What is this gross orange stuff in my stream?

An orange rust slime is often seen where groundwater seeps out of the ground. Sometimes it can be seen attached to branches, roots, and the cobble of stream beds. At times this slimy muck may be accompanied by an oily-looking, rainbow-colored sheen floating on the surface of the water, and a bad smell. Although this water condition looks strange, this process occurs naturally and is not toxic to fish and aquatic life and does not pose an environmental health risk. What is being observed is part of a natural bio-geologic process.

Iron is a common element and is one of the most ubiquitous materials on earth (35% of the planet) and is the fourth most abundant element in the Earth’s Crust (5% by mass). Some microbes are chemotrophs and obtain energy by the oxidation of electron donors in their environments and have the ability to use iron as an electron acceptor and electron donor to gain energy, effectively feeding on iron. Instead of getting energy from consuming reduced organic material, iron-oxidizing bacteria (Sphaerotilus-Leptothrix) get theirs through changing ferrous iron (Fe2+) into ferric iron (Fe3+).

Iron from the sediments, which is usually the oxidized or ferric form (Fe3+) which is insoluble, gets into the groundwater where there are iron-reducing bacteria  (FeRB) that eat organic matter in the water.  Since there is no oxygen in the groundwater to support aerobic respiration where they live, these iron-reducing bacteria use something other than oxygen to oxidize the organic matter they eat:  ferric iron.  This results in the release of reduced ferrous iron (Fe2+), which is soluble in water.  The groundwater carries this soluble ferrous iron to places where the groundwater comes to the surface in a spring or a creek bottom, and there it serves up the ferrous iron to the iron-oxidizing bacteria waiting to feed on it.  The iron-oxidizing bacteria (aka iron bacteria) combines the ferrous iron (Fe2+) from the groundwater with oxygen from the atmosphere and the creek, and produces iron oxide (Fe2O3) or rust, which is insoluble, and makes the orange rust-colored deposits called floc (aka flocculate) in stream beds.

So, the bright orange rusty substance that is observed in wetlands, lakes and streams stream is just that, rust (iron oxide deposits) observed a) associated with a gelatinous mass of oxidizing iron bacteria

Although these bacteria colonies can impart a musty unpleasant swampy, oily or petroleum, cucumber, sewage, rotten vegetation, or musty taste or odor to the water, they are not toxic. However, bad tasting water is a concern to water well users and the presence iron bacteria may lead to treatment (physical removal, chemical treatment, pasteurization and/or chlorination).

Also deceiving is that at times the orange mucky floc may also be accompanied by an oily-looking, rainbow-colored sheen floating on the surface of the water. This sheen is not caused by oil but is a biofilm produced by iron bacteria.  These sheens are different from those caused by petroleum products because they break apart when disturbed. Throw a stick into the water. If the sheen breaks apart into pieces, it is probably a result of iron bacteria. If it doesn’t, petroleum products, which are toxic, might be contaminating the water.

It’s important to note that harmless iron floc occurs primarily around a pH level of 6.5 or 7 in freshwater habitats. When bright orange discharge is accompanied by high acidity (pH between 2.5-5.0), this could be an indicator of drainage from exposed acid sulphate soils or other acidic/sulfidic material such as pyrite from marine sediments, which is dangerous to water quality and aquatic organisms and requires management considerations and mitigation.

Erick Burres

Clean Water Team Coordinator

California Water Quality Collaboration Network Facilitator

Safe to Swim Network Co-facilitator

Email: erick.burres@waterboards.ca.gov

Phone: 213 712 6862 mobile

Mailing address: Erick Burres – Clean Water Team C/O SARWQCB - 3737 Main Street, Suite 500, Riverside, CA 92501