LNAPL TRANSMISSIVITY AS A RECOVERY BENCHMARK: A CASE STUDY FOR A SAPROLITE AQUIFER

Operation of a fuel bulk storage and distribution terminal in the Piedmont Province of northwestern South Carolina from the 1940s through the late 1980s resulted in a large, light, non-aqueous phase liquid (LNAPL) plume, consisting primarily of kerosene/jet fuel, existing at a depth of approximately 45 feet below ground surface. A total of over 100 monitoring wells, recovery wells, and small-diameter piezometers have been installed in six or more discrete investigative events over the past three decades. Remediation via total fluids extraction (pump and treat) from 16 extraction wells has been ongoing since the early 1990s. Over 159,000 gallons of fuel have been recovered to date with a clear decline in recovery rate over time.

Recent efforts to clarify regulatory drivers and evaluate site strategy have led to a reappraisal of the LNAPL plume. Issues include the practicality of maintaining current recovery efforts, steps to optimize recovery operations, validity of apparent LNAPL thickness data derived from small-diameter monitoring points, and determining tools and techniques for use in understanding LNAPL behavior.

A recent LNAPL plume evaluation incorporated review of historical recovery data, well-specific yield testing for existing recovery wells, laser-induced fluorescence (LIF) analysis, core collection and lab analysis, and baildown testing at select locations to determine LNAPL transmissivity. These tests were undertaken to determine the extent of LNAPL, smear zone characteristics, and LNAPL mobility and recoverability. Results will be presented with discussion of their applicability and utility to this site in light of recent developments in understanding LNAPL behavior and practical recoverability endpoints. For example, LNAPL well thickness was determined to be a poor indicator of recoverability and oil transmissivity (T_o) proved to be a meaningful basis for gauging relative LNAPL recoverability.

Methods comparing LIF response, baildown test results, and lab results derived from core analyses will be discussed and correlations evaluated to streamline data collection and decision-making related to practical management strategies and development of technically defensible remedial endpoints.