Development of Partitioning Tracer Tests for Characterization of Nonaqueous-Phase Liquid-Contaminated Aquifers

Abstract

Abstract The partitioning interwell tracer test (PITT) was used by the petroleum industry to measure the residual oil saturation in water flooded oil fields. Extensive laboratory studies and field-scale tests have been conducted over the past years to transfer this technique to the environmental community for the purpose of nonaqueous phase liquid (NAPL) site characterization and remediation performance assessment. This technique was specifically enhanced and modified to address the new challenges related to NAPL site characterization problems. This paper describes the differences of PITTs between the oil field tracer applications and NAPL site characterization application, and the design and data interpretation strategy that is crucial for the successful field implementation. Both the method of first temporal moment analysis and the method of inverse modeling for the tracer data analysis are presented. To demonstrate the effectiveness of the PITT technique in NAPL site characterization application, the results from two field scale pilot tests are also presented. Introduction Groundwater is an important natural resource. In the United States, more than half of the population relies upon groundwater as the source of drinking water. Unfortunately, the contamination of groundwater reserves by nonaqueous phase liquids (NAPLs) has become a major and widespread problem and are, found in a wide variety of hydrogeological environments throughout the US. NAPLs can be either denser than water or lighter than water. Chlorinated hydrocarbons (CHCs) such as trichloroethene (TCE), tetrachloroethene (PCE), and carbon tetrachloride (CTET) are denser than water and frequently present in the subsurface as dense, nonaqueous phase liquids (DNAPLs) Petroleum hydrocarbons such as benzene are lighter than water and frequently present as light, nonaqueous phase liquids (LNAPLs). Fig. 1 shows a possible scenario of NAPL contamination. When NAPL is released at the surface, free-phase NAPLs move downward through soil under the force of gravity or laterally along the surface of sloping fine-grained stratigraphic units. As DNAPL travels downward through the soil, it leaves a trail of small globules of NAPL trapped within the pore space of the soil. These trapped globules can be located in either the vadose or saturated zones of the aquifer. Along with the trapped globules, there may be pools of NAPL which have collected within stratigraphic traps as shown in Fig. 1. Quantifying the amount and spatial distribution of NAPLs trapped in the soil like this is a very difficult task. However, in order for any NAPL remediation method to be effective, it is essential that the NAPL zones be properly characterized. The traditional methods in the last 15 years of hydrogeological practice for characterizing the zone containing DNAPL involve (1) analysis of soil samples, (2) observations during drilling and other field activities, and (3) NAPL detection in wells. The first of these methods is inadequate in that soil cores do not allow sufficiently large volumes of typical aquifers to be sampled in order to permit a meaningful estimate of the residual NAPL saturation and chemical composition. The other methods are based on random observations and luck in the choice of drilling site. For example, Savannah River, SC, is a site which has been known to be contaminated with NAPLs, Over three hundred boreholes were drilled within a five-acre plume before any free-phase DNAPL was actually located. This lack of NAPL site characterization techniques has prompted EPA to call for improved field methods for rapid and inexpensive detection of NAPL zones. P. 919^