2.1 Quantify annual nutrient loads from septic systems

Key Message: Properly sited and maintained conventional septic systems can reduce about 30% to 40% of nitrogen inputs. But all septic systems are not equal in their potential to impair surface water quality. Many factors affect nutrient loading from septic systems, including design, age and size; soil types and distance from drainfields; and how well the system has been maintained. Quantifying localized nutrient loads will help to prioritize areas most in need of upgrades or conversions to central sewer.

Importance

Properly sited and maintained conventional septic systems can reduce about 30-40% of nitrogen inputs (Toor et al., 2011b). Local hydrogeology can determine whether and how much additional nitrogen reduction occurs through natural denitrification before effluent reaches a water body. The magnitude of nutrient loading to surface waters from septic systems is subject to many variables:

• Appropriate design and size
• Functional condition of tank and drainfield (maintenance schedule, age, proper operation)
• Soil type and distance between drainfield water table (throughout the year)
• Soil type, denitrification potential, and distance between drainfield and surface water

As a result, all septic systems are not equal in their potential to degrade surface water quality. Quantifying the magnitude of nutrient loads under different localized hydrogeologic conditions is important in prioritizing upgrades, conversions to central sewer, and developing an overall nutrient budget and management plan.

Overview

The Florida Department of Health (FDOH) estimates that 30% of the state’s population is served by 2.6 million on-site sewage systems. FDOH regulates septic systems pursuant to Chapter 64E-6, F.A.C. In Sarasota County, Ordinance No. 2017-040 also applies. Among other things, these regulations provide for location, installation, and site evaluation criteria. 

The number of likely or known parcels using septic systems in Sarasota County is uncertain. The County Government estimate is 48,853 parcels (2019). FDOH’s estimate is 40,692 (FDOH 2016). Both estimates are based on inaccurate data. FDOH records show that an average of about 400 (1.1%) of septic systems in Sarasota County have been repaired each year over the past 25 years. Better accounting and metadata are needed to improve our understanding of the number of septic systems (see Chapter 2.3) and the variables that influence their efficiency to produce reliable estimates of nitrogen loading from septic systems.

Approach

To estimate nutrient loading from septic systems, the number of systems and their average annual unit loadings are needed. For Sarasota County, neither of these are accurately known. To demonstrate the approach, we used an estimate of 35,000 parcels using septic systems in Sarasota County, together with data available from FDOH and the U.S. Environmental Protection Agency (EPA) (Figure 2.1.1). Assuming an average household size of 2.2 for Sarasota County results in estimates of nitrogen loadings of 19.8 pounds and phosphorus loadings of 4.8 pounds per household per year. Septic systems that are failing or not functioning properly could be considered at or approaching the average gross nutrient loading for estimating purposes.

FDOH estimates that a properly constructed and functioning septic system can remove between 30% and 40% of the gross nitrogen load (FDOH 2015). This level of nitrogen reduction takes place primarily in the drainfield system where ammonia nitrogen is quickly and readily converted to nitrate through nitrification. However, treatment efficiency depends on many factors including maintenance regimes, the age of the system and most importantly, whether natural denitrification is likely to occur downstream before the effluent reaches a water body. While there is evidence that some areas in Sarasota County may be capable of providing as much as 90% reduction in nitrogen loads from septic systems due to natural denitrification downstream (Suau, unpublished data), a 30% reduction was utilized as a conservative assumption for these calculations. A 30% reduction equates to a net loading of 13.8 pounds per household per year or 483,000 pounds per year county-wide. A 90% reduction would equate to 1.98 pounds per household per year or 69,300 pounds per year county-wide.

A properly constructed and functioning septic system removes about 97% of the gross phosphorus load (Mechtensimer et al., 2017). This equates to a net loading of 0.14 pounds of phosphorus per household per year or 5,040 pounds per year county-wide. 

To identity the potential for natural denitrification, water quality data collected monthly over the past six years and published by Sarasota County Government were reviewed for areas predominantly served by septic systems (Table 2.1.1 and Figure 2.1.2).

These data suggest there are areas served by septic systems with elevated nitrogen levels, but some may be associated with legacy or current wastewater loads from a nearby central wastewater treatment facility. In other areas with septic systems that drain to water bodies where data do not exceed water quality thresholds, local hydrogeology may naturally facilitate denitrification. In particular, the area that drains to the Siesta Ditch is composed of parcels that are 100% served by septic systems. A preliminary investigation indicates that natural denitrification may explain an apparent average annual TN reduction of 88% to 90% (Suau, unpublished data). Understanding the role of localized hydrogeology in the Sarasota County region to facilitate natural denitrification is critical to informing public policy and investments. Greater knowledge of natural denitrification is also of value in areas where reclaimed water is used to irrigate landscapes (see Chapter 1.2).