Sometimes two separate theories can, when analyzed individually, make perfect sense but when you try and reconcile these two theories with each other a contradiction occurs.  When that happens scientists must perform more analysis to show that either one theory was false or come up with a third theory that can explain the inconsistency.

There was a story on NPR today that showcased that phenomenon perfectly.

The story was about how rivers in the US used to be acidic (pH < 7.0) but now a majority of them are alkaline (pH > 7.0).  As you’ll see, both theories make sense when analyzed separately but they don’t make sense when you combine them.

Why were our rivers more acidic decades ago?

The story started back in 1963 in a New Hampshire forest. A young scientist named Gene Likens found a stream there that was as acidic as tomato juice.

Likens eventually found the culprit: acid rain. Industrial air pollution was acidifying water that rained down from the sky, killing trees and the ecosystems of streams in the East.

OK, that sounds plausible.  Acid rain was a big problem 50 years ago and this theory passes the smell test.  All things being equal, acidic precipitation falling into a body of water would lower the pH.

But now?

Now — 50 years later — there’s less acid rain. But rivers aren’t neutral, they’re alkaline, and that seems to be the trend in lots of places. “The real shocker to me,” Likens says, “was [that] we found it from New Hampshire to Florida, and in rivers and streams that drained agricultural land, forest land and urban land.”

Two-thirds of the 97 streams and rivers his team studied in the East have been growing more alkaline — from the mighty Susquehanna to small urban streams, like Gwynns Falls in downtown Baltimore.

Why are our rivers more alkaline now?

Acid rain is largely behind the phenomenon, the scientists say. It’s been eating away chunks of rock, especially limestone rock, and the runoff produces carbonates that flow into rivers. “We’re basically dissolving the surface of the Earth,” says Kaushal. “It’s ending up in our water. It’s like rivers on Rolaids. There’s a natural antacid in these watersheds.”

Again, this theory by itself makes sense but surely you are asking the same question that I asked after hearing this story (and which NPR didn’t ask).

Why didn’t the limestone rock, eroded by acid rain, flow into the rivers 50 years ago and offset the acid rain falling into the rivers?

If there is still enough acid rain to erode limestone rock then there should be enough to fall into the rivers and lower the pH.

I would have to assume that acid rain falling into rivers has a greater impact on the river’s pH than waiting for that same acid rain to erode rocks and have that limestone run into the river.  Acid rain falling into the water has an immediate effect and it takes much more acidic rain to erode rock and then carry that rock into the river.  Why is a smaller amount of acid rain eroding enough rock now to swamp out the falling acid rain in the rivers but it didn’t do that 50 years ago?

Is there another theory that the heavier acid rain decades ago weakened the limestone only to have the rocks fall off now?  That would be the 3^rd theory that reconciles the other two.

Just one more reason why I love science!  It’s rarely ‘settled’ and new theories always generate new questions.