Performance assessment of NAPL remediation in heterogeneous alluvium

Abstract

Over the last few years, more than 40 partitioning interwell tracer tests (PITTs) have been conducted at many different sites to measure nonaqueous phase liquid (NAPL) saturations in the subsurface. While the main goal of these PITTs was to estimate the NAPL volume in the subsurface, some were specifically conducted to assess the performance of remedial actions involving NAPL removal. In this paper, we present a quantitative approach to assess the performance of remedial actions to recover NAPL that can be used to assess any NAPL removal technology. It combines the use of PITTs (to estimate the NAPL volume in the swept pore volume between injection and extraction wells of a test area) with the use of several cores to determine the vertical NAPL distribution in the subsurface. We illustrate the effectiveness of such an approach by assessing the performance of a surfactant/foam flood conducted at Hill Air Force Base, UT, to remove a TCE-rich NAPL from alluvium with permeability contrasts as high as one order of magnitude. In addition, we compare the NAPL volumes determined by the PITTs with volumes estimated through geostatistical interpolation of aquifer sediment core data collected with a vertical frequency of 5–10 cm and a lateral borehole spacing of 0.15 m. We demonstrate the use of several innovations including the explicit estimation of not only the errors associated with NAPL volumes and saturations derived from PITTs but also the heterogeneity of the aquifer sediments based upon permeability estimates. Most importantly, we demonstrate the reliability of the PITT in strongly heterogeneous aquifer sediments—a feature of particular importance at those sites where the volume of NAPL-contaminated aquifer sediments is too large to afford soil coring at a density of sampling that yields similar NAPL volumes as the PITT. Finally, the data support the proposed performance assessment protocol for NAPL remediation combining the use of PITTs and several aquifer sediment cores for vertical definition of the NAPL geometry.

Introduction

In recent years, there has been an increasing interest in the remediation of nonaqueous phase liquid (NAPL) source zones. Even though tremendous advances have been made to characterize NAPL source zones using partitioning interwell tracer tests (PITTs), the most common approach to assessing remediation performance is still the use of soil cores and/or monitoring ground water quality (Jackson et al., 2000).

When compared to the scale of typical NAPL source zones, samples of aquifer sediment (i.e., “soil samples”) can provide only point estimates of NAPL saturation and do not give good estimates of the total mass of NAPL (Bedient et al., 1999, p. 45). In addition, these point NAPL saturation estimates are only accurate if appropriate preservation techniques are employed to minimize contaminant loss due to volatilization (Hewitt et al., 1995) and there is no loss of NAPL by drainage during core recovery (Jackson et al., 2000). In particular, air entry into soil cores will change the NAPL saturation from that of a two-phase system (NAPL–water) to the lower saturations of a three-phase system (air–NAPL–water) (see Table 3 in Mercer and Cohen, 1990). Consequently, NAPL will drain and, therefore, be lost from the core. Furthermore, the representative elementary volume (REV) of experimentally measured NAPL blob size distributions has been shown to vary between 10⁻² and 10⁴ cm³ (Mayer and Miller, 1992). Though the lower end of this range of the NAPL blob sizes can be sampled by conventional soil cores, the upper end is well out of the range of most soil collection techniques. This indicates that the resulting point measurements may not be representative of the NAPL saturation even in the direct vicinity of the sample. Laboratory and field studies have demonstrated that even slight heterogeneities, such as small variations in grain size, have profound effects on the distribution and character of denser-than-water NAPL (DNAPL) migration pathways in the subsurface Kueper et al., 1993, Poulsen and Kueper, 1992. By contrast, the distribution of a lighter-than-water NAPL (LNAPL) is strongly influenced by its buoyancy and, consequently, the water table elevation.

Monitoring ground water concentrations is another popular means of assessing NAPL remediation (Jackson et al., 2000), but this rarely provides definitive indications of NAPL mass recovery. It is not uncommon to measure contaminant concentrations that are 1–2 orders of magnitude below the aqueous solubility. NAPL dissolution phenomena have been shown to depend on a number of system parameters such as aqueous-phase velocity Pfannkuch, 1984, Powers et al., 1992, NAPL saturation Miller et al., 1990, Powers et al., 1994, NAPL composition (Frind et al., 1999), textural heterogeneity (Mayer and Miller, 1996), length scale (Imhoff et al., 1994) and the aspect ratio between the monitoring well and the NAPL zone Anderson et al., 1992, Jackson and Mariner, 1995. Consequently, the use of ground water contaminant concentrations to draw conclusions about either the volume, mass or saturation of the NAPL zone involves an ill-posed inverse problem for which no unique solution exists even for the simplest of cases.

The ability of PITTs to estimate residual NAPL saturation under field conditions is documented in literature Jin et al., 1997, Rao et al., 1997, Rao et al., 2000, Annable et al., 1998, Jawitz et al., 1998, Jawitz et al., 2000, Young et al., 1999, Falta et al., 1999, Cain et al., 2000. The PITT has the advantage of providing a quantitative estimate of the errors involved in its estimation of NAPL saturation and volume (Dwarakanath et al., 1999). Furthermore, it estimates the NAPL volume in the swept pore volume at the interwell scale (meters) similar to the large-scale averaging that a pumping test measures in terms of hydraulic parameters. In conjunction with multilevel samplers, a PITT can also provide information on the spatial distribution of NAPL. Furthermore, PITTs may be used to see if previously uncontaminated zones have been contaminated by displaced NAPL. However, a conventional PITT does not provide detailed information on the distribution of NAPL at the core scale (centimeters) in the subsurface. We recommend the use of several continuously cored boreholes with subsampling every 5–10 cm to define the vertical profile of NAPL distribution.

Source zone remediation technologies are essentially divided into chemical flooding technologies, such as surfactant flooding, cosolvent flooding and in situ chemical oxidation, and thermal technologies, such as steam flooding and six-phase heating. Performance assessment of such remedial actions requires not only knowledge of the amount of NAPL removed but also that the remaining volume of NAPL and its distribution in the subsurface be accurately quantified. Quantifying both the final NAPL volume and its spatial distribution is essential to determine the potential risk for future contamination of the ground water because both factors control the absolute value and spatial distribution of ground water contamination over time. Such information will allow the determination of an optimum course of action such that risk-based remediation objectives are achieved. Despite the wealth of information in existing literature on the analysis of soil core data, ground water monitoring data and PITTs, a rigorous quantitative approach for assessing the performance of remedial technologies is not discussed in the current environmental literature. Such an approach is necessary to accurately assess the performance of the various NAPL zone remediation projects currently being conducted.

In this paper, we present an approach that combines PITTs, multilevel samplers and soil cores to assess the performance of remedial operations. We use PITTs to estimate the average NAPL volume in the swept pore volume of interest and multilevel samplers to measure the change in NAPL saturation at critical depths. Soil cores are used to determine the vertical NAPL distribution at the centimeter scale. For illustrative purposes only, we also compare the results of volume estimates derived from interpolating soil core data and from PITTs. Because the performance of a remediation technology may be limited by aquifer heterogeneities, it is necessary that the tools used to assess the performance be accurate within heterogeneous systems. In order to allay frequently raised concerns about the accuracy of PITTs in heterogeneous aquifer sediments, we demonstrate the similarity of NAPL volume estimates from many closely spaced soil cores and the PITT. Furthermore, we explicitly measure the heterogeneity of the aquifer materials at Hill AFB, a measurement that has been overlooked in other studies that have used both PITTs and soil cores.

While this combined approach can be used in conjunction with any remedial technology and NAPL of any density, we illustrate the effectiveness of the approach by assessing the performance of a surfactant/foam flood in a heterogeneous alluvial aquifer contaminated with DNAPL (Hirasaki et al., 1997). The results from both the pre-surfactant PITT and soil cores and post-surfactant PITT and soil cores are specifically used to determine whether cleanup was effective in the zones of lower permeability.