STUDY OF THE 25-YEAR-OLD NIPISI OIL-SPILL - PERSISTENCE OF OIL RESIDUES AND COMPARISONS BETWEEN SURFACE AND SUBSURFACE SEDIMENTS

During the years 1970-1972 the Nipisi, Rainbow, and Old Peace River pi peline spills occurred in the Lesser Slave Lake area of northern Alber ta. The Nipisi spill was by far the largest of the three spills and is also one of the largest land spills in Canadian history. The most rec ent field survey was conducted in 1995 in order to determine which cle anup methods were most successful and to provide up-to-date informatio n about any changes in residual oil and vegetative recovery 25 years a fter the spills. The comprehensive chemical analysis data of the Nipis i samples indicate that (a) the Nipisi samples can be categorized into three groups plus the background group, according to the contaminatio n level and degradation degree of the samples; (b) degradation of resi dual oil of surface samples (0-4 cm) had clearly occurred, and the wea thered percentages of this group of samples were estimated to be in th e range of 15-43%; (c) subsurface samples (10-40 cm) exhibited great q uantities of oil even 25 years after the spills, indicating that the n atural recovery rates were slow for this group of samples; (d) the ext ent of contamination and degree of degradation correlated strongly wit h sample depth; (e) oil trends were similar across the site, even thou gh different treatments had been used after the spill at the site.