E. Eric Adams

Field study of dispersion in a heterogeneous aquifer: 2. Spatial moments analysis

Analysis is performed of a 20-month natural gradient tracer study in the saturated zone of a highly heterogeneous aquifer. Graphical presentation of concentration distributions versus time and spatial moments analysis reveal dramatically non-Gaussian behavior and a systematic mass loss. Implications of the mass loss on plume moments is analyzed through sensitivity studies. The moments data are interpreted by applying two simple models: (1) pure advection from a continuous source, and (2) advection plus dispersion in a converging nonuniform flow field. A longitudinal dispersivity of 5–10 m is estimated from the latter model and is somewhat larger than the value of about 1.5 m calculated by Rehfeldt et al. (this issue) using the stochastic theory of Gelhar and Axness (1983) based on independent measurements of the spatial variation of hydraulic conductivity. The dispersivity of 5–10 m is an order of magnitude larger than values measured at recently studied field sites (Borden and Cape Cod) with less heterogeneity, but an order of magnitude lower than would be computed from the moments data if the flow is presumed to be uniform.

Field study of dispersion in a heterogeneous aquifer: 1. Overview and site description

Results are presented for a large-scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near Columbus, Mississippi. The study was initiated with a 48-hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers (pentaflourobenzoic acid, o-trifluoromethylbenzoic acid, and 2,6-diflourobenzoic acid). Over a 20-month period, seven comprehensive samplings of the tracer plume were performed at approximately 1- to 4-month intervals using an extensive three-dimensional sampling well network. The dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction. This asymmetry was produced by accelerating groundwater flow along the plume travel path that, in turn, resulted from an approximate 2-order-of-magnitude increase in the mean hydraulic conductivity between the near-field and far-field regions of the site. The Columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer, the large-scale spatial variations in groundwater velocity, and the extensive set of hydraulic conductivity measurements for the aquifer.

Modeling the release of CO2 in the deep ocean

In order to better understand the mechanics of ocean disposal of CO2 captured from power plants, a comprehensive plume model was developed to simulate the dynamic, near-field behavior of CO2 released in the water column as either a buoyant liquid or vapor. The key design variables in the model that can be controlled are: (1) release depth, zo (2) number of diffuser ports, N, and (3) initial bubble or droplet radius, ro. For a CO2 stream from a 500 MW power plant with 100% capture and zo=500 m, N=10, and ro=1 cm, the model predicts that the plume will rise less than 100 m. This will result in CO2 enrichment at depths greater than 400 m. Detailed predictions of local CO2 concentrations near the plume are presented and discussed. The issue of the residence time of the captured CO2 in the ocean is also addressed. We estimate a typical residence time of less than 50 years for releases of CO2 less than 500 m deep and, for a release depth of 1000 m, a residence time from 200 to 300 years. These residence times may be increased by releasing in areas of downwelling or by forming solid CO2-hydrates, which can sink to the ocean floor.

Impacts of ocean CO2 disposal on marine life: I. A toxicological assessment integrating constant‐concentration laboratory assay data with variable‐concentration field exposure

Feasibility studies suggest that the concept of capturing CO2 from fossil fuel power plants and discharging it to the deep ocean could help reduce atmospheric CO2 concentrations. However, the local reduction in seawater pH near the point of injection is a potential environmental impact. Data from the literature reporting on toxicity of reduced pH to marine organisms potentially affected by such a plume were combined into a model expressing mortality as a function of pH and exposure time. Since organisms exposed to real plumes would experience a time‐varying pH, methods to account for a variable exposure were reviewed and a new method developed based on the concept of isomortality. In part II of this paper, the method is combined with a random‐walk model describing the transport of passive organisms through a low pH plume leading to a Monte‐Carlo‐like risk assessment which is applied to several candidate CO2 injection scenarios.

Simulating Oil Droplet Dispersal from the Deepwater Horizon Spill with a Lagrangian Approach

An analytical multiphase plume model, combined with time-varying flow and hydrographic fields generated by the 3-D South Atlantic Bight and Gulf of Mexico model (SABGOM) hydrodynamic model, were used as input to a Lagrangian transport model (LTRANS), to simulate transport of oil droplets dispersed at depth from the recent Deepwater Horizon MC 252 oil spill. The plume model predicts a stratification-dominated near field, in which small oil droplets detrain from the central plume containing faster rising large oil droplets and gas bubbles and become trapped by density stratification. Simulated intrusion (trap) heights of ~ 310–370 m agree well with the midrange of Q1 conductivity-temperature-depth observations, though the simulated variation in trap height was lower than observed, presumably in part due to unresolved variability in source composition (percentage oil versus gas) and location (multiple leaks during first half of spill). Simulated droplet trajectories by the SABGOM-LTRANS modeling system showed that droplets with diameters between 10 and 50 μm formed a distinct subsurface plume, which was transported horizontally and remained in the subsurface for >1 month. In contrast, droplets with diameters ≥90 μm rose rapidly to the surface. Simulated trajectories of droplets ≤50 μ mi n diameter were found to be consistent with field observations of a southwest-tending subsurface plume in late June 2010 reported by Camilli et al. [2010]. Model results suggest that the subsurface plume looped around to the east, with potential subsurface oil transport to the northeast and southeast. Ongoing work is focusing on adding degradation processes to the model to constrain droplet dispersal.

Evolution of droplets in subsea oil and gas blowouts: development and validation of the numerical model VDROP-J.

The droplet size distribution of dispersed phase (oil and/or gas) in submerged buoyant jets was addressed in this work using a numerical model, VDROP-J. A brief literature review on jets and plumes allows the development of average equations for the change of jet velocity, dilution, and mixing energy as function of distance from the orifice. The model VDROP-J was then calibrated to jets emanating from orifices ranging in diameter, D, from 0.5 mm to 0.12 m, and in cross-section average jet velocity at the orifice ranging from 1.5 m/s to 27 m/s. The d50/D obtained from the model (where d50 is the volume median diameter of droplets) correlated very well with data, with an R(2)=0.99. Finally, the VDROP-J model was used to predict the droplet size distribution from Deepwater Horizon blowouts. The droplet size distribution from the blowout is of great importance to the fate and transport of the spilled oil in marine environment.

Environmental impacts of ocean disposal of CO2

Abstract This paper analyzes one of the most important environmental impacts of ocean disposal of CO 2 , the acidification around the release point. We present a methodology which allows us to quantify the effects of lower pH on marine organisms. Preliminary results show that some impacts are inevitable around the release point, but their severity will depend on the release technology.

Intercomparison of oil spill prediction models for accidental blowout scenarios with and without subsea chemical dispersant injection.

We compare oil spill model predictions for a prototype subsea blowout with and without subsea injection of chemical dispersants in deep and shallow water, for high and low gas-oil ratio, and in weak to strong crossflows. Model results are compared for initial oil droplet size distribution, the nearfield plume, and the farfield Lagrangian particle tracking stage of hydrocarbon transport. For the conditions tested (a blowout with oil flow rate of 20,000 bbl/d, about 1/3 of the Deepwater Horizon), the models predict the volume median droplet diameter at the source to range from 0.3 to 6mm without dispersant and 0.01 to 0.8 mm with dispersant. This reduced droplet size owing to reduced interfacial tension results in a one to two order of magnitude increase in the downstream displacement of the initial oil surfacing zone and may lead to a significant fraction of the spilled oil not reaching the sea surface.

The influence of droplet size and biodegradation on the transport of subsurface oil droplets during the Deepwater Horizon spill: a model sensitivity study

Abetter understanding of oil droplet formation, degradation, and dispersal in deepwaters is needed to enhance prediction of the fate and transport of subsurface oil spills. This research evaluates the influence of initial droplet size and rates of biodegradation on the subsurface transport of oil droplets, specifically those from theDeepwaterHorizon oil spill. A three-dimensional coupledmodel was employedwith components that included analyticalmultiphase plume, hydrodynamic and Lagrangianmodels. Oil droplet biodegradationwas simulated based onfirst order decay rates of alkanes. The initial diameter of droplets (10–300 μm) spanned a range of sizes expected fromdispersant-treated oil. Results indicate thatmodel predictions are sensitive to biodegradation processes, with depth distributions deepening by hundreds ofmeters, horizontal distributions decreasing by hundreds to thousands of kilometers, andmass decreasing by 92–99%when biodegradation is applied compared to simulationswithout biodegradation. In addition, there are twoto four-fold changes in the area of the seafloor contacted by oil droplets among scenarios with different biodegradation rates. The spatial distributions of hydrocarbons predicted by themodel with biodegradation are similar to those observed in the sediment andwater column, although themodel predicts hydrocarbons to the northeast and east of thewell where no observations weremade. This study indicates that improvement in knowledge of droplet sizes and biodegradation processes is important for accurate prediction of subsurface oil spills.

Near field impacts of reduced pH from ocean CO2 disposal

Abstract A methodology has been developed to quantify mortality suffered by marine zooplankton passing through a CO2-enriched sea water plume. Predicted impact depends on the mode of injection with scenarios which disperse the CO2 showing least impact. Benthic impacts also depend on injection mode, with localized effects expected for any scenario in which the plume contacts the bottom. Effects of scale were found whereby the predicted impact from 10 power plants is greater than 10 times the impact of one plant at the same site. Based on available data, our modeling suggests that mortality associated with exposure to low pH can be avoided by properly dispersing the CO2 and keeping the plume off of the seabed.