Journal of Hazardous, Toxic, and Radioactive Waste

This study quantified the sensitivity of advective contaminant arrival time to the porosity of hydrogeologic units (HGUs) for groundwater transport from the Nevada Test Site (NTS) to the Yucca Mountain (YM) area in the United States. Advective arrival time of contaminants associated with underground nuclear detonations was obtained by using particle tracking on the basis of flow velocities from the Death Valley Regional Groundwater Flow System (DVRFS) model and porosity information for the HGUs along the flowpaths. The objectives were to (1) determine which HGU’s porosity is influential in contaminant advective arrival time from the NTS to the YM, and (2) apportion uncertainties in contaminant advective arrival time to the uncertainty contributions from individual HGU porosity. Over 90% of the uncertainty in advective transport arrival time from the NTS to YM could be described by uncertainties in porosity values of two of the 27 HGUs present in the DVRFS model. The study was significant for optimizing resources in field characterization to reduce uncertainty by targeting HGUs contributing the most uncertainty.

Discrete geologic features such as faults and highly permeable embedded channels can significantly affect subsurface flow and transport characteristics. Therefore, they must be properly identified, parameterized, and represented in subsurface simulation models. In this work, we use an improved ensemble Kalman filter (EnKF) for history-matching fault network geometry from production data. EnKF is a sequential Monte Carlo data assimilation method that simultaneously propagates and updates an ensemble of model states, resulting in a set of calibrated model realizations that can be readily used for model prediction and uncertainty analysis. A pattern-based stochastic simulation algorithm was used to generate fault network realizations based on a priori fault trace data. The classic EnKF algorithm was enhanced with a grid-based covariance localization scheme to better handle non-Gaussian permeability distributions resulting from the presence of faults. Numerical experiments indicate that the modified EnKF can be a promising method for uncovering unmapped faults by using production data.

A modeling study is conducted to evaluate the sensitivity of acid gas evolution in a deep saline aquifer to expected variations of multiple geologic and engineering variables. Relative permeability hysteresis, aquifer heterogeneity variance, formation water salinity, permeability of caprock and leakage wells, injection rate, regional hydraulic gradient, and formation depth are evaluated as uncertain input parameters to a three-dimensional synthetic aquifer model with fully heterogeneous permeability. To understand the effect of conceptual model uncertainty on predicting gas flow and storage, permeability of the heterogeneous model is upscaled to equivalent permeability for three increasingly homogenized stratigraphic models: an eight-unit facies model, a three-unit depositional model, and a one-unit formation model. Two upscaling methods are used: a flow-based numerical method and an analytical averaging method. Over 120 years (20 years of injection and 100 years of monitoring), multiphase compositional simulation is conducted to model gas migration and trapping in the aquifer and its dissolution in the formation brine. Results suggest that among the variables evaluated, gas-relative permeability hysteresis, heterogeneity variance, and injection rate have the most significant impact predicting the total mobile gas in the storage system, whereas caprock permeability is the most important factor influencing the prediction of total gas leakage and thus the storage security. Over the simulation time scale, for the fixed amount of gas injected, regional hydraulic gradient, salinity, and formation depth have lesser impact on gas flow and storage predictions. Further, leakage through abandoned wells can occur when permeability of the wellbore is as low as 1 mdarcy (md), while caprock permeability becomes critical to storage security when it is more than $1\times {10}^{-4}\mathrm{md}$, in which case significant leakage occurs during the monitoring phase. Compared to the predictions of the heterogeneous model, the greater the number of stratigraphic units in the upscaled models, the better its accuracy in predicting gas storage and plume sweep. However, the accuracy of the stratigraphic models depends on aquifer variance, upscaling method, and type of prediction outcome that is being evaluated.

In this paper, a novel reactive diffusion-convection transport model is proposed to investigate the flow during different ${\mathrm{CO}}_2$ trappings in deep saline aquifers. Dissolved and immiscible ${\mathrm{CO}}_2$ have trapping times ranging from 1 million to several million years. The mineral trapping has a much longer temporal scale than the solubility trapping. The fine resolution of diffusion-to-advection characteristics is mapped by a corresponding implementation of finite-element numerical simulation. The diffusivity and reaction rate has prominent effects on diffusion-convection-reaction behaviors of the ${\mathrm{CO}}_2$-brine system. The 2-D numerical solutions give insight into the trapping evolution of injected ${\mathrm{CO}}_2$ in a deep subsurface environment.

The objective of this study is to investigate the effect of subsoil hardpan on the leaching of zinc and copper fractions in soil amended with swine manure (SM plot) and chemical fertilizer (CF plot) for 13 years. Soil samples were taken from the layered soil profile from each plot to examine the vertical distribution of fraction, mobility, and soil physical properties. Metal fraction and mobility were evaluated by using a six-step sequential extraction procedure. The water-soluble and exchangeable fraction (EXCH), ${\mathrm{CH}}_3\mathrm{COONa}$ extractable fraction (NaOAc), Me-Org-zinc, and Me-Org-copper (metal-organic complex fraction) increased in the plow layer of SM plot. The transition layer between the plow and subsoil hardpan was an obstacle to the leaching of NaOAc-Zn and exchangeable zinc owing to adsorption of metals and the underlying low-permeability layer. As a result, the migration of NaOAc-zinc and exchangeable zinc was negligible, whereas a significant amount of water-soluble zinc, Me-Org-zinc, and copper easily leached to the subsoil hardpan in the SM plot compared to the CF plot. This study provides a detailed knowledge about the leaching of high mobility heavy metals in layered soil and has implications for an accurate evaluation of environmental risk from manure treatment.

To investigate the efficiency of chemical stabilization on soil residuals after soil washing, laboratory batch experiments were performed on a clayish soil contaminated with Cu, Cr, Ni, Pb, Zn with lime, ${\mathrm{Na}}_2{\mathrm{HPO}}_4$, or a mixture containing ${\mathrm{FeSO}}_4$ under various concentrations. The instant and overall mobility and the fraction distribution of these metals after 7-day stabilization were determined by acid extraction and the modified Tessier’s sequential extraction procedures. Results show that the potential mobility of metals is orders of magnitude much more than instant mobility, and specific attention therefore should be paid to the potential mobility when evaluating the performance of chemical stabilization. Metal fraction analyses suggested that the metal stabilization involved various mechanisms dependent on the metal’s properties, species, and the interference between metals. Among the stabilization combinations, the mixture of ${\mathrm{FeSO}}_4$ and lime most effectively reduced the sum of the metal instant mobility, and $15\mathrm{mg}{\mathrm{g}}^{-1}$ ${\mathrm{Na}}_2{\mathrm{HPO}}_4$ achieved the highest efficiency in lessening the corresponding overall mobility. However, a more substantial decrement of instant mobility was observed.

Understanding colloid mobilization, transport, and deposition in the subsurface is a prerequisite for predicting colloid-facilitated transport of strongly adsorbing contaminants and further developing remedial activities. This study investigated the transport behavior of soil-colloids extracted from a red-yellow soil from Okinawa, Japan. Different concentrations of suspended-soil colloids (with diameter $<1\mu \mathrm{m}$) were applied, at different flow velocities and pH conditions, to 10-cm long water-saturated columns repacked with either Narita (mean diameter $D_{50}=0.64\mathrm{mm}$) or Toyoura (mean diameter $D_{50}=0.21\mathrm{mm}$) sands. The transport and retention of colloids were studied by analyzing colloid effluent breakthrough curves (BTCs), particle size distribution in the effluent, and colloid deposition profiles within the column. The results showed a significant influence of flow velocity: Low flow velocity caused tailing of colloid BTCs with higher reversible entrapment and release of colloids than high flow velocity. The finer Toyoura sand retained more colloids than the coarser Narita sand at low pH conditions. The deposition profile and particle size distribution of colloids in the Toyoura sand clearly indicated a depth-dependent straining mechanism. By fitting colloid transport models (one-site and two-site models) to the colloid effluent breakthrough curves, transport and deposition of colloids in Narita sand at low pH were best described by a one-site attachment-detachment model, whereas colloid transport and deposition in Toyoura sand at low pH were better captured by a two-site attachment, detachment, and straining model. The coupled effects of solution chemistry, colloid sizes, and medium surface properties have a dominating role in particle-particle and particle-collector interactions in colloid transport and deposition.

Gas exchange through the compacted final cover soil at landfill sites plays a vital role for emission, fate, and transport of toxic landfill gases. This study involved measuring the soil-gas diffusivity ($D_p/D_o$, the ratio of gas diffusion coefficients in soil and free air) and air permeability ($k_a$) for differently compacted soil samples (reddish-brown soil) from the final cover at the Maharagama landfill in Sri Lanka. The samples were prepared by either standard Proctor compaction or hand compaction to dry bulk densities of 1.60–$1.94\mathrm{g}{\mathrm{cm}}^{-3}$. Existing and modified models for predicting $D_p/D_o$ and $k_a$ were tested against the measured data. The simple, single-parameter Buckingham model predicted measured $D_p/D_o$ values across compaction levels equally well or better than a dry bulk density (DBD) dependent model and a soil-water retention (SWR) dependent model. The measured $k_a$ values for differently compacted samples were highly affected by the compaction level and the sample moisture preparation method. Also, for air permeability, a single-parameter Buckingham-type $k_a$ model was most accurate in predicting $k_a$ in the differently compacted soil samples. Equivalent air-filled pore diameters (the effective diameter of the drained pores active in leading air through the sample) for gas flow, $d_{eq}$, were calculated from the measured $D_p/D_0$ and $k_a$ values. The $d_{eq}$ increased with compaction level, suggesting that a very high compaction level creates well-connected macropores in the reduced total pore space of the cover soil. This is an important consideration when designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions from the waste layer to the atmosphere.

Researchers have performed experimental studies using ammonium citrate (AC) during the electrokinetic (EK) remediation process for the extraction of cadmium (Cd) and copper (Cu) from the contaminated soil. They evaluated the efficiency of ammonium citrate by considering it as a washing solution and a purging solution at the anode electrode compartment. The efficiency of electrokinetic extraction was observed to be significantly influenced by the pH and buffering nature of the soil medium. The experimental studies indicate that the removal of cadmium and copper was 48.9% and 30.0%, respectively, when ammonium citrate was used both washing and purging solution. The solubility of both cadmium (${\mathrm{Cd}}^{++}$) and copper (${\mathrm{Cu}}^{++}$) in EK-treated soils has also been estimated by sequential extraction studies with deionized (DI) water. The analytical techniques, X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscope (SEM) provide the evidence of migration of cations during treatment of contaminated soil by process of electroosmosis (EO). The SEM images of both cadmium- and copper-contaminated soils show that these soils have a fluffier and more porous structure. This might be caused by the change in surface charges of the clay particles as a result of introduction of heavy metals. The mineralogical compositions of soil are not altered significantly by electrokinetic process.

Bhandup Complex water treatment plant located in Mumbai, India, supplies water to most of Mumbai. The plant receives raw water from Vehar Lake. Alum has been used as a coagulant in the treatment process for the past 27 years. The backwashed water from the plant is released into Vehar Lake, which carries a significant amount of suspended matter. Therefore, a layer of sludge was deposited on the meeting point of backwashed water and Vehar Lake. The status of aluminum (Al) concentration in sludge deposits, banks, water, and sediment samples of Vehar Lake and its comparison with Al concentration of soil in the area surrounding the plant is presented in this paper. The comparison showed that the surrounding soil samples had the least Al concentration (16,394–$26,081\mathrm{mg}/\mathrm{kg}$), whereas the highest values of 53,882–$73,382\mathrm{mg}/\mathrm{kg}$ and 62,096–$76,496\mathrm{mg}/\mathrm{kg}$ were reported in upper and lower layers of sludge, respectively. This paper also discusses the detrimental effect of Al when it is available in toxic form at acidic pH.

In this study, electrical resistivity was related to the fundamental geotechnical properties of cement solidified/stabilized heavy metal–contaminated soils. Lead-contaminated soils were artificially prepared with various concentrations and treated by portland cement Type I at three content levels. The compacted cylindrical specimen was prepared with a diameter of 50 mm and height of 100 mm. The apparent electrical resistivity, electrical conductivity of pore fluid, unconfined compressive strength, and water contents were measured at different curing times. The result shows that the apparent electrical resistivity increases with the increase of curing time and the decrease of porosity and saturation degree as a result of cement hydration development. The effects of lead concentration, cement content, and curing time on the apparent electrical resistivity were discussed. The cementation index $m$ was used to evaluate the cementation degree of cement-treated lead-contaminated soils. Lead pollutant at high concentrations could suppress or retard the hydration of solidified/stabilized soils. Electrical resistivity measurements can be used as a nondestructive method to evaluate the hydration development of solidified/stabilized lead-contaminated soils.

Four pilot-scale lysimeters were used to study the benefits of landfill operation with and without leachate recirculation in tropical weather conditions. Young and old landfills were simulated by filling lysimeters with a segregated fraction of fresh municipal solid waste (MSW) and MSW mined from an open dump site, respectively, and periodically monitoring leachate quantity and quality and biogas quality. For each substrate, one lysimeter was operated as a bioreactor with leachate recirculation and another lysimeter was operated as a controlled dump, for a period of 10 months. Densities between 652 and $825\mathrm{kg}/{\mathrm{m}}^3$ could be achieved with fresh and mined MSW. Despite such compaction during waste placement, bioreactor technology helps in leachate management, especially in the case of the young landfill lysimeter operated in tropical weather. The benefits of leachate recirculation in the young landfill lysimeter were evident from the significant decrease in chemical oxygen demand (COD) (86%), biochemical oxygen demand (BOD) (82%), dissolved organic carbon (DOC) (85%), and volatile solids (75%) in leachates. However, ammonia nitrogen (amm-N) and chlorides in the leachates accumulated in bioreactor landfills. Operating an old landfill lysimeter as a bioreactor seemed to have no exceptional advantage in the context of leachate management, although leachate recirculation enhanced the methane potential of both fresh and mined MSW.