WATER RETENTION CHARACTERISTIC AND UNSATURATED HYDRAULIC PROPERTIES OF CONTAMINATED SOILS AT THE NATIONAL CRUDE OIL SPILL FATE AND NATURAL ATTENUATION RESEARCH SITE

Even forty years following the 1979 pipeline break at the NCOSFNARS near Bemidji, Minnesota the land surface within the zone sprayed by pipeline oil shows signs of oil contamination. The contamination has caused the surface to be water repellent, and this repellency is suspected to exert a strong influence on the spatial and temporal pattern of recharge and chemical loading to the shallow groundwater underlying the spray zone. This repellency also affects the establishment of vegetation on the site, resulting in very sparse vegetative cover. The water repellency affects the water retention and the unsaturated hydraulic properties of the soil at the surface and in the shallow subsurface. To quantify the water retention and unsaturated hydraulic properties of the soil in the spray zone we collected samples of the soil from different locations and depths, and in the laboratory measured the drainage cycle moisture retention curve using hanging water columns. For the same samples we also quantified the unsaturated hydraulic properties of the samples using a mini-infiltrometer device. The measurements with the infiltrometer were conducted using the samples without and with a mixture of ethyl alcohol. The ethyl alcohol is a surfactant that makes a water repellent soil, water wettable. These same samples were also examined using the MED (Molarity of Ethanol Droplet) water drop test where the applied water droplets represented a range of ethyl alcohol concentrations. The measured water retention characteristic and the unsaturated hydraulic properties of the soil samples will be related to the measured degree of water repellency. Also, the resulting characteristics/properties will be compared to the same characteristics/properties presented in the literature for soils that have been made water repellent following forest fire.