PNNL

Abstract

In the study of flow and transport in porous media, dual-energy gamma radiation is used to either simultaneously determine the bulk density and the volumetric liquid content in an air-liquid system (porous medium either rigid or non-rigid), the volumetric liquid contents of two liquids in a two-liquid system (rigid porous medium with or without air), or the salt concentration and volumetric liquid content in a saturated one-liquid-system (rigid porous medium). As was recently pointed out by Oostrom et al. (1998), the use of a single radiation source is sometimes sufficient and, in fact, provides more reliable data. An application of the use of one radioactive source is the determination of the volumetric liquid content values in two-liquid systems, such as a porous medium containing water and a non-aqueous phase liquid (NAPL). In this paper we will briefly review the principles involved in, and the possible errors associated with, the measurements of single- and dual-energy gamma radiation systems for the determination of volumetric liquid contents in rigid porous media containing water and a NAPL. The discussion will include an outline of an improved calibration procedure to determine the incident count rates. Subsequently, we will discuss the advantages and disadvantages (limitations) of gamma radiation measurements. Finally, we will present examples of experiments in which gamma radiation measurements were obtained to determine volumetric liquid content values of light and dense NAPLs after NAPL spills had occurred and during remediation processes. The use of gamma radiation is not only attractive for NAPL saturation measurements after spills and during remediation, but also for the measurements of capillary pressure-saturation curves. The latter are needed as input into computer models to predict NAPL behavior in porous media.