Dyes and isotopes track groundwater from sink to spring

The hydraulic connection between a sinkhole and a natural spring—the longest and largest yet documented—could help reduce the guesswork in mapping karst aquifers.

Beneath Florida’s cities and swamps lies a complex network of karst conduits. The same chemical weathering that carves truck-sized tunnels through the calcium carbonate rock also leads to sinkholes at the surface. For Florida insurance agents, sinkholes are a headache. But for the state’s hydrogeologists, every sinkhole is an opportunity to understand the aquifer below.

Sinkholes allow surface water, as well as contaminants, to flood into an aquifer. By mapping the network of entry points and exit springs, hydrogeologists can better understand the underground system and better protect drinking water at the source. That understanding is important to populations outside Florida: Karst aquifers provide drinking water for 25% of people on Earth.

Isotope analysis helps hydrogeologists trace water origins, but the technique’s use has generally been limited to sinkhole lakes and springs no more than 4 kilometers apart. Recently, however, a team in Florida used isotope ratios to connect points 32 kilometers apart. It’s the farthest hydraulic connection between a sinkhole and a spring yet documented and the first connection involving a first-magnitude spring (those discharging an average of 100 cubic feet—2.8 cubic meters—of water per second).