<title>Fluorescent response of fuels in soils: insights into fuel-soil interactions</title>

Abstract

We have carried out experiments to examine calibration issues for a sensor system for real- time, remote, in situ monitoring of POL pollutants in soils, consisting of a pulsed UV laser/photodiode array fiber optic fluorescence system installed in a truck-mounted cone penetrometer. We have studied the fluorescent response of fuels as a function of soil type and conditions, and found that the fluorescent response of diesel fuel marine (DFM) varies by an order of magnitude or more as a function of soil type, exposure time, and moisture. The controlling variation for fuel fluorescent response is available surface area of the substrate, although exposure time, grain size, mineralogy, moisture and degree of soil aggregation cause secondary effects. Experiments to study the effects of long term exposure to sands and clays to DFM, with and without the addition of water, have provided insight into the interactions of fuel with the soil materials. Given time, DFM will soak into available pore spaces in a high surface area soil material, decreasing the bulk fluorescent signal over the first 10 - 12 days of exposure, and then stabilize. While the addition of water displaces some fuel from the soil surface and pores, this effect is limited in soils which are given adequate time to equilibrate. Such soil-fuel interactions may inhibit remediation efforts in clay-rich soils. Results from these experiments allow for more effective calibration of in situ fluorescent measurements in the field.