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The Loam Ranger - Iron chemistry: Colours in the landscape

Dear Loam Ranger,

After the recent heavy rain we’ve noticed a slick of some orange-coloured slime running across paths and an oily film on the soil surface. Is it harmful?

We’ve had a lot of calls in the last 6 months from people concerned that they have oil slicks in their creeks or dams or running onto their property, or that their dams have become contaminated. Often this is associated with springs and water leaks they have not seen before. Typically these oily surface slicks refract light and show a rainbow of colours. The first thing we tell callers to do is lightly touch the oily film. If it swirls and flows together again it is likely due to oil, but if it breaks up like shattered icebergs it is due to iron.

Iron is a remarkable element in that it can exist in soil in both a fully oxidised condition (rust or ferric oxide) and a partially “reduced” form (ferrous oxide). It can become partially reduced in waterlogged soils owing to a lack of oxygen. (To become fully reduced back to the solid metal requires a blast furnace!) In the partially reduced or ferrous form it typically forms grey or greenish colours, which explains the colour of mangrove mud soils. When exposed to the air it rapidly oxidises and forms bright orange, yellow and red colours typical of dry landscapes such as deserts. These ferric oxides are extremely insoluble, whereas ferrous compounds are much more soluble.

Water flowing underground where oxygen is absent can dissolve ferrous salts. When this water meets the atmosphere, the ferrous salts will quickly oxidise, forming rusty weeps or, on very slow-flowing or stagnant water, a surface “slick” only a few molecules thick that refracts light. Unlike oil it is a rigid crystalline solid and will break up and not reform. In fast flowing water or in windy conditions a slick does not form, but rather a haze of suspended brown iron oxides forms, making the water look muddy.

Many bore waters contain ferrous iron and show constant problems of iron oxide depositing in tanks, in lines and on plants. It takes very little iron oxide to impart a strong colour; it is strongly staining, leaving dirty orange to brown stains on concrete paths, buildings and plant leaves.

In eastern Australia, soils oscillate between waterlogged and dry owing to the El Niño cycle. Under waterlogging, iron changes to the soluble ferrous form. When the soil dries out again, the iron precipitates again as ferric oxide, forming ironstone, a reddish brown, very hard, cement-like mineral. In some soils this forms little balls or “iron gravel”. In others it forms a plate-like deposit.

Sometimes the red is tinged with black, because manganese also oscillates between a soluble manganous form and black, insoluble manganic oxide. For the same reason, you may also see black balls of manganese oxide in subsoils and black weeps from drainage holes in retaining walls.

The reason we are seeing a lot of iron and manganese moving around lately is the unusually high rainfall following a “La Niña” period, which has brought springs and creeks to life again from groundwater flows. Retaining walls, basements and springs are commonly seen releasing rusty water, and even “oily” slicks. This is not harmful, and illustrates a soil chemical process that is fundamental to the way our soils have formed over time