1.1 Purpose and Scope
This publication provides an overview of proven technologies for recovering free-product petroleum releases to groundwater. An approach is given to optimize free-product recovery (FPR) for minimal water production and free-product smearing. Information and guidance for design and analysis of free-product recovery systems using trenches, skimmer wells, single and dual-pump wells, and vacuum-enhanced wells are provided. The principles that govern the distribution and movement of free-product petroleum hydrocarbons near the water table in an unconfined groundwater aquifer are reviewed. Information and data for parameter estimation are also provided.
This publication does not address the methods, procedures and equipment to be used in site assessment for locations of free-product releases. It does not address issues of efficiency of free-product recovery in relating recovery amounts to reduction in risk to groundwater receptors. Tools for characterizing petroleum hydrocarbon distribution are discussed only if they relate to design and analysis of free-product recovery systems. This publication does not address alternative remediation methods for petroleum-impacted soils and groundwater such as soil vapor extraction, bioventing, air sparging, pump-and-treat, or intrinsic bioremediation.
Discussion of petroleum hydrocarbon liquids in the subsurface environment involves the use of certain terminology. Petroleum hydrocarbon liquids generally are less dense than water. This implies that such liquids would float on the water table of an unconfined groundwater aquifer. This contrasts with liquids such as chlorinated solvents that have density greater than water, resulting in the potential to sink to the base of an aquifer. Petroleum hydrocarbons are an example of a "light" nonaqueous phase liquid (LNAPL), which means that they are lighter than and immiscible with water. If LNAPL is present in sufficient quantity, it will move in response to impressed forces. This condition of free-product or mobile LNAPL contrasts with the condition of residual product where the hydrocarbon liquid is present as discontinuous globules within the porous matrix. Residual LNAPL cannot be skimmed or pumped from a well.