INTEGRATED DESIGN OF A DNAPL PARTITIONING TRACER TEST

Partitioning tracer tests (PTTs) are now commonly used to estimate the location and volume of nonaqueous-phase liquid (NAPL) at contaminated groundwater sites. PTTs are frequently completed before and after remediation efforts as one means to assess remediation effectiveness. Design of a PTT can be quite complex. At a minimum, the design requires consideration of the following factors: type of contaminant; distribution of contaminant at the site, including location of hot spots; hydraulic characteristics of the site; appropriate selection of partitioning tracers based several factors; measurement of the partitioning coefficients for the various tracers; the time allotted to conduct the PTT; evaluation of the magnitude and arrival time of the tracer breakthrough curves; duration of the tracer input pulse; maximum tracer concentrations; analytical detection limits for the tracers; estimation of the capture zone of the well field to tracer ensure mass balance and to limit residual tracer concentrations left in the subsurface; effect of chemical remediation agents on the PTT results, and disposal of the extracted tracer solution. These design principles are applied to a chemical-enhanced remediation effort for a chlorinated-solvent DNAPL site in Virginia Beach, Virginia. For this project, hydrology and the pre-PTT contaminant distribution were characterized using traditional methods (slug tests, hydraulic-head measurements, groundwater and soil concentrations from monitoring wells, and geoprobe analysis), as well as membrane interface probe analysis. Additional wells were installed after these studies. Partitioning tracers were selected based on the primary DNAPL contaminants at the site, expected NAPL saturations, and costs. Partitioning coefficients were measured in the laboratory in water and in solutions containing expected post-remediation residual concentrations of the chemical remediation agent, which was shown to influence the partitioning coefficients. The numerical model was used to optimize the injection-extraction rates and distribution to most efficiently sweep the NAPL hot spot, to enable hydraulic capture of the tracers, and to predict the peaks and arrival times of the tracer breakthrough curves. Results of the field PTT are presented to evaluate the effectiveness of the PTT design approach.