Stability and Consolidation of Sediment Tailings Incorporating Unsaturated Soil Mechanics

Tailing dams are commonly used to safely store tailings without damaging the environment. Sand tailings (also called Sediment tailings) usually have a high water content and hence undergo consolidation during their placement. As the sediment tailings are usually placed above the ground water level, the degree of saturation and permeability of the sediment tailing is associated with the unsaturated condition due to the presence of negative pore-water pressure or suction. Current practices normally focus on the analyses saturated conditions. However, this consolidation process requires the flow of water between saturated and unsaturated zones to be considered. The objective of this study is to investigate the stability and consolidation of sediment tailings for the construction of road pillars considering the water flow between saturated and unsaturated zones. The scope of this study includes the unsaturated laboratory testing of sediments and numerical analyses of the road pillar. The results show that the analyses based on saturated conditions overestimate the time required to achieve a 90% degree of consolidation. The incorporation of the unsaturated soil properties is able to optimize the design of slopes for road pillars into steeper slope angles.

Variability of Bimodal Soil-Water Characteristic Curves under Different Confining Pressures

Soils with two subcurves of Soil-Water Characteristic Curve (SWCC) (dual porosity soils) might be found within various residual soils. Soils located in different depths have different confining pressure. Residual soils are found in the unsaturated zones due to the deep groundwater table. There is a linear correlation between the hydraulic properties of the soil in the unsaturated area and that of its unsaturated properties. This study aims to examine the influence of the confining pressure towards the SWCC of dual porosity soil. The scope of this study involves measurements of the drying and wetting SWCC using Tempe cells, pressure plates, and an advanced triaxial apparatus. In this study, the mathematical equations were developed to explain the effect of confining pressure on SWCC. The experimental results indicated that the dual porosity soil exhibits bimodal characteristics for the drying curve of SWCC and it exhibits unimodal characteristics for the wetting curve of SWCC. As the confining pressure increases, the air entry values, the inflection points, and the standard deviation of drying SWCC increase. In addition, the hysteresis of SWCC is becoming smaller with the increasing confining pressure.

Improve NMR estimation of water content and distribution in unsaturated bedrocks

<p>Nuclear magnetic resonance (NMR) has been widely used in near-surface geophysics due to its direct sensitivity to water. As a field form of NMR, borehole NMR has been applied to in situ hydrological investigations for decades. However, the recent implementations of borehole NMR to unsaturated zones face challenges due to the complex geology. Due to the fast operation speed and unsaturated conditions in critical zones, the raw NMR signals often suffer from limited relaxation time ranges and low signal to noise ratios. Such low quality of raw data can induce artifacts during inversion and following data interpretations. This study investigates the long-overdue evaluations of how the low borehole NMR data quality affects water distribution estimation in unsaturated zones. A synthetic analysis based on lab NMR data was first performed to simulate the inversion errors induced by the low-quality borehole NMR data. Lab NMR measurements were conducted on carbonate and shale samples from a well that has a corresponding borehole NMR profile. In order to match the low signal-to-noise ratio and data size of the low-quality borehole NMR data, lab NMR data points were reduced, deadtime was increased and normally distributed noise was added.  The inversion results of the synthetic data reveal that the low signal to noise ratio leads to an overestimation of signals at lower relaxation time while the limited relaxation time range does not significantly affect the total water estimation. To improve the water estimation from the low-quality borehole data, a peak decomposition and peak fusion method were then applied to the synthetic data. Relaxation time distribution of both lab and synthetic data were decomposed into multiple normally distributed peaks. The first peak with the shortest relaxation time from lab NMR was used to substitute the first peak of the synthetic borehole NMR relaxation time distribution. After peak decomposition and fusion, the predicted water contents were closer to lab NMR than original synthetic data. This study reveals the mispredictions of water distribution due to the low data quality of borehole NMR. The success of improving water content estimation on the synthetic study has clear implications that the peak decomposition and peak fusion method can be applied to actual borehole NMR data to improve water content and distribution estimation in unsaturated zones.   </p>

GROUNDWATER VULNERABILITY DETERMINATION OF NORTHEASTERN BOSNIA ACCORDING TO GLA METHOD

The paper assesses the vulnerability of 27 groundwater bodies in the basin of the river Bosna, ie Spreča, then the river basin of the Drina river (Drinjača), as well as parts of the immediate Sava river basin (Čelić area - Gnjica river and Turija basin) by the GLA method. For each water body, the determination of the effective soil capacity, degree of infiltration and rock type was made, as well as the final assessment of the protective efficiency of the unsaturated zones soil covering.

Effects of dissolved organic matter on sorption of benzotriazole to soil

<p>Dissolved organic matter (DOM) plays an important role in affecting the environmental behaviors of organic contaminants. Effects of two representative DOMs (dissolved humic acid (HA) and tannic acid (TA)) on sorption of benzotriazole (BTA) to a reference soil were investigated by batch experiments. The results indicated that TA had stronger sorption to soil than HA (initial solution pH=6.0±0.1). This is because that TA contains more carboxylic and phenolic groups than those of HA. In the solution with DOM, the enhanced sorption of BTA was caused by cumulative sorption resulting from sorbed DOM. Hydrogen bonding was proposed as the main binding mechanism between BTA and the sorbed DOM. When the solution pH at sorption equilibrium increased from 6.5 to 10.5, the electrostatic repulsion inhibited the sorption of BTA in solution with/without HA. In addition, less hydrogren bonds made the effect of HA in promoting BTA sorption decrease when solution pH changed from 6.5 to 10.5. Higher molecular weight fractions of HA could be preferentially sorbed by the soil, its enhancement on BTA sorption was more obvious than that of the low molecular weight fractions. These findings are conducive to a better understanding of environmental behaviors of BTA as well as other organic compounds with similar structure in the unsaturated zones.</p>

How deep do signals of surface conditions extend into the subsurface Critical Zone?

<p>Fluids (water and gases) connect surface and subsurface compartments of the Critical Zone by transporting matter, including chemical energy and organisms. In the AquaDiva Collaborative Research Center, one of our research goals is to use a variety of tracers to determine how the subsurface and the organisms inhabiting it reflect and depend on surface conditions.  This research is performed at the Hainich Critical Zone Exploratory (CZE), a hillslope transect in limestone and marlstone sedimentary rocks where a network of surface observations is linked to routinely monitored groundwater wells.   This CZE is especially interesting because its different rock units and hydrogeologic conditions create environments with different microbiomes and conditions.</p><p>This talk will synthesize information collected by AquaDiva researchers on how different kinds of ‘signals’ identify important mechanisms connecting surface and subsurface.   Biologically dominated signals, such as cell counts, metagenomics, metabolomics, the molecular composition and properties of dissolved organic matter,  change with distance from the surface.   While some individual compounds and organisms can be found across the different critical zone compartments, it is clear that that ground water and its inhabitants are not just diluted from the surface but reflect and co-evolve with microbial communities and subsurface environmental conditions.   Isotopic tools trace elements rather than chemical compounds and provide independent information on the timescales for surface-subsurface transport or the sources of energy or metabolites.  For example, we used bomb-radiocarbon as a tracer for surface carbon recently fixed by plants.  The 14C in dissolved or particulate organic matter and inorganic C demonstrate how newly fixed, recycled or even fossil (rock derived) C is incorporated into microbial food webs. Finally, surface conditions, including structure, influence the subsurface metabolism by regulating the transfer of electron acceptors like O2, excess nutrients from fertilizers, or reactive nanocrystalline Fe, through soils and unsaturated zones into groundwaters.  </p>

Estimation of evapotranspiration rate by using the Penman–Monteith and Hargreaves formulas for the loess in Northeast Bulgaria complex with HYDRUS-1D

Loess and loess-like sediments cover approximately 13% of the Bulgarian territory, mainly within the Danubian plain. From the Danube River to the Fore-Balkan, the loess soils form a loess complex where its depth varies from 50–60 meters in the north to few meters in the south, respectively. Widespread loess sediments possess a specific feature: they typically form deep unsaturated zones. Quantification of the near surface water balance is extremely important for evaluating land-atmosphere interactions, and the impact of land-use change on the subsurface flow and the evapotranspiration rate is an essential term in this quantification. In the frames of a scientific project, an automatic weather station was installed in a typical plain terrain of the loess complex in Northeast Bulgaria, recording meteorological data from September 2015 to February 2017. This study provides a mathematical description of processes (i.e., Penman-Monteith and Hargreaves Methods) used to estimate daily evapotranspiration rates implemented into the numerical model HYDRUS-1D, as well as a respective rate investigation of months with and without intensive rainfalls. Overall results indicate that using the Hargreaves formula for evaluation of the potential evapotranspiration leads to overestimation between 10% and 20%, respectively for a “wet” and “dry” month.