These researchers from Canterbury University have come up with an idea that they say can cheaply remove 3 million tonnes of CO₂ from the air each year. And it utilises existing infrastructure.

The TLDR is that geothermal water contains CO₂, and the stations here have systems that capture and dissolve that CO₂ into the water before it’s reinjected. This CO₂ rich water is heavier than the surrounding water, so it sinks to the bottom.

The heat from geothermal wells is not replenished at the speed it’s taken by the stations, so eventually the water coming out isn’t hot enough and new wells need to be drilled.

The scientists are suggesting that instead of simply drilling a new wells, we burn biomass from forestry to heat the warm water up the last bit. The carbon in this fuel has been gathered by the trees, and if it was released into the atmosphere it would be carbon neutral. But if you use the station’s existing CO₂ capture and dissolve systems, the carbon goes underground permanently. The operation becomes a carbon sink that also enables the use of geothermal energy that would otherwise be unusable.

They say, “in terms of buying ourselves out of an emissions liability, geothermal carbon removal is one of the cheapest options out there.” They compare it with the cost of switching from a petrol to electric car - US$700 for each tonne of CO₂ saved. With the existing infrastructure, they say their plan would remove CO₂ at a cost of about US$55 a tonne.

Their papers and a bunch more evidence and info is linked in the article.