Six Hours or Half a Million Years: How Larkin City's Water Sources Change

Planning Board Notes: Granite Lake and the East Wells

1Larkin City, a city of about 85,000 people in the hills of the southeastern United States, gets its water from two places. Most of it comes from Granite Lake, a reservoir that formed behind a dam on the Tolan River in 1960. Water flowing through the dam also spins turbines that make about a fifth of the city's electricity. The rest comes from wells in limestone east of town. This year the planning board is writing a 50-year plan for water and energy, and city engineer Ms. Henrietta Vu asked two seventh graders on the youth council, Pilar Ndiaye and Kasimir Oyelowo, to study how the land around these water sources is changing.

2Where the Tolan River enters the still water of Granite Lake, it slows down and drops the sand, silt and clay it carries. The sand has built a delta, a fan of sand that now reaches about 3 km into the upper end of the lake. Finer mud drifts farther and settles over the lake bottom toward the dam. When it was new, the lake held 400 million m³ of water. Every ten years, a boat with sonar maps the lake bottom to find how much of that space sediment has filled (Graph 1).

3In the summer of 2014, a wildfire burned about 20 km² of steep forest above the upper end of the lake. Burned soil soaks up very little water. The next spring, a heavy storm sent debris flows, fast-moving slurries of mud, rock and burned logs, down the bare slopes. In about six hours they carried roughly 2 million m³ of sediment into the lake.

4East of town, the wells draw water from limestone, a rock made from the shells of sea animals that piled up on an ancient sea floor. The limestone is about 400 m thick, lies under much of the region and took roughly 5 million years to build up. Rainwater picks up a little carbon dioxide from the air and soil, which makes it slightly acidic, and it dissolves limestone very slowly as it soaks through cracks. A camera lowered into one well showed a crack that has widened into a channel about as wide as a hand; geologists estimate this took roughly 1,000 years. Under the east side of town, mapped caves have about 20 km of connected passages, which formed over about 500,000 years. Each year the springs carry about 9,000 t of dissolved limestone out of the region, even though their water looks clear. On average, the limestone surface there is being lowered by about 0.05 mm a year.

5In 2022, during a dry summer when the wells were pumping hard and the water table dropped quickly, the ground under a parking lot gave way in less than an hour. The sinkhole was about 12 m wide and 8 m deep. Water filling a hollow underground helps hold up the soil and rock above it.