Quantitative information regarding the endmember composition of the gas and oil that flowed from the Macondo well during the Deepwater Horizon oil spill is essential for determining the oil flow rate, total oil volume released, and trajectories and fates of hydrocarbon components in the marine environment. Using isobaric gas-tight samplers, we collected discrete samples directly above the Macondo well on June 21, 2010, and analyzed the gas and oil. We found that the fluids flowing from the Macondo well had a gas-to-oil ratio of 1,600 standard cubic feet per petroleum barrel. Based on the measured endmember gas-to-oil ratio and the Federally estimated net liquid oil release of 4.1 million barrels, the total amount of C 1 -C 5 hydrocarbons released to the water column was 1.7 × 10 11 g. The endmember gas and oil compositions then enabled us to study the fractionation of petroleum hydrocarbons in discrete water samples collected in June 2010 within a southwest trending hydrocarbon-enriched plume of neutrally buoyant water at a water depth of 1,100 m. The most abundant petroleum hydrocarbons larger than C 1 -C 5 were benzene, toluene, ethylbenzene, and total xylenes at concentrations up to 78 μg L −1 . Comparison of the endmember gas and oil composition with the composition of water column samples showed that the plume was preferentially enriched with water-soluble components, indicating that aqueous dissolution played a major role in plume formation, whereas the fates of relatively insoluble petroleum components were initially controlled by other processes.Gulf of Mexico | subsurface plumes D uring the 3 mo following April 20, 2010, the Macondo well emitted several million barrels of gas and oil at the seafloor of the Gulf of Mexico following the sinking of the Deepwater Horizon drilling platform. Relative to oil spills occurring at the sea surface, petroleum hydrocarbons experienced a unique set of processes following their release at 1.5-km depth (1-4). This spill demonstrates the importance of interwoven chemical, physical, and biological processes in regulating the transport and fate of hydrocarbons in the deep marine environment. Compositional information for petroleum (gas and oil) released by the well at the seafloor is essential for evaluating the fates of hydrocarbons in the sea. Moreover, such information provides direct constraints on estimates of the total mass of individual hydrocarbons released to the environment and the flow rates at the site of the spill (5). Gases are of particular interest because the gas fraction represents a large component of the carbon released, and it was biodegraded rapidly in the water column (3, 4). Compositional data for released oil is also necessary for forensic analyses when distinguishing Macondo well oil from hydrocarbons released from other sources in the Gulf of Mexico.Numerous studies have examined factors that influence the compositional evolution of oil spilled at the sea surface (6-10), where evaporation and dissolution may simultaneously remove hydrocarbons fr...

Batch equilibrations were performed to investigate the ability of hydroxyapatite (Ca5(PO4)3OH) to chemically immobilize U in two contaminated sediment samples having different organic carbon contents (123 and 49 g kg(-1), respectively). Apatite additions lowered aqueous U to near proposed drinking water standards in batch equilibrations of two distinct sediment strata having total U concentrations of 1703 and 2100 mg kg(-1), respectively. Apatite addition of 50 g kg(-1) reduced the solubility of U to values less than would be expected if autunite (Ca(UO2)2(PO4)2·10H2O) was the controlling solid phase. A comparison of the two sediment types suggests that aqueous phase U may be controlled by both the DOC content through complexation and the equilibrium pH for a given apatite application rate. Sequential chemical extractions demonstrated that apatite amendment transfers U from more chemically labile fractions, including water-soluble, exchangeable, and acid-soluble (pH ≈ 2.55) fractions, to the Mn-occluded fraction (pH ≈ 1.26). This suggests that apatite amendment redirects solid-phase speciation with secondary U phosphates being solubilized due to the lower pH of the Mn-occluded extractant, despite the lack of significant quantities of Mn oxides within these sediments. Energy dispersive X-ray (EDX) analysis conducted in a transmission electron microscope (TEM) confirmed that apatite amendment sequesters some U in secondary Al/Fe phosphate phases.

During the disaster, a substantial fraction of the 600,000-900,000 tons of released petroleum liquid and natural gas became entrapped below the sea surface, but the quantity entrapped and the sequestration mechanisms have remained unclear. We modeled the buoyant jet of petroleum liquid droplets, gas bubbles, and entrained seawater, using 279 simulated chemical components, for a representative day (June 8, 2010) of the period after the sunken platform's riser pipe was pared at the wellhead (June 4-July 15). The model predicts that 27% of the released mass of petroleum fluids dissolved into the sea during ascent from the pared wellhead (1,505 m depth) to the sea surface, thereby matching observed volatile organic compound(VOC) emissions to the atmosphere. Based on combined results from model simulation and water column measurements, 24% of released petroleum fluid mass became channeled into a stable deep-water intrusion at 900- to 1,300-m depth, as aqueously dissolved compounds (∼23%) and suspended petroleum liquid microdroplets (∼0.8%). Dispersant injection at the wellhead decreased the median initial diameters of simulated petroleum liquid droplets and gas bubbles by 3.2-fold and 3.4-fold, respectively, which increased dissolution of ascending petroleum fluids by 25%. Faster dissolution increased the simulated flows of water-soluble compounds into biologically sparse deep water by 55%, while decreasing the flows of several harmful compounds into biologically rich surface water. Dispersant injection also decreased the simulated emissions of VOCs to the atmosphere by 28%, including a 2,000-fold decrease in emissions of benzene, which lowered health risks for response workers.

Quantum chemical implicit solvent models are used widely to estimate aqueous redox potentials. We compared the accuracy of several popular implicit solvent models (SM8, SMD, C-PCM, IEF-PCM, and COSMO-RS) for the prediction of aqueous single electron oxidation potentials of a diverse test set of neutral organic compounds for which accurate experimental oxidation potential and gas-phase ionization energy data are available. Using a thermodynamic cycle, we decomposed the free energy of oxidation into contributions arising from the gas-phase adiabatic ionization energy, the solvation free energy of the closed-shell neutral species, and the solvation free energy of the radical cation species. For aqueous oxidation potentials, implicit solvent models exhibited mean unsigned errors (MUEs) ranging from 0.27 to 0.50 V, depending on the model. The principal source of error was attributed to the computed solvation free energy of the oxidized radical cation. Based on these results, a recommended implicit solvation approach is the SMD model for the solvation free energy combined with CBS-QB3 for the gas-phase ionization energy. With this approach, the MUE in computed oxidation potentials was 0.27 V, and the MUE in solvation free energy of the charged open-shell species was 0.32 eV. This baseline assessment provides a compiled benchmark test set of vetted experimental data that may be used to judge newly developed solvation models for their ability to produce improved predictions for aqueous oxidation potentials and related properties.

Comprehensive two-dimensional gas chromatography (GCxGC) provides nearly complete composition data for some complex mixtures such as petroleum hydrocarbons. However, the potential wealth of physical property information contained in the corresponding two-dimensional chromatograms is largely untapped. We developed a simple but robust method to estimate GCxGC retention indices for diesel-range hydrocarbons. By exploiting n-alkanes as reference solutes in both dimensions, calculated retention indices were insensitive to uncertainty in the enthalpy of gas-stationary-phase transfer for a suite of representative diesel components. We used the resulting two-dimensional retention indices to estimate the liquid vapor pressures, aqueous solubilities, air-water partition coefficients, octanol-water partition coefficients, and vaporization enthalpies of a nearly complete set of diesel fuel hydrocarbons. Partitioning properties were typically estimated within a factor of 2; this is not as accurate as some previous estimation or measurement methods. However, these relationships may allow powerful and incisive analysis of phase-transfer processes affecting petroleum hydrocarbon mixtures in the environment. For example, GCxGC retention data might be used to quantitatively deconvolve the effects of water washing and evaporation on environmentally released diesel fuels.

HOBr, formed via oxidation of bromide by free available chlorine (FAC), is frequently assumed to be the sole species responsible for generating brominated disinfection byproducts (DBPs). Our studies reveal that BrCl, Br(2), BrOCl, and Br(2)O can also serve as brominating agents of the herbicide dimethenamid in solutions of bromide to which FAC was added. Conditions affecting bromine speciation (pH, total free bromine concentration ([HOBr](T)), [Cl(-)], and [FAC](o)) were systematically varied, and rates of dimethenamid bromination were measured. Reaction orders in [HOBr](T) ranged from 1.09 (±0.17) to 1.67 (±0.16), reaching a maximum near the pK(a) of HOBr. This complex dependence on [HOBr](T) implicates Br(2)O as an active brominating agent. That bromination rates increased with increasing [Cl(-)], [FAC](o) (at constant [HOBr](T)), and excess bromide (where [Br(-)](o)>[FAC](o)) implicate BrCl, BrOCl, and Br(2), respectively, as brominating agents. As equilibrium constants for the formation of Br(2)O and BrOCl (aq) have not been previously reported, we have calculated these values (and their gas-phase analogues) using benchmark-quality quantum chemical methods [CCSD(T) up to CCSDTQ calculations plus solvation effects]. The results allow us to compute bromine speciation and hence second-order rate constants. Intrinsic brominating reactivity increased in the order: HOBr ≪ Br(2)O < BrOCl ≈ Br(2) < BrCl. Our results indicate that species other than HOBr can influence bromination rates under conditions typical of drinking water and wastewater chlorination.

ABSTRACTnah River Site (SRS) near Aiken, SC, is a prime example, having been contaminated with a variety of metals. proximately 5.3 ha, and the potential to drastically disturb the Upper Three Runs/Tims Branch stream system and possibly enhance contaminant transport due to ero-

Biodegradation plays a major role in the natural attenuation of oil spills. However, limited information is available about biodegradation of different saturated hydrocarbon classes in surface environments, despite that oils are composed mostly of saturates, due to the limited ability of conventional gas chromatography (GC) to resolve this compound group. We studied eight weathered oil samples collected from four Gulf of Mexico beaches 12-19 months after the Deepwater Horizon disaster. Using comprehensive two-dimensional gas chromatography (GC × GC), we successfully separated, identified, and quantified several distinct saturates classes in these samples. We find that saturated hydrocarbons eluting after n-C22 dominate the GC-amenable fraction of these weathered samples. This compound group represented 8-10%, or 38-68 thousand metric tons, of the oil originally released from Macondo well. Saturates in the n-C22 to n-C29 elution range were found to be partly biodegraded, but to different relative extents, with ease of biodegradation decreasing in the following order: n-alkanes > methylalkanes and alkylcyclopentanes+alkylcyclohexanes > cyclic and acyclic isoprenoids. We developed a new quantitative index designed to characterize biodegradation of >n-C22 saturates. These results shed new light onto the environmental fate of these persistent, hydrophobic, and mostly overlooked compounds in the unresolved complex mixtures (UCM) of weathered oils.