Discovery Magazine

The Importance of Fertilizer Nutrient Leaching to Groundwater and Surface Waters

It is clear that the nutrient loading to the Indian River Lagoon (IRL) must be reduced. We believe residential fertilizer nutrient loading is an important source of fertilizer pollution in the lagoon, leading to widespread algal blooms. According to the SJRWMD, fertilizer pollution is the main source of nutrients to the IRL, accounting for up to 70% of the lagoon-wide influx of N and P, and homeowners routinely put down more than 10X the amount that agriculture does (MRC, 2011). One pound of fertilizer washing off a lawn can trigger the growth of 500 lbs of algae, according to the MRC (2011). Algal blooms, such as the recent brown tide bloom (2012) and the superbloom (2011), block sunlight to seagrass—the linchpin of the IRL ecosystem. When algae die, the resulting oxygen sags can trigger fish kills. If seagrasses continue to decline and suffer, the lagoon could eventually shift from a seagrass-based to an algal-based system — with disastrous ecological results.

The EPA wants N and P loading cut in half in 15 years, costing an estimated 1.4 billion in improvements. So far, TMDL nutrient loading reduction successes have been largely through expensive stormwater reduction and treatment projects by local governments, and more than 100 of these projects now exist in the IRL watershed. Other local plans for meeting new nutrient TMDL and EPA requirements could include septic tank removal, street sweeping and pet waste and fertilizer input reductions through education programs. The cheapest and easiest way to lower fertilizer pollution to the IRL may be through source control by enacting strict local government residential fertilizer pollution ordinances. The Florida legislature has required that local governments adjacent to waters impaired by pollution, such as the IRL, must pass an ordinance regulating the use of lawn fertilizers. The Florida DEP has created a model ordinance as a guideline for local governments, but this ordinance actually does very little to limit fertilizer pollution.

This study is focused on collecting data that counties could use in the IRL drainage basin, as well as other areas, to justify ordinance changes, such as the restriction or ban of quick-release fertilizer application or restriction or ban of fertilizer application in the wet season (June 1—Sept. 30). The rainy season is the worst time for fertilizer pollution, particularly with quick-release fertilizer applications. The major quick-release forms of nitrogen are ammonium sulfate, ammonium nitrate and urea, and these forms of nitrogen fertilizer pollution produce a rapid response in turf grass growth and color. In most soils the ammonium form is quickly converted to the nitrate form, which is completely water soluble and highly mobile in soil, and this form can readily leach downward. Although some quick-release fertilizers can wash off lawns with rain, probably more important in Florida is that quick-release forms easily leach through sandy soils into shallow groundwater, and then seep into surface water bodies. This avenue, often neglected in fertilizer pollution loading studies and discussions, is the subject of this research.

Data will be collected from residences in the IRL watershed over a 15-16 month period in order to indicate the fertilizer pollution impact on the IRL. The fertilizer pollution impact of granular slow-release vs liquid quick-release fertilizer applications, under different rainfall and water table scenarios, will be investigated at the three test sites. Data will either support or refute the need for more restrictive local government fertilizer ordinances, particularly in IRL drainage basin counties that have only adopted the DEP fertilizer pollution model ordinance. Liquid quick-release fertilizer is generally undesirable as it moves quickly through the soil into the water table and receiving water bodies. It is anticipated that the collected data will indicate the application of soluble quick-release fertilizer, in particular, represents a significant nutrient source to the IRL, and banning its use, via ordinances, may be the best course of action and would likely result in a significant water quality improvement.

Three fertilizer pollution residential research sites will be used in this study. One residence will serve as a control (no fertilization) while the other two test sites will receive precise applications of granular slow-release and liquid quick-release fertilizer, with measurement times reflecting different antecedent conditions. All three sites will be sampled four times over a 15-16 month period. Slow-release granular fertilizer will be applied at one residential test site by a rotary broadcast spreader with a deflector shield at a rate of 0.5 lbs N/1000 ft2. Quick-release soluble liquid fertilizer will be applied at the recommended rate of 0.5 lbs soluble N/1000 ft2 at another residential test site. The third residential test site will receive no fertilizer and serve as a fertilizer pollution control. This site will have no history of fertilization. Fertilizer will be applied once in the dry season (spring) and once in the wet season (summer), with sampling occurring approximately 2 and 10 days after fertilizer application.

At least five groundwater samples will be collected at each of the three residential sites using clean M.F.E. PushPoint sediment pore water sampling rods connected to a peristaultic pump, at a depth of 0.5 ft below the water table. Additional surface water grab samples will also be collected at each residential site, and groundwater samples will be collected at each residence prior to scheduled fertilizer applications (control). In addition, 10 percent of all samples will be collected in duplicate and 10 percent of the samples will be collected at two depths (0.5 and 1.5 ft, below the water table) per residential site. Nutrient samples will be analyzed for fertilizer pollution from NH4-N, NOx-N, and SRP. Rainfall (amounts and intensities), residential irrigation amounts (if any), sediment type and particle size, hydraulic gradients (horizontal and vertical) and depth to water table will be documented and used in data interpretation. The sprinkler systems may be manipulated during dry periods to test different rainfall scenarios. Near-shore nutrient concentration data will be used with seepage meter data to estimate the fertilizer pollution (nutrient loading) impact on the IRL.