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Hermione Cockburn: This is exactly what I’ve been looking for. This whole bed exposed here is limestone, and you can tell because it’s this beautiful pale, grey blue characteristic colour, and it’s also highly soluble compared to other rock types so the surface is often smooth and fluted where it’s been eroded by rainwater. But limestone is made up of the remains of tiny sea creatures, millions and millions of them that accumulated on the bed of a warm shallow sea millions and millions of years ago that was then compacted into this hard sedimentary rock.

And as well as being made up of just the remains of tiny creatures, you also commonly find fossils of bigger plants and animals. And this rock here has just got some beautiful crinoid fossils in. You can see them. This one’s amazing, this is the stem of a crinoid or sea lily here, and all these little discs that you can see here are parts of them. Now the bulk of the limestone is made up of calcium carbonate, and that’s an alkaline, so once I’ve crushed this up it’s going to be ideal for neutralising the acidic mine water as it comes pouring out of that mine, so I’m just going to collect a few bits and take it back to the mill.

Kate Humble: In theory what should have happened?

Hermione: Well I’ve left mine water sitting with limestone overnight and what I’m hoping is that the water will have neutralised and the pH will have gone up to 7.

Kate: Okay, and 7 was what you said that good, clean mountain stream water should be.

Hermione: Yes, that’s neutral.

Kate: Okay.

Hermione: So, and as we saw yesterday, the mine water is lower, so do you want to test the mine water?

Kate: Okay, we’ll put them in at the same time, in it goes.

Hermione: If this is 7 it should be yellowish.

Kate: Okay, and mine should still be that sort of dark orangey colour. So when you’re happy.

Hermione: Okay, ready?

Kate: Look! I can see the difference already.

Hermione: Wow, look at that, that’s pretty good.

Kate: That’s amazing!

Hermione: So, well mine’s sort of 6-ish.

Kate: 6 to 7.

Hermione: Yeah, so it’s pretty good. Yeah, that’s not bad. Actually it’s sort of turning more yellow as we watch. So, hmm.

Kate: So basically that indicates that the limestone is doing the right thing, is basically neutralising that acid.

Hermione: Yeah.

Kate: Now when you come to scale this up, which presumably you’re going to need to do,

Hermione: Yes.

Kate: is there a worry that it could tip it the other way and actually get the mine water to become alkaline, which presumably would be just as bad?

Hermione: Yes it would be, but that is the beauty of using a natural product like limestone and this passive system, because in a way the reaction is self-limiting.

Kate: Okay.

Hermione: The acidic mine water partially dissolves some of the calcium carbonate that makes up limestone. That releases carbonate components which neutralises the water, but once it’s neutral the reaction stops because there’s no more acidity to attack the limestone.

Kate: Oh brilliant.

Hermione: So, so long as I’ve got enough limestone it will just neutralise the water and leave it at that.

Kate: Great, okay, well you’re going to have to presumably go and dig out a lot of rocks.

Hermione: Yes.