API Publ 4715-2002 pdf download.Evaluating Hydrocarbon Removal from Source Zones and its Effect on Dissolved Plume Longevity and Magnitude.
For many decades, the oil production industry has recognized that significant limitations exist to complete extraction of oil from geologic formations. Attempts to recover fuels and crude oil (collec- tively known as light nonaqueous phase liquids or LNAPL) accidentally released to the subsurface encounter similar limitations. This report explains how multiphase fluid mechanics (mixed presence of LNAPL, water, vapor) relate to these recovery limitations. The report further explains how the endpoints to recovery relate to both the longevity of the LNAPL as a source of dissolved-phase and vapor-phase constituents and to the downgradient dissolved-phase concentrations. This work is focused on LNAPLs, but the general principles also apply to many aspects of dense nonaqueous phase liquid (DNAPL) recovery and risk. Release of an LNAPL to the subsurface introduces the potential of several risk factors to nearby receptors: (1) Vapor phase migration of volatile constituents from LNAPL in the vadose zone to the surface. (2) Dissolution of constituents from LNAPL in the vadose zone through infiltration of recharging waters, and subsequent downgradient movement of those constituents once those waters encounter the water table. (3) Release of sufficient LNAPL that it exceeds the capacity of the vadose zone to absorb it, resulting in the accumulation of a mobile LNAPL lens above and below the origi- nal groundwater table. (4) Upward vapor phase migration of volatile constituents from the above LNAPL lens to the land surface, and (5) Downgradient migration of dissolved-phase constituents resulting from dissolution of the LNAPL lens.
Because the solutions are analytic, they make many simplifying assumptions. Therefore, the linked suite of physical and chemical calculations will not provide a detailed representation of the site. The calculations described in this report, whether solved in a spreadsheet environment or using the software utility, are designed as screening tools only. The results of the calculations cannot be precisely calibrated to site conditions, just as the results of other screening models cannot. The toolkit described in this report is most properly considered as a quantitative concep- tual model to be used for screening decision-making. There is a deliberate compromise between screening analytic methods versus numerical calculations that can consider a more complete range of complexities. While it is clear that conditions not considered by the software utility, such as complex vertical and lateral variations in soil properties, seasonally varying groundwater elevations, and laterally varying groundwater flow velocities near the LNAPL are important, the parameters necessary for such evaluations are not often available. Further, the effort involved in numerical multiphase, multidimensional modeling is significant. The approach presented here is therefore designed to use available information in the best manner possible, but it should be clear that uncertainty will exist in the results. The recommended use of the toolkit is expected to produce conservative results. If more accurate or detailed assessment is needed, numerical mod- eling and/or advanced data collection will be warranted, consistent with the higher-tiered levels of effort in many risk assessment guidelines (e.g., Risk-Based Corrective Action, ASTM 1995; Risk Assessment Guidance for Superfund, EPA, 1995). Therefore, while simple to use, this screening conceptual evaluation method requires good user judgment and awareness of the limitations.
The presence of multiple phases (water, LNAPL, vapor) in porous material influences the movement and transport of each phase under ambient or remediated conditions. Multiphase fluid mechanics and other principles are used to estimate the pore fluid fractions and their mobility under a variety of conditions. The distribution and composition of the LNAPL then determines the equilibrium chemi- cal partitioning into groundwater and vapor. Ultimately, the application of these principles results in estimates of the time dependent concentration of soluble components partitioning out of the LNAPL and into groundwater, with a link to vapor flux under ambient flow and partitioning conditions. For instance, one could look at chemical partitioning from an LNAPL source that has had no remediation action, or one could consider the same source after some cleanup effort (but not during that effort). This toolkit does not directly consider institutional controls, such as plume containment, that are often an important component of risk management. However, one could use the toolkit to consider the time frame over which an institutional control might be appropriate.