HYDROLOGIC CHARACTERISTICS OF CONTAMINATED SOILS AT THE NATIONAL CRUDE OIL SPILL FATE AND NATURAL ATTENUATION RESEARCH SITE

Even thirty 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. The contamination also detrimentally affects the growth of non-woody and woody vegetation within the spray zone area. To evaluate the surface and shallow subsurface hydrology of the spray zone, and thereby provide information for future studies of the deeper unsaturated zone underlying the spray zone, a study has been initiated to characterize the water repellency of soil profiles within the zone. To conduct this study a surface grid of 80 points was established within the spray zone in late June, 2011. At the grid locations the water repellency of the surface has been characterized using the Water Drop Penetration Time (WDPT) test. The WDPT involves placing a drop of distilled water onto the soil surface and measuring the time required for the drop to infiltrate. The degree of water repellency is then classified based on this time into categories of wettable (<5 s), slightly water repellent (5-60 s), strongly water repellent (60-600 s), severely water repellent (600 –3600 s), and extremely water repellent (>3600 s). Many of the locations sampled on the grid have surface soils in the category of extremely water repellent. To assess the variation of water repellency with depth in the soil the WDPT will be measured at selected depths down to 15 cm at a subset of the grid locations. The infiltration characteristics for the soil will be quantified with a falling head method, the Modified Philip-Dunne infiltrometer, at six locations representing a range of surface water repellency. Samples of soil collected from the grid locations have been characterized in terms of texture. Generally the soils are composed of fine and coarse sand, with finer fractions being less than 20%. Other characteristics that will be derived in the laboratory from field samples include water retention and the critical water content. The critical water content is the threshold water content below which water repellency is manifested by a soil.