Tracer Experiments in Saturated Sand by Computed Tomography and Inductively Coupled Plasma

Abstract

The spreading of a tracer in saturated sand is important to the understanding of groundwater hydraulics and is fundamental to issues concerning groundwater quality. A thorough understanding of this spreading process is necessary for accurate forecasts of the behavior of groundwater resources. Computed tomography (CT) is a nondestructive testing technique developed for medical diagnostic purposes. Recently, soil scientists and petroleum engineers have used CT to measure a variety of soil properties including bulk density, porosity, water content, etc. Computed tomography scanning is based on attenuation measurements of X-ray beams which are directed through an object from many different angles. These measurements are numerically reconstructed with a digital computer to produce a tomogram or internal “picture” of the distribution of the X-ray attenuation in the object. This distribution can be correlated to the variation of other properties, such as those mentioned above. The time required to produce the tomogram is relatively short (seconds) compared to the duration of a typical flow experiment (minutes to hours); thus, CT scans can generate real-time data. The inductively coupled plasma (ICP) method is a standard method used in the analysis of groundwater for inorganic contaminants. It is based on the measurement of atomic emission by an optical spectroscopic technique. The objectives of this study are: (1) to use CT to develop a procedure that measures the spreading of saline tracers in saturated sand with X-ray attenuation measurements; (2) to test this procedure with laboratory flow experiments; and (3) to validate the CT flow experiments with chemical analyses of effluent samples using the standard ICP method. The results of statistical lack-of-fit analyses indicate that a linear model can be used to relate tracer concentration in saturated sand to X-ray attenuation. Breakthrough curves, plotted as relative concentration versus time, are obtained from flow experiments conducted with tracers and analyzed with CT technology. The results of this investigation indicate that existing medical CT technology can be used effectively to measure tracer concentration in flow experiments in saturated sand. The results of ICP analyses support the CT results. The ICP results also clarify the limitations of the CT procedure used here.