In-situ characterizations of dense non-aqueous phase liquids using partitioning tracers. Annual progress report, September 15, 1996--September 14, 1997

Abstract

This paper describes laboratory research conducted to investigate the use of partitioning tracers for the detection, volume estimation, and remediation performance assessment of vadose zones contaminated by nonaqueous phase liquids (NAPLs). These laboratory studies used Ottawa sand and field soil packed columns. The columns were saturated, then reduced to residual saturations of water and NAPL, creating a three phase system of air, water, and NAPL.Conservative and partitioning gas tracers were injected into the column and their elutions were analyzed. The method of moments was used to estimate partition coefficients between the air and NAPL phases for each of the tracers. The partition coefficients and retardation factors are used to select appropriate tracers for NAPL detection and volume estimation in the field. This research identified several perfluorocarbon tracers suitable for use in the field and demonstrated the feasibility of using partitioning tracers as a tool for NAPL detection and volume estimation in the vadose zone. INTRODUCTIONA widespread and recurring public health problem identified in recent years is the contamination of the subsurface by nonaqueous-phase liquids (NAPLs) (NRC 1994).Contamination at numerous private, government, and industrial facilities is well-documented and is clearly a problem for communities and populations located near these sites.1 Nonaqueous-phase liquid contamination of the subsurface occurs as NAPLs migrate from the surface, where in the past they were typically disposed in unlined pits, through the vadose zone to the water table. The migration of the NAPL though the vadose zone leaves a trail or streak of NAPL held in the pore space by capillary forces and is defined as residual or immobile NAPL.As water percolates through a contaminated vadose zone, NAPLs and other contaminants dissolve in the mobile water and may be carried to the saturated zone as dissolved constituents. Previously clean aquifers may receive harmful doses of pollutants without the bulk of the contaminants ever reaching the saturated zone.In many cases, because of the potential adverse impact of vadose-zone contamination on water supplies, remediation of the vadose zone may be essential. However, to design an effective remediation of an area, a detailed description of the site is necessary. Thus, the first step in site remediation is characterization. Two current methods using core sampling or cone penetrometer testing, which depend upon laboratory analysis, are time-consuming and expensive. To improve the site characterization process, better methods of detecting NAPLs are necessary (EPA 1992).A potentially better characterization method utilizes a technology developed in the petroleum industry. This technology is the partitioning inter-well tracer test (PITT). Partitioning, both single-well and interwell, tracer tests have been used for years to character& liquid residuals in oil-and gas-saturated reservoirs (Cooke 1971, Deans 1971, Allison et al. 1991, Tang and Harker 1991a. In a PITT...