Effects of Soils on Laser Induced Fluorescence of BTX Contaminated Pore Waters

A laboratory testing program was conducted to identify and interpret the fundamental factors affecting the performance of a new microchip-laser based fluorescence sensor in soil and ground water. Investigations were performed using a versatile experimental apparatus designed to simulate the in situ interface between the laser induced fluorescence (LIF) sensor and contaminated media while providing complete control of test conditions. Attempts were made to isolate the effects of soil properties such as grain size, soil type, color, mineralogy, and organic content on in situ LIF observations. Test results indicate that soil has no measurable effect on the determination of pore fluid fluorescence lifetimes or the general form of pore fluid emission wavelength profiles. However, for a given contaminant concentration in the pore space of a soil with a narrow grain size distribution, decreases in soil grain size are accompanied by a decrease in the magnitude and variability of observed LIF signals. For soils containing a wide range of particle sizes, in-soil LIF observations are primarily influenced by pore space geometry in relation to the smallest particles present in the soil. These trends were found to be a primary function of the volume of pore fluid in a soil specimen that is in the direct path of laser excitation energy. Soil organic content and optical characteristics such as reflectivity also have potential influence on in-soil LIF observations, although on a secondary basis. After reviewing these experimental results, which indicate the relative impact of soil properties on fluorescence observations, a simple geometric model is presented that captures the primary effects of soil on pore fluid fluorescence observations.