An “earthy” taste in Cape Town’s water supply is nothing to worry about, the City of Cape Town told residents yesterday.

“Over the past few weeks, water consumers in several parts of the city have experienced an earthy taste and odour in their drinking water,” the city said.

“We would like to reassure the residents of Cape Town that despite the slight taste and odour, the water is absolutely safe to drink.”

The city said the taste and odour were being caused by elevated concentrations of “geosmin” originating from two of the major dams supplying the city.

It said continuous water quality monitoring by an SA National Accreditation System-accredited laboratory had revealed elevated geosmin

concentrations greater than 10 nanograms per litre (ng/l).

In general, the human perception threshold was about 15 and 20ng/l for geosmin odour and taste.

People with a heightened sense of smell and taste would be the first to notice the presence of geosmin in their water supply.

“These are extremely low concentrations and it should be noted that a nanogram is a billionth of a gram,” said Peter Flower, the manager of the city’s bulk water branch.

Geosmin are naturally occurring compounds found in rivers and dams as organic molecules produced by blue-green algae.

Bright sun, warm temperatures and nutrients result in ideal growing conditions for the earthy smelling algae.

The compounds are produced inside the algae cells and are only released when the algae die.

Apart from the sunlight and high temperatures, algae require nutrients to grow and some of the dams have suffered from upstream negative environmental impacts that have raised the nutrient levels, which support these algal blooms.

This phenomenon usually only occurs once a year for a few weeks during the summer months.

Geosmin is typically earthy or musty in character and is sometimes described as smelling of fresh green mealies. – Sapa