Volumetric Water Research Articles

Surfactant enhanced removal of PCE in a nominally two-dimensional, saturated, stratified porous medium

Although surfactant enhanced remediation of nonaqueous phase liquids (NAPLs) by pump-and-treat technology has been studied extensively in the laboratory with one-dimensional columns, very few multi-dimensional investigations have been reported. In this study we focus on the removal of perchloroethylene (PCE) from a two-dimensional, saturated porous medium containing a low permeability sand layer situated in an otherwise high permeability sand. A PCE spill was applied at the surface of the porous medium and allowed to redistribute until static equilibrium was achieved. The porous medium was then flushed with various surfactant and co-solvent formulations injected at the PCE source location and extracted at the bottom of the porous medium using a configuration similar to that of Abdul and Ang [Abdul, S.A., Ang, C.C., 1994. In situ surfactant washing of polychlorinated biphenyls and oils from a contaminated field site: Phase II. Pilot study. Ground Water 32, 727–734]. Effluent samples were analyzed for dissolved PCE concentrations. Volumetric water and PCE content values were determined at a number of locations by means of dual-energy gamma radiation measurements. Once surfactant flushing had started, PCE moved as a distinct separate phase ahead of the surfactant front. Most of this downward moving PCE accumulated on top of the low permeability sand layer. Some PCE, however, passed quickly through this layer and subsequently through the high permeability sand below it. Movement of some of the PCE into and through the low permeability sand layer was attributed to local heterogeneities combined with reduced interfacial tensions associated with the surfactant formulation. Clean-up of PCE in most of the high permeability sand was considered to be effective. PCE accumulated on top of the fine layer, however, posed a significant challenge to remediation and required several pumping configurations and surfactant/co-solvent formulations before most of it was removed.

Soil moisture variability: a comparison between detailed field measurements and remote sensing measurement techniques

Surface soil moisture, defined here as the volumetric soil water content in the top 10 cm of the soil, shows a great deal of variability. The variance observed within a square metre can be as large as for a whole field. For that reason alone, point measurements in low quantities cannot be representative for the average soil moisture of a whole field. Soil moisture estimated by remote sensing techniques, especially by microwave sensors, might give adequate information on areally-averaged soil moisture. If these techniques are to be worthwhile in hydrological modelling, it will be necessary to link the various spatial resolutions of the different instruments to each other and to point-scale measurements. Despite differences in instrument configuration it is shown that it is possible, under certain circumstances, to downscale high resolution areal soil moisture estimates to low resolution soil moisture estimates using data from HAPEX-Sahel'92 and Washita'94.

LATERAL BROMIDE DISTRIBUTION IN A VERTIC CLAY SOIL

In order to control the transport oftoxic elements, dissolved salts, and nutrients in agricultural areas, information on the spatial variability of field-scale transport properties is needed. To evaluate this for an unsaturated layered clayey soil, tracer tests were conducted at the Cherfech experimental field research station in Tunisia. Bromide-tagged water was infiltrated under ponding conditions on a 21.7-m 2 horizontal field plot equipped with 60 solute samplers, 15 neutron probe access tubes, and 15 piezometers. Volumetric soil water content was measured by means of a neutron probe at five depths, and soil moisture samples were withdrawn through ceramic soil water samplers at four depths, each with 15 suction samplers. The results showed typical evidence of preferential flow, with a wide variety of travel times with depth. This was noted especially for deeper soil layers, which displayed a large horizontal variation. In two tracer experiments, the groundwater tracer concentration increased up to twice the concentration of the water in the unsaturated zone withdrawn from different depths. This shows that bypass directly to the groundwater, initially at 1.5 m depth, occurred under ponding with chemigation.

Field Determination of Unsaturated Hydraulic Conductivity of Forest Soils

This study introduces a field method to determine unsaturated hydraulic conductivity which is applicable to sloping terrain with a limited water supply. A single steel ring infiltrometer and an artificial rainfall simulator are used in this method to reduce the amount of water required to attain a steady-state flux condition. Six tensiometers and a time domain reflectometry (TDR) are employed as the soil capillary pressure head and the volumetric soil water content measurement devices, respectively. Water contents measured by the TDR are corrected using a simple calibration method suggested by Hook and Livingston (1996). Unsaturated hydraulic conductivities are computed based on the instantaneous profile method using capillary pressure head and water content changes measured during a drainage process. The proposed method was applied to a forest soil profile in Rokko Mountain range. Results showed that the relationships between the unsaturated hydraulic conductivity and capillary pressure head developed by the proposed method coincide well with those measured by the conventional steady-state laboratory experiment. The proposedin-situ method is the effective simple means to determine unsaturated hydraulic conductivities of forest soils, since this method is enough accurate and consuming less amount of water and time.

Infiltration and Redistribution of Perchloroethylene in Stratified Water‐Saturated Porous Media

AbstractAlthough infiltration, redistribution, and drainage of dense nonaqueous‐phase liquids (DNAPLs) in water‐saturated soils and aquifers are known to be sensitive to stratifications of the solid phases of the porous media, little experimental evidence is available to explain the behavior of DNAPL movement. To improve our understanding of DNAPL movement under these conditions, physical simulations were conducted with various perchloroethylene (PCE) spills in initially water‐saturated, stratified porous media in nominally one‐dimensional glass columns and in an intermediate scale, nominally two‐dimensional laboratory flow container. The columns and flow container were packed with two distinct layers of coarse sand, with a fine sand layer in between. Volumetric water and DNAPL contents, as well as bulk densities, were determined with a dual‐energy gamma radiation system. To obtain additional information required for the explanation of the displacement processes, in‐ and outflow rates of water and PCE were measured during each experiment. In the sand columns, PCE accumulated on top of the fine layer, while in the flow container, considerable lateral movement was observed just above the fine sand layer. In general, these phenomena were attributed to the small conductivity values of the fine layers relative to those of the coarse layers rather than to displacement pressures not being exceeded. Overall our results suggest that in field situations, continuous PCE spills are more likely to penetrate horizontal fine layers than short intermittent spills.

Effect of water-table management and nitrogen supply on yield, plant growth and water use of corn in undisturbed soil columns

The effect of three water-table depths (30, 60 and 80 cm below the soil surface) and four N rates (0, 45, 90 and 135 kg ha−1) on plant growth, yield and water use were evaluated for corn (Zea mays L.). Research was conducted in a greenhouse, using 36 undisturbed foil columns (20 cm i.d. and 90 cm length) collected with a Meta-Drill vibrating core sampler from a Fox sandy loam soil at Harrow Research Centre. Corn grown in the 80-cm water-table depth had the greatest degree of water stress, as indicated by low volumetric soil water content, low stomatal conductance and transpiration rates, and elevated soil-surface and leaf-surface temperatures. There was a substantial increase in plant dry weight and grain yields as the N rates increased from 0 to 135 kg ha−1 with the 30- and 60-cm water-table depths. Under our experimental conditions, maximum grain yields were obtained with a 60-cm water-table depth. Grain yields were significantly reduced with the 80-cm water-table depth. With this water-table depth, grain yield was also reduced by N addition. Key words: Water-table management, Zea mays, yield, stomatal conductance, leaf temperature

Differential Effects of Soil Water Content and Temperature on Nitrification and Aeration

AbstractEnvironmental concerns have stimulated increased interest in NO−3 accumulation in soils. The aeration status of the soil, which is mainly governed by the water content and temperature, is a central factor. The biological process responsible for NO−3 accumulation, nitrification, was measured to estimate the combined effects of water content and temperature and determine their joint effect on soil aeration. The effects of temperatures of 15, 20, 25, 30, and 35°C and water contents equivalent to 0.35, 0.42, 0.50, 0.52, 0.57, and 0.60 relative water content (volumetric water content/total porosity) on the nitrification activity of soil samples containing 2‐mm sieved soils taken at 0 to 20 or 20 to 40‐cm depth were determined by measuring NO−3 accumulation for 17 h. A descriptive model including three biological parameters, maximum nitrification rate (Nrmax), optimal relative water content (Θopt), and temperature, was developed. Maximum Nrmax occurred at 25.5°C in 0‐ to 20‐cm soil and at 20°C in 20‐ to 40‐cm soil, suggesting an adaptation of soil nitrifying populations to the temperature regime of the soil. The Nrmax value was negatively related to Θopt, and Θopt was dependent on temperature (T). This Θopt (T) relationship was parabolic in nature, with Θopt being at a minimum between 20 and 25°C. It could be simulated using O2 diffusion and respiration rates, inferring that these processes influenced Θopt and T correlation. The ranges of O2 concentrations favorable to maximum nitrification within an aggregate volume fraction were estimated for different temperatures. Nitrification was generally maximum when the intraaggregate pore spaces were saturated with water, with no water in the interaggregate pore space (i.e., 0.44 relative water content at 25°C and 0.36 at 20°C at 0‐ to 20‐ and 20‐ to 40‐cm depths).

Soil water dynamics in an oak stand

A model describing water uptake by plants with particular attention to the soil-root interface under transient conditions is derived and discussed. Field data on a daily scale enable the unknown parameters of the model to be determined with the help of an identification technique. The model is then used to analyse the experimental results presented in part I of this paper. The loss of total conductivity of the soil-tree system under drought conditions whereas the metabolism of the trees seems to remain unaffected can be explained by the increase of the soil-root resistance. In fact this resistance becomes the limiting factor when the volumetric soil water content decreases (below θ=0.33 for the superficial layer and 0.36 for the deeper one in the studied case). Such values can be frequently encountered at the end of summer.

Effects of soil type on soybean crop water use in weighing lysimeters

Different soils are known to affect the amount and distribution of both available water and roots. Optimising irrigation water use, especially when shallow water-tables are present requires accurate knowledge of the root zone dynamics. This study was conducted to determine the effect of two soil types on root growth, soil water extraction patterns, and contributions of a water-table to crop evaporation (E). Two weighing lysimeters (L1 and L2) with undisturbed blocks of soil were used. The soil in L1 had higher hydraulic conductivity and lower bulk density than that in L2. Well watered conditions were maintained by irrigation for the first 110 days from sowing (DFS). Root length density (RLD) was calculated from observations made in clear acrylic tubes installed into the sides of the lysimeters. Volumetric soil water contents were measured with a neutron probe. A water-table (EC = 0.01 S m-1) was established 1 m below the soil surface 18 DFS. RLD values were greater in L1 than L2 at any depth. In L1, maximum RLD values (3 × 104 m m-3) were measured immediately above the water-table at physiological maturity (133 DFS). In L2, maximum RLD values (1.5 × 104 m m-3) were measured at 0.42 m on 120 DFS and few roots were present above the water-table. From 71 to 74 DFS, 55 and 64% of E was extracted from above 0.2 m for L1 and L2, respectively. In L2, extraction was essentially limited to the upper 0.4 m, while L1 extraction was to 0.8 m depth. Around 100 DFS the water-table contributed 29% (L1) and 7% (L2) of the water evaporated. This proportion increased rapidly as the upper soil layers dried following the last substantial irrigation 106 DFS. Over the whole season the water-table contributed 24% in L1 and 6.5% in L2 of total E.

CORRECTING SOIL VOLUMETRIC WATER CONTENTS FROM A DIRECT-READING TIME DOMAIN REFLECTOMETRY INSTRUMENT (IRAMS)

Determination of volumetric soil water content (θ) using time domain reflectometry (TDR) is well established. A commercially available instrument (IRAMS) (the IRAMS (Instrument for Reflectometry Analysis of Moisture in Soils) is a trademark registered by Foundation Instruments Inc. of Ottawa) is now available which incorporates computer software, thus providing direct readouts of θ. A field study of the operation of the IRAMS showed that it operates consistently and repeatedly. The IRAMS values were higher but related linearly to those obtained using a TDR cable tester and manual calculations of travel times. A linear correction of the IRAMS readings is proposed and possible causes are suggested for the observed deviations from expected values. Key words: Time domain, reflectometry, soil water content, field

Physical environment near the surface of plowed and no-tilled soils

Mechanical tillage is a dynamic soil process that influences the physical environment near the surface, thus affecting biological processes in the soil. Water content, bulk density, air permeability, hydraulic conductivity and organic carbon were compared for moldboard plowed and non-tilled conditions at five locations from east-central U.S.A. to the Great Plains. Soils were Blount, Maury, Nicollet, Webster and Crete-Butler cropped to continuous corn ( Zea mays L.), and Alliance and Duroc cropped to wheat/fallow ( Triticum aestivum L.). Major differences in soil physical characteristics between tillage practices were largely confined to the top 75 mm of soil. The volumetric water and organic carbon (C) contents of the 0–150-mm layer at time of sampling ranged from 8 to 66% and 12 to 75% higher, respectively, in no-tilled than in plowed soils. Bulk density was greater and total porosity in the surface layer was as much as 10% less for no-tilled than for plowed treatments. Because of generally higher water contents and/or lower porosity, the water-filled pore space (WFPS) in the 0–150-mm layer of no-tilled soils was 6–28% higher than that of plowed soils. Air permeability in the surface layer of no-tilled soil was less than for plowed, but there were no differences due to tillage in the 75–150-mm depth. Physical soil characteristics influence the soil water regime, and thereby affect rate of biological reactions in the soil. At many points in the soil drying cycle, water added to a no-tilled soil usually creates a less aerobic environment compared to adding the same amount of water to plowed counterpart.

Influence of the water regime on denitrification and aerobic respiration in soil

The influence of soil moisture on denitrification and aerobic respiration was studied in a mull rendzina soil. N2O formation did not occur below −30 kPa matric water potential (Ψm), above 0.28 air-filled porosity (a) and below 0.55 fractional water saturation (Θv/PV ≙ volumetric water content/total pore volume). Half maximum rates of N2O production and O2 consumption were obtained between Ψm = −1.2 and −12 kPa,a = 0.05 and 0.23, and Θv/PV = 0.63 and 0.92. No oxygen consumption was measured at Θv/PC ≧ 1.17. O2 uptake and denitrification occurred simultaneously arounda = 0.10 (at Ψm = −10 kPa and Θv/PV = 0.81) at mean rates of 3.5 µl O2 and 0.3 µl N2 h−1g−1 soil. Undisturbed, field-moist soil saturated with nitrate solution showed constant consumption and production rates, respectively, of 0.6 µl O and 0.22 µl N2O h−1g−1 soil, whereas the rates of air-dried remoistened soil were at least 10 times these values. The highest rates obtained in remoistened soil amended with glucose and nitrate were 130 µl O2 and 27 µl N2O h−1g−1 soil.

Soil and xylem water potential and soil water content in contrasting Pinus contorta ecosystems, Southeastern Wyoming, USA.

The relationships between volumetric soil water content (ϕ), in situ soil water potential (Ίsoil) and predawn xylem pressure potential (Ίpredawn) were quantified in four contrasting lodgepole pine ecosystems in Wyoming, USA. On three of the sites, changes in Ίsoil correlated closely with Ίpredawn, but on a porous soil derived from coarse granitic parent material, Ίpredawn declines occurred much sooner than corresponding declines in Ίsoil, possibly because of local depletion of rhizosphere moisture and low molecular diffusivity of water in that soil. Exptrapolation of laboratory-derived characteristic curves for soil moisture to field conditions yielded different relationships between ϕ and Ίsoil than curves derived from in situ measurements, probably because of disruption of soil structure and porosity during sample collection and handling in laboratory studies. Although a close correlation between ϕ and Ίpredawn was observed, future efforts at modelling the soil-plant-atmosphere continuum should be directed towards a more detailed understanding of the complex relationships between Ίsoil at varying depths and plant water stress.