general objectives are (i) to quantify the macroscopic hydraulic properties of very coarse textured sediments in the near-zero (-10 to 0 kPa) matric potential region, and (ii) determine the microscale basis for fast unsaturated flow. Through these macroscopic and microscale studies, we intend to develop appropriate scaling relations for unsaturated flow in coarse-granular sediments. Gaining improved understanding of unsaturated flow in coarse granular sediments is important for practical concerns. Effective remediation and management of contaminated coarse-grained sediments such as those found at the Hanford Site requires knowledge of unsaturated fast flow. A better understanding of the failure mode for natural and engineered capillary barriers is alsomore » needed. This research proposal was developed to improve our understanding of the physics of unsaturated flow in coarse- to very coarse-textured sediments, and through this meet practical needs related to contaminant transport in such vadose environments.« less

The Hanford Site was one of the DOE's major nuclear weapons production sites from 1940 to 1989. Over time, 67 of the 149 single-shell tanks have leaked or are suspected of having leaked [1]. Contaminants such as 99Tc and U have been found in elevated concentrations in the vadose zone and groundwater beneath the single shell tank farms [2]. In order to make decisions on remedial actions, numerous scientists have been investigating these problems through the DOE's Environment Management Science Program Hanford Vadose Zone Project. The U.S. Department of Energy has on-going projects at the Hanford site to monitor existing contaminant plumes in groundwaters, and to characterize the subsurface distribution of contaminants in tank farms. These efforts include the Tank Farm Vadose Zone Characterization Project and the 200 Area Remedial Action Project.

The knowledge gap on vadose zone colloid transport limits predicting contaminant transport at many DOE sites, and remains an outstanding scientific challenge. Although the process of contaminant sorption at mineral surfaces has received much recognition as a major mechanism controlling contaminant behavior in subsurface environments, virtually little attention has been given to the possibility of contaminant sorption at gas-water interfaces, a major interface in the vadose zone. Moreover, little effort has yet been advanced to optimize such interactions for the purpose of facilitating in-situ remediation. Gas-water interfaces, unlike water-solid interfaces, are mobile. Therefore, associations of contaminants with gas-water interfaces can be very important not only in subsurface contaminant distributions, but also in contaminant mobilization, and potentially in remediation. The first objective of this project was to develop a fundamental understanding of interactions between contaminants and gas-water interfaces. For surface-active molecules, surface excesses can be determined through the Gibbs equation combined with measuring changes in surface tension with respect to changes in their solution concentration. However, for surface-active colloids, surface tension changes are too small to measure. Until initiation of this research project, there were no techniques available for quantifying sorption of colloids at gas-water interfaces. The second purpose of themore » proposed research, based on improved understanding gained in the first phase studies, was to develop a sorptive microbubble remediation technique, using surfactant stabilized microbubbles (fine gas-bubbles, 1-15 {micro}m in diameters) for subsurface in-situ remediation.. In the saturated zone, both pump-and-treat, and air sparging remediation methods are ineffective at displacing contaminants in zones that are ''advectively inaccessible''. Stable microbubbles might be able to migrate beyond preferential flow pathways through buoyant rise. Oxygen and nutrient delivery for promoting aerobic degradation of organic contaminants, and surfactant delivery for emulsifying NAPLs are potential benefits of microbubble injection.« less

[1] Some environmentally sensitive unsaturated zone sediments, such as those underlying radioactive waste tanks in Hanford (Washington State), contain large fractions of gravels and coarse sands. Coarse, granular media are also included in designs of engineered capillary barriers for subsurface waste isolation. Thus knowledge of the unsaturated hydraulic properties of gravels is needed to understand flow and transport in these critical settings. When standard methods for measuring moisture characteristics or water retention relations are used for gravels, corrections are needed in the near-zero region of matric (pressure) potentials. The need for correction results from gravity stratification of saturation profiles within even short sample columns. Such a correction method was developed and used to determine drainage curves for Hanford gravels having characteristic grain sizes of 8.0–9.5, 4.8–5.3, and 2.0–2.4 mm. In 30 mm tall sample columns, gravity corrections were essential for the 9 and 5 mm gravels and less significant for the 2 mm gravel. Validity of the correction method was demonstrated through accurately reconstructing average column saturation-potential relations from their predicted local saturation-potential relations. The method and results presented here are part of an ongoing study on Hanford gravels and on limits to classical unsaturated hydraulic scaling encountered at large grain sizes.