Ming-Kuo Lee

Occurrence of arsenic in core sediments and groundwater in the Chapai-Nawabganj District, northwestern Bangladesh.

Groundwater and core sediments of two boreholes (to a depth of 50m) from the Chapai-Nawabganj area in northwestern Bangladesh were collected for arsenic concentration and geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10-40m of depth) range from 2.8microgL(-1) to 462.3microgL(-1). Groundwater geochemical conditions change from oxidized to successively more reduced, higher As concentration with depth. Higher sediment arsenic levels (55mgkg(-1)) were found within the upper 40m of the drilled core samples. X-ray absorption near-edge structure spectroscopy was employed to elucidate the arsenic speciation of sediments collected from two boreholes. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. In addition, a pH-Eh diagram was drawn using the Geochemists Workbench (GWB) software to elucidate the arsenic speciation in groundwater. The dominant groundwater type is Ca-HCO(3) with high concentrations of As, Fe and Mn but low levels of NO(3)(-) and SO(4)(2-). Sequential extraction analysis reveals that Mn and Fe hydroxides and organic matter are the major leachable solids carrying As. High levels of arsenic concentration in aquifers are associated with fine-grained sediments. Fluorescent intensities of humic substances indicate that both groundwater and sediments in this arsenic hotspot area contain less organic matter compared to other parts of Bengal basin. Statistical analysis clearly shows that As is closely associated with Fe and Mn in sediments while As is better correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing arsenic into the groundwater.

Level and Degradation of Deepwater Horizon Spilled Oil in Coastal Marsh Sediments and Pore-Water

This research investigates the level and degradation of oil at ten selected Gulf saltmarsh sites months after the 2010 BP Macondo-1 well oil spill. Very high levels (10-28%) of organic carbon within the heavily oiled sediments are clearly distinguished from those in pristine sediments (<3%). Dissolved organic carbon in contaminated pore-waters, ranging up to hundreds of mg/kg, are 1 to 2 orders of magnitude higher than those at pristine sites. Heavily oiled sediments are characterized by very high sulfide concentrations (up to 80 mg/kg) and abundance of sulfate reducing bacteria. Geochemical biomarkers and stable carbon isotope analyses fingerprint the presence of oils in sediments. Ratios of selected parameters calculated from the gas chromatograph spectra are in a remarkable narrow range among spilled oils and initial BP crude. At oiled sites dominated by C(4) plants, δ(13)C values of sediments (-20.8 ± 2.0‰) have been shifted significantly lower compared to marsh plants (-14.8 ± 0.6‰) due to the inflow of isotopically lighter oil (-27 ± 0.2‰). Our results show that (1) lighter compounds of oil are quickly degraded by microbes while the heavier fractions of oil still remain and (2) higher inputs of organic matter from the oil spill enhance the key microbial processes associated with sulfate reducing bacteria.

CHARACTERIZATION OF COPPER BIOREDUCTION AND BIOSORPTION BY A HIGHLY COPPER RESISTANT BACTERIUM ISOLATED FROM COPPER-CONTAMINATED VINEYARD SOIL

Copper is an essential but toxic heavy metal that negatively impacts living systems at high concentration. This study presents factors affecting copper bioremoval (bioreduction and biosorption) by a highly copper resistant monoculture of Pseudomonas sp. NA and copper bioremoval from soil. Seven bacteria resistant to high concentration of Cu(II) were isolated from enrichment cultures of vineyard soils and mining wastes. Culture parameters influencing copper bioreduction and biosorption by one monoculture isolate were studied. The isolate was identified by 16S rRNA gene sequence analysis as a Pseudomonas sp. NA (98% similarity to Pseudomonas putida, Pseudomonas plecoglossicida and other Pseudomonas sp.). The optimal temperature for growth was 30 degrees C and bioremoval of Cu(II) was maximal at 35 degrees C. Considerable growth of the isolate was observed between pH 5.0 and 8.0 with the highest growth and biosorption recorded at pH 6.0. Maximal bioreduction was observed at pH 5.0. Cu(II) bioremoval was directly proportional to Cu(II) concentration in media. Pseudomonas sp. NA removed more than 110mg L(-1) Cu(II) in water within 24h through bioreduction and biosorption at initial concentration of 300mg L(-1). In cultures amended with 100mg L(-1), 20.7mg L(-1) of Cu(II) was biologically reduced and more than 23mg L(-1) of Cu(II) was biologically removed in 12h. The isolate strongly promoted copper bioleaching in soil. Results indicate that Pseudomonas sp. NA has good potential as an agent for removing copper from water and soil.

Implications of organic matter on arsenic mobilization into groundwater: Evidence from northwestern (Chapai-Nawabganj), central (Manikganj) and southeastern (Chandpur) Bangladesh

Boreholes (50 m depth) and piezometers (50 m depth) were drilled and installed for collecting As-rich sediments and groundwater in the Ganges, Brahmaputra, and Meghna flood plains for geochemical analyses. Forty-one groundwater samples were collected from the three areas for the analyses of cations (Ca(2+), Mg(2+), K(+), Na(+)), anions (Cl(-), NO(3)(-), SO(4)(2-)), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) were performed to characterize the major mineral and chemical contents of aquifer sediments. In all three study areas, results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescent intensity of humic substances in groundwater samples ranges from 30 to 102 (mean 58 ± 20, n = 20), 54-195 (mean 105 ± 48, n = 10), and 27-243 (mean 79 ± 71, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Arsenic concentration in groundwater (20-50 m of depth) ranges from 3 to 315 μg/L (mean 62.4 ± 93.1 μg/L, n = 20), 16.4-73.7 μg/L (mean 28.5 ± 22.4 μg/L, n = 10) and 4.6-215.4 μg/L (mean 30.7 ± 62.1 μg/L, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Specific ultra violet adsorption (SUVA) values (less than 3 m(-1) mg(-1) L) indicate that the groundwater in the Ganges flood plain has relatively low percentage of aromatic organic carbon compared to those in the Brahmaputra and Meghna flood plains. Arsenic content in sediments ranges from 1 to 11 mg/kg (mean 3.5 ± 2.7 mg/kg, n = 17) in the three flood plains. Total organic carbon content is 0.5-3.7 g/kg (mean 1.9 ± 1.1 g/kg) in the Ganges flood plain, 0.5-2.1 g/kg (mean: 1.1 ± 0.7 g/kg) in the Brahmaputra flood plain and 0.3-4.4 g/kg (mean 1.9 ± 1.9 g/kg) in the Meghna flood plain. Arsenic is positively correlated with TOC (R(2) = 0.50, 0.87, and 0.85) in sediments from the three areas. Fourier transform infrared (FT-IR) analysis of the sediments revealed that the functional groups of humic substances in three areas include amines, phenol, alkanes, and aromatic carbon. Arsenic and Fe speciation in sediments were determined using XANES and the results imply that As(V) and Fe(III) are the dominant species in most sediments. The results also imply that As (V) and Fe (III) in most of the sediment samples of the three areas are the dominant species. X-ray absorption fine structure (EXAFS) analysis shows that FeOOH is the main carrier of As in the sediments of three areas. In sediments, As is well correlated with Fe and Mn. However, there is no such correlation observed between As and Fe as well as As and Mn in groundwater, implying that mobilizations of Fe, Mn, and As are decoupled or their concentrations in groundwater have been affected by other geochemical processes following reductive dissolution of Fe or Mn-hydroxides. For example, dissolved Fe and Mn levels may be affected by precipitation of Fe- and Mn-carbonate minerals such as siderite, while liberated As remains in groundwater. The groundwaters of the Brahmaputra and Meghna flood plains contain higher humic substances in relative fluorescence intensity (or fluorescence index) and lower redox potential compared to the groundwater of Ganges flood plain. This leads to the release of arsenic and iron to groundwater of these three plains in considerable amounts, but their concentrations are distributed in spatial variations.

Effects of pH on Metals Precipitation and Sorption: Field Bioremediation and Geochemical Modeling Approaches

At the Sanders Lead car-battery recycling plant, near Troy, AL, groundwater is highly acidic (pH varies from 3 to 3.5) and carries high concentrations of Pb, Cd, Zn, Cu, and Fe. Pilot field experiments conducted at the site show that in situ metabolism of sulfate reducing bacteria (SRB) can produce desired geochemical effects to remove heavy metal Pb, Cd, Zn, and Cu from groundwater. A reaction path model of sulfate reduction shows the redox potential (Eh) effects on mineral precipitation and pH controls on the sorption of different metals. Lead strongly adsorbs to hydrous ferric oxide (HFO) present in the aquifer over a wide pH range. Both sorption (due to pH increase) and solid sulfide formation are important for removing Pb. Although theoretical modeling shows that the sorption of most cations is promoted as pH increases, HFO can only scavenge Zn, Cd, Co, and Ni at relatively neutral pH conditions. Thus concentrations of our primary contaminants Zn and Cd attenuate in acidic conditions primarily via precipitation or coprecipitation of solid sulfide phase as Eh drops. The modeling result explains why the Pb plume is retarded in migration with respect to the Cd plume under the low-pH conditions at the site. For As, arsenate sorbs strongly onto the protonated weak sites of ferric hydroxide for the pH range of calculation. Arsenite sorption is also favored by increasing pH, however, arsenite desorbs and becomes mobilized at very low oxidation state as it reacts with dissolved sulfide to form AsS complexes. In addition, intermittent rainfall events could cause short-term Eh increases, potentially leading to oxidation of sulfide solids and subsequent pH decrease, and the remobilization of metals. This study argues for the importance of accounting for pH changes when evaluating the fate, transport, and long-term stability of metals at shallow contaminated sites.

Effects of inorganic nutrient levels on the biodegradation of benzene, toluene, and xylene (BTX) by Pseudomonas spp. in a laboratory porous media sand aquifer model

The effect of inorganic nutrients (sulfate, phosphate, and ammonium chloride) on the aerobic biodegradation of benzene, toluene, and xylene (BTX) by Pseudomonas spp. was studied in the laboratory using a glass sand tank. The increase of nutrient levels resulted in enhanced bacterial growth and BTX degradation. Sulfate and phosphate serve as key electron acceptors in the microbiological processes degrading BTX. The observed bacterial morphological changes during BTX degradation reveal that the filamentous bacteria were the dominant species at low temperatures about 20 degrees C. The spherical and rod-shaped cells became dominant at higher temperatures ranging from 25 degrees C to 28 degrees C. When the BTX mixture was allowed to be biodegraded for longer incubation periods of 21-42 h at high phosphate concentrations, large amounts of rod-shaped cells were clustered. The morphological adaptation appears to be controlled by the temperature and nutrient levels in the sandy medium where Pseudomonas spp. thrives.

Paleohydrology of the Delaware Basin, Western Texas: Overpressure Development, Hydrocarbon Migration, and Ore Genesis

This study integrates quantitative modeling techniques with field observations to establish a paleohydrologic framework of the Delaware basin, western Texas. The reconstructed paleohydrologic models allow for a better understanding of the development and maintenance of anomalous overpressures, hydro carbon generation and migration, and ore genesis in the basin. Results of numerical modeling show that disequilibrium compaction and oil generation might generate excess fluid pressures during the Late Permian in response to the rapid deposition of evaporite beds. The preservation of this overpressure to the present, however, requires the presence of an extremely low-permeability (<10-11 d) top seal. Most shaly sediments, with permeability ranging from 10-4 to 10-8 d, thus may be too permeable, by several orders of magnitude, to preserve overpressure for more than 250 m.y. The predicted present-day gas window is located within the overpressure zone, suggesting that the volume increase associated with the oil-to-gas conversion may be attributed to present overpressures. The native sulfur deposits likely formed in a fluid mixing zone resulting from the Laramide uplift of the western basin during the Tertiary. In our model, meteoric water recharged along the basins uplifted western margin and discharged basinward. Hydrocarbons migrated landward by pressure gradients and buoyancy and discharged upward along faults in the western basin, where they mixed with meteoric water. Many oil and mineral reservoirs may have formed in the fluid mixing zone, where extensive chemical reactions take place. In the Culberson sulfur ore district, for example, fluids including hydrocarbons and meteoric water migrated upward through faults from underlying carrier beds, into the Permian Salado limestone. There, the mixture of fluid drives biochemical reactions that precipitate native sulfur.

Effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development: a numerical study

This study outlines numerical experiments to investigate the effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development in evolving sedimentary basins. The model integrates predicted groundwater flow and temperature and pressure distribution with thermal maturation simulations. The programme uses the Arrhenius kinetic model to simulate the kerogen–oil or oil–gas conversion processes. Such conversion processes result in an increase in fluid volume and overpressure development since oil and gas generated are less dense than their precursors. The model integrates an equation of state to calculate gas densities for the CH4–CO2–H2O system over a wide temperature–pressure (T–P) range expected in sedimentary basins; this approach allows for prediction of the rate of pore volume increases and fluid pressure changes due to gas generation. Sample calculations of compaction of kerogen-rich shales in the Delaware Basin shed light on the magnitudes of overpressures created by hydrocarbon generation from the Late Pennsylvanian to Middle Permian. Oil generation can cause excess pore pressure (c. 425 atm) up to c. 40% of that generated by compaction only (c. 300 atm). Oil and CH4 gas generation together yield the maximum excess pressure (c. 750 atm) up to about 150% of that generated by compaction only. There is much greater pore pressure build-up from oil to CH4 conversion (c. 325 atm) than oil to CO2 conversion (c. 75 atm) because density of CH4 gas is less than that of CO2 under the same P and T conditions. Sensitivity analyses also show that lower activation energy and higher pre-exponential factor lead to faster thermal cracking that allows oil or gas to reach peak generation earlier. Moreover, a basin experiencing a high heat flow throughout the burial history reaches hydrocarbon generation and overpressure development earlier. Calculation results also show that the oil and gas windows become deeper as the sedimentation rate increases. Thus, a basin experiencing high sedimentation rates would exhibit higher levels of thermal maturity and excess pore pressure over the deeper section. This also implies that greater overpressure may be expected at shallower depths in a basin with relatively low sedimentation rates. The modelling results demonstrate that kinetic parameters, basal heat flow and sedimentation rates all influence the timing, duration and depth of oil and gas generation, which in turn, profoundly affects the spatial and temporal distribution of overpressure.

Analysis of fluid pressure propagation in heterogeneous rocks: Implications for hydrologically-induced earthquakes

Mathematical models are developed to study the propagation of excess pore pressure in heterogeneous and fractured rocks. For a homogeneous rock, the time needed for a pore pressure front to migrate downward is directly proportional to the square of the depth, and inversely proportional to the permeability of the rock. Variational studies show that pore pressure propagation is highly influenced by the permeability heterogeneity of the crust. The results argue for the importance of accounting for geologic complexity when using mathematical models to estimate the extent of downward transmission of increases in hydraulic heads. The models presented in the paper are used to estimate a range of possible hypocentral depths of periodic seismicity observed near Mt. Ogden on the Alaska-British Columbia border. The time lag between these earthquakes and hydrologic loading is on the order of days or weeks, indicating a quick response of seismicity to the increased surface water input from rainfall or glacial discharge, if such a causal relationship exists. Our models estimate that the hypocentral depths of these earthquakes could be on the order of several kilometers, if a high degree of vertical interconnectivity of fractures exists.

Characterisation of organic matter associated with groundwater arsenic in reducing aquifers of southwestern Taiwan

Arsenic (As) in groundwaters extensively used by people across the world constitutes a serious public health threat. The importance of organic matter (OM) as an electron donor in microbially-mediated reduction of As(V) or Fe(III)-bearing As-host minerals leading to mobilisation of solid-phase arsenic is widely recognised. Notwithstanding this, there are few studies characterising OM in such aquifers and, in particular, there is a dearth of data from the classic arsenic bearing aquifers in southwestern Taiwan. Organic geochemical analyses of sediments from a known groundwater arsenic hot-spot in southwestern Taiwan revealed contributions of thermally mature and plant derived origin, consistent with OM sources in all other Asian groundwater aquifer sediments analysed to date, indicating comparable sources and routes of OM transfer. The combined results of amended As(V) reduction assays with the organic geochemical analysis revealed that the microbiological process of dissimilatory As(V) reduction is active in this aquifer, but it is not controlled by a specific source of analysed OM. These indicate that (i) part of the OM that was considered to be less bio-available could still be used as an electron donor or (ii) other electron donors, not analysed in present study, could be controlling the rate of As release.