The intent of geologic carbon dioxide (CO2) sequestration, also known as "geologic carbon sequestration," is to prevent anthropogenic CO2 from entering the atmosphere, thereby maintaining acceptable levels of atmospheric CO2. Ideally, the CO2 will be safely transported to sequestration sites and injected deep underground, where it will be permanently trapped, with no negative impact on the terrestrial environment or on underground resources such as groundwater or fossil fuel deposits. In practice, carbon sequestration will be neither perfectly effective nor risk-free. There are risks associated with geologic carbon sequestration, and evaluation of these risks will be important for making decisions concerning where and how sequestration should be performed. The goal of a risk assessment is to quantify the likelihood of harm (or loss) and to present the results in a format that assists decision makers who must act to tolerate, mitigate, or eliminate the potential harm. The goal of risk management is to establish the practical significance of the assessed risks, compare the costs of reducing these risk to benefits gained, compare the risks to the societal benefits derived from incurring the risk, and to establish political and institutional processes of reducing risks. This section summarizes the risk assessment approaches and risk management methods that can be applied to study geologic carbon sequestration in California, including CO2 capture, transportation and storage operations. It includes consideration of human exposure limits, pathways for human and environmental exposure during operations and post-operations, and how risks change over the time scales associated with storage as the CO2 migrates and reacts in the subsurface environment.

In this paper we assess dermal exposure and dose resulting from intermittent contact with residue-contaminated surfaces. These estimates require an understanding of (1) the quantitative relationship between exposure and absorbed dose; (2) the impact of intermittent exposure on absorbed dose; and (3) the temporal resolution required of exposure data to model absorbed dose. We apply a numerical model of dermal absorption to evaluate integrated dermal uptake from a series of contacts with residue-contaminated surfaces. A range of exposure scenarios and contaminant properties are considered. We use results from dermal transfer studies in which dermal loading, resulting from a series of controlled contacts with surface residues, was measured using fluorescent imaging techniques. These experimental results allow us to define archetypal temporal boundary conditions with varying skin loading and removal (via rinsing and pressing). We apply these boundary conditions in a numerical model of contaminant transport through the stratum corneum (SC) and viable epidermis (VE). This model uses a Crank-Nicholson approach to solve the transient Fickian diffusion problem with an explicit approach to connect the SC and VE. We successfully tested the model against analytical solutions of single-contact exposure scenarios for a range of compound properties. The model allows us to investigate accumulated doses from short-term and intermittent contacts, thereby avoiding important limitations of analytical solutions. Our results indicate that increasing the partition coefficient between the SC and VE (ksv) decreases the time-integrated mass transferred through the VE as total contact time increases. This effect is caused by residual contaminant in the SC that reduces the concentration gradient (that drives transport from the skin surface into the SC) for subsequent contacts. Because of this ?residual effect,? multiple single-contact estimates cannot accurately capture integrated dose. The extent to which the single-contact estimate differed from the true dose varies with contaminant and skin properties. Below a ksv of about 0.1 the time-integrated mass transferred through the VE is relatively unaffected by changes in ksv, because the skin clears quickly with low ksv. We also describe the sensitivity of the time-integrated uptake (i.e., through the VE) to (1) the thickness and diffusivity of both the SC and the VE and (2) the contaminant-to-SC partition coefficient. Our results indicate that estimates of integrated dose resulting from a series of intermittent exposures can be practically obtained from the numerical model and related to compound properties. Results of this study can be used to guide dermal exposure model refinements and exposure measurement study design.

Achieving consensus among scientists is often a challenge?particularly in model development. In this article we describe a recent scientific consensus-building process for Life Cycle Impact Assessment (LCIA) models applied to chemical emissions?including the strategy, execution, and results of a process that used model comparison to achieve parsimony. This process has succeeded in establishing a transparent LCIA consensus model. We present the lessons that may be adapted by similar consensus processes in other fields. LCIA characterizes potential impacts on human health and the environment attributable to chemical emissions over the life cycle of a product. LCIA relies on substance-specific characterization factors (CFs) that combine exposure potential and toxicity to represent the relative contribution of the substance to health and environmental impacts (1). LCIA focuses on comparative assessment, using approaches adapted from risk assessment. In 2003, in response to large variations in available methods, an international model comparison/consensus process was initiated. This process was under the umbrella of the Life Cycle Initiative, a joint effort of the United Nations Environment Program (UNEP) and the Society of Environmental Toxicology and Chemistry (SETAC) (2). The process encompassed an international group of model developers responsible for the most commonly-used worldwide LCIA characterization models and focused on characterization of human and ecosystem health impacts. It also involved disciplinary experts in fate and transport, exposure assessment, health risk assessment, and ecotoxicology. The comparison/consensus process fostered a common understanding among the participants of which model elements contribute most to the relative magnitude of LCIA characterization factors. It became clear that with a careful focus on the most influential model elements a consensus model could be established. Experience dictated that a more transparent model would be more likely to gain and retain acceptance and wide-spread use. The need for consistent documentation and transparency led the participants to create an entirely new model, building on contributions from the existing models. This required consensus on essential model elements, provided robust results consistent with existing models, and made parsimony a guiding principle. The tangible outcome is "USEtox", named in recognition of the UNEP-SETAC Life Cycle Initiative under which it was developed. The model is supported by all participating model teams as a basis for future global recommendations of LCIA characterization factors.

Achieving consensus among scientists is often a challenge?particularly in model development. In this article we describe a recent scientific consensus-building process for Life Cycle Impact Assessment (LCIA) models applied to chemical emissions--including the strategy, execution, and results of a process that used model comparison to achieve parsimony. This process has succeeded in establishing a transparent LCIA consensus model. We present the lessons that may be adapted by similar consensus processes in other fields. LCIA characterizes potential impacts on human health and the environment attributable to chemical emissions over the life cycle of a product. LCIA relies on substance-specific characterization factors (CFs) that combine exposure potential and toxicity to represent the relative contribution of the substance to health and environmental impacts (1). LCIA focuses on comparative assessment, using approaches adapted from risk assessment. In 2003, in response to large variations in available methods, an international model comparison/consensus process was initiated. This process was under the umbrella of the Life Cycle Initiative, a joint effort of the United Nations Environment Program (UNEP) and the Society of Environmental Toxicology and Chemistry (SETAC) (2). The process encompassed an international group of model developers responsible for the most commonly-used worldwide LCIA characterization models and focused on characterization of human and ecosystem health impacts. It also involved disciplinary experts in fate and transport, exposure assessment, health risk assessment, and ecotoxicology. The comparison/consensus process fostered a common understanding among the participants of which model elements contribute most to the relative magnitude of LCIA characterization factors. It became clear that with a careful focus on the most influential model elements a consensus model could be established. Experience dictated that a more transparent model would be more likely to gain and retain acceptance and wide-spread use. The need for consistent documentation and transparency led the participants to create an entirely new model, building on contributions from the existing models. This required consensus on essential model elements, provided robust results consistent with existing models, and made parsimony a guiding principle. The tangible outcome is "USEtox", named in recognition of the UNEP-SETAC Life Cycle Initiative under which it was developed. The model is supported by all participating model teams as a basis for future global recommendations of LCIA characterization factors.