Grantee Research Project Results

1997 Progress Report: Partitioning Tracers for In-Situ Detection and Measurement of Nonaqueous Phase Liquids in Porous Media

EPA Grant Number: R825407
Title: Partitioning Tracers for In-Situ Detection and Measurement of Nonaqueous Phase Liquids in Porous Media
Investigators: Brusseau, Mark
Institution: University of Arizona
EPA Project Officer: Hahn, Intaek
Project Period: November 1, 1996 through October 31, 1999
Project Period Covered by this Report: November 1, 1996 through October 31, 1997
Project Amount: $246,339
RFA: Environmental Fate and Treatment of Toxics and Hazardous Wastes (1996) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management , Safer Chemicals

Objective:

The central objective of this research is to investigate the impact of nonideal transport upon the use of partitioning tracers. Partitioning tracer tests can be used for the detection and volume estimation of nonaqueous phase liquids (NAPL's) in the "swept zone" of an aquifer. The theory depends upon the chromatographic separation of two or more selected tracers injected in a single pulse into a NAPL-contaminated aquifer. The degree of temporal separation between the solutes is dependent upon the extent of partitioning of the reactive tracers from the advecting water into immobile NAPL ganglia and pools. Partitioning coefficients are used to relate equilibrium concentrations of the tracers in the NAPL and aqueous phases. Sets of conservative and reactive tracers are often used to have the greatest degree of separation between tracer pairs. This temporal displacement between the various tracer pairs permits the calculation of a NAPL saturation required to produce the degree of retardation exhibited by the partitioning tracers.

Given that the degree of retardation of a partitioning solute is dependent upon the volume of NAPL present in the system, the equation used to calculate NAPL saturation (S_N = vol. of LNAPL / vol. of void space) can be readily derived from the expression for the retardation factor of the advection-dispersion equation. Furthermore, since the retardation factor may be viewed as a ratio of reactive tracer to conservative tracer travel times, an equation for S_N can be derived which requires only a) the partitioning coefficients for the two tracers being compared and b) a representative travel time for each tracer.

In particular, the effects of rate-limited partitioning and solute biodegradation upon the volume estimation of non-aqueous phase liquids (NAPLs) are to be investigated. If such nonidealities are not taken into consideration when analyzing data from a partitioning tracer study, the study results my not be accurate.

Progress Summary:

(1) A series of field-scale interwell (forced gradient) partitioning tracer studies have been previously performed in a severely contaminated sand and gravel aquifer. The majority of the contamination consisted of petroleum products and organic solvents. The data derived from these partitioning tracer studies has been used to determine the amount of NAPL present in the study zones. Some of these data sets have recently been analyzed in greater detail to determine if rate-limited partitioning (mass transfer) influenced the estimation of NAPL saturation. A series of one-dimensional mathematical transport models were used to fit the field data, produce calibrated transport coefficients, and interpret the results. From the initial results of this analysis, it was determined that equilibrium partitioning dominated tracer retardation. This finding was significant because the study was conducted under a forced gradient, and interstitial velocities were much greater than under natural gradient conditions.

(2) The impact of solute biodegradation on the efficacy of the partitioning tracer method has been examined. It was discovered that tracer biodegradation can be significant under experimental field conditions, and the corresponding loss of tracer mass can significantly alter estimates of NAPL saturations. Determination of a solute travel time from a breakthrough curve tends to be very sensitive to the data contained in the "tail" of the curve (at later study times). It was discovered that the biodegradation of the organic tracers could remove a significant amount of mass from the tail of the breakthrough curves. This process often reduced tracer concentrations to values below detection limits. Due to physical heterogeneity, the tails of the tracer profiles were substantial, and the loss of this data skewed the tracer's travel times, which were derived from a moment analysis technique. The degradation of a conservative organic tracer caused the overestimation of NAPL saturations. Likewise, the degradation of the reactive organic tracers caused the underestimation of NAPL saturations.

(3) Alternative conservative and reactive tracers have been examined for use in future tracer experiments. Among the considerations for these new tracers was method of detection and analysis, toxicity, and detection limits. These tracers should be available for use in the future partitioning tracer studies described below.

(4) Currently, we are in the planning stages of a field-scale partitioning tracer experiment, to be conducted in the spring of 1998. This study will also address the issues of contaminant biodegradation and rate-limited partitioning at a nearby historically contaminated site. A suite of tracers will be injected into a contaminated portion of the aquifer using an injection/extraction well couplet.

Accomplishments and Research Results:

From the previously mentioned field data, it was concluded that the studies were likely conducted at or near equilibrium partitioning conditions at the local (pore) scale. These findings suggest that the partitioning tracer studies conducted at the field site under a set of specific conditions were relatively unaffected by local scale rate-limited partitioning. However, it should be noted that these findings are only valid for selected sampling locations. It is possible that other sampling locations within the study zone could exhibit a significant degree of rate-limited behavior.

The field studies were severely impacted by in situ biodegradation. Due to the reduction of solute concentrations at later study times for some of the tracers, significant error was introduced into the calculation of the tracer travel times and the corresponding NAPL saturations. To plan a successful field partitioning tracer study, the possibility of solute degradation must be considered, and tracers must be carefully selected to avoid significant losses of tracer mass.

Future Activities:

(1) Planned activities for this research topic include laboratory "column" studies with NAPL contaminated porous media laboratory and further analyses of additional sampling locations scattered throughout the study regions. The column studies will investigate the impact of pore water velocity and NAPL saturation (volume of NAPL present) on the degree of rate-limited partitioning of the tracers. These studies should provide more insight into the nature of rate-limited partitioning at an actual contaminated field site, and allow better understanding of the results from past field studies. Additional sampling locations from the field experiments are to be analyzed using 1-D transport models to determine if there is some spatial variation in the degree of rate-limited partitioning of the tracers at the field site. Newly developed two and three-dimensional transport models will also be applied to the field site to investigate the effects of local scale rate-limited partitioning upon the flux-averaged data from extraction wells.

(2) Further analysis and quantification of the degree of tracer biodegradation during field experiments will be conducted. The relationship between this source of mass loss and NAPL volume/saturation estimates will continue to be investigated.

(3) Newly selected tracers will be used in both laboratory and field-scale partitioning tracer studies. Their behavior in various porous media and relative ease of analysis will be monitored and assessed.

(4) A preliminary field-scale partitioning tracer study will be conducted in the spring of 1998. Additional field studies at this location will be conducted in the future (a) at a larger (spatial) scale and (b) after an extended application of a remediation system.

Journal Articles:

No journal articles submitted with this report: View all 7 publications for this project

Supplemental Keywords:

RFA, Scientific Discipline, Waste, Ecosystem Protection/Environmental Exposure & Risk, Health Risk Assessment, Environmental Chemistry, Remediation, Chemistry, Fate & Transport, Hazardous Waste, Hazardous, hazardous waste treatment, fate and transport, oils spills, fate and transport , NAPL, contaminant transport, risk characterization, chemical speciation, transport contaminants, partitioning tracers, adverse human health affects, chemical contaminants, ecological impacts, hazardous chemicals, porous media, contaminant transport models, restoration planning

Progress and Final Reports:

Original Abstract

1998 Progress Report

Final Report