Yucks! What is that thing in my water?

All right, hands up, those of you who have seen "oily" patches like those in the 2 photos below. Let's count: 1, 2, 3...

Yes, they seem to be rather common in Singapore, whether in mangroves, streams, canals or ponds. And they appear throughout the year.

What are they? Are they natural or man-made? What are the ecological implications? Are they an indicator of water quality?

(Photo of Sungei Buloh Wetland Reserve (SBWR) by Anuj Jain 2009)

(Photo of Sungei Buloh Wetland Reserve (SBWR) by Anuj Jain 2009)

From the looks of the above photos, these "oil" patches could be caused by either real oil or certain metals (especially iron and manganese). To differentiate between them, try using a stick to disturb the patch. If it breaks up into smaller angular patches, it is likely to be metals. If the patch does not break but instead appears to follow your swirling, oil is present.

Iron and manganese are usually from the surrounding soils so yes, I would consider this natural.

Oils can also be from natural sources especially if nearby vegetation contains oils and somehow get into the water (pine oil, anyone?). A submerged dead animal can also produce oil from its fats.

Oils can be anthropogenic though. Without performing a lab analysis, it is difficult to differentiate between oils of natural and anthropogenic origins.

Ecologically, natural patches are probably harmless. Anthropogenic oil CAN be a problem, depending on its exact nature. Is it cooking oil, petrol, diesel, fuel oil or grease?

Yes, the appearance of oil in water can be an indicator of poor water quality but for most purposes, water quality is measured by a suite of parameters that can be quantified e.g. DO, oil content

Compare to the photo below. Even though there is a hint of an "oily" sheen, its characteristic colour (orangey) and angular fragments identify it as probably iron oxide. No prizes for guessing that it is similar to rust.

(Photo of a stream near Choa Chu Park by Robin 2009)