6.1 Estimate annual nutrient loads in surface water

Chapter 6. Stormwater Design and Maintenance
Science Solutions
Activities to Remove Nutrients

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.

Update (March 2023): An expanded analysis of the dissolved inorganic nitrogen (DIN) concentrations at 72 sites across the Sarasota County region using water quality and quantity data spanning the last 15 years has enabled the development of a decision-support metric to assist in the prioritization of management activities to reduce nitrogen loads in the area. See Progress section for details.

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. In particular, analysis of water quality and quantity data can provide useful metrics for the management of dissolved inorganic nitrogen (DIN). The relationship of DIN as a fraction of total nitrogen can provide a reliable indication of pollution hotspots, increasing the potential for improved DIN source identification as well as targeted investments for DIN reduction.

 

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. Sarasota County’s network of Automated Rainfall Monitoring System (ARMS) stations provides stream stage data but not discharge data. A number of USGS stations in the Sarasota County region do have field measured rating curves relating stream stages and discharges.

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). (See Progress section for an updated analysis including data from 2006-2021 at 72 sites across Sarasota County.)

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. For this analysis, DIN is the sum of ammonium (NH4), nitrite (NO2), and nitrate (NOᴣ) nitrogen. These forms of nitrogen serve as “instant food” for plant and algae growth. DIN in elevated concentrations can be indicative of anthropogenic sources such as wastewater by-products, synthetic fertilizers, and possibly areas where natural landscapes have been removed and replaced with compacted material (sands, silts, and clays). 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).

Progress

Update (March 2023):

Nitrogen Concentration
The water quality data analysis of dissolved inorganic nitrogen (DIN) as a fraction of total nitrogen (TN) was expanded to include available data from 2006-2021 at 72 sampling sites across Sarasota County. As a fraction of TN, DIN ranged from eight percent to sixty-six percent across sampling sites. Within this range there appears to be a correlation between watershed areas with anthropogenic alterations and increasing percentages of DIN (Figure 6.1.5). Conversely, watershed areas with limited anthropogenic alterations or certain management strategies exhibit low DIN percentages, generally below 10% (provisional methods and data, Suau communication).

Percent DIN could be used as a management metric to help prioritize locations that may require detailed site evaluations to identify potential sources of elevated DIN and suggest potential management actions (Table 6.1.3). Sites with confirmed sources of DIN that are also sources of elevated levels of total nitrogen on or near a waterway/estuary could be further prioritized.

Figure 6.1.5. Dissolved inorganic nitrogen (DIN) as a fraction of total nitrogen (TN) expressed as a management grade ranging from A+ (less than 10% DIN) to F (greater than 50% DIN).

Table 6.1.3. An example of the DIN management metric applied to select monitoring locations with a subjective initial suggestion of potential DIN sources. A more thorough review and site evaluation will likely reveal additional and more direct sources of elevated DIN.

Nitrogen Load
The annual TN and DIN loads defined as pound per acre may also be a useful metric for relative DIN management prioritization and investment. To determine pollutant loading from the watersheds, USGS stream flow volume, nitrogen concentrations, watershed drainage area were used to calculate TN and DIN loading for eleven sites throughout the Sarasota region (provisional methods and data, Suau communication). there does not appear to be a clear correlation between watershed with anthropogenic alterations and unit TN loads. However, watersheds with anthropogenic changes such as Whitaker Bayou and Phillippi Creek appear to have significantly higher unit DIN loads than more natural or managed watershed areas (Table 6.1.4).

Table 6.1.4. Average annual total nitrogen and dissolved inorganic bioavailable nitrogen loads (pounds per acre) for select monitoring locations in Sarasota County.

Updated Recommendations

  • The relative DIN percentage should be used as a meaningful metric to prioritize strategic reduction investigations, management actions, and investments. Proximity to waterways should be considered for prioritization.
  • Where available, utilize measured nitrogen loads as the basis for planning purposes and loading model calibration/verification (see Chapter 10.2).
  • Reinstate discontinued monitoring locations where DIN percentages for their period of record exceeded 15%.
  • The following sampling locations should be added to address significant watershed area gaps:
    • South Creek at U.S.41
    • Fox Creek at I-75
    • Fox Creek upstream of confluence with Shakett Creek
    • Forked Creek at S.R. 776
    • Wares Creek
    • Palma Sola Creek
    • Unnamed drainage ditches at El Conquistador Parkway.
  • Partnerships and coordination opportunities to augment Sarasota County Government monitoring with other municipalities, estuary programs, and entities such as the Suncoast Waterkeepers should be leveraged (see Chapter 9.2).
  • Unit DIN loads for watershed areas should be developed as a metric for relative comparison and management of DIN.
  • Measured nitrogen loads should be used as the basis for planning purposes as well as pollutant load model calibration/verification.
  • Engage the USGS or qualified hydrologist to develop rating curves and manage the ARMS data base (see Chapter 10.1).

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 9.2, Chapter 9.4, Chapter 10.1, Chapter 10.2

 

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