To safeguard Broward and Miami-Dade against rising seas, planners need to understand which areas within each county are most vulnerable.

That’s one takeaway from a study released last week by the RAND Corporation. Researchers looked at how changes in sea level and rainfall will impact private property and drinking water wells in Miami-Dade and Broward from now till 2040.

The study found that Broward’s freshwater wells are much more vulnerable than Miami-Dade’s.

"As the tide comes up or water levels come up, you have more salty water that’s moving towards the west, towards these wellfields," said Jennifer Jurado, chief resilience officer for Broward County.

Credit Kate Stein / WLRN / WLRN Broward's chief resilience officer, talks to city officials from throughout Broward County at a resilience roundtable in May.

Jurado and other South Florida planners say rising seas are a risk for the cities with wells close to the coast.

"There have been a number of cities that have needed to look at real estate outside of their municipality for wellfield placement," Jurado said.

County maps show saltwater has intruded underground past the Fort Lauderdale airport, threatening wells for several cities, including Hallandale Beach and Fort Lauderdale (which has wells providing water to several other municipalities).

Jurado said, in total, the 31 municipalities in Broward have 25 different water providers.

The risk aren't the same in Miami-Dade. The county has one main water provider, the Miami-Dade Water and Sewer District, and decades ago planners moved all the county’s wells to the west.

The study found Miami-Dade wells are unlikely to be at risk from rising seas in the next 20 years.

Read more: Broward Leaders Are Worried About How To Pay For Sea-Level Rise

The researchers also say both counties have billions of dollars in vulnerable assets: today, $12 billion dollars' worth in Broward; $2.1 billion dollars in Miami-Dade. Those numbers are expected to grow because of future development, in addition to sea-level rise and expected changes in rainfall patterns and the subsequent impact on groundwater.

The study authors write that, "directing growth toward areas of higher ground" in Miami-Dade, "or concentrating it in certain transportation corridors (Broward) could mitigate assets at risk."