Inline gas chromatographic tracer analysis: An alternative to conventional sampling and laboratory analysis for partitioning tracer tests

Partitioning tracer techniques have been demonstrated to be effective for characterizing the amount of nonaqueous phase liquid (NAPL) contaminants present in aquifers. Partitioning interwell tracer tests (PITTs) typically involve the collection of a large number of groundwater samples for subsequent laboratory analysis. This paper describes the implementation of inline methods for gas chromatographic (GC) analysis of tracers in both the laboratory and in the field. Benefits of inline analysis include reduced costs associated with sampling, transport, storage, and laboratory analyses, minimization of tracer mass loss during sample collection and transport, and immediate access to data for on-site decision making. A series of laboratory PITT experiments were conducted in a two-dimensional flow chamber packed with various combinations of clean sand and NAPL, resulting in a range of average residual saturation, S-N, values. Inline chromatographic analysis enabled automation of these experiments while maintaining accurate S-N determinations. At the field scale, inline data from a PITT conducted at Hill Air Force Base (AFB), UT were compared to data from traditional sample collection, transport, and laboratory analysis. For each of the tracers investigated, inline analysis reported higher mass recoveries than laboratory methods, indicating that some mass loss of the tracers occurred during sample transport and laboratory analysis. Estimated S-N values determined from field inline analysis were consistent with those determined from laboratory analysis.