On a 33-mile stretch of the Suwannee River between Dowling Park and Branford, Florida, every drop of water that joins the river comes from underground — from springs, and from groundwater seeping up through the riverbed. In July 1995, at low flow, U.S. Geological Survey scientists measured the river and 11 springs along it. The river gained 950 cubic feet per second over the reach, and its load of nitrate grew by 3,700 kilograms a day. The springs were carrying nitrate from the land into a stretch of river prized for its ecology and recreation.
That finding came from the USGS's National Water-Quality Assessment (NAWQA) of the Georgia-Florida Coastal Plain, sampled from 1992 to 1995: nearly 62,000 square miles of southern Georgia and Florida, home to about 9 million people.
A region that drinks groundwater
About 80 percent of the people in the study area get their drinking water from the ground, and nearly 94 percent of that comes from the limestone and dolomite of the Upper Floridan aquifer. Across much of the area the aquifer is unconfined and riddled with karst — sinkholes and springs — so what happens on the land reaches it quickly. More than half the land is forest grown for paper and lumber; about 25 percent is farmland, with cotton, peanuts, corn, soybeans and wheat in the middle of the region, vegetables throughout, citrus in the south, and dairy and poultry farms in the north and center.
The Upper Floridan aquifer is poorly confined, and so vulnerable to contamination, across much of the study area. Image from the U.S. Geological Survey's report.
Large farming areas and many large cities share the study area. Image from the U.S. Geological Survey's report.
Nitrate: high under farms, low under cities
In groundwater, nitrate exceeded the EPA drinking-water standard of 10 milligrams per liter in more than 20 percent of samples from agricultural areas. The median under farmland was 4.2 mg/L; in the row-crop area of south-central Georgia, nitrogen isotopes pointed to both chemical fertilizer and animal waste as sources. The highest reading of all, 33 mg/L, came from a well in a forest — beside confined animal feedlots. Sandy, well-drained soils, heavy fertilizer use, plentiful recharge and karst all put the region's farmland groundwater at risk.
Deep water and city water were cleaner. In urban areas, the median in the Upper Floridan aquifer was below 0.05 mg/L, against 0.95 mg/L in the shallow surficial aquifer, and nitrate generally fell with the depth of the well.
Nitrate was related to the land use at the well and tended to fall with depth. Image from the U.S. Geological Survey's report.
Streams never approached the drinking-water standard: 95 percent of samples held less than 1 mg/L of nitrate, and the highest, 2.5 mg/L, was in the Withlacoochee River in southwestern Georgia. Phosphorus was the bigger concern for streams — 28 percent of dissolved-phosphorus samples were above 0.1 mg/L, the maximum the EPA recommends for total phosphorus to keep aquatic plants in check.
The river and the aquifer trade water
Along the lower Suwannee, river and aquifer swap roles. At low flow, groundwater carrying nitrate pours in through springs and the riverbed; measured springs supplied about 40 percent of the river's gain in that reach, and other groundwater about 60 percent. Nitrate in the springs ran from 1.3 to 8.2 mg/L, and in the river it almost doubled through the lower segment, where 89 percent of the added load entered. Troy Spring, the largest, delivered the most.
At high flow it runs the other way. When the Suwannee flooded Little River Springs, the river stood about 10 feet above the spring's usual level and poured into the aquifer — and nitrate in the nearby groundwater fell.
Measured springs supplied nearly half of the increase in nitrate load along the reach. Image from the U.S. Geological Survey's report.
At high flow, April 10, 1996, water levels in wells stood 10 feet higher. Image from the U.S. Geological Survey's report.
Pesticides: the city creek surprised
Of 85 pesticides and breakdown products analyzed, 32 turned up in streams and fewer in groundwater, but none exceeded a drinking-water standard. The herbicides atrazine, metolachlor and prometon were found most often, and no insecticide appeared in groundwater. Two insecticides, diazinon and malathion, did sometimes exceed criteria for protecting aquatic life — and more often in an urban stream in Tallahassee, Lafayette Creek, than in three streams draining farmland. In the farm stream, detections followed the planting and harvest seasons; in the city creek they did not, and from June through August insecticides were found about as often as herbicides. Even in forests, herbicides showed up in 4 of 19 well samples.
Diazinon in an urban stream exceeded aquatic-life criteria far more often than in farm streams. Image from the U.S. Geological Survey's report.
Radon and old pesticides
Radon in groundwater ranged from 50 to 40,000 picocuries per liter — among the highest in the nation. It comes naturally from uranium in the rocks, and is highest where phosphate deposits lie: the median was 1,150 pCi/L in the surficial aquifer of the Central Florida Ridge, and 720 pCi/L in the Upper Floridan aquifer. There was no drinking-water standard for radon; as a rough guide, 10,000 pCi/L in household water adds about 1 pCi/L to indoor air.
In streambeds, organochlorine pesticides long out of use still persist: DDT's breakdown product DDE was found in 45 percent of samples, and in 22 percent of samples organochlorine pesticides — mostly chlordane and DDT and their products — exceeded aquatic-life criteria. Streambeds in urban basins held the most organic compounds — 40 or more in two of them.
Radon was high in both the surficial and Upper Floridan aquifers. Image from the U.S. Geological Survey's report.
Sources
- U.S. Geological Survey, Water quality in the Georgia-Florida coastal plain, Georgia and Florida, 1992–96, Circular 1151, a contribution of the National Water-Quality Assessment Program.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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