Low Hydraulic Conductivity Research Articles

Riverbed Temperature and Heat Transport in a Hydropeaked River

AbstractHydropeaking, the alternating storage and release of water from reservoirs for hydropower generation, perturbs the thermal regime of many large rivers. While its effects on river temperature have been long studied, impacts on the thermal regime of riverbeds remain mostly unknown, despite riverbed temperature affecting rates of nutrient cycling and habitat suitability for benthic organisms. This study combines detailed field observations and flow and heat transport modeling to assess the impact of hydropeaking on riverbed temperatures in a large regulated river. The field site was 12 km downstream from a dam that induces large daily flow variations. Vertical thermistor arrays were used to collect riverbed temperature data across the entire 70 m‐wide channel. The riverbed near the left bank was highly dynamic thermally, transitioning between river and groundwater temperatures over daily hydropeaking cycles. In contrast, the rest of the riverbed, including near the right bank, was similar in temperature to the river and had relatively stable temperatures. Modeling showed that the temperatures near the banks are explained by advective heat transport driven by hydrostatic changes in river level, while the temperatures over the rest of the channel can be explained mostly by conductive heating. Gaining groundwater conditions and high sediment hydraulic conductivity favor thermally dynamic zones near banks, while low hydraulic conductivity (below 1 m/d) and neutral or losing groundwater conditions result in muted temperature fluctuations, as observed at the right bank. These spatial patterns can help predict thermally sensitive processes in the riverbeds of hydropeaked or flooding rivers.

Swell-shrink and hydraulic behaviour of compacted red soil-bentonite mixture

Clay liners are provided in waste landfills to prevent the leachate from percolating into underlying soil and ground water and polluting it. Hence, soils used as landfill liners must possess low hydraulic conductivity (< 10−7 cm/s). In the initial as-compacted state, liners satisfy this design criterion. However, in the field, liners are subjected to alternate wet-dry cycles and the initial microstructure of liners change, thereby affecting their hydraulic properties. The present study was conducted on a locally available red soil mixed with bentonite so that it satisfies the requirement of hydraulic conductivity in the as-compacted state. To achieve this objective, identical specimens compacted at optimum moisture content to its maximum dry density were inundated with distilled water under a surcharge load of 12.5 kPa. Upon attainment of maximum swelling, hydraulic conductivity tests were conducted at swollen state under a hydraulic gradient of 20. The soil specimens were then dried at a temperature of 45±2 °C. The weight, height and diameter of specimens were measured continuously during drying to study the shrinkage behaviour of the soil. The dried specimens were subjected to subsequent wet-dry cycles until the specimens reached an equilibrium state and the hydraulic conductivity was determined after each wetting cycle. SEM images were also captured to analyse the changes in the soil structure in the as-compacted state and at the end of different wetting cycles.

Clay mineral formation in Permian rocks of a geothermal borehole at Northern Upper Rhine Graben, Germany

Hydrothermally altered rhyolite rocks in the Permian Donnersberg Formation of a geothermal borehole in the Northern Upper Rhine Graben (Germany) were investigated to find out answers for the low hydraulic conductivity of the rocks. The composition of clay minerals and the temperature of smectite–illite transformation were carried out using X-ray diffraction, X-ray fluorescence, transmission electron microscopy, Fourier transform infrared spectroscopy, and polarized-light microscopy analyses. Clay mineral (CM) composition includes illite/muscovite (1M and 2M1 polytypes), illite–smectite interstratifications (IS-ml), smectite, and chlorite; and non-clay minerals such as quartz, feldspars, epidote, calcite, dolomite, and hematite were detected. The 2M1-polytype mica might be the only primary sheet silicates from the parent rocks, while the others occur as authigenic neo-formed CMs under heat flow and geothermal gradient. The development of CMs indicates different mechanisms of illitization and smectitization. Based on the texture, morphology, structure/polytype, and chemistry of rocks and minerals, in particular CMs, the study grouped the CM formation into three transformation processes: smectitization during magma cooling and possible contact metamorphisms with decreasing and low temperature, smectite illitization controlled by burial diagenesis and hydrothermal alteration, and illite smectitization followed exhumation and Cenozoic subsidence with decreasing temperature. The rhyolites were altered to all of the orders IS-R0, IS-R1, and IS-R3 by the dissolution-precipitation and layer-to-layer mechanisms. The first one supported small xenomorphic plates and flakes of 1Md, elongated particles of 1M, and pseudo-hexagonal forms of 2M1. The second one could lead to the platy particles of 1Md and 2M1 polytypes. The dominant temperature range for the transformation in the area has been 140–170 °C– ~ 230 °C.

Macrostructural and Microstructural Properties of Residual Soils as Engineered Landfill Liner Materials

ABSTRACT This study assessed the performance of residual soils with regard to their macrostructural and microstructural properties and compatibility with leachate in pursuit of exploring alternative cost-effective and efficient landfill liner materials. A series of laboratory investigations was conducted on three residual soil samples by using tap water and leachate as permeation fluid to achieve the objectives of the study. The zeta potential measurements revealed that the presence of multivalent cations in the leachate decreased the diffuse double layer (DDL) thickness around the soil particles. The reduced DDL thickness caused a decrease in Atterberg limits of soil-leachate samples and changes in the classification of fine fractions. Additionally, the effects of pore clogging attributed to chemical precipitation and bioclogging were responsible for the reduction in measured hydraulic conductivities of soil-leachate samples. These effects can be clearly observed from the field-emission scanning electron microscopy images of soil-leachate samples with the appearance of less visible voids that led to a more compact and dense structure. The formation of new non-clay minerals and associated changes in the Al and Si ratio as reflected in the x-ray diffraction diffractograms and energy-dispersive x-ray analyses, respectively, were attributed to the effects of chemical precipitation. This study concluded that S1 and S2 residual soil samples are potential landfill liner materials because they possess adequate grading characteristics, adequate unconfined compressive strength, low hydraulic conductivity, and good compatibility with leachate. In contrast, the S3 sample requires further treatment to enhance its properties in order to comply with the requirements of landfill liner materials.

The importance of the hydrological pathways in exporting nitrogen from grazed artificially drained land

When considering hydrological export pathways from drained pastoral fields, the shallow groundwater underlying the artificial drainage system may fulfil an as important role as the artificial drainage system itself. However, the important split between artificial drainage and groundwater flows at a field site is usually unknown. Consequently, the effects that the groundwater system’s redox status may have on the forms and total amounts of nitrogen (N) exported from the site cannot be confidently assessed. We addressed these deficiencies by investigating the export of various forms of N in the artificial drainage and the shallow groundwater beneath two dairy farming sites (Tatuanui, Waharoa) within the Piako River catchment in New Zealand.Due to the very low hydraulic conductivity of the degraded peat in the saturated zone perched on a clay aquiclude, no significant water or contaminants were exported at Tatuanui via the unconfined shallow groundwater. Accordingly, artificial drainage discharge into the receiving surface collector drain was the only export pathway for N at this site. The sealed nature of the groundwater zone resulted in the accumulation of organic-N and ammonium-N in the peaty shallow saturated zone underlying the mineral soil. In contrast, the Waharoa site featured a more mobile shallow groundwater system, which on average conveyed approximately equal volumes of water offsite as the artificial drainage system.Nitrate (NO3-N) was at both sites the predominant form of N (76%) leached from the aerobic soil profile and discharged through the artificial drainage. As the shallow groundwater was at both sites in a reduced redox state, any NO3-N leached from the rootzone into the saturated zone was consumed. Accordingly, even the mobile shallow groundwater at Waharoa did not convey significant NO3-N. The NO3-N consumption in the reduced groundwater presumably occurred due to dissimilatory NO3-N reduction (concomitant increases in ammonium-N concentrations, with no organic material available for mineralisation in the saturated zone) and microbial denitrification (producing gaseous forms of N). The NO3-N reductions in the groundwater resulted in the average total N exports via the shallow groundwater pathway at the Waharoa site being substantially lower (31% of the exported N) than in the artificial drainage (69%). This is despite both pathways exporting similar water volumes. Because of the NO3-N consumption in the reduced shallow groundwater at Waharoa, over 98% of the N exported through this pathway was either as ammonium-N or organic-N.

Effect of Chia Seed Mucilage on the Rhizosphere Hydraulic Characteristics

The rhizosphere is one of the major components in the soil–plant–atmosphere continuum which controls the flow of water from the soil into roots. Plant roots release mucilage in the rhizosphere which is capable of altering the physio-chemical properties of this region. Here, we showed how mucilage impacted on rhizosphere hydraulic properties, using simple experiments. An artificial rhizosphere, treated or not with mucilage, was placed in a soil sample and suction was applied to mimic the negative pressure in plant xylem. The measured water contents and matric potential were coupled with numerical models to estimate the water retention curve and hydraulic conductivity. A slower loss of water was observed in the treated scenario which resulted in an increase in water retention. Moreover, a slightly lower hydraulic conductivity was initially observed in the treated scenario (8.44 × 10−4 cm s−1) compared to the controlled one in saturated soil. Over soil drying, a relatively higher unsaturated hydraulic conductivity was observed. In summary, we demonstrated that mucilage altered the rhizosphere hydraulic properties and enhanced the unsaturated hydraulic conductivity. These findings improve our understanding of how plants capture more water, and postulate that mucilage secretion could be an optimal trait for plant survival during soil drying.

Cation Filtration of Montmorillonite on Hydraulic Conductivities of Some Bentonites in Artificial Seawater

AbstractBentonite is planned for use as a buffer for repositories of high-level radioactive waste (HLW) because the buffer must have extremely low hydraulic conductivity to seal wastes. A Japanese ...

A procedure to estimate cutoff wall transport properties from monitoring wells

AbstractOwing to their capability in limiting the transport of pollutants in the subsoil, cutoff walls are popular solutions for the confinement of contaminants. These barriers are often made of soil‐bentonite or cement‐bentonite mixtures, which are characterized by low hydraulic conductivity, low hydrodynamic dispersion, and long‐term durability. However, the aggressive chemical environment to which these walls are subjected might negatively impact on their performance. Assessment of their performance with time is thus a crucial issue in wall design. The use of dedicated monitoring wells, cast in place inside the wall during construction when the bentonitic mixture is still fluid, can be particularly suitable for both intercepting and detecting the fluids flowing through the barrier. In this research, the results of a numerical study aimed at providing a methodology to estimate the transport properties of the backfill material at the site scale are presented. The methodology relies on abaci and only requires (i) flow and concentration of contaminant data within a monitoring well inside the barrier and (ii) hydraulic head at inlet and outlet of the barrier to be known.

Estimation of 3D hydraulic conductivity distribution of unconfined aquifer using the geostatistical method and its relation to water quality and flooding in Upper Citarum River

The 3D distribution of hydraulic conductivity in the unconfined aquifer is needed to understand the interaction between groundwater and Citarum River, which may relate to water contamination and flooding. In this study, it was estimated using the geostatistical method. The data used are the hydraulic conductivity parameter from thirteen large-diameter dug wells determined using the slug test method. Further analysis was conducted using groundwater and Citarum River physical parameters, i.e. total dissolved solids, pH, and temperature. The results show that the hydraulic conductivity of the unconfined aquifer in the Upper Citarum River ranges from 8.34 x 10−7 to 2.19 x 10−5 m/s. The area with relatively high hydraulic conductivity, i.e. Ciparay and Solokan Jeruk Sub-districts, will have a greater possibility of groundwater or river water contamination depending on their interaction. Meanwhile, the area with relatively flat topography and low hydraulic conductivity, i.e. Majalaya and Baleendah Sub-districts, will be more susceptible to flood. Therefore, the mitigation of water contamination and flooding in the Upper Citarum River should consider the 3D hydraulic conductivity distribution, e.g. by monitoring the groundwater quality in some dug wells located in high hydraulic conductivity area and installing recharge or injection well to eliminate the artificial runoff.

Soil Factors are the Drivers for Wetlands Colonization by Pneumatopteris afra in Nigeria

The relationships between soil factors and plant community characteristics of some wetlands invaded by Pneumatopteris afra and non-invaded ones were investigated. Sixty soil samples were obtained from six wetlands comprising three invaded and three non-invaded in Lafia, Nigeria using sixty quadrants arranged on six 200 m transects. The samples, after air-dried and sieved using 2 mm mesh were analysed for the physico-chemical properties which include pH, organic matter (OM), percentage nitrogen (% N), phosphorus (P), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), exchangeable acidity (EA), percentage base saturation (% BS), particle size, porosity, bulk density (BD), hydraulic conductivity (HC), and moisture content (MC) using standard methods. Direct ordination in canonical correspondence analysis was used to determine the influence of these soil factors on P. afra abundance, Shannon diversity, and species richness of both invaded and non-invaded sites. All sites differ from each other in terms of their physico-chemical parameters. The invaded sites appeared to be more acidic (pH = 3.22), less sandy, more porous (38.11%), low HC (1.23) as compared with non-invaded ones. Soil factors that favoured abundance of P. afra(% OM and EA) correlated negatively with Shannon diversity index of invaded sites which was positively influenced by % N, pH, and cation exchange capacity (CEC). At the non-invaded sites, Shannon index and density were influenced positively by % BS, pH, AP, and % N. All these observations showed that the soil factors played significant roles in the establishment of P. afra at the invaded sites, and also on the plant diversity at non-invaded sites.

A Basin-Scale Groundwater Flow Model of the Columbia Plateau Regional Aquifer System in the Palouse (USA): Insights for Aquifer Vulnerability Assessment

Bedrock aquifers are vulnerable to contamination due to the preferential movement of pollutants via rock discontinuities and porous layers. In this research, we propose an approach to assess vulnerability in three dimensions by combining stable isotope values and particle tracking in a vertically anisotropic aquifer of basaltic-fluvial origin at the basin-scale. A steady-state flow and particle tracking model is presented for the Columbia River Basalt aquifer in the South Fork Palouse Basin. Backward particle analysis combined with the distribution of δ2H, δ18O and 4He values vs. depth shows how the aquifer is characterized by two separated zones. A shallow ( 150 mBGL) aquifer zone is characterized by much higher particle travel times as well as a distinctive isotopic fingerprint. At such depths, penetration of particles is partially impeded by the low hydraulic conductivity of the sedimentary layers and recharge preferentially occurs in correspondence of the basin margin. Along this margin, the vulnerability is higher for the contaminants to enter the aquifer system and reach the pumping wells. Thus, following this research, we envisage efforts to combine stable isotope techniques with particle tracking analysis in three dimensions to define areas exposed to contamination risk in fluvio-volcanic bedrock aquifers. These research efforts can represent an approach to integrate with two-dimensional GIS tools that are commonly used to assess aquifer vulnerability.

The feedback interaction between biomass accumulation and heterogeneous flow in porous media: Effect of shear stresses

Bioclogging, resulting from excess biomass growth in porous media, can influence flow, solute transport, and biogeochemical reactions in natural and remedial subsurface systems. In this study, we investigated bioclogging in heterogeneous porous media using a reactive-transport model, with a focus on the effect of shear stresses on the spatial changes of biomass accumulation and hydraulic properties. The results show that, when the shearing-induced detachment was negligible, the accumulation of biomass was mainly controlled by substrate transport. Newly generated biomass was then accumulated in the regions with high hydraulic conductivity due to the fast substrate supply, resulting in a homogenization of hydraulic conductivity. When the shearing-induced detachment was significant, newly generated biomass was less accumulated in the flow paths with higher hydraulic conductivity, and more in the pore regions with relatively low hydraulic conductivity, resulting in the increase of heterogeneity in hydraulic conductivity. The results were then used to establish a macroscopic correlation between the reduction of porosity and the reduction of hydraulic conductivity as a result of biomass growth, and to identify factors controlling such a correlation. A correlation was established and was found to be significantly affected by flow velocity and shearing-induced biomass detachment. Overall, this study indicated that flow-induced shearing is important in understanding bioclogging and its effect on hydraulic conductivity and water flow in porous media.

Bone diagenesis in the medieval cemetery of Vratislavs’ Palace in Prague

Diagenetic modifications in human bones from the early-medieval cemetery discovered in the garden of Vratislavs’ Palace, in the central Mala Strana district of Prague, have been investigated combining histological analysis and instrumental analysis with X-ray diffraction, infrared, and 31P NMR spectroscopy. A total of 15 ribs samples were collected for the study. One sample belonged to a child, whereas, of the other samples from adults, 7 belonged to males, 5 to females, and for 2 the sex attribution was uncertain. A diagenetic pathway common to most of the studied samples was considered the result of a burial environment characterized by a nearly static water regime, with limited temperature excursions, moderately oxic to suboxic, and with pH fluctuations around the limit of apatite recrystallization window, in agreement with the fine textured clay-rich soil, its low hydraulic conductivity, and the measured soil pH. A second pattern, related to variations in the microenvironment, interested a limited number of samples with poorer histological preservation. This was interpreted as the result of higher pH and a better oxygenated environment, which favoured mineral recrystallization. Further reactivation of deterioration processes probably occurred later in some of the graves perturbed by works conducted in the seventeenth century. This work highlights the complementarity of the information obtained from the adopted techniques in order to gain insights into the post-mortem fate of the human remains and their sedimentary environment. In this respect, the quantification of the amount of phosphorus in the amorphous hydrated layer of apatite provided a unique type of information on the mineral component of bone and its reorganization during diagenesis, revealing that a relevant fraction can survive diagenesis, at variance with what previously supposed.

Direct Observation of the Depth of Active Groundwater Circulation in an Alpine Watershed

AbstractThe depth of active groundwater circulation is a fundamental control on stream flows and chemistry in mountain watersheds, yet it remains challenging to characterize and is rarely well constrained. We collected hydraulic conductivity, hydraulic head, temperature, chemical, noble gas, and 3H/3He groundwater age data from discrete levels in two boreholes 46 and 81 m deep in an alpine watershed, in combination with chemical and age data from shallow groundwater discharge, to discern groundwater flow rates at different depths and directly observe active and inactive groundwater. Vertical head gradients are steep (average of 0.4) and thermal profiles are consistent with typical linear conductive continental geotherms. Groundwater deeper than ∼20 m is distinct from shallow groundwater and creek water in its chemistry, noble gas signature, and age (dominantly &gt;65 years compared to &lt;9 years). Together these results suggest low vertical groundwater flow velocities and a relatively shallow active circulation depth of ∼20 m. This hypothesis is tested with a simple 2‐D numerical fluid flow and heat transport model representing a hillslope transect through the two boreholes. The modeling indicates that the subhorizontally bedded sedimentary rocks underlying the basin are highly anisotropic with low vertical hydraulic conductivity, and at most ∼10% of bedrock recharge (equivalent to &lt;2% of stream baseflow) flows below a depth of 20 m. The study demonstrates the considerable value of discrete‐depth hydrogeologic, chemical, and age data for determining active circulation depth, and illustrates an approach for maximizing the utility of individual boreholes drilled for mountain bedrock aquifer characterization.

Assessing benzene and toluene adsorption with peat depth: Implications on their fate and transport.

After a hydrocarbon spill in a peatland, dissolution of water-soluble compounds including benzene and toluene introduces a dissolved-phase plume to the peatland groundwater system, while the adsorption of these solutes onto the peat matrix restrains their distribution velocity. The adsorption of benzene and toluene and its dependency on peat depth, thus degree of decomposition, are investigated. The batch adsorption experiments revealed that benzene and toluene adsorption isotherms in peat are linear, with adsorption coefficients ranging from 16.2 to 48.7L/kg and 31.6-48.7L/kg, respectively. In a vertical peat profile benzene adsorption decreased with depth, while toluene adsorption increased. Considering toluene adsorption onto cellulose is significantly less than toluene adsorption onto humic substance, the increase in toluene adsorption was attributed to decreasing cellulose and increasing humic substances with depth. Negligible competition for adsorption was observed between benzene and toluene at the measured concentrations. The retardation factors of benzene and toluene ranged respectively from 3.5 to 10.7 and from 5.4 to 17.7, both increasing with depth. Higher retardation in deeper peat coupled with lower hydraulic conductivity will lead to a weaker solute velocity in deeper peat, thus preferential migration of these dissolved-phase contaminants in shallow layers. The results can help predict the behavior of dissolved hydrocarbons in peatlands after a hydrocarbon spill.

Arsenic sequestration by iron oxide coated geopolymer microspheres

A green, low-cost adsorbent based on geopolymer “microspheres” is reported for the first time for arsenic adsorption. A simple two-step process involves synthesizing geopolymer microspheres by inverse suspension polymerization and impregnating them with iron oxide. The low hydraulic conductivity of fine iron oxide powders demands the use of suitable support materials. Iron oxide modified geopolymer microparticles synthesized here, are small, spherical, porous, mechanically, and chemically stable, have high surface area of 270 m 2 /g and can be easily separated from water after adsorption is complete. A comprehensive adsorption study was conducted by employing environmentally critical arsenic species, viz. As(III), As(V), monomethyl arsenate and dimethyl arsenate. The role of contact time, initial arsenic concentration, temperature, adsorbent dosage, pH and competing ions was evaluated. Highest arsenic adsorption was observed at an iron content of 20 wt%. High percent removal of ∼86%, 100%, 95% and 96% for As(III), As(V), DMA and MMA respectively, were observed at initial arsenic concentration of 0.5 mg/L, an adsorbent dosage of 10 g/L and in just 60 min. Low pH favored adsorption of As(V), monomethyl arsenate and dimethyl arsenate while high pH favored adsorption of As(III). The applicability of the novel adsorbent in column mode, its high regeneration capacity and, facile, green, and cheap production are few of its advantages. Moreover, unmodified geopolymer microspheres, introduced here, can be employed as adsorbents for other toxic heavy metals such as Pb, Cd and Hg. • Inverse suspension polymerization was used to synthesize geopolymer microspheres. • Geopolymer microspheres were infused with iron oxide for arsenic adsorption. • Highest amounts of arsenic was removed by geopolymer containing 20 wt% iron. • Removal of As(III), As(V), monomethyl arsenate and dimethyl arsenate was evaluated. • New adsorbent showed excellent regeneration, hence making it economically viable.

Evaluation of sand-clay-anionic polyacrylamide blends for alternative compacted clay landfill liner design

In this project, an innovative low hydraulic conductivity material for landfill cover and liner construction was studied. The material is a blend of natural clayey soil from Comodoro Rivadavia city (Chubut province, Argentina) mixed with fine uniform sand and anionic polyacrylamide (APAM). The research emphasizes understanding the influence of APAM addition on the soil water retention capacity (SWRC), unsaturated hydraulic conductivity, and swelling behavior. APAM is a super absorbent polymer that swells when immersed in water. SWRC was evaluated through the filter paper method. The unsaturated hydraulic conductivity and swelling behavior were determined using two fluids: distilled water and brine (C = 2 M). Results showed that APAM addition reduced the blends' microporosity, increased the water retention capacity, and reduced the hydraulic conductivity of the system. These promising results encourage further research on these blends' behavior to determine the most efficient blend formulation to enhance its hydro-mechanical performance and its chemical compatibility with landfill leachates for cover and low hydraulic conductivity liner layer construction.

Hydraulic Conductivity Measurement for Three Frozen and Unfrozen Soils in the Red River of the North Basin

HighlightsHydraulic conductivity was measured in frozen and unfrozen soil conditions by a minidisk infiltrometer.In the RRB, frozen sandy loam and silty clay soils had the highest and lowest hydraulic conductivity, respectively.Three simple equations were developed for the three soils to predict frozen soil hydraulic conductivity.Freeze-thaw cycles reduced soil hydraulic conductivity.Abstract. Hydraulic conductivity (k) is a key parameter in describing water movement through a soil profile. In the Red River of the North basin (RRB), the hydraulic properties of frozen soils vary with temperature, water content, and other factors. In this study, a minidisk infiltrometer was used to measure the k values of three soils from the RRB: Colvin silty clay loam, Fargo silty clay, and Hecla sandy loam. The k values were measured for frozen and unfrozen soils with five different initial soil water contents: oven dry, permanent wilting point, field capacity, midway between permanent wilting point and field capacity, and saturation. The results showed that the mean k value of a frozen soil increased with an increase in initial soil water contents. Hecla soil had the highest k values and Fargo soil had the lowest k values for frozen soils. Three equations were fitted with the measured k values of Colvin silty clay loam, Fargo silty clay, and Hecla sandy loam soils. The k values were also estimated using the Motovilov model. When evaluating model performance, the fitted regression models agreed more closely with the measured k values (index of agreement, d, values of 0.96, 0.94, and 0.94 for Colvin, Fargo, and Hecla soils, respectively) than Motovilov models. Based on overall considerations of statistical measures, the fitted regression models predicted the k values better than Motovilov models for all three frozen soils. It was also found that the k values decreased with an increase in the number of the freeze and thaw cycles that changed the soil properties. Keywords: Frozen soil, Hydraulic conductivity, Mini disk infiltrometer, Red River Valley.

Potential impacts of shale and tight gas developments on unconfined aquifers – a chemical screening framework

The Geological and Bioregional Assessment Program examines the potential impacts of shale, tight and deep coal gas development on water and the environment. A key part of the project was quantifying the potential for a decline in the water quality of unconfined aquifers due to unintentional chemical release at the soil surface. To assess this hazard, a quantitative analysis of chemical migration pathways was undertaken, which involved the estimation of contaminant attenuation by dilution and dispersion in soil and groundwater. This provided a conservative screening approach to identify areas for further analysis. Attenuation calculations involved one-dimensional advection-dispersion (AD) simulations through the unsaturated zone, and three-dimensional AD solute transport within the surficial aquifers. Dilution factor (DF) relationships for the combined effect of attenuation in the unsaturated and saturated zone were used to construct spatial maps of the potential for impact on aquifer properties after accidental chemical spills. A higher DF (therefore lower consequence of the surface contamination) was associated with deeper unsaturated zones characterized by heavier soils near the surface, and lower ground water velocities due to lower hydraulic conductivity and/or hydraulic gradient in the saturated zone. The framework was applied across the Cooper Basin and Beetaloo Sub-basin and resulted in two types of maps. The first identified areas being more susceptible to contamination if soil remediation does not occur within a 10-year period. The second map shows the spatially variable combined DFs for a ground water receptor, which may be used to develop site-specific management plans and mitigation measures.

Effect of Ammonium on the Hydraulic Conductivity of Kaolin and Bentonite as Clay Liners

: Landfill liners are underlying materials with low permeability whose main function is to mitigate the infiltration of toxic contents into ground water lying beneath. Landfill liners are primarily made of bentonite clay. Bentonite has a very low hydraulic conductivity, that might not be readily accessible, unlike kaolin which is found to have a lower hydraulic conductivity compared to that of bentonite and can be extensively obtained from numerous different sources. Explored, for the purposes of the present research paper, were various ratios of bentonite and kaolin and their hydraulic conductivity, in particular ratios of 90:10 kaolin to bentonite, 80:20 kaolin to bentonite, 70:30 kaolin to bentonite, 60:40 kaolin to bentonite and 50:50 kaolin to bentonite in an effort to achieve an acceptable barrier suitable as a liner / where tap water and ammonium solution were used as permeants. It was concluded that the ratios not lower than 20% bentonite (80:20, 70:30, 60:40 and 50:50) all had their hydraulic conductivity value reduced compared to the 100% kaolin.