Coarse Sand Layer Research Articles

Geo-temperature response to reinjection in sandstone geothermal reservoirs

To study the evolution rules and behaviors of heat transport in a sandstone geothermal reservoir caused by cooled water reinjection, this research focuses on the quantitative relationship among reinjection parameters and the thermal breakthrough time of production wells. The permeation, tracer, and reinjection tests were conducted in a simulation model using a large sand tank in conjunction with the numerical simulation method based on COMSOL Multiphysics. Subsequently, sensitivity analysis and nonlinear fitting were performed to investigate the effects of fluid viscosity and density on the reinjection process, and to analyze the impact of reinjection parameters on the thermal breakthrough time of production wells, along with their underlying mechanisms and law. The results indicate that the migration velocity of reinjection water is greater in coarse sand layer compared to that in medium sand layer, and the thermal breakthrough time t is linearly correlated with reinjection rate (Q) raised to the power of − 0.85, temperature difference (ΔT) raised to the power of − 0.21, and spacing between the production and reinjection wells (R) raised to the power of 1.4. The correlation equation and analysis show that when the temperature difference between production and reinjection ΔT is more than 30 ℃, the influence of ΔT on the thermal breakthrough time of production well becomes weak, because ΔT exerts an effect on the thermal breakthrough time of production well t by influencing the relative position of the 18.5 ℃ isotherm in the temperature transition region. The error in reinjecting high-temperature fluid into low-temperature fluid may be corrected by introducing a viscosity correction coefficient αμ.

Enhancing Water Quality Management in Ornamental Fish Aquariums with Pot and Bamboo Filter Systems

The filtration of aquarium wastewater composed of a pot and bamboo is constructed for the purification of microbial activities and to reduce turbidity. The aquarium wastewater is treated with sand to reduce BOD and COD; the usage of coconut husk and vetiver act to decrease the turbidity, nitrite, and pH while raising DO level. Initially, sand was sieved to produce 12 size ranges between 180 and 2000 m. The effectiveness of each range in removing turbidity, BOD, and COD through sand beds 4 or 5 cm deep were tested. The critical size (300 & 425 m) at which no solid particles could penetrate through the sand bed was observed at 3 and 4 cm in depth, respectively. Moreover, the flow of solids through sand filter beds had all particles smaller than 180 m removed from them. A 2 cm bed of sand allowed 0.2-0.5% of solids to flow through, and a 4 cm bed allowed 0.07-0.1 %. Both fine and coarse sand layers are supported by a layer of gravel. In the column approach, the thickness and flow rate of each layer were fixed. The thickness of the layers of sand and charcoal were fixed at 4 cm and 6 cm, respectively. The findings of the column analysis, the combined filter showed decreases in turbidity by 48%, COD by 52%, BOD by 30%, and increasing DO by 48%, respectively. The results reveal that absorbing nutrients can serve as bio-filters to create better conditions for fish growth. The final weight, length, and SGR of fishes in the three aquariums were significantly different after seven weeks of trial, the treated aquarium showed the highest value than the control.

Case Study of Pumping and Recharging in the Floodplain by Yangtze River in Jiang Bei Nanjing

For a project in Nanjing Jiangbei New Area, after excavation of foundation pit and foundation plate pouring, water inrush between side wall of foundation plate and wall occurred in foundation pit. At this time, the water level of confined water observation wells in 2-4 and 2-5 silty fine sand layers in foundation pit can meet the requirements of safe water level control. According to the analysis, the inrush water is inrush water from 3-4e and 3-4 coarse sand layers in deep layer. It is difficult to treat the floor slab in the pit due to the completion of construction. In order to solve the problem of water inrush in the pit, the measures of precipitation outside the pit are adopted, but precipitation causes rapid settlement of the ground in adjacent areas. In order to reduce the ground settlement, a recharge well is specially designed, the recharge test is organized, the structure of the recharge well is optimized, and the spacing between the two layers is arranged according to the test results. The problem of recirculation between pumping and recharging outside the pit is solved. Finally, settlement was successfully controlled. Without water-stop curtain, the project carries out precipitation and recharge outside the pit at the same time, which not only solves the inrush water, but also controls the settlement. In addition, the foundation pit is deep and inrush water is large. During the process, the analysis is carried out first, and then the test is carried out, so that the purpose of recharge can be achieved. It has reference value, so it is concluded in writing.

Seismic Characteristics of a Geotextile Tube-Reinforced Embankment and Shallow Foundations Laid on Liquefiable Soil

The ground in Saemangeum has a high water level and is mostly composed of silty soil and sand, which makes it susceptible to liquefaction and seepage effects. To investigate the seismic response of a geotextile tube-reinforced embankment and shallow foundations laid on a liquefiable soil, a simple spring type shaking table apparatus was developed. The variation in the response acceleration and shear stress-strain relationship were investigated, and the effect of soil improvement and reinforcement were explored, wherein one of the shallow foundations was laid on a coarse sand layer and reinforced by a polyester geotextile. The results showed that the main cause of damage to the embankment was seepage-induced liquefaction. Excessive surface accelerations were observed in the embankment soil due to lateral spreading, indicating the importance of analyzing the liquefaction potential of soils not only at the site area but also near embankments. Lastly, the inclusion of geotextile reinforcement and soil improvement only resulted in the slight reduction of shallow foundation settlement.

Soil heterogeneity influence on the distribution of heavy metals in soil during acid rain infiltration: Experimental and numerical modeling

Acid rain is a global environmental problem that mobilizes heavy metals in soils, while the distribution and geochemical fraction of heavy metals during acid rain infiltration in heterogeneous soils are still unclear. In this study, we performed column experiments to investigate the distribution and geochemical fraction of Cu, Pb, Ni and Cd in heterogeneously layered soils during continuous acid rain infiltration. Chloride ion used as a conservative tracer was found to be uniformly distributed during acid rain infiltration, showing insignificant preferential flow effects in the column. In contrast, however, the distribution of heavy metals was highly non-uniform, especially in the silty soil at the lower part of the column, indicating a heterogeneous distribution of adsorption capacity. In addition, in the control experiments with neutral rain infiltration, uniform distribution of heavy metals was observed, indicating that the heterogeneous distribution of adsorption coefficient during acid rain infiltration was mainly caused by different pH buffering capacities. A numerical model considering water flow and solute transport was developed, where the average water-solid distribution coefficient (Kd) in Layer 2 was only 1.5–12.5% of that in Layer 1 during acid rain infiltration. The model could predict the variation of heavy metal concentrations in soil with the majority of error less than 35%, confirming that different Kd induced the heterogeneous distribution of heavy metals. In addition, the geochemical fraction of heavy metals in the upper coarse sand layer remained stable, while the acid-extractable fractions in the lower loam and silt loam layer gradually increased. Our findings suggest that soil heterogeneity, especially chemical heterogeneity affected by rainfall acidity, has an important influence on the infiltration, migration and geochemical fraction of heavy metals in soils. This study could help guide the risk assessment of heavy metal-contaminated sites that were polluted by acid rain or landfill leachate.

2-D RESISTIVITY IMAGING AND HYDROGEOCHEMICAL ANALYSIS FOR LEACHATE MIGRATION AT DUMPSITES IN SOUTH-SOUTH NIGERIA

Vertical electrical sounding, 2-D tomography and hydrogeochemical analysis was applied to investigate dumpsite leachate plume generation, migration and its impact on the surrounding soil and groundwater aquifer at a municipality in the Western Niger Delta. Twelve vertical electrical sounding (VES) was investigated using the Mini-Res Resistivity Meter with the Schlumberger array. The VES survey result was interpreted with Winglink software. Five hand-dug well water and 5 tap/borehole water samples was collected for hydrogeochemical analysis using the sterile plain bottles and 1-litre plastic bottles. Resistivity results delineated 5 layers of lateritic topsoil, sandy clay, fine coarse-sand, medium coarse-sand and coarse sand. The VES and 2-D tomography mapped and identified 2 distinct zones viz:zone of low resistivity of 91Ωm and 394Ωm at depth of 5m to >28m indicating area of leachate contaminant plume and zone of high resistivity value of 422Ωm and 5102Ωm suspected to be dumpsite gases such as NH3, CH4, SO2 and CO2. The 2-D resistivity imaging also revealed that the generated dumpsite leachate contaminant plume was migrating from the Northern part of the burrow-pit dumpsite towards its Eastern part and to the Northern part of the dumpsite at Scot road/Sakponba road. Hydrogeochemical parameter results showed that colour, pH, iron, Lead and coliform bacteria exceeded permissible standard indicating that the leachate plume has migrated to the groundwater aquifer. Closed municipal landfill whose base is made of concrete and paved surfaces should be adopted in the area to prevent leaching of poisonous substances into groundwater aquifer.

Influence of coarse‐textured soil layers under crop root zone on soil water and salt dynamics and crop yield in shallow groundwater areas

AbstractCoarse‐textured soil layers under crop root zone could modify soil water and salt dynamics and affect crop growth. However, the influence is not thoroughly investigated, especially under shallow groundwater table and soil salinization conditions. For this purpose, we calibrated and validated a one‐dimensional agro‐hydrological model, LAWSTAC, for soil water and salt movement, and their interactions with maize growth in Northwest China. Simulation results fitted well with the measured values of soil water content, salt concentration, leaf area index, and crop yield for the maize growing seasons of 2012 and 2013. Further simulations were conducted to evaluate the influence of buried depths (80, 90, 100, and 110 cm) and thicknesses (10, 20, 30, 40, and 50 cm) of a coarse sand layer under root zone on soil water storage, salt leaching, and crop yield. Results demonstrated that soil water and salt status in the root zone was related to the water content of the coarse‐textured soil layer. Water percolation increased rapidly when water content in the coarse layer was higher than a critical value, which could be beneficial for salt leaching but unfavorable for water retention in the root zone. These results could help understand the influence of the coarse‐textured soil layer on root zone water storage and salt leaching and provide guidance on the irrigation scheduling for salt leaching and crop growth in areas with salinization problems and shallow groundwater tables.

Study on control measures of the influence of shallow buried tunnel excavation on the subgrade settlement of high speed railway in operation

A finite element model is established to study the settlement control technology of the ballastless track of high-speed railway under the shield of shallow buried tunnel. In particular, the reinforcement mechanism and construction control technology of the Metro Jet System (MJS) method for horizontal reinforcement under complex geological conditions (medium coarse sand and strong permeable layer) are discussed. The results showed that when the ground is not reinforced, the shield tunnel directly passes through the high-speed railway, and the settlement caused by the construction of the railway gene tunnel directly above the tunnel has greatly exceeded the limit of the settlement control standard. Due to the limitations of site, planning and construction conditions, the horizontal jet grouting pile of MJS construction method is selected as an auxiliary measure through comprehensive comparison. After the completion of the MJS construction, the maximum settlement of the subgrade is 30 mm, and the settlement of the high-speed railway subgrade is about 25 mm.

Analytical Solutions of Vertical Airflow in an Unconfined Aquifer with Rising or Falling Water Table

The rise and fall of the groundwater level can drive air flow in the vadose zone. In turn, the air flow can interact with the water flow. When the unconfined aquifer is covered by a low-permeability media, the coupling of the water flow and the air flow is more obvious. In this study, a conceptual model is established for coupling of air flow and water flow in the vadose zone in response to rapid fluctuations of the water table. Water injection and drainage experiments are conducted in a double-layered sand column with a thick layer (80.5 cm) of coarse sand and a thin layer of fine sand as a low-permeability cap. Different cap thickness (2 cm, 5 cm, 7.5 cm) and different thickness of the vadose zone (30 cm, 40 cm) are set for the experiments. Negative pressure (NP)/positive pressure (PP) of the air in the vadose zone is observed in the drainage/injection experiments, with higher pressure in experiments of thicker cap layer. In each experiment, NP or PP increases rapidly to reach a maximum in the early stage, and gradually becomes zero in the late stage. Analytical solutions on three subdivided stages indicate the permeability and thickness of the cap layer, as well as permeability and porosity of the media in the vadose zone are the key controlling factors on the process of coupling of air flow and water flow. The solutions also reveal the formation mechanism of air pressure in the vadose zone with a low-permeability cap. This study has both theoretical significance and engineering applications.

Investigation of groundwater potential of the eastern part of Usmanu Danfodiyo University permanent site, Sokoto, using vertical electrical sounding technique.

Geoelectric survey was carried out in the eastern part of Usmanu Danfodiyo University main campus with the aim of determining areas that are suitable for drilling boreholes. Thirty Vertical Electric Soundings were made at different parts of the area using the Schlumberger electrode spread arrangement with maximum half-electrode spacing of 100 m. Qualitative interpretation of the curves suggested thirteen different curve types highlighting that the area is underlain by three to six layers; the top soil predominantly lateritic in composition and occasionally wet, the fresh laterite, shale and limestone layer , the shale, limestone and sandstone layer, clayey sand, coarse sand, shale, limestone and sandstone layer and the fine sandstone and limestone layer. Using the measured current and voltage, apparent resistivity, thickness, depth, hydraulic conductance, transmissivity and protective capacity maps of the aquifers in the area were generated. Based on comparison of groundwater potential zones on these maps, an integrated groundwater potential map of the area was inferred and used to classify the area into low, moderate and high groundwater potential zones. Three zones in the south-central, east-central and extreme northeastern parts were inferred to be suitable for drilling boreholes in the study area.

Removal of Heavy Metals from Stormwater Using Porous Concrete Pavement

This study aimed to investigate the heavy metals, i.e. Cu, Pb, Ni, and Zn removal efficiency from stormwater runoff of a porous concrete pavement (PCP). A model of PCP was designed with the porosity and co-efficient of permeability of the pavement were 27.2% and 1.83 cm/sec, respectively. Artificial stormwater containing heavy metals are passed through the pavement at a constant rainfall rate to mimic the stormwater rainfall-runoff condition. The artificial stormwater infiltrated through the pavement were then collected at two different pavement layers at different time instances. From the experimental investigations, it is observed that Cu, Pb, Ni and Zn concentrations are significantly reduced in the treated stormwater. At the first collection point which is located below the sub-base layer and coarse sand layer of the pavement, the concentrations of Cu, Pb and Zn reduced 56%, 67% and 93% respectively compared to their initial concentration, Ni concentration reduced only 20%. At the second collection point which is located below the coarse and fine sand layers beneath the pavement, the concentrations of Cu, Pb, Zn, and Ni are reduced 92%, 89%, 100%, 100%, respectively.

Recent and historical tsunami deposits from Lake Tokotan, eastern Hokkaido, Japan, inferred from nondestructive, grain size, and radioactive cesium analyses

Geological evidence of recent tsunamis from sediment samples collected from Lake Tokotan, a coastal lagoon in eastern Hokkaido, northern Japan, was detected using computed tomography (CT) and soft X-ray images, grain size, and radionuclide profiles. Initial field observations revealed that sediments had no discernable sedimentary structures at the top of the core. However, results of CT imaging, soft X-ray, and grain size analyses show evidence for three invisible sand layers that are intercalated with mud layers. These sand layers exhibit trends of landward fining and thinning. Furthermore, the distribution of sand layers was limited to the center and seaward parts of the lake. Vertical profiles of cesium and lead concentrations in combination with recent eyewitness accounts indicated that these sand layers are correlated with the 1973 Nemuro-oki, 1960 Chilean, and 1952 Tokachi-oki tsunami events. The deeper part of the sediment cores includes three volcanic ash layers and three prehistoric coarse sand layers. The prehistoric layers are correlated with unusually large tsunamis that were geologically identified in previous studies from eastern Hokkaido. These findings suggest that nondestructive techniques, in combination with radionuclide analysis, allow for detection of frequent but faint tsunami deposits. This technique allows for an improved understanding of the history of tsunami inundation in Lake Tokotan and of other locations for which stratigraphic evidence for faint tsunamis layers is not readily apparent from field assessments.

The age of the oldest aeolian sand in the Otindag sandland

The Otindag sandland, located at the boundary of the East Asian monsoon area, is sensitive to climate change and is therefore an ideal place to investigate past environmental evolution. Due to frequent volcanic eruptions that have occurred since the Cenozoic, numerous interbeds of multi-stage basalt and sand exist in the stratigraphic sequence, which allowed older sand to be preserved due to the protective effect of the overlying rock. Previous studies of climate change in the sandland mainly focused on the time interval since the last glacial period, and information is lacking regarding climate change over longer timescales and younger volcanic activity. The key reason for this is that suitable geological carriers are rare, which prevents the expansion of the desired sediment chronological scale. In order to establish the chronological framework, samples of the sandy paleosoil, aeolian sand, lacustrine sand and volcanic baked layer were collected and dated by the optically stimulated luminescence (OSL) method. The conventional SAR, TT-OSL and pIRIR(200,290) procedures were used to obtain equivalent doses of quartz and feldspar. To ensure the accuracy of the dating data, the age of the feldspar dating of the oldest aeolian sand sample (TBB) was compared with the quartz result, and the quartz and feldspar ages were found to be consistent with each other within the error range. Considering the error of the age measured by TT-OSL (384.2±68.0 ka) exceeded 10%, the age measured by the pIRIR(200,290) method (321.2±11.5 ka) was adopted to represent the deposition age of the aeolian sand sample. In addition, the age of the oldest sand deposition in the three sections of the Otindag sandland may reach 300 ka, mainly due to the different thermal history and provenance from other areas such as Loess Plateau, resulting in the different saturation dose and dose rate lever of quartz or feldspar. The sedimentary age framework established by OSL dating enabled the reconstruction of the evolution process of the Otindag Sandland. The climate was cold and dry during the periods corresponding to 321.2, 160.3−151.6 and 4.4 ka, and the aeolian sand material quickly accumulated during these times, as dunes were activated and the sandland expanded. During the periods corresponding to 5.0 ka and 2.4−1.3 ka, sandy soil developed efficiently, implying the dunes were fixed. The oldest lacustrian sand age is 320.4±16.0 ka for quartz dated by the SAR method. Hence, aeolian sand and lacustrine sand developed in the same period, which indicates that there was a landscape where dunes and lakes coexisted around 320.4 ka, such as at present. The Optical Stimulated Luminescence dating results of the volcanic baking layer and the underlying coarse sand layer confirm that volcanic activity occurred around 321.2 and 155.0 ka. Combined with results of previous research, this study indicates that the northeast and northwestward basement faults of the northern margin of the North China Craton and the Central Asian orogenic belt are characterized by extensional tectonic properties in the background setting of plate subduction, and relative uplift and settlement occur in the sandland formation zone.

A bilayer coarse-fine infiltration system minimizes bioclogging: The relevance of depth-dynamics

Bioclogging is a main concern in infiltration systems as it may significantly shorten the service life of these low-technology water treatment methods. In porous media, biofilms grow to clog partially or totally the pore network. Dynamics of biofilm accumulation (e.g., by attachment, detachment, advective transport in depth) and their impact on both surface and deep bioclogging are still not yet fully understood. To address this concern, a 104 day-long outdoor infiltration experiment in sand tanks was performed, using secondary treated wastewater and two grain size distributions (GSDs): a monolayer system filled with fine sand, and a bilayer one composed by a layer of coarse sand placed on top of a layer of fine sand. Biofilm dynamics as a function of GSD and depth were studied through cross-correlations and multivariate statistical analyses using different parameters from biofilm biomass and activity indices, plus hydraulic parameters measured at different depths. Bioclogging (both surface and deep) was found more significant in the monolayer fine system than in the bilayer coarse-fine one, possibly due to an early low-cohesive biofilm formation in the former, driven by lower porosity and lower fluxes; under such conditions biomass is favorably detached from the top layer, transported and accumulated in depth, so that new biomass might colonize the surface. On the other hand, in the bilayer system, fluxes are highest, and the biofilm is still in a growing phase, with low biofilm detachment capability from the top sand layer and high microbial activity in depth, resulting in low bioclogging. Overall, the bilayer coarse-fine system allows infiltrating higher volume of water per unit of surface area than the monolayer fine one, minimizing surface and deep bioclogging, and thus increasing the longevity and efficiency of infiltration systems.

Enhanced reductive dechlorination of trichloroethene in an acidic DNAPL impacted aquifer

A field pilot test was conducted using an emulsified vegetable oil (EVO) and colloidal magnesium hydroxide [Mg(OH)2] formulation to enhance reductive dechlorination of dense non-aqueous phase liquid (DNAPL) trichloroethene (TCE) in an acidic (pH < 4), heterogeneous aquifer. The field test consisted of i) a single well injection test to evaluate Mg(OH)2 distribution and ii) installation of two EVO-Mg(OH)2 permeable reactive barriers (PRBs; PRB-1 & PRB-2) at varying distances downgradient of the DNAPL source area. Distribution of Mg(OH)2 was observed up to 2.3 m away from the injection point within a permeable coarse sand layer; however, Mg(OH)2 transport in the overlying clayey-silty sand was minimal. Downgradient of the PRBs, colloidal Mg(OH)2 increased the pH of the coarse sand to levels appropriate for biological reductive dechlorination (pH >∼5); however, some settling of Mg(OH)2 in the injection wells generated persistent high pH (∼9–10) within the PRBs. A redesigned suspension of colloidal Mg(OH)2 was tested and proved to be more effective at raising aquifer pH without an excessive rise in pH within the PRBs. At PRB-1 (located closest to the DNAPL source area), limited TCE biodegradation was observed due to the influx of high TCE concentrations (up to 400 mg/L) and inhibition of dechlorinating bacteria. At PRB-2 (located 25 m downgradient of the DNAPL source area), TCE concentrations were much lower (13–26 mg/L) and production of cis-1,2-dichloroethene (cDCE) and some vinyl chloride (VC) was observed. Subsequent bioaugmentation with a commercial dechlorinating culture at PRB-2 improved conversion of cDCE to VC and ethene at downgradient monitoring wells over the duration of the study. These results emphasize the importance of PRB location (relative to the DNAPL source), base selection for pH adjustment, source strength, and local heterogeneities for the design and long-term performance of ERD in acidic DNAPL-impacted aquifers.

Measurements and simulation of leaf xylem water potential and root water uptake in heterogeneous soil water contents

The relationship between leaf water potential, transpiration rate and soil water potential is complex, particularly when the soil water potential in the root zone is not uniform, which is the rule rather than the exception in soils. Our objectives were: 1) to measure the effect of heterogeneous soil water potentials on the relation between leaf water potential and transpiration rate and 2) to test whether root water uptake models could predict this relation. To this end, we combined the root pressure chamber technique, which allows measuring the suction in the leaves of transpiring plants, with two models of root water uptake, a simple one where soil and roots are presented as resistances in series and a more detailed 3D root architecture model. The experiments were carried out with lupines grown in sandy soil, for which the root architecture and root hydraulic properties had been previously estimated. The soil was partitioned in two layers separated by a coarse sand layer that allowed the roots to grow through but limited the water redistribution between the layers. Three scenarios (wet-wet, dry-wet, dry-dry) were tested. The results showed that the relation between transpiration and leaf water potential was linear in all scenarios. As the upper soil layer severely dried, the conductance of the soil-plant system decreased by ca. 1.65 times compared to the conductance of the plant-soil system in a uniform wet soil. As both layers dried, the conductivity was 8.26 times lower compared to the uniform-wet case. The combination of the experiment and modelling showed that a simple model is capable to reproduce the relation between transpiration, leaf water potential and soil water potential (despite an offset in the leaf water potential). Both simplified and the 3D root architecture models were capable of reproducing the measured changes in hydraulic conductance of the plant-soil system due to the soil drying. However, both models overestimated the measured leaf water potential by 0.1 MPa, probably because of a gradient in osmotic potential between the xylem and the soil. The simulations predicted the occurrence of hydraulic lift, even at day time conditions, although the hydraulic lift was relatively more important at low transpiration rates. The simulation suggested that a root architecture model is needed to estimate the variations of water uptake along the individual roots and this might be crucial to properly model hydraulic lift.

Experimental and theoretical study of bed load particle velocity

ABSTRACTThis paper reports an experimental study of sediment particle velocity in the Hydraulic Engineering Laboratory at the University of Arizona. The experiments were performed in a rectangular channel filled with a layer of coarse sand or gravel. The instantaneous particle velocity was measured by using the images captured with a commercial camera. An innovative image processing technique was developed for tracking individual particle motion, which leads to the determination of instantaneous particle velocity vector. Both the mean particle velocity and the distribution of particle velocities were analyzed. It was found that the instantaneous bed shear stress dominated the particle velocity as well as the probability density function of its distribution. This observation is consistent with the theoretically derived equations for particle velocity and its distribution. Furthermore, the Kolmogorov–Smirnov test confirmed the agreement between the distribution of experimental data and that obtained from the theoretical analysis.

Numerical analysis of soil water dynamics in a soil column with an artificial capillary barrier growing leaf vegetables

AbstractAn artificial capillary barrier (CB), which consists of two layers of gravel and coarse sand, was used to improve the soil water retention capacity of the root zone of sandy soil for the cultivation of Japanese spinach (Brassica rapa var. perviridis). The performance of a CB under specific conditions can be evaluated using numerical simulations. However, there have been relatively few numerical studies analyzing soil water dynamics in CB systems during crop growth. The objectives of this study were (i) to evaluate the performance of a CB during the cultivation of Japanese spinach irrigated at different rates and (ii) to investigate the effect of the irrigation schedule on root water uptake. Numerical analysis was performed using HYDRUS‐1D after the soil hydraulic properties of the CB materials were determined. In most cases, the HYDRUS‐1D results agreed well with the experimental soil water content data without any calibration when the dual‐porosity model describing soil hydraulic properties was used for gravel and coarse sand. We found that the dual‐porosity model was able to attenuate the unrealistically steep reduction in the unsaturated hydraulic conductivity predicted by the single‐porosity model. The numerical simulations also showed that the CB played an important role in maintaining plant‐available water in the root zone while maximizing the water use efficiency. The numerical simulations revealed that the irrigation frequency could be reduced by half during the early growth stage, and the water use efficiency could be greatly improved with the CB layer installed.

Rhizosphere hydrophobicity limits root water uptake after drying and subsequent rewetting

Recent experiments showed that rhizosphere of several plant species turns temporarily hydrophobic after severe drying and subsequent rewetting. Whether or not such hydrophobicity limits root water uptake is not known. A set of experiments was performed to test whether rhizosphere water repellency negatively affects root water uptake. To this end, a commercial surfactant was used as a rewetting agent to facilitate the wettability of the rhizosphere of lupins (Lupinus albus) in a sandy soil. Lupin plants were grown in rhizoboxes and were subjected to a severe drying cycle. Then half of the plants were irrigated with water and half with the surfactant solution. Time-series neutron radiography technique was used to monitor water redistribution in the rhizosphere during irrigation. In a second experimental set-up, lupins were grown in a sandy soil partitioned in five vertical compartments separated by a 1-cm layer of coarse sand (acting as a capillary barrier). Water and surfactant were injected in different compartments and the rehydration of the root tissues beyond the irrigated compartments was monitored with neutron radiography for 2–3 h. Root rehydration rates were used to estimate the water fluxes across the root-soil interface. The rhizosphere of lupin roots in sandy soil irrigated with water remained partly dry for at least 2–3 h, while it was rapidly rewetted when irrigated with surfactant. Water flow into the roots irrigated with surfactant solution was 6.5 times faster than into the roots irrigated with water. These results prove that water repellency of the rhizosphere of lupins in sandy soils limited the water fluxes into the roots and root rehydration during the first two to three hours after irrigation. This might not always be negative, because it can limit water losses from roots to dry soil and therefore avoid severe root dehydration.

Estimation of Hydraulic Parameters from Vertical Electrical Resistivity Sounding

This study presents the assessment of the groundwater potential in Iyesi, Ota, southwestern Nigeria. Thirty (30) vertical electrical soundings (VESs) were conducted using Schlumberger array with a maximum half-current electrode spacing \((AB/2)\) of 420 m. The apparent resistivity data observed were interpreted first by partial curve matching and then by computer iteration technique using WinResist program. Eight eoelectric layers were denoted as top soil (sandy clay), lateritic clay, silty clay, silty sand, mudstone, medium grain sand, coarse sand and clay mud were delineated. The sixth (medium grain sand) and seventh (coarse sand) layers delineated form the aquifer unit with the overlying mudstone (fifth layer) serving as the confining bed. The geoelectrical and hydrogeological characteristics of the delineated aquifer in the study area was evaluated. The study shows that the aquifer is highly productive and consequently a good groundwater potential. The litho-facies of the aquifer units are well sorted and graded; this accounts for the observed decrease in the model resistivity with depth and thus, increasing porosity with depth in the aquifer unit.