Time Frame for NAPL Recovery

The length of time required for recovery of LNAPL products is based on the volume of product present in the subsurface and is limited by numerous factors. Most often based on an educated guess, factors regarding volume estimates have innate compounding errors in relation to  

After the initial volume estimate has been determined, testing of a pilot recovery system should be initiated to evaluate recovery rates. However, factors that significantly affect recovery rates include the areal distribution and geometry of the free-hydrocarbon product plume, type(s) and design of recovery system selected, and the performance and efficiency of the system with time. 

Volume determinations and subsequent time frame for recovery of LNAPL product can be estimated. However, a large number of compounding errors are associated with these calculations. Thus, a reasonable time frame for remediation is clearly an estimate. 

The progress of recovery efforts cannot be based confidently on LNAPL product thickness maps. Although these maps provide quantification of overall trends, the numerous factors that impact hydrocarbon thicknesses make accurate quantification difficult. An estimate of effectiveness thus is based on volume recovered to date divided by the total volume that is considered recoverable. Furthermore, as the recovery project progresses and new data are introduced, the volume and time frame for recovery should be continually reevaluated and revised. 

Littlefield et al. (1984) provides a method of estimating the volume of recoverable product after recovery has been initiated. Contouring the NAPL thickness in monitoring wells and calculating the apparently oil-saturated soil volume are completed at two separate times. The difference in apparent oil-saturated soil volumes at these times is attributable to the quantity of oil recovered during that period. By using that relationship, an extrapolation can be used to estimate the remaining volume of recoverable oil. This value, in most situations, results in an overstatement of the recoverable volume of LNAPL in place. 

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