Saltwater intrusion is just one of the risks facing South Florida's drinking water.

The Biscayne Aquifer, a 4,000-mile sponge-like rock formation that filters and stores the region's clean groundwater, is also being polluted by sewage runoff and other contaminants.

Christopher Flavelle, who covers climate change and adaptation policy for Bloomberg News, told Sundial that protecting the aquifer will cost billions of dollars.

WLRN: What are the risks and threats to South Florida's drinking water and how are these risks connected to climate change?

Flavelle: It's a series of different risks, all tied to climate change. The risk people are most familiar with is saltwater intrusion -- this notion that as seas rise you'll get more salty water being pushed into the aquifer. Over time, though no one knows over how much time, the saltwater will start making it hard if not impossible to draw from the various well fields that are in the county.

What I found in my reporting is that is not the only risk. It might not even be the worst risk. There's also this question of septic tanks. There are about 90,000 septic tanks throughout Miami-Dade County, others in the rest of the region, and those septic tanks depend on a very delicate balance. As the groundwater level rises with sea-level rise, those tanks don't work as well. They don't manage to filter and clean the human waste that goes into them and as a result, you get more bacteria, medication and other stuff you don't really want in your drinking water going into the aquifer.

The third one is toxic contaminants getting pushed out of Superfund sites, other industrial sites by more intense rain events and flooding. Those get washed into the aquifer.

The fourth is this rock mining activity in sort of the northern part of the county where the Everglades start. People say that the concern with that rock mining is it creates sort of a superhighway from the surface into the heart of the aquifer. So whether it's contaminants from the mining or washed in from elsewhere in the county it goes into the aquifer making it harder to treat.

If you step back you get a pretty serious threat.

Do we know how much Miami-Dade County [relies] on septic tanks?

The number that I got from county officials was 90,000 countywide and to be clear no one said that the tanks were not of concern. There was a universal acceptance septic tanks were a problem. What no one knows is what to do about them. The figure I got for removing them or connecting all those homes to the main sewer system is $2-3 billion. You could do that it would be expensive but the second question is: Is it a good idea? Because a lot of those communities are by the coast and in 20, 30 or 40 years they could well be underwater and abandoned. So do you want to spend that money on neighborhoods that won't exist anymore?

All of these are a risk to our water but there is the problem of getting the money. Do we have to prioritize? Is one of those at the front of the list?

I think I'm the last person to weigh in to ... rank the risks. I think one of the problems is you don't really have a good sense of how to scale these risks and how to rank them. I get the sense that people are familiar with the septic tanks issue, but they don't seem especially concerned. I would defer to others who know the situation better to say if there should be more concerned that there is. But again, $2-3 billion ... is a huge amount of money. Is it the right thing to spend those limited resources on?

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