Probabilistic Assessment of Above Zone Pressure Predictions at a Geologic Carbon Storage Site.

Carbon dioxide (CO2) storage into geological formations is regarded as an important mitigation strategy for anthropogenic CO2 emissions to the atmosphere. This study first simulates the leakage of CO2 and brine from a storage reservoir through the caprock. Then, we estimate the resulting pressure changes at the zone overlying the caprock also known as Above Zone Monitoring Interval (AZMI). A data-driven approach of arbitrary Polynomial Chaos (aPC) Expansion is then used to quantify the uncertainty in the above zone pressure prediction based on the uncertainties in different geologic parameters. Finally, a global sensitivity analysis is performed with Sobol indices based on the aPC technique to determine the relative importance of different parameters on pressure prediction. The results indicate that there can be uncertainty in pressure prediction locally around the leakage zones. The degree of such uncertainty in prediction depends on the quality of site specific information available for analysis. The scientific results from this study provide substantial insight that there is a need for site-specific data for efficient predictions of risks associated with storage activities. The presented approach can provide a basis of optimized pressure based monitoring network design at carbon storage sites.

Predicting toxicity to Hyalella azteca in pyrogenic-impacted sediments-Do we need to analyze for all 34 PAHs?

Polycyclic aromatic hydrocarbons (PAHs) are major drivers of risk at many urban and/or industrialized sediment sites. The US Environmental Protection Agency (USEPA) currently recommends using measurements of 18 parent + 16 groups of alkylated PAHs (PAH-34) to assess the potential for sediment-bound PAHs to impact benthic organisms at these sites. ASTM Method D7363-13 was developed to directly measure low-level sediment porewater PAH concentrations. These concentrations are then compared to ambient water criteria (final chronic values [FCVs]) to assess the potential for impact to benthic organisms. The interlaboratory validation study that was used to finalize ASTM D7363-13 was developed using 24 of the 2-, 3-, and 4-ring PAHs (PAH-24) that are included in the USEPA PAH-34 analyte list. However, it is the responsibility of the user of ASTM Method D7363 to establish a test method to quantify the remaining 10 higher molecular weight PAHs that make up PAH-34. These higher molecular weight PAHs exhibit extremely low saturation solubilities that make their detection difficult in porewater, which has proven difficult to implement in a contract laboratory setting. As a result, commercial laboratories are hesitant to conduct the method on the entire PAH-34 analyte list. This article presents a statistical comparison of the ability of the PAH-24 and PAH-34 porewater results to predict survival of the freshwater amphipod Hyalella azteca, using the original 269 sediment samples used to gain ASTM D7363 Method approval. The statistical analysis shows that the PAH-24 are statistically indistinguishable from the PAH-34 for predicting toxicity. These results indicate that the analysis of freely dissolved porewater PAH-24 is sufficient for making risk-based decisions based on benthic invertebrate toxicity (survival and growth). This reduced target analyte list should result in a cost-saving for stakeholders and broader implementation of the method at PAH-impacted sediment sites. Integr Environ Assess Manag 2016;12:493-499. © 2015 SETAC.

Quantifying the benefit of wellbore leakage potential estimates for prioritizing long-term MVA well sampling at a CO2 storage site.

This work uses probabilistic methods to simulate a hypothetical geologic CO2 storage site in a depleted oil and gas field, where the large number of legacy wells would make it cost-prohibitive to sample all wells for all measurements as part of the postinjection site care. Deep well leakage potential scores were assigned to the wells using a random subsample of 100 wells from a detailed study of 826 legacy wells that penetrate the basal Cambrian formation on the U.S. side of the U.S./Canadian border. Analytical solutions and Monte Carlo simulations were used to quantify the statistical power of selecting a leaking well. Power curves were developed as a function of (1) the number of leaking wells within the Area of Review; (2) the sampling design (random or judgmental, choosing first the wells with the highest deep leakage potential scores); (3) the number of wells included in the monitoring sampling plan; and (4) the relationship between a well's leakage potential score and its relative probability of leakage. Cases where the deep well leakage potential scores are fully or partially informative of the relative leakage probability are compared to a noninformative base case in which leakage is equiprobable across all wells in the Area of Review. The results show that accurate prior knowledge about the probability of well leakage adds measurable value to the ability to detect a leaking well during the monitoring program, and that the loss in detection ability due to imperfect knowledge of the leakage probability can be quantified. This work underscores the importance of a data-driven, risk-based monitoring program that incorporates uncertainty quantification into long-term monitoring sampling plans at geologic CO2 storage sites.

Comparison of alkaline industrial wastes for aqueous mineral carbon sequestration through a parallel reactivity study.

Thirty-one alkaline industrial wastes from a wide range of industrial processes were acquired and screened for application in an aqueous carbon sequestration process. The wastes were evaluated for their potential to leach polyvalent cations and base species. Following mixing with a simple sodium bicarbonate solution, chemistries of the aqueous and solid phases were analyzed. Experimental results indicated that the most reactive materials were capable of sequestering between 77% and 93% of the available carbon under experimental conditions in four hours. These materials - cement kiln dust, spray dryer absorber ash, and circulating dry scrubber ash - are thus good candidates for detailed, process-oriented studies. Chemical equilibrium modeling indicated that amorphous calcium carbonate is likely responsible for the observed sequestration. High variability and low reactive fractions render many other materials less attractive for further pursuit without considering preprocessing or activation techniques.

Comparison of polymeric samplers for accurately assessing PCBs in pore waters.

Assessing the hazard posed by sediments contaminated with hydrophobic organic compounds is difficult, because measuring the freely dissolved porewater concentrations of such low-solubility chemicals can be challenging, and estimating their sediment-water partition coefficients remains quite uncertain. We suggest that more accurate site assessments can be achieved by employing sampling devices in which polymers, with known polymer-water partition coefficients, are used to absorb the contaminants from the sediment. To demonstrate the current accuracy and limitations of this approach, we compared use of three polymers, polydimethylsiloxane, polyoxymethylene, and polyethylene, exposed to a single sediment in two modes, one in which they were exhaustively mixed (tumbled) with the sediment and the other in which they were simply inserted into a static bed (passive). Comparing porewater concentrations of specific polychlorinated biphenyl (PCB) congeners with results obtained using air bridges, we found the results for tumbled polymers agreed within 20%, and the passive sampling agreed within a factor of 2. In contrast, porewater estimates based on sediment concentrations normalized to f(OC)K(OC), the weight fraction of organic carbon times the organic-carbon normalized partition coefficient, averaged a factor of 7 too high. We also found good correlations of each polymer's uptake of the PCBs with bioaccumulation by the polychaete, Neanthes arenaceodentata. Future improvements of the passive sampling mode will require devices that equilibrate faster and/or have some means such as performance reference compounds to estimate mass transfer limitations for individual deployments.

An evaluation of the ability of chemical measurements to predict polycyclic aromatic hydrocarbon-contaminated sediment toxicity to Hyalella azteca.

The present study examined the ability of three chemical estimation methods to predict toxicity and nontoxicity of polycyclic aromatic hydrocarbon (PAH) -contaminated sediment to the freshwater benthic amphipod Hyalella azteca for 192 sediment samples from 12 field sites. The first method used bulk sediment concentrations of 34 PAH compounds (PAH34), and fraction of total organic carbon, coupled with equilibrium partitioning theory to predict pore-water concentrations (KOC method). The second method used bulk sediment PAH34 concentrations and the fraction of anthropogenic (black carbon) and natural organic carbon coupled with literature-based black carbon-water and organic carbon-water partition coefficients to estimate pore-water concentrations (KOCKBC method). The final method directly measured pore-water concentrations (pore-water method). The U.S. Environmental Protection Agency's hydrocarbon narcosis model was used to predict sediment toxicity for all three methods using the modeled or measured pore-water concentration as input. The KOC method was unable to predict nontoxicity (83% of nontoxic samples were predicted to be toxic). The KOCKBC method was not able to predict toxicity (57% of toxic samples were predicted to be nontoxic) and, therefore, was not protective of the environment. The pore-water method was able to predict toxicity (correctly predicted 100% of the toxic samples were toxic) and nontoxicity (correctly predicted 71% of the nontoxic samples were nontoxic). This analysis clearly shows that direct pore-water measurement is the most accurate chemical method currently available to estimate PAH-contaminated sediment toxicity to H. azteca.

Improving risk assessments for manufactured gas plant soils by measuring PAH availability.

Remediation of soils at oil-gas manufactured gas plant (MGP) sites is driven primarily by the human health risks posed by the carcinogenic polycyclic aromatic hydrocarbons (PAHs), particularly benzo[a]pyrene (BaP), that are associated with lampblack residues. Although PAHs on lampblack are tightly sorbed, risk assessments do not account for this reduced availability. A multi-investigator study of 7 oil-gas MGP site soil samples demonstrated that the dermal and ingestion absorption factors are far lower than current default assumptions used in risk assessments. Using these sample-specific absorption factors in standard risk assessment equations increased risk-based cleanup levels by a factor of 72 on average (with a range from 23 to 142 times the default level). The rapidly released fraction of the BaP in each sample, as measured by supercritical fluid extraction, was closely correlated (r2 = 0.96) to these calculated cleanup levels. The weight of evidence developed during this research indicates that the risks posed by PAHs on lampblack are far less than assumed when using default absorption factors and that a tiered evaluation protocol employing chemical analyses, chemical release data, and in vitro bioassays can be used to establish more realistic site-specific criteria.