Low Hydraulic Conductivity Research Articles

A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity

A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.

Aerial root hydraulic conductivity increases with plant size for the aroid vine Rhodospatha oblongata (Araceae)

Rhodospatha oblongata (Araceae) is an aroid vine which reaches maturity at trees canopies. The beginning of R. oblongata’s ascension towards the canopy occurs when one of the branches reaches the stem of a host, being able to reach eight to ten meters in height. Throughout this ascendant path R. oblongata develops two types of aerial roots: the anchor roots, which is shorter and adhered to the host, never reaching the soil; and the feeder roots, which is long, also adheres to the host but connects the vine to the forest soil. Both roots are here compared in morpho-physiological aspects related to the efficiency of axial hydraulic conductivity. Two hypotheses are tested: i) both roots present distinct xylem hydraulic conductivity; ii) hydraulic conductivity of both roots vary with plant size. The characterization of the roots was based on crescent R. oblongata individuals divided in five size classes. Thirty specimens of each anchor and feeder roots were analyzed along plant size increase. Both roots gradually increase in number and external diameter while the R. oblongata vertically ascends to reach plant canopies. The stele of both roots increase in diameter, in order to accommodate xylem vessels that became larger. The increase in these morpho-anatomical parameters has a positive influence on the xylem hydraulic conductivity, that also increases along the ascendant way of R. oblongata. Comparative measurements show that in general anchor roots present smaller morpho-anatomical structures and lower hydraulic conductivity in comparison to feeder roots. Xylem diameter distribution is unimodal for anchor roots, but bimodal for feeder ones. While all feeder roots present a great concentration of vessels around 60 mm of diameter, the second peak occurs at xylem diameter values that increase with plant size. These modifications optimize the root water transport while the vegetative body of R. oblongata increases in size, connecting its leaves at canopies to the soil water with elevated hydraulic efficiency

Impact of biological clogging on the barrier performance of landfill liners

The durability of landfill mainly relies on the anti-seepage characteristic of liner system. The accumulation of microbial biomass is effective in reducing the hydraulic conductivity of soils. This study aimed at evaluating the impact of the microorganism on the barrier performance of landfill liners. According to the results, Escherichia coli. produced huge amounts of extracellular polymeric substances and coalesced to form a confluent plugging biofilm. This microorganism eventually resulted in the decrease of soil permeability by 81%–95%. Meanwhile, the increase of surface roughness inside the internal pores improved the adhesion between microorganism colonization and particle surface. Subsequently, an extensive parametric sensitivity analysis was undertaken for evaluating the contaminant transport in landfill liners. Decreasing the hydraulic conductivity from 1 × 10−8 m/s to 1 × 10−10 m/s resulted in the increase of the breakthrough time by 345.2%. This indicates that a low hydraulic conductivity was essential for the liner systems to achieve desirable barrier performance.

Spatial and temporal variation of soil organic carbon in the North China Plain.

The organic carbon, permeability test, grain size, chemical composition, and mineral composition were analyzed for 147 samples collected from the Luan River catchment, Hebei province, China, to quantitatively characterize the effects of land use, climate change, sedimentary environment, mineral composition, and chemical composition on the spatial and temporal variation of soil organic carbon (SOC). The results indicate that there was higher SOC content and stronger variation in the south plain than in the northern low mountain. The effects of land use, climate change, and sedimentary environment on SOC distribution were greater than the effects of mineral composition and chemical composition. The cropping systems in the Luan River catchment resulted in significant difference in SOC concentration between the south plain and north mountain. The precipitation mainly transmitted its effects through the sedimentary environment to SOC, which caused the stronger temporal variation in SOC from June to October in the south plain. The north mountain did not have significant temporal variation because of the lower hydraulic conductivity of the sedimentary sequence. The spatial variation of SOC was correlated with land use, and their temporal variation was attributed to climate change and sedimentary environment. Apart from land use, the decision maker can also affect the organic carbon mineral and sequence through the sedimentary environment.

Effect of hydrogeomorphic setting on calcareous fen hydrology

AbstractCalcareous fens are species‐rich peatlands that are dependent on minerotrophic water sources for wetland functioning, with current conceptual models suggesting the water source is ubiquitously groundwater upwelling. By quantifying the water balance and subsurface water flow paths and fluxes over 3 growing seasons for calcareous fens in 3 different hydrogeomorphic settings (Riparian, Trough, and Basin), we show evidence that challenges this conceptual model. The Riparian Fen received an order of magnitude more water inputs than the Trough or Basin Fens and was dominated by stream recharge inputs and groundwater outputs. Precipitation and evaporation dominated the water balance of the Trough Fen whereas only the Basin Fen received sizeable groundwater inputs. Indeed, subsurface water fluxes were low at all fens due to weak hydraulic gradients and low saturated hydraulic conductivity in some areas of each wetland, though variations in growing season precipitation led to subsurface flow reversals in all 3 fens. Our results demonstrate the importance of understanding landscape position, or hydrogeomorphic setting, on calcareous fen hydrology for improving conservation, management, and restoration efforts of these important ecosystems.

Compression behavior of reconstituted soils mixed with bentonite for a cutoff wall in a landfill site

Soil-bentonite (SB) backfill is used extensively in cutoff walls at landfill sites; the walls are used as engineered geotechnical barriers for contaminant control. With increasing bentonite content, the coefficient of consolidation and hydraulic conductivity of the SB decrease. However, when the bentonite content is increased beyond a certain percentage, the hydraulic conductivity of the SB decreases very little. One of the aims of this paper is to introduce the concept of optimal bentonite content (OBC) for SB cutoff walls, in which the hydraulic conductivity (kh) is expected to be lower than 1 × 10−9 m/s. Additionally, the paper introduces a new index consolidation stress ratio, cvσ′, which is used to obtain the OBC. For this study, the initial water contents of the SB backfill material are selected to be 0.8, 1.0, and 1.2 times their corresponding liquid limits. The clayey soils are amended with different bentonite contents, 0, 5, 8, and 10% (by dry weight basis) for the oedometer tests. Then, piezocone penetration test (CPTU) is applied in SB cutoff wall at a landfill site in Jingjiang city, China. The results of the laboratory and field studies show that the introduction of a new index, cvσ′, is very useful for calculating the OBC and for evaluating the coefficient of consolidation and hydraulic conductivity of SB backfill. The advantage of SB backfill with OBC is that it can achieve the design requirement of very low hydraulic conductivity and improve the safety reserves.

Laboratory-based characterization of shallow silty soils in southwest Christchurch

Cyclic triaxial test data are presented to characterize the cyclic response of silty soils at three no-liquefaction case history sites in southwest Christchurch. Stress-strain response and axial strain accumulation demonstrate nuanced, transitional responses of silty soils. Post-liquefaction reconsolidation volumetric strains are within the range expected for clean sands. However, there are clear differences in the post-liquefaction response of silts from that of sands. Low-plasticity silts undergo time-dependent reconsolidation whereas sands undergo immediate reconsolidation. Simplified liquefaction triggering procedures estimate significant liquefaction at these sites; yet, no liquefaction manifestations were observed during the Canterbury earthquake sequence. Laboratory estimates of cyclic resistance are consistent with estimates from simplified liquefaction triggering procedures, and both estimates are well below the estimated seismic demand. Thus, liquefaction is likely triggered at the element-level in the silty soil deposits. Post-liquefaction reconsolidation test results suggest water and ejecta may not necessarily accumulate in these stratified silty soils as they would accumulate in thick deposits of liquefiable clean sands. Thus, manifestations of liquefaction may not be observed at stratified silt/sand sites with delayed reconsolidation responses and lower hydraulic conductivities. Additional mitigating factors may also have contributed to the discrepancy between simplified procedure estimates of liquefaction and the lack of liquefaction observed at these sites.

Simulating crop yield, surface runoff, tile drainage and phosphorus loss in a clay loam soil of the Lake Erie region using EPIC

Useful in evaluating best management practices and nutrient management planning, the prediction of phosphorus (P) transfer from agricultural lands to water bodies via surface runoff and tile drainage remains as a challenge, as few models can provide reasonably accurate P loss simulations. The EPIC (Environmental Policy Integrated Climate) model was firstly applied to simulate crop yields, surface runoff, tile drainage, and dissolved reactive P (DRP) losses under a corn-soybean rotation grown on a “cracking” Brookston clay loam soil (Vertisol) in the Lake Erie basin, Ontario, Canada. Different potential evapotranspiration and curve number equations were compared to determine the most suitable equations (Penman-Monteith equation and variable Daily Curve Number with soil moisture index) for this region. A crack flow coefficient was used to deal with inflow partitioned to cracks. A soil layer below tile drain with low saturated hydraulic conductivity was employed in simulating the experimental site, where most water was leaving the field through tile drain. Lateral subsurface flow was used to substitute for drainage. Annual simulations of crop grain yield, cumulative surface runoff, and cumulative drainage closely matched observed data. Over shorter periods (months), surface runoff (NSE = 0.78), tile drainage (NSE = 0.57), and relevant DRP loss (NSE > 0.5) simulations were satisfactory, except for two periods of DRP loss in surface runoff, where most DRP moved downward through lateral flow and deep percolation due to limitations in the crack flow coefficient. For this vertic soil, EPIC generally simulated crop yields and flow volumes well, while DRP losses were only adequately simulated.

Conceptualization of flow and transport in a limestone aquifer by multiple dedicated hydraulic and tracer tests

Limestone aquifers are of great interest as a drinking water resource in many countries. They often have a complex crushed and fractured geology, which makes the analysis and description of flow and transport processes in such aquifers a challenging task. In this study, the solute transport behavior including fracture-matrix interaction in hydrogeological units of a limestone aquifer in eastern Denmark was characterized by designing, conducting and interpreting six depth-specific tracer tests involving natural- and forced-gradient conditions with multiple tracers representing different diffusion properties. To determine flow parameters, the tracer tests were complemented by a comprehensive set of depth-specific borehole and hydraulic tests.Based on the tests, a new and stronger conceptual understanding was developed for the different aquifer units. The investigated limestone aquifer is composed of a glacially crushed unit and two fractured units, with calcarenitic and bryozoan limestone of similar hydraulic properties. Hydraulic tests revealed that the crushed unit has a lower hydraulic conductivity than the fractured limestone units, likely due to the crushed conditions with small limestone clusters and small-aperture fractures potentially filled with fine material.In the fractured limestone units, a distinct preferential flow and primary transport along major horizontal fractures was inferred from the tracer tests under forced-gradient conditions. The dominant horizontal fractures were identified on impeller flow logs and appear connected between wells, having an extent of up to several hundred meters. Connectivity between the aquifer units was investigated with a long-term pumping test and tracer tests, revealing restricted vertical flow and transport. A very pronounced hydraulic conductivity contrast between major fractures and matrix could also be inferred from the borehole and hydraulic tests, which is consistent with the findings from the tracer tests. The difference in the matrix diffusion behavior of the simultaneously injected tracers and a long tailing in the breakthrough curves revealed that matrix diffusion has a strong influence on the solute transport in the fractured limestone.

Hydration kinetics of bentonite buffer material: Influence of vapor pressure, bentonite plasticity, and compaction density

Abstract Bentonites are extensively studied for their application in radioactive waste repositories as potential buffer material to isolate high-level nuclear waste owing to its high swelling capacity and low hydraulic conductivity. The field-scale studies on bentonite buffer material revealed the presence of various humidity conditions when the compacted specimens are subjected to repository conditions in the underground facility. Study on hydration kinetics of compacted bentonite clays under different humidity conditions is very important for the performance evaluation of the facility. The vapor hydration kinetics of candidate buffer material was presented in this work. The influence of bentonite plasticity, compaction density, and vapor pressure on the sorption kinetics was explained with the underlying mechanisms. The hydration kinetics of powder bentonite specimens subjected to saturated vapor pressure environment was also studied to understand the presence of different hydration mechanisms. Rectangular hyperbola method was used for linearizing the kinetic sorption data. The procedure for predicting sorption rate and equilibrium water content using the initial trend was given in this work. The proposed method was validated on different measured data sets of sorption isotherms. The proposed method was satisfactory in linearizing the sorption data and estimating the equilibrium water content under different compaction densities and humidity conditions on the studied bentonites.

Utilization of waste products as alternative landfill liner and cover materials – A critical review

ABSTRACTLandfill sealing technologies comprise low hydraulic conductivity engineered layers as part of covers and bottom liners, for which clays and geosynthetic materials are conventionally used. In last decades, an increasing number of diverse wastes have been suggested as alternative landfill barrier materials. A comprehensive critical review on the different wastes proposed in the literature as components of landfills barriers is presented. The geotechnical and hydraulic properties, the advantages and disadvantages of the studied wastes are discussed, while their environmental impact issues are emphasized. Overall, several wastes were identified as technically suitable to be used in landfill barriers, nonetheless it is worth to mention that some of them also show concerning leaching of hazardous contaminants that could restrict their practical application in real landfills.

Soil physicochemical properties and their significance for sustainable sugarcane production in Kesem Allaideghe plains irrigation project area, Eastern Ethiopia

A feasibility study was carried out to assess the suitability of Allaideghe plains, located in middle awash valley, for the cultivation of sugarcane plantation. The project area, covering 38000 ha, lies between 9o 07’ to 9o 26’ N latitude and 40o 30’ to 40o 50’ E longitude. A detailed soil survey was carried out on 500 m x 500 m grid consisting 1520 auger holes and 76 soil profile observations. The soils of the entire project area were very deep (>200 cm). The textural classes included silty clay loam, clay and heavy clay which revealed that texture varied from fine to very fine with high clay content. Soil reaction (pH), electrical conductivity of saturated extract (ECe), exchangeable sodium percentage (ESP), cation exchange capacity (CEC), organic carbon (OC), CaCO3 ranged between 7.7 to 8.2, 0.9 to 8.0 dS m-1, 9.9 to 42.7%, 40.6 to 61.7 cmol (+) kg-1, 0.3 to 1.2 g kg-1, and 8.3 to 18.3 %, respectively. Soluble cation contents of Na, Mg, Ca and K varied from 8.5 to 20.2, 0.2 to 2.6, 4.7 to 12.6, and 0.1 to 0.2 meq l-1, respectively. Major hydraulic properties influencing water availability and irrigation scheduling for sugarcane included infiltration rate and hydraulic conductivity. Basic infiltration rate varied from 0.2 to 6.2 cm h-1 and in-situ hydraulic conductivity varied from 0.07 to 0.60 m d-1. Low hydraulic conductivity may cause waterlogging in the project area. In order to improve soil structure and water availability, addition of gypsum, plant residues and organic matter are recommended.Keywords: Soil survey, physical and chemical soil properties, infiltration rate, hydraulic conductivity

Analysis of Tyrrell Sea Deposits from the Vicinity of the Victor Diamond Mine: Comparison of Three North American Clay Deposits

he De Beers Victor Mine is an open pit diamond mine, and is located in the James Bay lowlands. The lowlands are characterized by extensive peatlands overlying Tyrell Sea sediments. One of the potential impacts of open pit mining, and the focus of the current work, is the potential for differential subsistence in the Tyrell Sea sediments owing to continuous groundwater withdrawal from the underlying limestone aquifers. To fully understand the potential effects of subsistence, a better understanding of the nature and properties of the Tyrell Sea sediments is needed. This will be achieved by analyzing various properties of samples collected from the Victor Diamond Mine, and comparing those properties with values from Lake Agassiz sediments and Bearpaw Shale. Properties such as hydraulic conductivity, grain size, plastic and liquid limit, and mineralogy will be compared. It is expected that the samples from the Victor Mine are a rock flour dominated by clay fraction, composed mostly of finely ground carbonates. The sediments are expected to have high plasticity, low hydraulic conductivity, and moisture content too low for that of true clay.

Structure-based hydro-mechanical properties of sand-bentonite composites

For the geological disposal of highly contaminated wastes, medical or other sorts, clay barrier systems are commonly designed and used. The engineered liners contain buffer material which is often carefully proportioned mixtures of pure bentonite and sand. Bentonite is an active clay mineral with very low hydraulic conductivity and extremely high expansive properties, which benefits in controlling the downward migration of hazardous contaminants to groundwater. In the design of such composite buffer geomaterial, deformation and pore-flow analysis is a pivotal matter and has therefore been thoroughly investigated in the decades past. When unsaturated, the coupling hydraulic-mechanical behaviour of sand-bentonite mixtures are complex. Among possible reasons behind this complex behaviour is the dependency of hydraulic hysteresis and consolidation properties on size, shape and sorting of solids and pores in the soil's skeleton, which are also rarely accounted for in most of the commonly used soil models.In this contribution, the hydro-mechanical behaviour of saturated and unsaturated sand-bentonite soil is investigated in the context of the recently developed Concept of Double Porosity (CDP). The geomaterial under study is assumed to consist of an incompressible, rigid, elastic solid skeleton surrounded by viscous water and gas fluids, and connected via a network of elastoplastic clayey bridge/buttress units. Roundness and sorting are varied for the sand constituent. The clay fraction (CF) is also varied across testing specimens. The experimental work here introduces two micromechanical models (small clay and large clay) which facilitates interpretation of macro-scale coupled hydromechanical behaviour of composite sand-bentonite geomaterials. The findings from this work will aid design practitioners through a tentative decision support system proposed in closing remarks.

Effects of three morphometric features of roots on soil water flow behavior in three sites in China

Soil physical properties, infiltrability, and water flow behavior are closely associated with root morphology. In this study, dye tracer experiments were conducted in the field to assess the effects of roots with various morphometric features on soil water flow behavior and soil infiltrability. Preferential flow is the dominant type of water flow in the three study sites (Minqin, Dongtai, and Mengla). The presence of roots caused noncapillary porosity and saturated hydraulic conductivity to increase by 26% and 252% in the Minqin, 92% and 93% in the Dongtai, and 234% and 96% in the Mengla, respectively, relative to that in in soils without roots. The various morphometric features of roots affect water flow behavior in soil. The fibrous roots of maize induced water to flow and distribute evenly throughout the plough layer of the study site in Minqin. Furrows and ridges in this site exhibited different soil physical and hydrological properties. Furrows could store higher amounts of water than ridges, thereby increasing the likelihood of water absorption by plants. In Dongtai, ponding and surface runoff occurred when water infiltration was inhibited by the mud layer, which exhibited a high soil bulk density value of 1.43 g cm−3 and a low saturated hydraulic conductivity value of 1.67 × 10−5 cm s−1. These phenomena were not observed in plots the smooth roots of Spartina alterniflora penetrated the mud layer. In this site, preferential flow and lateral flow, which is triggered by the sandy loam layer, are important for water discharge from the beach to the ocean. In Mengla, water flowed evenly on hard soil, which exhibited a high bulk density value of 1.43 g cm−3. The fine roots of rubber could guide water infiltration into deep soil layers (73.54 cm), thereby redistributing water to the root zones of rubber trees. Therefore, our findings indicated that water infiltration behavior, which is crucial for water distribution, is affected by the various morphometric features of roots.

Surface drainage in leveled land: Implication of slope

ABSTRACT In the lowlands of Rio Grande do Sul, land leveling is mostly carried out with no slope for the purpose of rice production. In this environment, soils with a low hydraulic conductivity are predominant owing to the presence of a practically impermeable B-horizon near the surface. Land leveling leads to soil accommodation resulting in the formation of depressions where water accumulates after heavy rainfalls, subsequently leading to problems with crops implanted in succession to rice, such as soybeans. The objective of this research was to quantify the areas and volumes of water accumulation in soil as a function of the slope of land leveling. Five typical leveled lowland areas were studied as a part of this research. The original areas presented slopes of 0, 0.20, 0.25, 0.28 and 0.40%, which were used to generate new digital elevation models with slopes between 0 and 0.5%. These newly generated digital models were used to map the depressions with surface water storage. In conclusion, land leveling with slopes higher than 0.1% is recommended to minimize problems with superficial water storage in rice fields.

Experimental study on the artificial recharge of semiconfined aquifers involved in deep excavation engineering

Artificial recharge measures have been adopted to control the drawdown of confined aquifers and the ground subsidence caused by dewatering during deep excavation in Tianjin, Shanghai and other regions in China. However, research on recharge theory is still limited. Additionally, confined aquifers consisting of silt and silty sand in Tianjin have lower hydraulic conductivities than those consisting of sand or gravel, and the feasibility and effectiveness of recharge methods in these semiconfined aquifers urgently require investigation. A series of single-well and multiwell pumping and recharge tests was conducted at a metro station excavation site in Tianjin. The test results showed that it was feasible to recharge silt and silty sand semiconfined aquifers, and, to a certain extent, the hydrogeological parameters obtained from the pumping tests could be used to predict the water level rise during single-well recharge. However, the predicted results underestimated the water level rise near the recharge well (within 7 m) by approximately 10–25%, likely because the permeability coefficient around the well was reduced during the recharge process. Pressured recharge significantly improved the efficiency of the recharge process. Maintaining the recharge and pumping rates at a nearly equal level effectively controlled the surrounding surface and building settlement. However, the surrounding surface subsidence tended to rapidly develop when recharge stopped. Therefore, the recharge process should continue and gradually stop after the pumping stops. The twin-well combined recharge technique can be used to control the head loss of an aquifer when one of the recharge wells requires pumping to solve the associated clogging problems.

Removal of metals from industrial wastewater and urban runoff by mineral and bio-based sorbents

The study was performed to evaluate chemically modified biosorbents, hydrochloric acid treated peat (HCl-P) and citric acid treated sawdust (Citric acid-SD) for their metal removal capacity from dilute industrial wastewater and urban runoff and compare their efficiency with that of commercially available mineral sorbents (AQM PalPower M10 and AQM PalPower T5M5 magnetite). Batch and column experiments were conducted using real water samples to assess the sorbents' metal sorption capacity. AQM PalPower M10 (consisting mainly of magnesium, iron and silicon oxides) exhibited excellent Zn removal from both industrial wastewater and spiked runoff water samples even at low dosages (0.1 g/L and 0.05 g/L, respectively). The high degree of Zn removal was associated with the release of hydroxyl ions from the sorbent and subsequent precipitation of zinc hydroxide. The biosorbents removed Ni and Cr better than AQM PalPower M10 from industrial wastewater and performed well in removing Cr and Cu from spiked runoff water, although at higher dosages (0.3–0.75 g/L). The main mechanism of sorption by biosorbents was ion exchange. The sorbents required a short contact time to reach equilibrium (15–30 min) in both tested water samples. AQM PalPower T5M5 magnetite was the worst performing sorbent, leaching Zn into both industrial and runoff water and Ni into runoff water. Column tests revealed that both HCl-P and AQM PalPower M10 were able to remove metals, although some leaching was witnessed, especially As from AQM PalPower M10. The low hydraulic conductivity observed for HCl-P may restrict the possibilities of using such small particle size peat material in a filter-type passive system.

The effect of soil physical amendments on reclamation of a saline-sodic soil: simulation of salt leaching using HYDRUS-1D

Poor soil physical conditions such as low hydraulic conductivity can limit salt depletion from surface soil. Altering the pore system by addition of organic and inorganic amendments may improve salt leaching as a reclamation strategy. Column studies were conducted to investigate salt leaching in amended and non-amended soil profiles. A one-dimensional water and solute transport model (HYDRUS-1D) was also assessed for its applicability to simulate salt leaching for amendment strategy. Columns of length 300mm were filled with saline-sodic soil at the lower end (100–300mm) and then covered with soil amended with 40% (wt/wt) fine sand and 20% (wt/wt) wood chips, separately. A control column was filled with saline-sodic soil only. One rainfall scenario typical for a location in south-west Queensland (Australia) was applied to the columns. Water potentials were monitored using tensiometers installed at three depths: 35, 120 and 250mm. The concentrations of individual cations (Na+, Ca2+, Mg2+ and K+), electrical conductivity and sodium adsorption ratio of the soil solutions were also monitored for the investigated depths. A reduction in surface salinity (up to 28.5%) was observed in the amended soil profiles. This study indicated that the addition of wood chips to surface soil improved salt leaching under the tested conditions. The simulation successfully predicted both hydrology and chemistry of the columns. This study also concluded that HYDRUS-1D is a powerful tool to simulate salt leaching in the amended soil profiles, and can be applied to predict the success of amendment strategy under natural climatic conditions.

Examining the Potentials and Limitations of Using Temperature Tracing to Infer Water Flux through Unsaturated Soils

Core Ideas Temperature‐based methods have been primarily restricted to saturated flow conditions. There is a minimum water flux that can be monitored with temperature methods. Temperature methods will be restricted to relatively high fluxes in coarse soils. Irrigation agriculture is the primary consumer of water worldwide. Because groundwater resources are being depleted, there is an increasing need to measure subsurface flux to quantify consumptive use and return flow. This study used HYDRUS‐1D to simulate one‐dimensional vertical movement of water and heat. Temperature time series were analyzed for different materials, depths, and applied fluxes to understand the limitations of temperature methods for monitoring steady‐state water flux under unsaturated conditions. The conditions used in this analysis were intended to explore the minimum standards needed for unsaturated, steady‐state flux estimation. If these conditions cannot be met, then it is unnecessary to look at more challenging conditions (e.g., heterogeneity, transient flux, and unknown hydraulic and thermal properties). Previous work has suggested that there is a minimum flux that is measurable with temperature methods. The low saturated hydraulic conductivities of fine‐grained materials limit the applicability of temperature methods to high fluxes that lead to near‐saturated conditions for these materials. Although coarser soils have higher hydraulic conductivities, thermal diffusivity increases significantly with decreasing water content and may still complicate flux measurements for unsaturated conditions. As a result, we conclude that temperature‐based methods in unsaturated conditions will most likely fail even with the most simplifying assumptions for most soil types. Therefore temperature methods should be restricted to monitoring steady‐state fluxes in coarse soils and/or conditions near saturation.