Environmental UCMs result from highly degraded petroleum hydrocarbons and once formed they can stay largely unchanged in sediments for many years.  For example, in 1969 a diesel oil spill contaminated saltmarsh sediment within Wild Harbor River, US; by 1973 only a baseline hump was observed, which remained largely unchanged within the anaerobic sediment for the next 30 years.  In a study of the potential for UCM-dominated oil to be further degraded, it was concluded that even using bacteria specifically adapted for complex UCM hydrocarbons in conjunction with nutrient enrichment, biodegradation rates would still be relatively slow.  Bacterial degradation of hydrocarbons is complex and will depend on environmental conditions (e.g. aerobic or anaerobic, temperature, nutrient availability, available species of bacteria etc.).