Water Quality

Oil Spill or Decaying Vegetation?

In the late summer and fall, normal seasonal changes in the Utah mountain streams result in a natural plant decay process. This decaying plant matter produces a film or sheen on the surface of the water, similar in appearance, to the oily sheen produced when petroleum is spilled into water. As a result, concerned citizens have mistakenly reported their observation of this natural decay process, as a petroleum spill. The reports describe black residue, an obvious sheen on the water surface, and odors. Site inspection of the reported spills reveal a recurring cause related to lower seasonal water flows, which result in dead pools of water with low dissolved oxygen concentrations. The black residue is typically senescent (dying) algae or macrophytes that are abundant in the flowing streams. Reduced flows expose the algae, then drying or freezing of the plant cells produce a black appearance (similar in appearance to petroleum); however, the decaying vegetation has an “earthy- musty” odor rather than a more pungent petroleum odor.
The small dead pools can become anaerobic, creating an environment suitable for an iron-oxidizing, chemolithotrophic bacteria (Gallionella ferruginea).The bacteria derive needed energy by oxidizing dissolved ferrous iron or manganese. The resulting ferric oxide is insoluble, and appears as brown gelatinous slime on the bottom of the pools. Organic metabolic by-products often produce a visible sheen on the water surface, but unlike sheens caused by petroleum products, the iron bacteria sheens break apart when disturbed.

The sheen produced by this natural decaying process can be mistaken for the sheen produced by petroleum, but unlike petroleum, there are no environmental concerns for this natural decaying process.
 Decaying vegetation checklist:       The sheen is on the water surface of stagnant or standing water, the sheen breaks apart when disturbed, there is no petroleum odor.
  Oil spill checklist:     The sheen is on the surface of flowing water, the sheen does not break apart or disperse when disturbed, there is a petroleum odor.