Leachates were collected during the period of 1981 to 1983 from several depths in two surface piles of raw, mined oil shale in the Piceance Basin of northwest Colorado. Although both piles were subject to similar climatic conditions, the composition of major inorganic ions in the leachates differed considerably because of different shale compositions. The study demonstrated that toxic mechanisms occurring within complex chemical mixtures cannot always be identified on the basis of single-compound toxicity values, and that reconstituted mixtures of principal chemical components are useful in identifying toxicants and their interactions with other mixture constituents.