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At least 37 records · Page 2Linked to original sources
USGS1987

Determination of Fe(III) and Fe(II) in oxalate extracts of sediment

A method was developed to determine the concentrations of oxalate-extractable Fe(III) and Fe(II) in sediments. Sediment was extracted in acid ammonium oxalate under N 2 . Iron(II) in the extract was determined with ferrozine. Iron(III) in the extract was reduced with hydroxylamine-hydrochloride, and total Fe in the extract was determined with ferrozine. Oxalate-extractable Fe(III) was calculated as the difference between Fe(II) and total Fe in the extract. Iron(II) was not oxidized, and Fe(III) was not reduced during the extraction. For an accurate estimate of oxalate-extractable Fe(III), fresh samples had to be analyzed, because air-drying or freeze-drying the samples oxidized Fe(II) to oxalate-extractable Fe(III). With a minimal increase in analytical effort, the method yields far more information on Fe geochemistry than the standard aerobic oxalate extraction method.

Soil Science Society of America Journal↗
USGS1987

Selenium adsorption by goethite

The adsorption of Se by goethite was studied as a function of time (10 min–24 h), temperature (295.5 and 303.5 K), pH (4–11), particle concentration (3–300 mg/L), total Se concentration (0.02–5 × 10 −5 M ), oxidation state [Se(IV) and Se(VI)], and competing anion concentration [(anion)/(Se(IV) = 0.25 to 50 000] in order to assess the influence of these factors on Se mobility. The data indicate that (i) the surface sites of goethite are heterogeneous, (ii) the adsorption of selenite [Se(IV)] reaches equilibrium in 2 h and is completely reversible with respect to pH, (iii) the removal of selenite from solution increases with decreasing pH and increasing particle concentration, (iv) the adsorption edge for selenite shifts to lower pH values with increasing total selenite concentrations, (v) selenite adsorbs much more strongly than selenate [Se(VI)], and (vi) the influence of additional anions on selenite adsorption depends on the relative affinity of the anions for the surface and the relative concentrations of the anions. For a given anion concentration ratio, the competition sequence with selenite is phosphate > silicate ≥ citrate > molybdate > bicarbonate/carbonate > oxalate > fluoride > sulfate. Therefore, the conditions that favor the mobility of Se in the environment with respect to adsorption are alkaline pH, high Se concentrations, oxidizing conditions, and high concentrations of additional anions that strongly adsorb.

Soil Science Society of America Journal↗
USGS1988

Air encapsulation during infiltration

A series of field and laboratory experiments were performed to measure the effects of air encapsulation within the soil's transmission zone upon several infiltration properties. In the field, infiltration rates were measured using a double-cap infiltrometer (DCI), and soil-water contents were measured using time-domain reflectometry (TDR). Before half of the infiltration experiments, CO 2 was injected through the DCI into the soil to reduce the amount of air encapsulation in the soil's transmission zone. For a gravelly loam as steady infiltration rates were approached, the average volumetric water content was 0.38 cm 3 cm −3 for control experiments and 0.43 cm 3 cm −3 for CO 2 experiments. The average steady infiltration rate was 0.42 cm min −1 for the control experiments compared to 4.40 cm min −1 for the CO 2 experiments. For a sandy loam as steady infiltration rates were approached, the average volumetric water content was 0.43 cm 3 cm −3 for control experiments compared with 0.45 cm 3 cm −3 for CO 2 experiments. The average final infiltration rate was 0.09 cm min −1 for the control experiments compared with 0.42 cm min −1 for the CO 2 experiments. In the laboratory, infiltration experiments were performed using repacked soil columns (15-cm i.d. by 140 cm long), again using TDR and CO 2 flooding. For a medium sand as steady infiltration rates were approached, the average volumetric water content was 0.29 cm 3 cm −3 for the control experiments and 0.36 cm 3 cm −3 for the CO 2 experiments. The average steady infiltration rate was 0.25 cm min −1 for the control experiments and 1.23 cm min −1 for the CO 2 experiments. For a loam as steady infiltration rates were approached, the average volumetric water content was 0.45 cm 3 cm −3 for the control experiments and 0.50 cm 3 cm −3 for the CO 2 experiments. The average steady infiltration rate was 0.02 cm min −1 for the control experiments and 0.10 cm min −1 for the CO 2 experiments. These results suggest that a significant portion of the total encapsulated air resided in interconnected pores within the soil's transmission zone. For the time scale considered, this residual air caused the effective hydraulic conductivity of the transmission zone to remain at a level no greater than 20% of the saturated hydraulic conductivity of the soil.

Soil Science Society of America Journal↗
USGS1988

Hydraulic conductivity of a sandy soil at low water content after compaction by various methods

To investigate the degree to which compaction of a sandy soil influences its unsaturated hydraulic conductivity K , samples of Oakley sand (now in the Delhi series; mixed, thermic, Typic Xeropsamments) were packed to various densities and K was measured by the steady-state centrifuge method. The air-dry, machine packing was followed by centrifugal compression with the soil wet to about one-third saturation. Variations in (i) the impact frequency and (ii) the impact force during packing, and (iii) the amount of centrifugal force applied after packing, produced a range of porosity from 0.333 to 0.380. With volumetric water content θ between 0.06 and 0.12, K values were between 7 × 10 −11 and 2 × 10 −8 m/s. Comparisons of K at a single θ value for samples differing in porosity by about 3% showed as much as fivefold variation for samples prepared by different packing procedures, while there generally was negligible variation (within experimental error of 8%) where the porosity difference resulted from a difference in centrifugal force. Analysis involving capillary-theory models suggests that the differences in K can be related to differences in pore-space geometry inferred from water retention curves measured for the various samples.

Soil Science Society of America Journal↗
USGS1988

The feasibility of using sequential extraction techniques for As and Se in soils and sediments

Laboratory experiments were conducted with well-characterized minerals to test the applicability of selective extraction schemes for Se and As partitioning in soils and sediments. Two specific steps were tested: the reductive dissolution of amorphous iron oxides and the oxidation of organic material. Selenium and As associated with amorphous iron oxides were usually not found in solution after reductive dissolution, due to readsorption onto other minerals unaffected by the extractant. Oxidants intended to dissolve organic material also oxidized Se(IV) adsorbed on a mineral to Se(VI), causing the release of Se(VI) to the extractant solution.

Soil Science Society of America Journal↗
USGS1988

Sorption characteristics of organic compounds on hexadecyltrimethylammonium-smectite

When hexadedyltrimethylammonium (HDTMA) ion is exchanged for metal cations like calcium in smectite, the sorptive properties of the clay are greatly modified. The resultant HDTMA-smectite complex behaves as a dual sorbent, in the sorption of organic compounds, in which the mineral fraction functions as a solid adsorbent and the organic (HDTMA) phase as a partition medium. Capacities of mineral adsorption and partition uptake by HDTMA in the HDTMA-smectites are illustrated by sorption of benzene, trichloroethene (TCE), and water as vapors on the dry sample and by sorption of benzene and TCE from water. The exchanged HDTMA in clay is found to be a much more powerful partition medium than ordinary soil organic matter in the uptake of benzene and TCE. Based on this finding, HDTMA-smectite appears to be an effective sorbent for removing organic contaminants from water. It is suggested that such sorptive organo-clay complexes could be used to enhance the containment capabilities of clay landfill liners and bentonite slurry walls.

Soil Science Society of America Journal↗
USGS1988

Distribution of selenium in soils of agricultural fields, western San Joaquin Valley, California

Soils from three agricultural fields in the Panoche Creek alluvial fan area in the western San Joaquin Valley, California, were analyzed for soluble, adsorbed, and total concentrations of selenium (Se) to assess the distribution and forms of Se in relation to the leaching of Se from soils. This assessment is needed to evaluate the importance of soil Se in affecting ground water concentrations. Soil samples were collected from three fields with drainage systems of different ages (6, 15, 1.5 yr) and different Se concentrations in drain water (58, 430, 3700 µg L −1 , respectively). Concentrations of soluble Se and salinity were highest in soils from the field drained for 1.5 yr and lowest in the field drained for 6 yr. Of the total concentration of soil Se from all three fields, the proportion of adsorbed and soluble Se ranged from 1 to 11% and <1 to 63%, respectively. Most of the soluble Se was present as selenate and most of the adsorbed Se was found to be selenite. Consistent with a model of evaporative concentration of Se and salinity in soils and shallow ground water in this area, most of the variance in soluble Se is explained by salinity (r 2 > 0.68) in saturation extracts of soils sampled from below the water table. In contrast, most soluble salts and Se apparently have been leached from the unsaturated soils in the fields drained for 6 and 15 yr. For the leached soils, dissolution and precipitation of evaporite minerals containing Se may no longer control concentrations of soluble Se.

California↗
USGS1989

Development rates of Late Quaternary soils, Silver Lake playa, California

Soils formed on alluvial fan deposits that range in age from about 35 000 to 200 yr BP near Silver Lake playa in the Mojave Desert permit study of the rates of soil development in an arid, hyperthermic climate. Field-described properties of soils were quantified and analyzed using a soil development index that combines properties and horizon thicknesses. We compared the variability in index values from five independent descriptions of the same soil profile with the variability in values from four soils of the same age described by different people. The variability due to description accounts for between 30 and 80% of the total variability (that due to both the soils and description) for the properties described, which emphasizes the need for consistent soil descriptions. Pedogenic CaCO 3 (as indicated by color), pH increase, and dry consistence appear to change with age at linear rates, whereas rubification appears to change at a logarithmic rate. The linear rates are best attributed to the progressive accumulation of CaCO 3 - and salt-rich eolian dust derived from the playa and other more distant sources. The total-texture values of soils on fans older than 10 000 yr BP are similar, which suggests that playas in this area may have been wet enough to restrict the availability of fines from these sources for many thousands of years prior to 10 000 yr BP. Index values for most properties of soils on the lower fans adjacent to the playa increase at faster rates than those of soils of the same age on the upper fans farther from the playa, probably due to the proximity to this local dust source. Equations derived from regressions of soil age and properties can be used to estimate ages of undated, lithologically similar deposits in similar climates and geomorphic settings.

California↗
USGS1990

Selenium speciation methods and application to soil saturation extracts from San Joaquin Valley, California

Methods to determine soluble concentrations of selenite, selenate, and organic Se were evaluated on saturation extracts of soil samples collected from three sites on the Panoche Creek alluvial fan in the western San Joaquin Valley, California. The methods were used in combination with hydride-generation atomic-absorption spectrometry for detection of Se, and included a selective chemical-digestion method and three chromatographic methods using XAD-8 resin, Sep-Pak C 18 cartridge, and a combination of XAD-8 resin and activated charcoal. The chromatography methods isolate dissolved organic matter that can inhibit Se detection by hydride-generation atomic-absorption spectrometry. Isolation of hydrophobic organic matter with XAD-8 did not affect concentrations of selenite and selenate, and the isolated organic matter represents a minimal estimation of organic Se. Ninety-eight percent of the Se in the extracts was selenate and about 100% of the isolated organic Se was associated with the humic acid fraction of dissolved organic matter. The depth distribution of Se species in the soil saturation extracts support a hypothesis that the distribution of soluble Se and salinity in these soils is the result of evaporation from a shallow water table and leaching by irrigation water low in Se and salinity.

California↗
USGS1991

Variability of an unsaturated sand unit underlying a radioactive- waste trench

Properties of soils vary considerably within any field. This study was conducted to investigate the variability in properties of an unsaturated sand unit that lies at a depth of 13 m below land surface. Four-hundred-forty soil core samples, obtained from a 1.75 by 18-m horizontal plane within a sand unit underlying a waste trench, were used to describe the variability of moisture content, tritium concentration, and several physical properties. A simple model based on unit-gradient theory was used to calculate fluxes of water and tritium through the study plane. The effects of including spatial variability of properties in model calculations were examined. The sand was uniform in terms of median particle size, bulk density, and proposity, but several variables, including saturated hydraulic conductivity ( K s ) and tritium concentration, showed considerable variability. In general, correlation coefficients calculated between variables were low. Spatial correlation structures were identified for moisture content, tritium concentration, and a few particle-size variables, but were absent for K s and most other variables. Variances of K s and field-measured volumetric moisture content (θ) were lower than published values for soils. Model-calculated fluxes of water and tritium that accounted for spatial variability of properties were considerably higher than those that did not account for the variability. Model results were more sensitive to changes in θ and residual moisture content (each parameters of the relative hydraulic conductivity function) than to changes in K s .

Illinois↗
USGS1991

Collimated neutron probe for soil water content measurements

The use of uncollimated (undirected) neutron moisture meters is common in the biological and geophysical sciences. A collimated neutron probe was designed to enable measurements in specific directions from the access tube. To determine the size and shape of soil volume affecting the neutron counts, experiments were conducted to evaluate: (i) the vertical distance of soil above and below the probe that influences neutron counts, (ii) the horizontal distance away from the probe into the soil that influences neutron counts, (iii) the angle of soil viewed by the probe from the collimator, and (iv) the three-dimensional thermal-neutron density field. The distance and the angular dimensions of the volume of influence were defined as the horizontal distance of neutron penetration from the edge of the probe, the vertical distance above and below the center of the effective measurement point of the probe, and the angle from the center of the probe, which would allow the determination of relative water content to within 95%. The vertical distance was ≈0.5 m, the horizontal distance was ≈0.2 m, and the angle of soil viewed by the probe from the collimator was ≈120°. Thermal neutrons detected from distances or angles larger than these values influence the determination of relative water content by 5% or less.

Soil Science Society of America Journal↗
USGS1991

Balloon and core sampling for determining bulk density of alluvial desert soil

Samples were collected from major strata in the upper 5 m of an alluvial soil profile in the Amargosa Desert of southern Nevada to compare rubber-balloon and drive-core bulk-density measurement methods. For strate where the fine soil was <82% sand and <15% clay, differences between total and fine-soil bulk-density values determined by the two methods were typically <10 and 15%, respectively, even where rock-fragment content was as great as 48% by volume. Outside this range of fine-soil texture, where soil consistency was either very loose or very hard, the core method appeared to sample inaccurately, resulting in bulk-density values >0.30 Mg m −3 less than those determined by the balloon method. Under the severe sampling conditions encountered, large decreases in the relative accuracy of the core method were not directly related to rock-fragment content, but were related to extremes in the cohesiveness of the strata sampled.

Nevada↗
USGS1991

Selenium mobility and distribution in irrigated and nonirrigated alluvial soils

Dissolution and leaching of soil salts by irrigation water is a primary source of Se to shallow groundwater in the western San Joaquin Valley, California. In this study, the mobility and distribution of selenite and selenate in soils with different irrigation and drainage histories was evaluated using sorption experiments and an advection-dispersion model. The sorption studies showed that selenate (15–12400 µg Se L −1 ) is not adsorbed to soil, whereas selenite (10–5000 µg Se L −1 ) is rapidly adsorbed. The time lag between adsorption and desorption of selenite is considerable, indicating a dependence of reaction rate on reaction direction (hysteresis). Selenite adsorption and desorption isotherms were different, and both were described with the Freundlich equation. Model results and chemical analyses of extracts from the soil samples showed that selenite is resistant to leaching and therefore can represent a potential long-term source of Se to groundwater. In contrast, selenate behaves as a conservative constituent under alkaline and oxidized conditions and is easily leached from soil.

Soil Science Society of America Journal↗
USGS1992

Semiempirical model of soil water hysteresis

In order to represent hysteretic soil water retention curves accurately using as few measurements as possible, a new semiempirical model has been developed. It has two postulates related to physical characteristics of the medium, and two parameters, each with a definite physical interpretation, whose values are determined empirically for a given porous medium. One parameter represents the fraction of the pore space that is not subject to Haines jump hysteresis. Its associated postulate is that a single value of this parameter characterizes a given medium, or, equivalently, that the medium is self-similar with respect to the division between hysteretic and nonhysteretic pore space. The second parameter is the effective body-to-neck size ratio of the medium's largest pore. The second postulate specifies a particular relation between the size distributions of pore bodies and of pore necks. Tests of the model show that it provides high-quality optimized fits to measured water content vs. matric pressure wetting curves for a wide variety of media. The parameter values obtained through optimization correlate plausibly with such media properties as uniformity of particles, complexity of structure, and degree of compaction. A practical use of this model is to provide a complete simulated main wetting curve for a medium where only a main drying curve and two points on the wetting curve have been measured. With additional development, it may be possible to do without the two measured wetting points if parameters can be evaluated from other measurements or known properties of the medium.

Soil Science Society of America Journal↗
USGS1992

Improved apparatus for measuring hydraulic conductivity at low water content

A modification of the steady-state centrifuge method (SSCM) for unsaturated hydraulic conductivity ( K ) measurement improves the range and adjustability of this method. The modified apparatus allows mechanical adjustments to vary the measured K by a factor of 360. In addition, the use of different flow-regulating ceramic materials can give a total K range covering about six orders of magnitude. The increment of K adjustment is a factor of about 1.6. This makes it potentially useful for measuring targeted values of K or, through a trial and error procedure, of water content (θ). The range extension afforded by this modification has led to the lowest steady-state K measurement to date: 1.1 × 10 −11 m/s at θ of 0.068 m 3 water/m 3 for a sandy soil of the Delhi series (mixed, thermic Typic Xeropsamment).

Soil Science Society of America Journal↗
USGS1993

An improved method for quantifying soil macroporosity

Quantitative information on macroporosity is needed to predict water flow and solute transport in field soils. A method was developed for determining the number, shape, and size distribution of soil macropores. Horizontal serial sections sawed from paraffin-impregnated soil cores were photographed under ultraviolet (UV) light. Anthracene, mixed with the paraffin, fluoresces a bright bluish white under UV light and provides a sharp contrast between the soil matrix and the paraffin-filled pore space. Section photographs were converted to 256 level, grey-scale digital images using a flat-bed scanner. Image processing was used to classify each pixel in a digital image as pore space or soil matrix, to group the pore space pixels into pores, and to measure the area and perimeter of each pore. The method was able to measure pores with an equivalent radius ≥85 µm. Macroporosity in soil cores sampled form a tillage path and from an adjacent, undisturbed (notill) region was quantified. Tillage sections contained, on the average, 9.4 macropores/cm 2 with an equivalent macroporosity of 8%, while no-till sections contained 0.8 macropores/cm 2 with an equivalent macroporosity of 0.3%. Computed intrinsic permeabilities for tillage sections that included macropore information were significantly larger than values computed using micropore information alone, suggesting that macropores must be included in permeability calculations when the number of macropores is large. The developed method should be useful for quantifying macroporosity in nonskeletal soils.

Soil Science Society of America Journal↗
USGS1994

The feasibility of recharge rate determinations using the steady-state centrifuge method

The establishment of steady unsaturated flow in a centrifuge permits accurate measurement of small values of hydraulic conductivity ( K ). This method can provide a recharge determination if it is applied to an unsaturated core sample from a depth at which gravity alone drives the flow. A K value determined at the in situ water content indicates the long-term average recharge rate at a point. Tests of this approach have been made at two sites. Unsaturated K values were measured easily for sandy core samples from a site in the San Joaquin Valley of California. The results indicate that a better knowledge of the matric pressure profiles is required before a recharge rate can be determined. Fine-textured cores from a site in southeastern Washington required new developments of apparatus and procedures, especially for making centrifuge measurements with minimal compaction of the samples. Measured K values led to preliminary recharge rate determinations that are reasonable considering the known hydrology and topography of the site.

Soil Science Society of America Journal↗
USGS1994

Variations in water balance and recharge potential at three western desert sites

Radioactive and hazardous waste landfills exist at numerous desert locations in the USA. At these locations, annual precipitation is low and soils are generally dry, yet little is known about recharge of water and transport of contaminants to the water table. Recent water balance measurements made at three desert locations, Las Cruces, NM, Beatty, NV, and the U.S. Department of Energy's Hanford Site in the state of Washington, provide information on recharge potential under three distinctly different climate and soil conditions. All three sites show water storage increases with time when soils are coarse textured and plants are removed from the surface, the rate of increase being influenced by climatic variables such as precipitation, radiation, temperature, and wind. Lysimeter data from Hanford and Las Cruces indicate that deep drainage (recharge) from bare, sandy soils can range from 10 to >50% of the annual precipitation. At Hanford, when desert plants are present on sandy or gravelly surface soils, deep drainage is reduced but not eliminated. When surface soils are silt loams, deep drainage is eliminated whether plants are present or not. At Las Cruces and Beatty, the presence of plants eliminated deep drainage at the measurement sites. Differences in water balance between sites are attributed to precipitation quantity and distribution and to soil and vegetation types. The implication for waste management at desert locations is that surface soil properties and plant characteristics must be considered in waste site design in order to minimize recharge potential.

Soil Science Society of America Journal↗