Arsenic and Antimony in Groundwater Flow Systems: A Comparative Study

Arsenic (As) and antimony (Sb) concentrations and speciation were determined along flow paths in three groundwater flow systems, the Carrizo Sand aquifer in southeastern Texas, the Upper Floridan aquifer in south-central Florida, and the Aquia aquifer of coastal Maryland, and subsequently compared and contrasted. Previously reported hydrogeochemical parameters for all three aquifer were used to demonstrate how changes in oxidation–reduction conditions and solution chemistry along the flow paths in each of the aquifers affected the concentrations of As and Sb. Total Sb concentrations (Sb_T) of groundwaters from the Carrizo Sand aquifer range from 16 to 198 pmol kg⁻¹; in the Upper Floridan aquifer, Sb_T concentrations range from 8.1 to 1,462 pmol kg⁻¹; and for the Aquia aquifer, Sb_T concentrations range between 23 and 512 pmol kg⁻¹. In each aquifer, As and Sb (except for the Carrizo Sand aquifer) concentrations are highest in the regions where Fe(III) reduction predominates and lower where SO₄ reduction buffers redox conditions. Groundwater data and sequential analysis of the aquifer sediments indicate that reductive dissolution of Fe(III) oxides/oxyhydroxides and subsequent release of sorbed As and Sb are the principal mechanism by which these metalloids are mobilized. Increases in pH along the flow path in the Carrizo Sand and Aquia aquifer also likely promote desorption of As and Sb from mineral surfaces, whereas pyrite oxidation mobilizes As and Sb within oxic groundwaters from the recharge zone of the Upper Floridan aquifer. Both metalloids are subsequently removed from solution by readsorption and/or coprecipitation onto Fe(III) oxides/oxyhydroxides and mixed Fe(II)/Fe(III) oxides, clay minerals, and pyrite. Speciation modeling using measured and computed Eh values predicts that Sb(III) predominate in Carrizo Sand and Upper Floridan aquifer groundwaters, occurring as the Sb(OH)₃⁰ species in solution. In oxic groundwaters from the recharge zones of these aquifers, the speciation model suggests that Sb(V) occurs as the negatively charged Sb(OH)₆⁻ species, whereas in sufidic groundwaters from both aquifers, the thioantimonite species, HSb₂S₄ ⁻ and Sb₂S₄ ²⁻, are predicted to be important dissolved forms of Sb. The measured As and Sb speciation in the Aquia aquifer indicates that As(III) and Sb(III) predominate. Comparison of the speciation model results based on measured Eh values, and those computed with the Fe(II)/Fe(III), S(-II)/SO₄, As(III)/As(V), and Sb(III)/Sb(V) couples, to the analytically determined As and Sb speciation suggests that the Fe(II)/Fe(III), S(-II)/SO₄ couples exert more control on the in situ redox condition of these groundwaters than either metalloid redox couple.     

Modelling of intact and jointed mudstone samples under uniaxial and triaxial compression

Rock mass is a highly complex entity where the strength and deformation behaviour can be significantly affected by its secondary structures such as joints, fissures and bedding planes. Whilst many research works have been conducted to study the behaviour of a specific rock mass, a thorough understanding of its strength and deformation behaviour incorporating different joint sets has not been established. In this study, a comprehensive numerical modelling using a three-dimensional distinct element code, 3DEC, was undertaken to study the strength and deformation behaviour of a mudstone, locally found in Melbourne, in unconfined and confined states. The initial unconfined model established for intact mudstone was calibrated against the well-established laboratory-based empirical strength relationships and subsequently compared with some strength test data available for field samples. The intact unconfined model was then extended to study the strength behaviour in the confined state. The results obtained from this confined intact model were compared with existing strength criteria and were found in good agreement. The confined model was extended further to investigate the effects of joint sets and dip angles on the rock mass strength and deformation behaviour by incorporating two different joint configurations (one-joint and two-joint) with varying dip angles (0°–90°). This study found that the rock mass strength in a confined state varied significantly between the two joint configurations.     

Stability Analyses of Municipal Solid Waste Landfills with Decomposition

A bioreactor landfill is operated to enhance refuse decomposition, gas production, and waste stabilization. Some of the potential advantages of bioreactor include rapid stabilization of waste, increased landfill gas generation, gain in landfill space, enhanced leachate treatment, and reduced post closure maintenance period. Due to the accelerated decomposition and settlement of solid waste, bioreactor landfills are gaining popularity as an alternative to the conventional Subtitle D landfills. However, the addition of leachate to accelerate the decomposition changes the physical and engineering characteristic of waste and therefore affects the geotechnical characteristics of waste mass. The changes in the physical and mechanical characteristics of solid waste with time and decomposition are expected to affect the shear strength of waste mass. The objective of this paper is to analyze the stability of solid waste slopes within the bioreactor landfills, as a function of time and decomposition. The finite element program PLAXIS is used for numerical modeling of bioreactor landfills. Stability analysis of bioreactor landfills was also performed using limit equilibrium program STABL. Finally the results from finite element program PLAXIS and limit equilibrium program STABL are compared. GSTABL predicted a factor of safety of more than 1 in all the cases analyzed, whereas PLAXIS predicted a factor of safety of less than 1 at advanced stages for a slope of 2:1. However, the interface failures between solid waste and landfill liners have not been considered in this paper.     

A full-scale field study for performance evaluation of axially loaded large-diameter cylinder piles with pipe piles and PSC piles

This paper presents the results from a pile load testing program for a bridge construction project in Louisiana. The testing includes two 54-in. open-ended spun cast concrete cylinder piles, one 30-in. open-ended steel pile and two (30- and 16-in.) square prestressed concrete (PSC) piles driven at two locations with very similar soil conditions. Both cone penetration tests (CPTs) and soil borings/laboratory testing were used to characterize the subsurface soil conditions. All the test piles were instrumented with vibrating wire strain gauges to measure the load distribution along the length of the test piles and measure the skin friction and end-bearing capacity, separately. Dynamic load tests were performed on all test piles at different times after pile installations to quantify the amount of setup with time. Static load tests were also performed on the PSC and open-ended steel piles. Due to expected large pile capacities, the statnamic test method was used on the two open-ended cylinder piles. The pile capacities of these piles were evaluated using various CPT methods (such as Schmertmann, De Ruiter and Beringen, LCPC, Lehane et al. methods). The result showed that all the methods can estimate the skin friction with good accuracy, but not the end-bearing capacity. The normalized cumulative blow counts during pile installation showed that the blow count was always higher for the PSC piles compared to the large-diameter open-ended cylinder pile, regardless of pile size and hammer size. Setup was observed for all the piles, which was mainly attributed to increase in skin frictions. The setup parameters “A” were back-calculated for all the test piles and the values were between 0.31 and 0.41.     

Editorial

Editorial Introduction

Editorial     

Concentrations and speciation of arsenic along a groundwater flow-path in the Upper Floridan aquifer, Florida, USA

Arsenic (As) concentrations and speciation were determined in groundwaters along a flow-path in the Upper Floridan aquifer (UFA) to investigate the biogeochemical “evolution“ of As in this relatively pristine aquifer. Dissolved inorganic As species were separated in the field using anion-exchange chromatography and subsequently analyzed by inductively coupled plasma mass spectrometry. Total As concentrations are higher in the recharge area groundwaters compared to down-gradient portions of UFA. Redox conditions vary from relatively oxic to anoxic along the flow-path. Mobilization of As species in UFA groundwaters is influenced by ferric iron reduction and subsequent dissolution, sulfate reduction, and probable pyrite precipitation that are inferred from the data to occur along distinct regions of the flow-path. In general, the distribution of As species are consistent with equilibrium thermodynamics, such that arsenate dominates in more oxidizing waters near the recharge area, and arsenite predominates in the progressively reducing groundwaters beyond the recharge area.     

Arsenic Geochemistry of the Great Dismal Swamp,Virginia, USA: Possible Organic Matter Controls

Surface water samples for arsenic (As) concentration and speciation analysis were collected from organic matter-rich blackwaters of the Lake Drummond portion of the Great Dismal Swamp in southeastern Virginia, USA. Arsenic concentrations and speciation were determined by selective hydride generation, gas chromatography with photoionization detection. Surface waters from the Great Dismal Swamp are high in dissolved organic carbon (DOC) concentrations (445–9,600 μmol/kg) and of low pH (4.2–6.4). Total dissolved As concentrations [i.e., As(III) + As(V)], hereafter As_T, range from 2.2 nmol/kg to 21.4 nmol/kg. Arsenite, As(III), concentrations range from ∼1 nmol/kg to 17.7 nmol/kg, and As(V) ranges from ∼1 nmol/kg to 14.1 nmol/kg. Arsenate, As(V), is the predominant form of dissolved As in the inflow waters to the Great Dismal Swamp, whereas within the swamp proper arsenite, As(III), dominates. Arsenite accounts for 8–37% of As_T in inflow waters west of the Suffolk Scarp, and between 54% and 81% of As_T in Lake Drummond and Great Dismal Swamp waters east of the scarp. Arsenite is strongly correlated to DOC (r = 0.94) and inversely related to pH (r = −0.9), both at greater than the 99% confidence level. Arsenate is weakly related to pH and DOC (r = 0.4 and −0.37, respectively), and neither relationship is statistically significant. No statistical relationships exist between As(V) or As(III) and PO₄ concentrations. The predominance of As(III) and its strong correlation with DOC in Great Dismal Swamp waters suggest that DOC may inhibit As(III) adsorption or form stable aqueous complexes with As(III) in these waters. Alternatively, phytoplankton and/or bacterially mediated reduction of As(V) may be important processes in the organic-rich blackwaters and/or sediment porewaters of the swamp, leading to the prevalence of As(III) in the water column.     

Fatal landslides in Europe

Landslides are a major hazard causing human and large economic losses worldwide. However, the quantification of fatalities and casualties is highly underestimated and incomplete, thus, the estimation of landslide risk is rather ambitious. Hence, a spatio-temporal distribution of deadly landslides is presented for 27 European countries over the last 20  years (1995–2014). Catastrophic landslides are widely distributed throughout Europe, however, with a great concentration in mountainous areas. In the studied period, a total of 1370 deaths and 784 injuries were reported resulting from 476 landslides. Turkey showed the highest fatalities with 335. An increasing trend of fatal landslides is observed, with a pronounced number of fatalities in the latest period from 2008 to 2014. The latter are mostly triggered by natural extreme events such as storms (i.e., heavy rainfall), earthquakes, and floods and only minor by human activities, such as mining and excavation works. Average economic loss per year in Europe is approximately 4.7 billion Euros. This study serves as baseline information for further risk mapping by integrating deadly landslide locations, local land use data, and will therefore help countries to protect human lives and property.     

Assessment of Red Bed Groundwater in the Jinqu Basin,Southeastern China: Its Enrichment Regularity and Emergency Exploitation Potential

Natural Resources Research - Due to unbalanced spatial distribution and insufficient capacity allocation of water resources in the Jinqu Basin, developing and utilizing the red bed groundwater...     

A Novel Testing Apparatus for Hydromechanical Investigation of Rocks: Geo-Sequestration of Carbon dioxide

This paper presents the design, development and application of a new multi-phase high-pressure and elevated temperature rock hydromechanical testing apparatus for the investigation of reservoir and cap rock behaviour in carbon geo-sequestration projects. The triaxial apparatus is designed to support high confining stress, injection pressures and higher temperatures to imitate the natural thermo-hydro-geomechanical conditions of deep underground geological formations. The apparatus also includes an acoustic emission device for the study of the mechanical failure behaviour of rocks under compression. The apparatus is designed to support a range of different rock specimen sizes from 34 to 54?mm in diameter. Since sequestration projects involve the injection of supercritical carbon dioxide, which is extremely sensitive to temperature and pressure, and is highly corrosive in nature, special precautions were taken in the design and manufacture of the apparatus. The data acquisition system is powered and calibrated in accordance with each of the sensors and is guided by a series of in-house developed and commercial softwares for data storage and analysis. The methodology for conducting advanced testing on cap rock and reservoir rocks with the injection of water and supercritical CO₂ is presented with the appropriate theory. Some preliminary tests have been carried out on sandstone specimens sourced from the Melbourne region using the newly designed apparatus and the results are presented in this paper.