Groundwater is important in many ways.

It's an essential part of the water cycle - supplying rivers and lakes with freshwater.

It's crucial for wildlife and vegetation.

It cleans our surface waters through natural processes.

It can act as a vital buffer for freshwater - for humans living in a world with increasing demands for water.

When water seeps through the layers of the subsurface - it'll be filtered and cleaned by natural processes.

The cleaning process comes in a variety of forms.

There's a physical part where large substances are held back - and other compounds like iron will precipitate and settle on grain surfaces.

There's also a bacterial part -  where a variety of bacteria will interact with compounds in the water.

Some bacteria interact with organic compounds in the water - and decompose these into simpler forms.

Eventually, they'll end up as pure CO2 and water.

The chemical processes and environment will affect the composition of the water.

Anoxic conditions, where there's no free oxygen in the pore space - will be very efficient in removing harmful bacteria such as E. coli.

The cleaning effect starts from the moment the water enters the ground.

But the efficiency depends on the type of rock, soil and sediment - that the water moves through.

For bacterial activity to have an effect on the degradation of compounds - time is an important factor.

The longer the time, the more complete the degradation will be.

This means that the cleaning effect is less in coarse sands - where the water flows fast, than in more clay-rich materials - where the water flow is very slow.

In this manner, a shallow clay-rich layer - may protect a deeper water-bearing sand layer - from harmful substances seeping down from the surface.

So, the subsurface cleans the water making it a safe and valuable resource.

On top of being a clean resource, it also plays a role for a robust water supply.

Surface water supplies are widely used both for irrigation and drinking water.

But they bear the risk of drying out if we have extended drought periods - as we saw recently in Cape Town as a prominent example.

Groundwater, on the other hand, reacts slowly and in a delayed manner - to these annual variations in rainfall.

This means that groundwater can be used as a safe resource for drinking water - during dry periods.

In areas where surface water plays an important role in the fresh water supply - groundwater can have an important role acting as a buffer in dry seasons.

To put it simply: groundwater is very important for the water cycle in nature.

It's also a very efficient water purifier.

In that context, groundwater can act as a sustainable water resource in dry regions.