Previous studies have documented enhanced rates of oil removal from marine sediments by physical dispersion and biological degradation processes following the formation of oil-mineral aggregates (OMAs), which are microscopic particles of oil stabilized by fine minerals. In January 2000, approximately 29,000 bbl of crude oil were accidentally released from the OSSA II pipeline in the Bolivian Altiplano at a point crossing the Rio Desaguadero. Mineralogical analysis of sediments from the Rio Desaguadero basin revealed the presence of clay minerals known to readily interact with oil to form OMA. In support of laboratory tests that showed a significant amount of OMA formation in low salinity waters (0.35), OMA formation was observed when samples of Rio Desaguadero water and sediment were mixed with pipeline oil. Oil dispersion and enhanced biodegradation rates facilitated by rapid OMA formation after the spill incident may explain the oil fraction (27-37%) that was unaccounted in mass balance models that considered factors such as evaporative loss and oil recovered by clean up operations. (C) 2002 Elsevier Science Ltd. All rights reserved.