By: Tommy Cook, P.E., Mark Thomas, P.E., and Robert L. Farley
Like many states, Florida uses millions of tons of fertilizer every year for crop agriculture and landscaping. However, what’s good for the plants has proven to be adverse for water quality. Runoff from fields, lawns and roadways enable chemical fertilizers with highly soluble nitrogen (NH3) to percolate into groundwater, leach into aquifers and damage sensitive environmental ecosystems. In some Florida locations, the public’s drinking water sources are being threatened.
Since 1980, the concentrations of nitrates in the waters of Wakulla Springs, near Tallahassee, have increased from approximately 80,000 kg annually to 270,000 kg in 2005. Florida’s Department of Environmental Protection (DEP) found that high nitrate levels were caused by sewage sludge extruded in sprayfields from Tallahassee wastewater plants, an increased number of septic tanks due to urban sprawl and fertilizers used in agriculture and landscaping.
In 2005, the DEP contacted the Florida Department of Transportation (FDOT) District 3 for help in minimizing the use of fertilizer application along roughly 200 miles of roadway in the Wakulla Springs basin.
At first, FDOT District 3 considered not fertilizing at all. But after much deliberation, it was decided that some measure of fertilization was needed in the basin area. Fertilizers are used to stimulate grass root growth, which in turn holds topsoil in place to stabilize roadway shoulders and reduce erosion. Every two years, the FDOT District 3, which encompasses 16 counties across the northwest panhandle of Florida, had applied close to 218 tons of chemical fertilizer in three counties alone. To help find a solution, District 3 called in its landscape consultant, PBS&J.
Mysterious waters
Woodville Karst Plain topology, which underlies much of the area south of Tallahassee, is porous limestone eroded with sinkholes and underground streams that have produced one of Florida’s major sources for drinking water: the Wakulla Springs basin. The basin is a 600-sq-mile area crisscrossed with underground water channels that form one of the world’s longest and deepest freshwater cave systems. In some places, the caves reach heights of more than 300 ft from floor to ceiling (compared to the height of the Grand Canyon at 175 ft) and hold the remains of mastodon bones from around 12,000 B.C.E.
Wakulla purportedly means “mysterious waters,” and divers are still mapping the enormous cave system that acts as an underground highway transporting water from various springsheds—some still unidentified—that feed into the Wakulla Springs water system. Water emerging from a central cave feeds the popular recreational area at Edward Ball Wakulla Springs State Park and the Wakulla River that flows to the Gulf of Mexico. At the park location, 175,000 gal of water per minute pour through the underground cave into the Wakulla Spring.
A highlight of visiting the park may include rides in glass-bottom boats to view the cave entrance 25 ft below the water surface, along with the lush Wakulla Springs ecosystem that supports fish, alligators, deer and birds. As the water has become more polluted, it has also become darker, so much so that visitors can only see below the water surface at certain times of the year. Increased nitrates have caused dense colonies of algae and hydrilla to stifle native plant growth, and some native species have vanished.
Roadway litter redefined
Crisscrossed above ground and over the channels lie U.S. Route 319 and State Road 263; both fall under the auspices of the FDOT District 3. The District is responsible for road management and expansion as well as maintenance and signage for these and other roads in the Wakulla Springs basin area.
Prior to the DEP’s request, District 3 was applying a chemical fertilizer every two years. The fertilizer had a nitrogen (N) -phosphorus (P)-potassium ratio designated as 13-13-13 and was spread at a rate of 450 lb per acre. Nitrogen is used to promote growth above ground, while phosphorus builds healthy plant roots, and potassium encourages overall growth. The 13-13-13 fertilizer delivered elemental nitrogen at roughly 6.5 lb per acre, with an unknown quantity lost to runoff and leaching. The sandy soil conditions required two applications.
After receiving the DEP’s request, District 3, together with PBS&J, began hunting for an innovative solution to the nitrate problem. PBS&J’s research turned up an intriguing alternative: organic chicken litter fertilizer—feather meal—with ammonium-based (NH4) nitrogen sources. It was surely worth investigating given the nitrate damage to the unique geological formation of the Wakulla Springs basin area.
Chicken litter-based fertilizers typically have NPK ratios in the range of 5-5-5, and are released slowly in the soil, making the nutrients much more available to the plants over time. While chemical fertilizers are artificially produced and use highly soluble nitrates (NO3), organic-based fertilizers are derived from natural sources and rely on ammonium based (NH4) nitrogen sources to provide plant nutrients. NH4 requires microbial activity in the soil to break down slowly, thus reducing the percolation of nitrates into the aquifer below. Humic acid may also be added to hold water in poor soils by increasing the percentage of organic matter held between soil particles. PBS&J’s research disclosed that the cost of organic-based nitrogen fertilizer was approximately $400 per acre. Though slightly higher in cost than the chemical fertilizer District 3 had been using, the decision was made to apply the alternative fertilizer only when needed, rather than follow a routine bi-annual application. In the future, District 3 will perform periodic tests to determine nutrient levels in the soil, and fertilizer will be applied only if plants need an infusion to grow and stabilize road shoulders.
Growing awareness
U.S. Route 319 runs through the Wakulla Basin about two miles west of Edward Ball Wakulla Springs State Park. The two-lane road is heavily traveled and, in adverse weather, serves as an evacuation route. As Tallahassee sprawl has grown, the pressure to add lanes has increased. District 3 is now in the planning stage of widening the road and will begin building an additional two lanes in 2008.
From the initial stages of the project, the Florida Springs Task Force (FSTF)—an umbrella organization for various local groups and agencies including Florida’s DEP—together with the FDOT District 3 have been mindful of the sensitive aquifer. The FSTF has provided District 3 with the best maps available to the underground cave system so that when Rt. 319 is widened, the location of new runoff retention ponds will be positioned at least 300 ft away from caves and active sinkholes. This way, runoff carrying silt, chemicals and oil from the road will not have a direct path to the aquifer below. In one stretch, FDOT will elevate the road by three feet to redirect runoff flow away from sensitive areas. In the meantime, FDOT will reduce shoulder work near drainage outfalls whenever possible to decrease runoff from leaching the spring.
The FSTF is continually developing outreach programs to educate the public. One of its focal points in the area is Munson Slough, a primary conduit carrying urban stormwater from central Tallahassee. The slough is located directly north of Wakulla Spring and has become dangerously polluted.
To help FSTF stop illegal dumping of various waste materials into the slough, FDOT developed signage that has been posted along U.S. 319, one state road and four county roads reading “Munson Slough Drains to Wakulla Springs.” It is hoped that this public awareness campaign will make travelers think twice before dumping unwanted items that could degrade the aquifer.
The proactive responses that FDOT and PBS&J have taken to help Florida protect its spring ecosystems and improve water quality have not gone unnoticed by FSTF. In recognition of its commitment, FDOT District 3 was presented with the Florida Springs Protection Award by the DEP Florida Spring Task Force on February 2.
As many states grapple with the environmental problems affecting sensitive ecosystems and aquifers, it is hoped that these innovative solutions may be a catalyst for others, and prompt newfound respect for what the chicken left behind after crossing the road.
About The Author: Tommy Cook is the district maintenance engineer. He can be contacted at [email protected]. Mark Thomas, P.E., is the assistant district maintenance engineer for FDOT District 3 located in Chipley, Fla. He can be contacted at [email protected]
