6.1 Estimate annual nutrient loads from stormwater

Key Message: Robust long-term monitoring and modeling programs that take into account surface and subsurface runoff, stream flows, and organic and inorganic nitrogen concentrations are essential for accurately estimating and managing nutrients in watersheds.

Importance

Since surface and subsurface runoff conveys excess nutrients (and other pollutants) to receiving water bodies including estuaries and bays, identifying the location of primary outfalls and quantifying their pollutant loads is essential to develop data-driven nutrient budgets and specific management strategies for watersheds or bays.

Overview

Sarasota County Stormwater Environmental Utility has identified and mapped primary outfalls to one of seven coastal bay segments (Figure 6.1.1). In addition, there are smaller secondary stormwater outfalls serving the coastal fringe and barrier islands. The eastern portion of Sarasota County including, most of the City of North Port, is located in the large Myakka River watershed which drains to Charlotte Harbor.

Figure 6.1.1. Major stormwater outfalls discharging to canals, creeks, and bay waters. Watersheds corresponding to the six primary bay segments and the Myakka River indicate outfall receiving waters. Source: Sarasota County Government

The most direct and accurate means to estimate average annual nutrient loads from primary stormwater outfalls is to measure them. Annual nutrient loads are the product of annual runoff volumes and nutrient concentrations. Therefore, data on both water quantity (water flow) and water quality (nutrient concentrations) are needed. Longer periods of record, for which both water quality and water quantity monitoring data are available, yield greater precision and reliability for estimating variability from year to year and long-term averages.

Sarasota County Government has an extensive monitoring program for its primary watersheds that contribute to the Sarasota Bay and Charlotte Harbor systems (see Chapter 10.1). The program was phased in from 2004-2007, providing the potential for more than a decade of annual nutrient load determinations. Southwest Florida Water Management District (SWFWMD), Florida Department of Environmental Protection (FDEP) and U.S. Geological Survey (USGS) monitoring stations provide historical water quantity or quality data. For example, USGS has operated a continuous stream flow gage on the Myakka River north of State Road 72 since 1939.

Figure 6.1.2. Water quality and water flow monitoring stations in Sarasota County. Source: Sarasota County Water Atlas

One data limitation is that many local stream gage stations require development or confirmation of the rating curve to accurately calculate average annual flow volumes. A rating curve models the relationship between stream water height (stage) as measured by the gage and volume of water flow (discharge), unique to the hydraulic characteristics at a particular stream gage location (Fondriest Environmental, Inc. 2015). Furthermore, flow must be measured at upstream gages independent of tidal influence.

Coordination among a variety of agencies conducting environmental water quality sampling in Southwest Florida surface waters occurs through the Regional Ambient Monitoring Program (RAMP) with standardized field sampling and laboratory analysis protocols that results in consistent data methods used by all parties. This ensures that water quality data meet stringent state quality assurance standards before being submitted to the Watershed Information Network (WIN), a common statewide public database maintained by FDEP. Sarasota County data is published through WIN on the Sarasota County Water Atlas (see Chapter 9.4)

Approach

To demonstrate the approach for estimating annual nutrient loads from monitoring data, multiple years (2006–2018) of water quantity and quality data were evaluated for a representative urban/suburban watershed (Whitaker Bayou) and contrasted with a more natural watershed (Deer Prairie Slough).

Deer Prairie Slough Source: Sarasota Water Atlas

Whitaker Bayou Source: Sarasota Water Atlas

Water quantity data has been collected continuously by a USGS site since 1995 (Figure 6.1.3), and water quality data has generally been collected monthly since 2006 by Sarasota County Government. Since nitrogen is the limiting nutrient in the region, total nitrogen (TN) and dissolved inorganic nitrogen (DIN) were analyzed. Nitrogen in the form of DIN is readily available for uptake by phytoplankton and/or algae. Nitrogen loads were computed for each month and totaled to obtain the annual load.

Figure 6.1.3. Observed rainfall and calculated runoff (USGS gage 02299861) for the Whitaker Bayou watershed 1995-2018. In some years more than 50% of rainfall runs off the land into the bayou. Source: Sarasota County Water Atlas.

Table 6.1.1. Annual nitrogen loads 2006-2018 for Whitaker Bayou as calculated from runoff volume and water quality sampling data.

The average annual TN unit load for the period 2006-2018 was 3.43 pounds TN/acre of which 28% was DIN for the Whitaker Bayou watershed (Table 6.1.1).

Whitaker Bayou’s average annual nitrogen load was contrasted with a more natural or rural watershed in Sarasota County (Deer Prairie Slough). Although data are available for the same time period as Whitaker Bayou, a significant effort is needed to reduce the data. In addition, a rating curve would need to be developed/updated for the Deer Prairie Creek location. Instead, data available for a different period of record (1999–2002 and 2005) were evaluated to preliminarily contrast TN and DIN trends between these rural and urban watersheds.

The average annual TN load for Whitaker Bayou is only 6% higher than Deer Prairie Slough, but the percentage of DIN for Whitaker Bayou is approximately four times higher (Table 6.1.2 and Figure 6.1.4). This preliminary evaluation suggests that nutrient management should target DIN as well as TN, at least for the Whitaker Bayou watershed. As nitrate is typically the primary component of DIN, projects that facilitate denitrification could be prioritized.

 

Figure 6.1.4. Comparison of organic nitrogen versus dissolved inorganic nitrogen in urban (Whitaker Bayou) and rural (Deer Prairie Slough) watersheds.

Table 6.1.2. Comparison of nitrogen loading between a suburban (Whitaker Bayou) and a rural (Deer Prairie Slough) watershed.

Analyzing available data for concurrent periods of record for Deer Prairie Slough and other watersheds could yield average annual nutrient loads for a range of hydrologic and land use conditions, as well as provide additional guidance on the management of specific nitrogen constituents, such as nitrate.

The importance of having a decade’s worth of water quantity and quality monitoring data available cannot be overstated. Since nutrient loads vary annually with natural variations in rainfall and runoff, multiple years of data are necessary to compute statistical averages precisely and reliably. Most critically, these data provide a basis for calibrating and verifying nutrient load models to assure they accurately reflect actual observations for scenario planning. Model calibration could and should be performed at the watershed or watercourse scale but could also be scaled for bay management (Figure 6.1.1). Once calibrated, nutrient load models could be used to predict and test load reductions and evaluate the costs and benefits of public policy and investments (e.g., regulatory, BMPs, capital projects).

Resources

  • Sarasota County Water Atlas
  • Sarasota County Stormwater Environmental Utility
  • Southwest Florida Water Management District
  • Florida Department of Environmental Protection
  • United States Geological Survey

Status

No Activity

Performance Measure

  • Quality control and assurance of water quantity flow data to identify and address data gaps or anomalies. Since the data are typically reported as water elevations, rating curves would need to be applied or developed to convert water elevations into average daily discharges and runoff volumes.
  • Quality control and assurance of nutrient water quality data to identify and address data gaps or anomalies and reduce and render the data usable.
  • Monthly and annual total phosphorus and total nitrogen loads, as well as dissolved inorganic nitrogen and nitrate loads, for each primary outfall and their watershed.

Experts or Leads

USGS, John Coffin; Sarasota County Environmental Stormwater Utility; New College of Florida Data Science Program; University of South Florida Water Institute Water Atlas; Stephen Suau

Cost Estimate

$100,000-$1,000,000

Related Activities

Chapter 9.4, Chapter 10.1

 

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Other Stormwater System Activities