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Nitrogen transport and transformation beneath storm-water infiltration basins in karst areas, Marion County, Florida

Project Chief: Andrew M. O’Reilly
Cooperator: University of Central Florida
Period of Project: October 2006 - July 2011

Problem Statement

Figure 1. Study area showing locations of the South Oak and Hunter’s Trace stormwater infiltration basins and monitoring sites.

Nitrate concentrations have increased in many Upper Floridan aquifer springs since the 1950s, exceeding 1 mg/L in recent years in some springs. Stormwater runoff is one possible source of nitrogen, among others including septic tanks and land-based application of reclaimed water or fertilizer, which can contribute to elevated nitrate concentrations. Previous studies have collected water-quality data to describe the effects of various stormwater best management practices (BMPs) on groundwater quality. However, in general, little research is available to develop a process-based understanding of the effects of stormwater on groundwater from a nutrient/biogeochemical cycling perspective.

Objectives

1) Identify and evaluate the natural processes (physical, chemical, and biological) that control the nitrogen cycle in soil and groundwater beneath stormwater infiltration basins

2) Develop, implement, and monitor a new stormwater infiltration BMP for nutrient (nitrogen and phosphorus) reduction

Figure 2. δ15N and δ18O of nitrate (NO3–) in precipitation, stormwater, soil water, and groundwater at the South Oak and Hunter’s Trace sites showing samples plotted relative to typical source ranges. Results indicate that denitrification is naturally occurring in the subsurface at the South Oak basin but not at the Hunter’s Trace basin.

Approach

Two stormwater infiltration basins in the karst/high recharge areas of Marion County, Florida, were monitored to identify subsurface biogeochemical processes (Figure 1). Chemical and hydrologic data were collected from 2007 to 2010, including: major elements, nutrient, organic carbon, and trace metals; dissolved gases; stable oxygen and hydrogen isotopes of water, and oxygen and nitrogen isotopes of nitrate and nitrogen gas; soil mineralogy and chemistry; nitrite reductase gene density by real-time polymerase chain reaction; and hydroclimatic data (rainfall, basin stage and groundwater level, soil moisture and temperature). Water samples were collected from ponded stormwater, suction lysimeters, and shallow wells near the water table.

Existing and alternative infiltration BMP design criteria were identified by the University of Central Florida (UCF). The existing design criteria for a typical stormwater infiltration basin involve excavation of a basin above the water table without modification of the underlying soil. Alternative design criteria, developed by UCF (http://stormwater.ucf.edu/), focus on different media (natural soil and amendment mixtures using recycled waste materials) that could facilitate nitrate removal. Laboratory soil column experiments performed by UCF identified soil amendments that may be effective in attenuating nutrient transport. The biosorption activated media are designed to mimic natural physicochemical and biogeochemical processes, leading to enhanced nitrate and phosphorus removal. The USGS assisted in interpreting laboratory testing of the biosorption activated media and conducted monitoring and interpretation of nitrogen cycling in an existing and an alternative BMP design in a full-scale field environment (fig. 1).

Results

• Oxygen, nitrate, manganese, iron, and sulfate reduction and methanogenesis were observed beneath the South Oak basin, whereas aerobic conditions persisted beneath the Hunter’s Trace basin leading to nitrate leaching to groundwater.
• The presence or absence of oxygen in the subsurface at both sites was strongly influenced by the interaction of soil texture and hydroclimatic conditions, causing wet soil moisture conditions conducive to denitrification beneath the South Oak basin and dry soil moisture conditions conducive to ammonification/nitrification beneath the Hunter’s Trace basin (fig. 2).
• Cyclic biogeochemical processes coincided with wet-dry hydrologic conditions at the South Oak site.
• Pollution control utilizing biosorption activated media was integrated with flood control and incorporated into a new BMP at the Hunter’s Trace basin.
• Biosorption activated media increased sorption capacity and soil moisture retention, mimicking the natural conditions beneath the South Oak basin.
• Decreases in subsurface concentrations of nitrate and orthophosphorus occurred after implementation of biosorption activated media.

Information Products

O’Reilly, A.M., Chang, N.B., Wanielista, M.P., and Xuan, Zhemin, 2011, Identifying biogeochemical processes beneath stormwater infiltration ponds in support of a new best management practice for groundwater protection. In: Schirmer, M., Hoehn, E., and Vogt, T. (eds.), GQ10: Groundwater Quality Management in a Rapidly Changing World, Proceedings of the 7th International Groundwater Quality Conference, Zurich, Switzerland, June 13-18, 2010. International Association of Hydrological Sciences Publication 342, p. 437-440.
http://www.iahs.info/redbooks/a342/abs_342_0437.pdf

O’Reilly, A.M., Wanielista, M.P., Chang, N.B., Harris, W.G., and Xuan, Zhemin, 2012. Soil property control of biogeochemical processes beneath two subtropical stormwater infiltration basins: Journal of Environmental Quality. doi:10.2134/jeq2011.0204