Fragipan Horizons Research Articles

Mineralogical differences between seam and prism material in loess‐derived fragipans in western Kentucky

AbstractFragipan horizons are common diagnostic subsurface features typically identified by their dense, brittle nature under field conditions. Fragipan horizons are often hypothesized to contain various silicate bonding agents, including phyllosilicates, amorphous aluminosilicates/silica, and silica sorbed to iron oxyhydroxides. However, the association of silica (Si) with other minerals in these horizons remains uncertain. To fill this knowledge gap, we sampled the prism and seam material of four loess‐derived fragipan horizons in western Kentucky. We characterized and compared the clay mineral composition of the prism and seam material using X‐ray diffraction (XRD), selective extractions, and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Fragipan prism and seam material exhibited a mixed clay mineralogy comprised of vermiculite, mixed layer minerals (hydroxyl‐interlayered vermiculite), illite, kaolinite, and quartz. We identified goethite in fragipan prisms in much greater quantities when compared to seam material, confirmed by both XRD and DRIFTS and corroborated by higher dithionite‐extractable iron in prisms. DRIFT spectra showed a peak near 3180 cm−1 assigned to OH‐stretching mode of goethite, which was prominent in prism materials. Goethite 110 XRD peak positions were negatively correlated to dithionite‐extractable aluminum (Al) and Si content of the prism material, which may suggest greater Si sorbed to the surfaces of Al‐substituted goethite. These results demonstrate the potential importance of Al‐substituted goethite in fragipan prisms and provide insight about possible associations with Si.

Fragipan Horizon Changes Using Annual Ryegrass and Other Admendments

Fragipan Horizon Changes Using Annual Ryegrass and Other Admendments

Atterberg limits fail in recognizing fragipan horizons

Fragipan is a subsurface soil horizon characterized by high bulk density, coupled with an abundance of fine, blind pores. We hypothesized that this distinctive porosity pattern could affect the Atterberg limits, with significant deviations from those of otherwise comparable soil horizons. Additionally, longer pre-wetting times might be needed to determine the liquid and plastic limits in fragipan, due to restrictions in water movements. Based on these hypotheses, our research focused on (1) assessing the most suitable fragipan-specific sample preparation time for liquid and plastic limit determination; (2) evaluating if the Atterberg limits reflect the presence of fragic properties; (3) exploring the relationships between Atterberg limits and other relevant soil properties through Artificial Neural Networks. Fragipan horizons (F) had a slightly higher liquid limit, much higher bulk density, and specific surface area than nonfragipan (NF). The plastic limit was however comparable for F and NF. Thus, fragipans had a larger interval of plasticity, which is in contrast with slaking tests and well-known field evidence. Despite the expected differences between F and NF, three hours’ pre-wetting was enough for all samples. This might be due to the size fraction used in the analysis, more sensitive to collapse upon water saturation than bigger clods. Atterberg limits did not help in discriminating F from NF, despite the relevant differences in other soil physical properties. Neural Networks showed that the transition from the plastic to the liquid state was mainly driven by texture and specific surface area, while the plasticity interval was mainly affected by the bulk density. These results provided further insights into the mechanical properties of fragipan horizons.

Fragipan Horizons: Definition, Properties, Genesis, and Influence on Soil Behavior

Many Missouri forest soils exhibit fragipans, which influence soil productivity, ecosystem services and land management. Fragipan bearing soils tend to occur where loess thickness is moderate (1 to 2 meters) or where the soil profile exhibits evidence of mass wasting of weathered limestone residuum. Consensus is consolidating around the self-weight collapse of loess and residuum after repeated wetting and desiccation. The use of gravel as an indicator of parent material differences and its correlation with fragipan development is not perfectly aligned, thus although most fragipans do exhibit a bisequal soil profile, the placement of the lithologic discontinuity is difficult given mass wasting, eluviation-illuviation, side slopes, and other soil processes that contribute to increasing the bulk density and conferring strength. Fragipan genesis is evolving; however, research involving Ecosystem Site Descriptions are a fusion of a land parcel’s soil properties, vegetational community, hydrology, and climate to guide land management. Ecological Site Descriptions associated with fragipan bearing soils are necessary, especially when making land management decisions.

Estimation of soil subsurface hydraulic conductivity based on inverse modelling and soil morphology

Soil hydraulic properties play a primary role in water infiltration and movement, and transport of nutrients and contaminants through the soil profile. However, the determination of soil hydraulic properties, especially the hydraulic conductivity at lower depths within the soil profile is very challenging. We investigated the use of inverse modelling based on APSIM (Agricultural Production Simulation Model) to determine likely ranges of the hydraulic conductivity at saturation (Ksat) and field capacity (KDUL) for a slowly permeable subsurface horizon. Simulations were run for the poorly drained Otokia silt loam, in the Otago region of New Zealand. APSIM simulations were set up either according to the site specific soil profile description or based on the national soils database (S-map) pedotransfer functions. Simulated temporal soil moisture data were compared to measurements from a field experiment. Within a sensitivity analysis Ksat values of the subsurface soil were varied from 2.4 to 192 mm/d, and values for KDUL from 0.05 to 1 mm/d. Finally simulations were run over 45 years using three climatic conditions, with annual rainfall amounts of 582, 628 and 730 mm. The effect of varying the hydraulic conductivity of the subsurface fragipan horizon (Cx) on the wetness status of the overlying B2g horizon was determined by calculating the average annual duration of soil moisture above field capacity. These were then used to estimate ranges of the hydraulic conductivity that contributes to a soil being imperfectly or poorly drained. The results highlight the significant influence that KDUL has on the reliability of modelling the hydraulic behaviour of soils with slowly permeable subsoils.

Influence of ryegrass on physico-chemical properties of a fragipan soil

Fragipan horizons are naturally occurring impermeable layers found in certain soil types which constrain crop yields. While abundant literature exists regarding the processes of fragipan genesis, there is little recognition given to potential alterations in the pan with surface management, particularly important in light of the fact that these horizons can occur near the surface. We collected soil cores from a Zanesville silt loam (Typic Fragiudalf) and seeded them to annual ryegrass in the greenhouse and determined relevant soil properties after one growing cycle. The presence of ryegrass lowered bulk densities in the Btx1 and Btx2 horizons by 8% and 6% when compared with the counterpart horizons in the control, suggesting an increase in porosity. These findings were paralleled by a 62% and 52% reduction in tensile strength of ryegrass-grown aggregates in these same horizons when compared with control aggregates. These trends are likely due in part to solubilization of cementing agents by ryegrass root exudates. Glycolate, azelaic acid, and diphenolics appeared as components of the water soluble organic fraction (WSOC) pool and the diphenolics were in greater concentrations in ryegrass soil cores. Diphenolic concentrations were associated with elevated water-soluble aluminum and iron levels, suggesting that ryegrass might be solubilizing these cementing agents and affecting bulk density. Biological drilling by ryegrass roots might also be involved to increase porosity in fragipan horizons. Our findings raise questions about the ability of ryegrass to ameliorate impermeable subsurface fragipan horizons and how the improvement in porosity might impact subsequent row crop production.

Simulating the effects of wet and dry on aggregate dynamics in argillic fragipan horizon

Fragipan is a dense and usually brittle subsurface soil horizon, limiting the penetration of roots and the infiltration of water. The genesis of fragipan is still unclear, but a general agreement exists on the importance of wet-dry cycles. In addition, in argillic Bx horizon, the presence of cations affecting the clay dispersion/flocculation behaviour might be a key factor in fragipan dynamics. In order to gain knowledge on the specific effect of alternating moisture conditions on the evolution of aggregates collected from a Btx horizon, we evaluated the variations of physical properties caused by a wet-dry cycle using both deionized and Ca-enriched water on the 1–2mm aggregate, and on newly formed aggregates (i.e. 2–5 and >5mm size classes) obtained after a lab experiment. The results were compared with a Bt horizon that did not show fragipan properties. Btx and Bt samples were collected from a Typic Fragiudalf developed on fluvio-glacial terraces in NW Italy. The two horizons had comparable clay content (around 13%), and their mineralogical composition was dominated by hydroxy-interlayered vermiculite and smectite. The fragipan 1–2mm aggregates showed a low clay dispersion ratio (11.5%) before the treatments, together with low volume of macropores (74mm3g−1), high volume of mesopores (111mm−3g−1), high slaking (30.1%), close packing of coarse particles and open arrangement of fine particles. The water treatment increased the amount of flocculated-clay in the new-aggregates, and Ca-treatment enhanced clay flocculation both in the newly formed and in the remaining 1–2mm aggregates. The clay flocculation induced a denser arrangement of clay particles (≥0.44), and a consequent reduction of mesopores (from 56.2 to 66.1mm−3g−1), combined with the opening of the coarser particles packing (≤0.78). This new particle arrangement did not correspond to the specific combination of coarse/fine particles arrangement of fragipan. The relative percentage of slaking also decreased. Therefore, upon both deionized water and CaCl2 wetting and drying, the newly formed aggregates (i.e., 2–5 and >5mm) from disturbed fragic materials, as well as those that did not take part in the new aggregate formation (1–2mm), did not show the specific physical properties of fragipan. The simulation of wet and dry cycle acted on clay colloidal behaviour, and the results suggested the degradation, rather than the enhancement, of Btx physical properties measurable in laboratory.

Clay mineralogy and genesis of fragipan in soils from Southeast Brazil

Fragipan horizon has a hard consistency when dry but is brittle when moist. Such a horizon restricts root growth and water infiltration due to the low volume of macropores and discontinuous voids. In Rio de Janeiro state in Brazil, neighboring soils were developed from different materials (sediments and granite/gneiss) and were subject to the same environmental conditions; one manifested fragipan characteristics in the subsurface and the other did not. The main objective of this study was to characterize and quantify clay minerals and relate their properties to the genesis of fragipan horizons. The fragipan in the studied soils show high bulk density (1.67gcm−3) and low average macroporosity (0.03cmcm−3), total porosity (0.42cmcm−3) and hydraulic conductivity (1.43cmh−1). Sequential and interrelated causes favored the formation of fragipan horizons: 1) face to face adjustment of kaolinite (Ka) filling of larger spaces occurring between sand grains, favored by low goethite contents and the absence of hematite and gibbsite; and 2) mineral binding and cementation of sand, silt and clay fractions by amorphous materials. Higher amounts of goethite and lower amounts of amorphous materials in the clay fraction were associated with horizons with higher total porosity.

Loess and fragipans: Development of polygonal-crack-network structures in fragipan horizons in loess ground

One of the defining features of the fragipan horizon is the presence of a blocky polygonal network structure. In loess soils, this network structure can be explained by contraction forces (due to drying) operating after hydrocollapse due to loading and wetting, as in the Bryant hypothesis for fragipan formation. Three stages are identified in the formation of a fragipan horizon in loess ground. There is a deposition phase in which the aeolian deposition of loess material produces certain ground properties. A collapse stage allows the soil structure to deform under the influence of loading and wetting. This collapsed material develops internal tensile forces as drying contraction proceeds and these cause the development of a characteristic crack network. The crack network can be modelled using a very simple Monte Carlo approach and the two dimensional structure produced gives a good representation of fragipan cracking. The collapse-contraction process for fragipan formation offers explanations for the strength and hardness of fragipans, the constant depth to fragipan horizons, the slaking in water (predominance of short range contact bonds) and the mineralogical similarities throughout the system. The fragipan horizon impedes drainage, and this becomes increasingly important as land use becomes more widespread.

Fragipan horizon fragmentation in slaking experiments with amendment materials and ryegrass root tissue extracts.

Slaking experiments were conducted of fragipan clods immersed in solutions of poultry manure, aerobically digested biosolid waste (ADB), fluidized bed combustion byproduct (FBC), D-H2O, CaCO3, NaF, Na-hexa-metaphosphate, and ryegrass root biomass. The fragipan clods were sampled from the Btx horizon of an Oxyaquic Fragiudalf in Kentucky. Wet sieving aggregate analysis showed significantly better fragmentation in the NaF, Na-hexa-metaphosphate, and ryegrass root solutions with a mean weight diameter range of 15.5–18.8 mm compared to the 44.2–47.9 mm of the poultry manure, ADB, and FBC treatments. Dissolved Si, Al, Fe, and Mn levels released in solution were ambiguous. The poor efficiency of the poultry manure, ADB, and FBC treatments was attributed to their high ionic strength, while the high efficiency of the NaF, Na-hexa-metaphosphate, and rye grass root solutions to their high sodium soluble ratio (SSR). A slaking mechanism is proposed suggesting that aqueous solutions with high SSR penetrate faster into the fragipan capillaries and generate the critical swelling pressure and shearing stress required to rupture the fragipan into several fragments. Additional fragmentation occurs in a followup stage during which potential Si, Al, Fe, and Mn binding agents may be released into solution. Field experiments testing these findings are in progress.

Distribution, Morphology, and Chemical Composition of Fe-Mn Nodules in Albeluvisols of the Carpathian Foothills, Poland

Iron and manganese oxides are common components of soils. They frequently occur in nodules constituting important soil sorbents and play a crucial role in a number of chemical reactions in the soil environment. In addition, the oxides are very sensitive to environmental changes (moisture, pH, Eh) constituting important indicators of soil-forming processes and water movement in the landscape. The objectives of the study were: i) to examine the distribution and size of Fe-Mn nodules in Albeluvisols containing a fragipan horizon; ii) to determine the morphology and chemical composition of the nodules, and iii) to estimate the effect of the fragipan horizon on water movement within the soil profile. An investigation was carried out on five soil profiles containing a fragipan horizon and classified as Fragic Albeluvisols within the Carpathian Foothills in Poland. In the Albeluvisols studied, the maximum concentration of Fe-Mn nodules occurred in the horizon lying directly on the fragipan horizon. This suggests that the pan acts as a natural barrier restricting water infiltration and leads to the seasonal development of a perched water table promoting redox processes. The most common are coarse (1–0.5 mm) and medium (0.5–0.25 mm) irregular Fe-Mn nodules showing a gradual boundary and undifferentiated internal fabric. Eluvial horizons contained also larger amounts of round and dense nodules with a sharp boundary, suggesting frequent oxidation and reduction. The fragipan horizon contained mainly irregular and soft nodules, suggesting longer saturation with water during the year. Concentrations of trace elements (Cu, Zn) and P were higher within Fe-Mn nodules than in the surrounding soil materials, showing that iron and manganese oxides adsorbed and immobilised these elements.

Origin of reversible cementation and brittleness of the fragipan horizon in Albeluvisols of the Carpathian Foothills, Poland

Abstract Fragipan horizon is a natural, subsurface soil layer exhibiting higher bulk density and lower porosity in comparison with the overlying horizons significantly restricting infiltration of water and root penetration. In contrast to hard and cemented horizons (e.g. duripan), clods of the fragipan horizon (Btx) slake or fracture immediately when placed in water. The most interesting issue concerning genesis and properties of the fragipan horizon is its reversible cementation, i.e. very hard consistence in dry state and brittle consistence in moist state. The main objective of the present study was to explain and improve the understanding of the specific consistence of the fragipan horizon in moderately humid climate on the example of Albeluvisols of the Carpathian Foothills in Poland basing on mineral and chemical composition of bridges linking coarse soil material. Quantitative X-ray diffraction shows that the studied fragipan horizon contains high concentration of swelling clay minerals. Soil material from the fragipan horizon exhibits numerous clay and iron–clay coatings and clay infillings indicating translocation of colloidal clay and iron oxides from overlying horizons. In addition, bridges between coarse grains of the soil material occur. The fragipan cementing material is composed mainly of SiO2, Al2O3, and Fe2O3 with lower concentration of K2O, MgO, CaO, and Na2O. The chemical composition of bridges within the pan is similar to that of clay coatings and clay infillings suggesting that clay minerals are major bonding agents and are responsible for brittleness and more friable consistence of the pan in the moist state. Additionally, free iron, aluminium, and silica may play some role in specific cementation of the investigated fragipan horizon but these elements are not main bonding agents.

Fragipan horizon degradation and bleached tongues formation in Albeluvisols of the Carpathian Foothills, Poland

A fragipan horizon is defined as a natural, dense subsurface soil horizon, which restricts the infiltration of water and the penetration of roots. Most fragipans show evidence of degradation, which manifests itself in the formation of bleached tongues along vertical cracks. Despite the fact that fragipans have been extensively studied since the 1940s, the problem of their genesis, degradation and vertical tongues formation remains obscured. The main aim of the present study was to explain the genesis of bleached tongues in fragipan horizon using the example of Albeluvisols from the Carpathian Foothills in Poland. Undisturbed soil material was subjected to detailed micromorphological analysis using optical microscopy. Bulk soil material and soil clay fractions were analyzed quantitatively using X-ray diffraction. Basing on the obtained results, it is concluded that the formation of the vertical cracks is likely to be a result of seasonal drying, which promotes the opening of cracks. The prevalence of swelling clay minerals within illuvial horizons promotes periodic swelling and shrinking due to moisture changes. Once vertical cracks are formed, the next phase of fragipan degradation i.e. bleached tongues formation takes place. Tongues formation is most likely caused by the eluviation (microerosion) of weathering products. Eluviation is indicated by a lower concentration of clay minerals and iron oxides in material from vertical tongues when compared with material from prismatic clods and Btx horizon. In the studied soils, the formation of tongues due to the infilling of vertical cracks with material from an upper, eluvial horizon is also likely but only in the upper parts of the cracks.

Pore‐size distribution and particle arrangement in fragipan and nonfragipan horizons

AbstractFragipan is a widely distributed subsoil horizon that induces severe limitations to plant growth and land use, mainly because of its high bulk density. In this work, we evaluated the pore‐size distribution through the analysis of the cumulative curve of intruded mercury volume in some soils with fragipan horizons. This approach provides information also about the arrangement of particles, thus we compared the results obtained for fragipan and nonfragipan horizons to relate porosity and particle arrangement with the specific physical properties of fragipans. The total volume of intruded mercury did not allow to discriminate between fragipan and nonfragipan horizons. However, from the variation of the pore volume as a function of the radius, two modal classes of pores were found, coarse and fine, respectively. The fine‐pore class arose from the arrangement of clay particles, and its volume was correlated to clay contents (r = 0.787) and to clay packing density (r = –0.621). The clay fractions in fragipans were less densely packed than in the other B horizons, even if they had similar clay contents. The coarser‐pore modal class is known to arise from the interactions between clay, silt, and sand particles, and its volume was different among horizons. Fragipan had a low volume of this modal pore class. In addition, a packing density for the coarser phase of 0.74, corresponding to a rhombohedral packing, was found only in fragipans. Thus, the low permeability and high bulk density of fragipans are linked to specific arrangements of the particles: an open packing of the clay phase is associated to an extremely dense packing of silt and sand. This combination is not present in any other soil horizon.

Investigation of the near subsurface using acoustic to seismic coupling

AbstractAgricultural, hydrological and civil engineering applications have realized a need for information of the near subsurface over large areas. In order to obtain this spatially distributed data over such scales, the measurement technique must be highly mobile with a short acquisition time. Therefore, some type of remote sensing or geophysical technique must be utilized. Geophysical measurements are sensitive to the distribution of physical properties, such as the electrical conductivity, dielectric constant, mass density, mechanical properties, etc., of the ground. In most instances, the geophysical properties must be reconciled with physical properties used by the soil scientist. This paper presents a preliminary investigation of the use of acoustic to seismic (A/S) coupling measurements for measuring the depth to the top of the fragipan horizon. Fragipans influence the hydrology and ecohydrology of the soil on a field scale. A suite of traditional geophysical measurements was taken to characterize the soils at two sites with different depths to the fragipan horizon. Data at these two field sites indicate that this A/S technique is sensitive to the spatial variability of the depth to the fragipan horizon. At present, inversion of the A/S data for the fragipan depth requires use of data from a separate geophysical measurement or soil cores to provide a field calibration. Copyright © 2009 John Wiley & Sons, Ltd.

Geochemical Assessment of Iron and Vanadium Relationships in Oxic Soil Environments

Geochemical assessment has become a cost-effective and highly accurate tool for estimating metal contamination, especially in those cases where the level of contamination is not considered severe. Difficulties frequently arise in attempting to discriminate between pristine metal concentrations and low-level environmental impacts. As an example, Vanadium contamination is frequently associated with coal and petroleum bi-products; however, air and water contamination pathways are also possible. The purpose of this investigation was to characterize the V and Fe concentration relationship among a wide variety of soil types and to formulate an estimate of the pristine V concentrations in these soils. If a linear relationship may be established between Fe and V, then geochemical analysis of impacted soils may discriminate between V as a natural background component and anthropogenic V. Forty-six moderately well-drained to well-drained soil profiles having cambic, argillic or calcic soil horizons were characterized for Fe and V using an aqua-regia digestion to determine if these soils exhibited a one-to-one correspondence between V and Fe. Such a correspondence was authenticated for the majority of these soils and may be used to discriminate between natural and anthropogenic V. The presence of argillic, calcic or fragipan horizons did not reduce the one-to-one correspondence between V and Fe, suggesting that these soil processes did not selectively partition either V or Fe. The method needs to be further evaluated for soils having anoxic soil conditions, lithologic discontinuities and other specific pedogenic processes.

Range of Fragipan Expression in Some Michigan Soils

Many of Michigan's loamy soils exhibit varying degrees of fragipan expression, which may be variously influenced by their parent materials. We examined three soils in northern Michigan with varying degrees of fragipan expression to assess development of fragipans formed in acidic and calcareous glacial drift. To accomplish this, soil characterizations were made through field, physical, chemical, and micromorphological observations and analyses. The soils have bisequal horizonation: an upper sequum associated with podzolization processes and lower sequum associated with lessivage. Protofragipans and fragipans are found only in the lower sequum. The soils experience periodic episaturation and contain at least one lithologic discontinuity, often near the protofragipan or fragipan. The protofragipan and fragipans have loamy sand to loam textures, weak platy to subangular blocky structure, higher bulk densities (1.5-1.9 g cm -3 ) and lower pH values (4.8-6.8) than adjacent horizons, brittle failure, and fine vesicular pores. Eluvial protofragipan and fragipan horizons contain albic materials that tongue into underlying argillic horizons. Illuvial fragipans exhibit clay coats, flows, and bridging. Thin-section characterizations confirm the presence of closely packed fabrics, intergrain bridging by clays, and void pedofeatures. Scanning electron microscopy (SEM) provides evidence of eluviation, reorganization of silt and clay, fluctuating redox conditions, degraded void pedofeatures, and the presence of surficially amorphous (bonding) materials in the protofragipan and fragipans. Soil extraction data do not preclude the presence of a fragic-property (brittleness) agent. Results, therefore, indicate that these protofragipans and fragipans are pedogenic and can form, with variable expression, in both acidic and calcareous glacial parent materials.

Concentration and Distribution of Six Trace Metals in Northern Kentucky Soils

Concentration and distribution of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) were determined in 26 soil profiles (n = 78) of northern Kentucky in response to environmental concerns about increasing anthropogenic inputs in a fast-paced urbanizing area. The selected sites represent alluvial, glacial till or residual soils that have not received any biosolid- or industrial-waste applications. Mean concentrations of Zn (53.8 mg kg−1) and Ni (25.9 mg kg−1) were the highest in the soil profile, whereas Cd (0.21 mg kg−1) was present only in trace amounts. All metals were within the low to middle range of baseline concentrations reported for US soils, suggesting minimal anthropogenic inputs. The distribution of Cu, Cr, Ni, and Zn increased with soil depth, whereas Cd and Pb concentrations were unaffected throughout the soil profile. Alluvial soils had the highest overall metal accumulations, particularly in surface soil horizons, indicating potential metal enrichment through depositional processes. The presence of a fragipan horizon or depth to bedrock did not significantly affect metal retention. Single correlation and multiple regression analyses indicated OM and pH as the most influential soil parameters for metal retention, followed by cation exchange capacity (CEC) and CEC/clay. Single correlations among metals suggested strong covariance of Zn with most metals throughout the soil profile, but weaker for Pb and Ni.

HYDROLOGIC PROCESSES IN VALLEY SOILSCAPES OF THE EASTERN PALOUSE BASIN IN NORTHERN IDAHO

Vadose zone hydrology in the eastern Palouse Basin of northern Idaho is poorly understood because loess deposits often contain multiple hydraulically restrictive horizons that impede water flow. Valley soilscapes are of particular interest from a hydrologic perspective, because during the winter months, most of the precipitation is redistributed as runoff and throughflow into these landscape positions. Understanding the relationship between near-surface perched water table dynamics and vadose zone hydraulic processes in valley soilscapes is necessary to assess the sustainability of the groundwater resource. We implemented a combined approach to assess hydrologic processes in valley positions using hydrometric measurements, natural tracers, and stratigraphic observations. Hydrographs of near-surface monitoring wells indicate that valley positions maintain a thicker zone of saturation for longer duration compared with adjacent upland positions. Deep tensiometers demonstrate that multiple zones of seasonal saturation develop within the vadose zone of valley soilscapes in response to paleosol fragipan horizons and sediments of contrasting hydraulic conductivity. In some instances, the saturated thickness of vadose zone water tables was greater than 2.0 m and, because they are confined, displayed a positive pressure head. On adjacent uplands, seasonal saturation occurs only above the uppermost fragipan. Eluvial horizons having low Mn d correspond to zones of saturation, whereas aquitards reflect Mn d maxima. The recharge rate calculated using natural Cl - mass balance was 2.4 mm y -1 and did not correspond to measurements of saturated thickness by tensiometers. In addition, natural chloride profiles of other valley soilscapes display differences in recharge rates according to regional patterns in soil development. Together, deep tensiometer readings, secondary Mn distributions, and Cl - profiles suggest that groundwater recharge does not occur via piston flow. Detailed stratigraphic analysis illustrates that preferential flow is a possible recharge mechanism. Results suggest that valley soilscapes play an important role in both surficial and deep regolith hydrological processes in the Palouse Basin.

Spatial distribution of lepidocrocite in a soil hydrosequence

AbstractThree terrace soils comprising a hydrosequence were examined to determine how the spatial distribution of lepidocrocite was related to depth and duration of saturation. Vertical relief was 1.0 m with well drained, moderately well drained, and somewhat poorly drained pedons spaced ∼60 m apart. All soils contained brittle, slowly-permeable subsoil horizons and were acidic with <35% base saturation throughout the upper sola. The well drained soil (Fragic Hapludult) had no morphological indicators of wetness within a depth of 180 cm, and water was perched above a brittle horizon at 82 cm for a total of only 41 days during the 3.4 year observation period. Nevertheless, trace amounts of lepidocrocite were detected in the subsoil. The moderately well drained soil (Typic Fragiudult) was saturated at a depth of 180 cm for 6% of the time, and water was perched on top of a fragipan at 74 cm for 13% of the time. Lepidocrocite was most abundant in this pedon and reached maximum concentrations below the fragipan in the capillary fringe of the regional water table (150–183 cm). The somewhat poorly drained member of the hydrosequence (Aeric Fragiaquult) was saturated at a depth of 180 cm for 96% of the observation period and also contained perched water above a fragipan for >90% of the time. Lepidocrocite occurred throughout this pedon but was most concentrated in fragipan horizons (86–135 cm) between the perched and regional zones of saturation. These horizons were saturated from 22 to 48% of the observation period. The results of this study suggest that lepidocrocite formation was favoured in horizons that were saturated for 5–50% of the time when soil temperatures exceeded 5°C.