Series: Phosphorus and the Environment, 3. Protecting our lakes: shoreline septic system concerns.

Karl J. Czymmek
PRO-DAIRY Program, Cornell University

Introduction
Phosphorus (P) chemistry is very complex in farm fields as well as streams and lakes. Only a small portion of the total quantity of P in the environment is bioavailable, meaning that it is readily available to living organisms. In this article two methods or tests that water chemists use to measure P are referred to: Total Phosphorus (TP) and Soluble Reactive Phosphorus (SRP). The TP represents most or all of the various forms of P that are present, while SRP is a fraction of TP, representing what is immediately available to organisms in the lake. The SRP acronym is often used as shorthand for bioavailable P. There are other forms of P considered by scientists, such as Dissolved Organic P (mostly available) and Particulate P (partial or limited availability), but these are topics for another time.

In the temperate freshwater ecosystems found in this region, P is usually the nutrient that limits algae growth. As water temperatures rise in the summer, SRP tends to be in such high demand that it is very rapidly used by lake organisms. The natural P cycle in a lake creates a continuous source of SRP used by water life. Lake-recycled SRP in the upper waters is supplemented when a summer storm carries a fresh surge of P, some of which is SRP, from the land (watershed) that drains into the lake. While it is critical to manage all forms of P that reach a lake (readily available or not), for the most part, it is the quantity of that bioavailable SRP supplied from and to a lake that feeds the organisms and drives algae blooms. Paying attention to and understanding all SRP sources is an important part of lake management.

It is well established in the scientific literature that runoff from a watershed, including farms and forests, contributes to the TP and SRP loading in lakes. This has been well publicized in communities across the Finger Lakes region as well. While significant attention has been devoted to agricultural contributions, the serious nature of water quality challenges that have been observed in recent years requires a better understanding of all watershed sources of P. This third article in the Phosphorus and Environment Series focuses on P sources from septic systems on lake shores.

Septic systems
Many people dismiss the notion that septic systems can have an impact on the lakes. After all, the quantity of nutrients shed by any individual human directly to lakes is small and local agencies may report rigorous testing and a record of high compliance for shoreline septic systems. While it is widely believed that a septic system is working properly so long as effluent does not show up on the surface, what goes on underground, unseen, may be a real concern. Septic related outbreaks to the yard surface are not the only indicator of poorly functioning or failing systems, and for a variety of reasons, the situation for shoreline septic systems may be more complicated. For a general review of shoreline septic system issues, see: http://waterquality.cce.cornell.edu/septic/CCEWQ-YourSepticSystem-Shoreline.pdf. A broad description of P and onsite wastewater systems is provided in an article by the National Environmental Services Center (2013) and can be found here: http://www.ct.gov/dph/lib/dph/environmental_health/pdf/pipeline-wastewater_issues_explained_to_the_public.pdf). This second article indicates that many shoreline communities with closely sited homes and leach fields in well-drained soils that are close to the shoreline have experienced problems with noxious algal blooms (page 6).

For septic systems, part of the issue lies in how they are designed to work. For many non-sewered homes, all the drainage from toilets, showers, laundry, sinks and dishwashers flow into a septic tank. Liquids are held here temporarily, while the solid materials settle in the tank. The solids should be removed every 1-3 years (if not, then the system risks failure or is in such porous soil that it is likely not properly treating the waste). Liquids pass through the tank and are distributed to a leach or drain field through pipes with drainage holes that distribute the liquids into what should be moderately permeable soil. In the right conditions, soil chemistry and biological activity are expected to treat the nutrients and bacteria released from the system. To ensure proper treatment, much of the focus for septic system function relates to making sure the soil drains sufficiently well that the liquids do not rise to the surface, yet does not drain so rapidly that poorly treated liquids reach the water table.

Problems can arise when septic systems are installed into well-drained situations, especially on shorelines, where the water table is often close to the soil surface and where the separation distance of 100 feet from the leach field to the surface water (New York State Department of Health, 2016) cannot be met. Lakeshore soils can be variable but there are many areas of gravelly, well-drained soil types near shorelines with unsuitably rapid percolation rates that are close to the water table. Other shoreline locations may have shallow bedrock or rock outcrops with cracks that allow liquids to pass with little or no treatment. Lakeside property owners have reported finding older cottages and camps with perforated 55 gallon drums for disposal systems with little or no pipe distribution system for a leach field at all. The only way these systems could have worked, often for decades, is if they were (or are) sitting in very porous material which implies that septic flows could be in direct contact with the lake. In other cases, old systems, overuse and other factors suggest that septic systems along local lakes can contribute SRP to the water that promote near-shore algae and nuisance aquatic vegetation growth. Some newer full collection systems may contribute as well if they have difficult to detect (and illegal) overflow/bypass connections.

Another key part of the issue lies in the characteristics of the P in human urine and the P content of septic outflow. First, about 2/3 of the P that humans excrete is in urine (Meinzinger and Oldenburg, 2009) and this P is highly bioavailable (Kirchmann and Pettersson, 1995). According to the National Environmental Services Center (2013), the median TP level in the liquid that flows out of the typical septic tank measures about 10 parts per million (ppm or milligram/l). It is unclear how much of septic outflow is in the SRP form, but since the P we excrete is highly bioavailable, it seems probable that a portion of the P in septic outflow is also highly bioavailable.

Changes have occurred in intensity of use of shoreline septic systems over the years. Many seasonal camps have been removed and replaced with larger, year-round homes. Properties that were single family with 3 or 4 children in the 70’s and 80’s are now shared by multiple families. Also, many properties are rented and now occupied up to 7 days per week, sometimes week after week. The increased “person days” on shorelines may be contributing to the changes in water quality that have been observed in some locations.

Considerations for Owasco Lake
A review of the USDA soil survey for Cayuga County shows that all of the major Owasco lakeshore points sitting at the mouth of streams south of Buck Point and Martin Point are mapped as having well-drained soils, and many of these locations consist of soils that are described as having a significant gravel content. Such soil conditions are identified as a risk for water pollution by various reference sources, and there may be other locations along the shore with soil conditions that are not well suited to septic treatment.

According to the 2016 Owasco Lake Report (Halfman et al., 2016), the NYSDEC threshold for impairment is 20 parts per billion (ppb or microgram/l) TP, and the lake-wide summer average TP has been approximately 15 ppb for the last few years. For background, note that 1 ppm is 1,000 times greater than 1 ppb. In comparison, 10 ppm TP median septic outflow is 500 times the TP impairment threshold of 20 ppb, and approximately 660 times the lake-wide summer average TP of approximately 15 ppb (Halfman et al., 2016). Given that septic outflow P concentration is hundreds of times higher than lake-wide summer average TP, and combined with the high bioavailability of P in our urine, it seems very possible that poorly functioning septic systems can be an important contributor to lakeshore hot spots of SRP that support algae and nuisance weed growth.

It should also be noted that several lakes with TP levels below 10 ppb, much lower than Owasco Lake, have also experienced algal blooms in recent summers, suggesting that TP for the lake may not be the best algae growth indictor and that SRP from shorelines or other sources could be involved. A better understanding of P in our water bodies is critical.

In Summary
In the short term, when it comes to growing algae and weeds in our lakes, the quality of P may be more important than the quantity: SRP represents the main form of immediately available P that is used by nuisance weeds and microorganisms such as the cyanobacteria that contribute to harmful algal blooms. Human urine contains a high proportion of bioavailable P and shoreline septic systems are often in close proximity to the water and may be situated in unsuitable soil conditions. The series of factors described here suggest that shoreline septic systems can contribute to elevated levels of SRP in our lakes and further investigation is warranted. As we work to understand and manage P from all sources, including agriculture, addressing shoreline septic contributions will be an important part of the solution.

References

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