5.1 Estimate nitrogen loading from atmospheric deposition using multiple monitoring stations within urban areas

Key Message: Atmospheric deposition—both direct and indirect—is a major nitrogen source to land and water in Sarasota County. Additional monitoring stations along urban-to-rural and coastal gradients would greatly improve understanding of nitrogen loading to priority water bodies.

Importance

Atmospheric reactive nitrogen (hereafter “atmospheric nitrogen”) can reach land or water through wet deposition (precipitation) or dry deposition (dust). Total nitrogen deposition includes both. It can enter waterbodies directly or indirectly—either falling directly onto water or settling on land and being carried by stormwater or groundwater into waterways. Atmospheric nitrogen deposition contributes to water quality degradation, loss of fish and wildlife, impacts on human health and quality of life, and ecosystem acidification (see Chapter 5.2).

To build accurate nutrient budgets and guide management, it’s important to quantify both total and relative contributions from atmospheric nitrogen. This requires multiple monitoring stations placed throughout urban areas and along rural-to-urban gradients.

Overview

The National Atmospheric Deposition Program (NADP) is a cooperative network of government agencies, educational institutions, private organizations, and NGOs. Its National Trends Network publishes annual U.S. maps of total nitrogen deposition (wet + dry). From 2002–2022, national nitrogen deposition—especially oxidized nitrogen and nitrate—declined in response to EPA regulations and market-based cap-and-trade programs (Figure 5.1.1). Sulfur dioxide emissions declined even more sharply. However, wet deposition of ammonium nitrogen (not directly regulated) has generally increased since 1989. NADP maps show nitrogen loads in rural eastern Sarasota County to be about 40% lower than in more urbanized western areas.

Local monitoring remains limited. Two networks operate stations in Sarasota County, but only one currently tracks nitrogen deposition. The NADP station at Verna Wellfield in northeast Sarasota County has collected data since 1983 on total nitrogen, nitrate, ammonium, and deposition rates. Recent records show declining atmospheric nitrate concentrations but rising deposition levels (Figure 5.1.2). However, the station’s rural location may limit its relevance to coastal and urbanized areas.

Florida’s Ambient Air Quality Monitoring Program, managed by the Florida Department of Environmental Protection (FDEP), operates 100 stations across the state. The monitoring data are used to maintain and improve air quality, understand temporal variability of air pollution, and investigate pollution exposure by humans and the environment. Four are located in Sarasota County—Paw Park, Lido Park, Bee Ridge Park, and Jackson Road—but none currently measure nitrogen concentrations or deposition rates (Figure 5.1.3). The Paw Park station collected nitrogen data until mid-2019.

Approach

NADP indicates that wet deposition is likely underestimated in urban areas due to underrepresentation—only 7% of sites are urban (Walker & Beachly 2019). More monitoring sites in Sarasota County are needed along a rural-to-urban transect, particularly in coastal and developed areas. The Verna Wellfield site should also be upgraded to include dry deposition measurements. In addition, the EPA’s Watershed Deposition Tool (WDT) should be applied to Sarasota County, as was done for Tampa Bay. This should incorporate variable land-to-water nitrogen transfer rates by land use and impervious surface coverage.

Three approaches to estimating nitrogen loading are presented and compared here.

• Atmospheric deposition of nitrogen to Tampa Bay and its watershed was estimated using 2002-2006 data from the EPA’s WDT, a user-friendly interface for extracting and analyzing model output data (Poor et al., 2013a). This effort incorporated atmospheric deposition data from the Verna Wellfield monitoring site in Sarasota County, the only NADP station in the region. From 2002-2006, the area-weighted average direct and indirect unit N loads to Tampa Bay were 9.3 kg/ha (8.3 lb/ac) and 12.02 kg/ha (10.7 lb/ac, respectively. Average indirect (land-to-water) deposition was 2.18 kg/ha (1.9 lb/ac). The largest source of error in estimating deposition using this method came from uncertainty in the N export coefficient, the rate at which N is transferred from watershed to bay and set to 18% for the entire Tampa Bay watershed (Poor et al., 2013b). Consideration of variable transfer rates as a function of land use/impervious coverage would be a reasonable next step.
• Using a concentration-based method, the South Florida Water Management District (SFWMD) utilized average nitrogen and phosphorus concentrations of 0.54 mg/L and 0.0159 mg/L to estimate atmospheric deposition for the Lake Okeechobee Watershed (SFWMD, 2010). Atmospheric deposition was computed as the product of these nutrient concentrations and annual precipitation, seemingly representing only wet deposition. Using these nutrient concentrations for Sarasota County area and average annual rainfall of approximately 54 inches results in a unit nitrogen deposition load of 7.3 kg/ha (6.5 pounds per acre) and a unit phosphorus deposition load of 0.21 kg/ha (0.19 pounds per acre), somewhat lower than the WDT estimates that include both wet and dry deposition.
• Finally, a third method used existing estimates to calculate the direct and indirect N deposition load to Sarasota Bay accounting for land use. Direct over water deposition rates were taken from 2006 Tampa Bay estimates (Poor et al., 2013a). Direct (gross) over land deposition rates were taken from 2017 NAPD maps of rural, suburban, and urban areas of Sarasota County, then converted to indirect (net) rates using the typical 18% land-to-water transfer rate (Table 5.1.1). The rates were applied to the 2017 Land Use/Cover information for the Sarasota County area and for the Whitaker Bayou watershed.

For the Sarasota County region, total gross and net indirect nitrogen deposition loads were estimated at 2,261,967 pounds and 415,984 pounds, respectively. For comparison, the approximation method utilized by the SFWMD for the Lake Okeechobee Watershed (SFWMD, 2010) yielded a gross indirect nitrogen deposition load of 2,463,293 pounds when applied to the Sarasota County region and using an average annual precipitation of 54 inches.

For the Whitaker Bayou watershed, total gross and net indirect nitrogen deposition loads were estimated at 24,434 pounds and 4,626 pounds, respectively. For comparison, the approximation procedure utilized by the SFWMD for the Lake Okeechobee Watershed (SFWMD, 2010) yielded a gross indirect nitrogen deposition load of 22,910 pounds when applied to Sarasota County and using an average annual precipitation of 54 inches.