NATURAL DISPERSION OF OIL IN A FRESHWATER ECOSYSTEM: DESAGUADERO PIPELINE SPILL, BOLIVIA

During a flood event in January 2000, approximately 29,000 barrels of mixed crude oil and condensate was accidentally released from a fracture in the OSSA II pipeline at a crossing point on the Rio Desaguadero, Bolivia. The resultant downstream impact zone included over 400 km of river banks and 500 km2 of flooded lowlands within the Bolivian Altiplano. Analysis of stranded oil samples recovered from the riverbanks suggested that significant oil loss occurred by evaporation (>40%). With additional data on the amount of surface oil remaining (<0.2%) and oil recovered (3–13%) at the end of cleanup operations, it became apparent that a large fraction of the spilled oil remained unaccountable (27–37%). Laboratory tests were undertaken to assess if oil-mineral aggregate (OMA) formation—a natural process believed to promote rapid oil dispersion—may provide part of the explanation for this unaccounted loss. OMA were generated from local oil, sediment and water samples under simulated field conditions. Epi-fluorescence microscopy analysis verified the presence of flake aggregates that would be formed typically by the interaction of oil with smectite minerals (e.g. montmorillonite). On the basis of these microscopy observations and of mineral analysis of suspended material in water from the spill site, which revealed abundant smectite, it is hypothesized that OMA formation occurred, and that this process facilitated significant removal of residual oil from the ecosystem by enhancing biodegradation rates.