Sensitivity Models and Design Protocol for Partitioning Tracer Tests in Alluvial Aquifers

Abstract

Zones of dense, nonaqueous phase liquids (DNAPLs) are difficult to characterize as to their volume, composition, and spatial distribution using conventional ground‐water extraction and soil‐sampling methods. Such incompletely characterized sites have negative consequences for those responsible for their remedial design, e.g., the uncertainties in the optimal placement of ground‐water extraction wells and in the duration of remediation. However, the recent use of the partitioning interwell tracer test (PITT) to characterize DNAPL zones at sites in New Mexico [unsaturated alluvium] and in Ohio, Texas, and Utah [saturated alluvium] demonstrates that the volume and spatial distribution of residual DNAPL can be determined with accuracy. The PITT involves injection of a suite of tracers which reversibly partition to different degrees between the DNAPL and the ground water or soil air resulting in the chromatographic separation of the tracer signals observed at the extraction well(s). The design of a PITT requires careful consideration of the hydrostratigraphic, hydraulic, and certain geochemical properties of the alluvium being tested. A three‐dimensional, numerical model of a heterogeneous alluvial aquifer containing DNAPL has been developed for use with the UTCHEM simulator to demonstrate partitioning tracer testing and to address questions that are frequently raised in its application. The simulations include (1) the estimation of DNAPL volume for the simple case where only residual DNAPL is present in heterogeneous alluvium, (2) sensitivity studies to demonstrate the effect of increasingly low residual DNAPL saturation on the tracer signal, and (3) the effect of free‐phase DNAPL on the estimation of the volume of DNAPL present. Furthermore, the potential interference of sedimentary organic carbon as a DNAPL surrogate on the tracer signal is considered and shown to be readily resolved by the careful choice of tracers. Finally, a protocol for the use of PITTs in alluvial aquifers is presented.

Partitioning Gas Tracer Tests for Measurement of Water in Municipal Solid Waste

“…To estimate the accuracy of partitioning tracer tests for measuring nonaqueous-phase liquids in subsurface media, computational experiments were conducted in a hypothetical aquifer contaminated with nonaqueous-phase liquids. 32 The investigators concluded that the retardation factor should be between 1.2 and 4 to obtain the response curve of the partitioning tracer in a reasonable sampling period and to ensure that good separation was achieved between the conservative and partitioning tracers. Based on this study, similar constraints were later assumed to apply for PGTTs for the detection of nonaqueous-phase liquids in the vadose zone.…”

“…In most field applications of partitioning tracer tests for the detection of nonaqueous-phase liquids, multiple partitioning tracers are injected simultaneously to extend the sampling range of the technology. 32 Water flow within landfills is known to follow preferential flow paths, which may result in isolated zones of very high water saturations. Because water reduces the relative permeability to gas flow, injected gases may bypass these low permeability zones and, thus, not detect such water pockets.…”

Critical Review of Environmental Forensic Techniques: Part I

“…The approach is based on comparing the transport of one or more partitioning tracers, such as calcium bromide, with a non reactive tracer such as 2,4-dimethyl-3-pentanol that is introduced into a NAPL contaminated zone (James et al, 1997). The breakthrough of the tracers with dierent NAPL anities is then used to infer the magnitude of retardation and residual NAPL saturation (Nelson and Brusseau, 1996;Jin et al, 1997;Annable et al, 1998;Dwaranakanath and Pope, 1998). Tracer techniques are limited to applications where the NAPL locations are reasonably well known.…”

Characteristics and influence factors for oil displacement efficiency determination in low-permeability and complex reservoir simulation

“…Young et al (1999) report DNAPL saturations ranging from 0.1% to 0.2% saturation prior to surfactant flooding and 0.06% residual DNAPL saturation upon completion of the surfactant flood. Jin et al (1997) cite three case studies where PITT determined the residual DNAPL saturations to range from 0.02 to 0.10 (2 to 10%).…”

“…Jin et al (1997) and Young et al (1999) have used the technique of partitioning inter-well tracer testing (PITT) to determine residual DNAPL (and LNAPL) saturations both prior to and following surfactant enhanced waterflood NAPL recovery efforts. Young et al (1999) report DNAPL saturations ranging from 0.1% to 0.2% saturation prior to surfactant flooding and 0.06% residual DNAPL saturation upon completion of the surfactant flood.…”