REAL TIME MONITORING OF NAPL SOURCES USING PHOTON ATTENUATION TECHNIQUES ON CHLORINATED SOLVENTS

Slow dissolution of nonaqueous-phase liquids (NAPL) trapped within aquifer soils provides log-term sources of groundwater contamination. In heterogeneous media, NAPL is entrapped as pools with high NAPL saturation and dissolution occurs predominantly at interfaces between pools and bulk aqueous phase. Generally, in most modeling and dissolution studies of DNAPL (dense NAPLs) source zones containing pools, it has been assumed that the saturation distribution within the pools is homogeneous, when in reality the saturation within the pool changes with depth. One of the most significant problems in source depletion characterization in experimental studies has been the absence of a technique to study this zone in a non invasive way to avoid physical changes of the source as well as the flow field. Principal objective of an ongoing research project is to use two photon attenuation measurement devices (X-ray and Gamma-ray) to obtain the changing saturation distribution in source zones containing pools undergoing natural as well as surfactant enhanced dissolution. The goal is to evaluate the capability of exiting modeling methods to predict mass transfer from pools. As the process of mass transfer from pools occur at very slow rates, monitoring of spatial and time distribution of saturation changes require automation. This paper presents the development and testing of an automated dual-gamma system to measure NAPL saturations from a pool during dissolution. The accuracy of the measurement is compared with another photon attenuation technique based on x-rays.