Written By: Drew Morson, PE, CFM
Nutrient Pollution and its Impact on the Environment, Human Health, and the Economy
Nutrient pollution is the excess of certain nutrients (typically nitrogen and phosphorous) in the environment. This excess of nutrients in the environment can cause numerous negative effects on environmental health, human health, and the economy. In Florida, one of the most visible results of nutrient pollution is algae blooms, also known as red tide. These red tide algae blooms can be toxic to not only wildlife but humans as well and have a direct impact on local economies that suffer from impacts to tourism. Data shows a decline in tourism during years with higher red tide concentration.
There are many sources of nutrient pollution. These include fertilizer, animal waste, sewage treatment plants, failing septic systems, cars, and various industrial sources. Stormwater runoff is often listed as a primary source of nutrients, but it mainly transports nutrients as opposed to being a source of them. Rainwater contains some nutrients from the atmosphere, but the excess nutrients in stormwater come from the collection of nutrients from the ground surface as runoff flows over the land.
The amount and type of nutrients carried by the runoff depend on what type of land use is present (agricultural, residential, industrial, commercial, etc.) with higher concentrations as a result of increased human activity. So, while stormwater itself is not necessarily the cause of nutrient pollution, it is often the vehicle for spreading downstream.
Stormwater Treatment Requirements in Florida and Their Evolution
Florida sets general stormwater treatment requirements for new projects at the state level through the Florida Department of Environmental Protection (FDEP) and regionally through the Water Management Districts (WMD). The WMDs have similar requirements to meet standards for stormwater quantity and quality by evaluating the differences between the existing and proposed conditions.
Quantity standards prevent new sites from increasing flows downstream and causing new flooding issues. Quality standards seek to limit the spread of excess nutrients downstream. Typical stormwater treatment practices include a stormwater management system such as a dry pond, wet pond, or underground storage to meet state and regional standards.
The quantity and quality standards are met by designing the stormwater management system and its outfall structure in specific ways. Quantity standards are met by holding back stormwater and limiting flows downstream with adjustments including modification of the size of the system and/or the outfall structure dimensions. Quality standards are met by providing minimum treatment volumes with adjustments made to specific design factors to maximize efficiency or by changing the type of treatment used.
The minimum quality requirements allow for the calculation of the minimum treatment volume to be provided given the size of the project and the type of treatment proposed. Since the current standards were initially adopted, this minimum treatment requirement has become known as presumptive treatment. At the time of its adoption, these standards were very progressive. Though since that time, more detailed nutrient loading analysis has been required for projects in certain areas.
Evaluating the Effectiveness of Stormwater Treatment Systems in Nutrience Load Reduction
A nutrient load analysis takes the land use and specific site layout into account for determining the predevelopment and proposed development nutrient loads from stormwater runoff. This allows for a direct comparison of nutrient loads for existing and proposed conditions to better show adherence to quality standards.
Studies have been conducted to review the real-world effectiveness of systems designed to meet the presumptive treatment requirements. The results have better quantified the actual treatment efficiencies of stormwater treatment options and, in conjunction with nutrient loading analysis, found that the treatment volumes provided to meet the requirements did not necessarily correlate with the treatment volume needed to reduce nutrient loads discharged downstream. In some cases, nutrient loading analysis show systems designed for presumptive treatment to be oversized; but in many cases, systems designed for presumptive treatment are undersized.
Addressing Nutrient Loading through Basin Management Action Plans and Proposed Changes to Statewide Requirements
The figure below illustrates the level of impairment across the state of Florida per FDEP data. A Basin Management Action Plan (BMAP) is a specific framework for water quality restoration that contains local and state commitments to reduce pollutant loading. While not all areas identified are specifically impaired for nutrients, all impairments relate to nutrients and stormwater such as dissolved oxygen or fecal coliforms.
As the State continues to identify impaired watersheds, the need to more directly address nutrient loading has increased. At this point, a thorough nutrient loading analysis is only required if a site falls into an impaired or otherwise identified area with nutrients or associated parameters listed as the parameter of concern. For years it has appeared that this trend would continue toward eventual full statewide coverage of impaired areas if no additional changes are made to development practices.
To this end, FDEP has recently published a draft change to the statewide requirements (http://publicfiles.dep.state.fl.us/dwrm/draftruledocs/stormwater/noc/Updated%20AH_I_thru%20CompareCourtesy.pdf) for stormwater management systems that includes a minimum 80% nutrient removal efficiency for Total Nitrogen and Total Phosphorous. The draft is currently available to the public to provide comments and feedback. It is unclear when this draft change will be adopted by the State and if any changes to specific requirements are made before then. What is clear, however, is that the State is moving towards updating standards to directly address nutrient loading.
Additionally, the State is reviewing the use of stormwater treatment credit banks that would function similarly to wetlands mitigation credit banks. Rather than including the level of stormwater treatment needed for a site, the requirement could be offset by the purchase of credits in the appropriate credit bank. This is a key step in providing more effective treatment systems.
It is certainly important to address and contain pollution at the source whenever possible, but single-site treatment systems are often neglected by property owners and become less effective over time. The use of larger regional treatment systems would not only help in addressing nutrients from existing developed areas but also help maintain the effectiveness of treatment over time. Like existing wetlands mitigation banks, these regional stormwater credit banks would be subject to more stringent monitoring requirements to ensure a system’s effectiveness is better maintained.
Conclusions: The Importance of Addressing Nutrient Pollution and Keeping Up with Changing Standards
Nutrient pollution in the form of excess nutrients in the environment is a direct result of increased human activity. Stormwater runoff is one of the main vehicles for the spread of this type of pollutant. While it is important to address pollutants at their source wherever possible, it is also important to utilize stormwater management systems to address the transport of cumulative nutrient loads from general human activity. With the State moving towards standards that address nutrient loading more heavily, it is important to be aware of changes to standards and subsequent design requirements as they are adopted.
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About the Author
Drew Morson, PE, CFM, has over 10 years of experience working for municipal and private sector clients in the areas of Civil and Environmental Engineering. His project experience includes design and permitting for commercial developments, industrial developments, roadway projects, stormwater retention facilities, and recreational parks with his main area of expertise being stormwater management/permitting. He has also performed project management and preventative maintenance for reverse osmosis water purification systems projects and water sampling for wastewater discharge permits for federal government agencies.
About Chastain-Skillman
CS is a leading engineering firm headquartered in Lakeland, FL, with satellite offices in Orlando, FL, and Nashville, TN. Established in Lakeland in 1950, our company provides Civil Engineering, Water/Wastewater Engineering, Land Surveying, Geology/Hydrogeology, and Construction Management/Inspection services.
At CS, we treasure our role in creating thriving communities, always respecting the impact our work has on their foundations and their futures. For more information, visit chastainskillman.com.
