Cape Town, South Africa, is highly reliant on groundwater for its rapidly growing population. But one of the main sources under the city, long in the spotlight for its dwindling water supplies, is at serious risk of being polluted.

Run-off from small-scale farms, landfills, cemeteries, factories and informal settlements risks leaching into the Cape Flats aquifer, which lies under much of Cape Town, found a recent study from the United Nations Environment Programme (UNEP).

Cape Town isn’t the only major city struggling with groundwater contamination. Urban centres from Pakistan to the United States have in recent years seen pollution creep into important aquifers, potentially threatening water supplies for millions of people.

“Groundwater has always been critically important but not fully recognized in sustainable development policymaking,” says Leticia Carvalho, head of the Marine and Freshwater Branch of UNEP.

“Exploring, protecting and sustainably using groundwater aquifers, like other freshwater ecosystems, will be central to surviving and adapting to climate change, conserving ecosystems and meeting the needs of a growing population.”

World Water Day on 22 March is highlighting the hidden resource that stores 98 per cent of the world’s liquid freshwater – groundwater.

Underground cache

Freshwater, though it makes up a relatively small portion of the world’s global water supplies, packs an outsize punch in terms of its importance for human and ecosystem health. Most of it, however, is not easily accessible: much of it is locked up in glaciers and snow and only 1.2 per cent is found in water bodies, including wetlands. The rest – around 30 per cent – is stored underground in water-holding rock or clay chambers called aquifers.

Groundwater in aquifers is often thought of as pristine.

However, aquifers are recharged by rain which, if mixed with soluble pollutants, such as petrol or fertilizers, can infiltrate porous rocks and pollute the groundwater below. Because it is filtered through rocks, groundwater can also naturally hold high amounts of minerals, such as fluoride or arsenic, which can be harmful to human health.

That’s especially problematic in the developing world. Many countries rely on naturally clean groundwater as advanced water treatment is economically infeasible, according to a paper by the “Friends of Groundwater” expert group of the World Water Quality Alliance. The Alliance was launched by UNEP and the European Commission in 2019.  

In the deep

UNEP has a global mandate to help countries measure and improve water quality, including by leading the development of the Global Water Quality Assessment and leading on the monitoring of Sustainable Development Goals (SDGs) relating to water quality and water ecosystem health.

While it is relatively easy to check the water quality of lakes and rivers, assessing the quality of “invisible” groundwater presents challenges.

For the UNEP study on Cape Town, researchers used satellite and aerial imagery to map potential pollution hotspots. That data, combined with on-the-ground measurements and numerical modelling, allowed researchers to chart the flow of groundwater and predict groundwater quality under future scenarios.

This ”triangulation approach” represented a significant achievement that could help guide a response to pollution in the Cape Flats aquifer, says Nina Raasakka, Coordinator of the World Water Quality Alliance.

A growing problem

According to the World Water Quality Alliance, groundwater supplies almost half of all drinking water and 43 per cent of water used for irrigation.

Experts say safeguarding groundwater quality is key to achieving the Sustainable Development Goals, especially those related to water (SDG 6), health (SDG 3), and food production (SDG 2).

That’s especially true amid a changing climate. In some arid and semi-arid regions, droughts may turn groundwater into the only reliable water resource.

But aquifers themselves are not immune to the climate crisis. More frequent and intense flooding associated with climate change could wash more faecal matter and toxic chemicals into groundwater aquifers (especially more vulnerable, shallow hand-dug wells), while in the longer term, sea-level rise could increase the salinity of coastal groundwater.

The UN-Water Summit on Groundwater in December 2022, and the International Year of Groundwater, also this year, aim to bring attention to groundwater at the highest international level, while highlighting the central role of freshwater in tackling the climate, biodiversity and pollution crises.