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Evaluating Hydrocarbon Removal

Evaluating Hydrocarbon Removal from Source Zones and its Effect on Dissolved Plume Longevity and Concentration

API Publication 4715

Light nonaqueous phase liquids (LNAPL; a.k.a. petroleum fuels, “product” and crude oil) are common sources of hydrocarbons in both water and vapor phases, with all phases presenting potential health, resource, and environmental risks. Ample environmental and oil reservoir data have shown that complete recovery of oil from geologic formations is not generally feasible. Even at residual saturation (trapped and immobile), the LNAPL phase has a mass that is typically several orders of magnitude greater than normally present in sorbed, water, or vapor states, implying the potential for long-term impacts.

Regulatory agencies usually require a responsible party to “remove free product to the maximum extent practicable.” Lacking in ways to easily assess the mobility and long-term risks of LNAPL pools from site to site, arbitrary maximum LNAPL thickness in wells (e.g., 0.1 feet) is often adopted as endpoints for free product recovery. However, observed LNAPL thickness in wells has little or no relationship to the magnitude of risk presented before or after cleanup attempts. Often, free product recovery efforts are undertaken with little understanding of how much product can be recovered, how long the recovery will take and whether the site conditions will change significantly after the recoverable product is removed. This report is intended, in part, to assist in evaluating these issues.

This study provides quantitative theory and tools to evaluate LNAPL sources, their chemistry, and the effects various remediation strategies may have on groundwater and vapor exposure pathways. These exposure pathways are often a critical component of quantitative risk assessment. The study was designed to link the multiphase and chemical processes controlling in situ LNAPL distribution, mobility, and cleanup to quantify estimates of the time dependent concentrations within and downgradient of the LNAPL source. This work considers active flow of groundwater through the LNAPL impacted interval, which has generally not been considered in other analytic evaluations.

The results of this work suggest that the physical limitations to hydraulic free-phase recovery using current technology are such that significant health or resource risk reduction is gained only within a narrow range of chemical and geologic conditions. The greatest risk reductions, as measured by concentration reduction along an exposure pathway, occur in highly permeable soils and for volatile and soluble fuel components. The least effective risk reductions occur in low permeability materials and for fuels that have small volatility and solubility. Under many conditions, it is likely that LNAPL-sourced dissolved and vapor plumes will be present for decades to centuries unless treatment options improve significantly. The evaluation methods presented here can be used to show that the field endpoint, both chemical and physical, of any given cleanup method controls the long-term residual risk.

Download Options

The API-LNAST software utility Version 1.5 referenced in this document has been superceded by Version 2.0 found in the API Interactive LNAPL Guide. However, Version 1.5 can be obtained at the Aqui-Ver website. Version 2.0 users should find the Version 1.5 User's Guide and example problems helpful for problem conceptualization.

Complete Report (6 MB pdf)
Executive Summary (250 KB pdf)
Report without Appendices (5.6 MB pdf)
Appendices Only (400 KB pdf)
API-LNAST Version 1.5 User's Guide (1.3 MB pdf)
API-LNAST Version 1.5 Example Problems (2.1 MB pdf)

Additional information can be found in "Persistence of LNAPL sources: relationship between risk reduction and LNAPL recovery." By David Huntley and G.D.Beckett, Journal of Contaminant Hydrology 59 (2002) 3-26 (November 2002).