MICROSCOPIC OBSERVATION OF A NAPL IN A 3-FLUID-PHASE SOIL SYSTEM

The focus of this study was to observe the location and form of a NAPL in a three-fluid-phase (air-NAPL-water) soil system utilizing an adva nced microscopic technique, cryo-scanning electron microscopy (cryo-SE M) with X-ray analysis. A sandy aquifer material was brought to residu al water (NaCl saturated) saturation (approximately 4% by pore volume) followed by NAPL (iodobenzene) saturation, ranging from approximately 1% to 80%, using modified Tempe(R) pressure cells. A small intact soi l core was obtained from the pressure cell and quickly frozen in liqui d nitrogen. The core was fractured and chromium coated at a high vacuu m and - 130-degrees-C. Secondary electron images (SEI) and X-ray dot m aps (Si, Cl and I) were made of samples with different NAPL saturation s. Photomicrographs and X-ray dot maps confirmed the existence of cont inuous NAPL films on soil (containing a residual water saturation) at high NAPL saturations. Photomicrographs revealed v-shaped wedges, pend ular rings and films on irregular shaped sand grains. At low NAPL satu rations (approximately 1% by pore volume), iodine was virtually nondet ectable in the overall X-ray analysis of the sample. This suggests tha t the small quantity of NAPL present in the sample probably existed as thin films or small isolated lenses or blobs.