How Installers Can Attack the Growing Problem of Algal Blooms

Waterfront homeowners and environmental officials issue growing concerns about the impact of phosphorus on water quality for recreational areas.

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Over the past year, I have had a couple of readers/homeowners in lakeshore areas ask why septic system setbacks and requirements are based at least in part on the potential for phosphorus inputs to the lake. So it’s a good time to address the topic and useful ways service providers can discuss phosphorus with their customers. 

Phosphorus is the limiting nutrient for algal growth in most freshwater lakes. Phosphorus can result in large algal blooms, sometimes resulting in development of toxic types of algae. Large blooms also affect use of the lake for swimming and fishing. Aesthetically the lake does not look or smell good during these episodes. It is something every lakeshore owner wants to avoid.

It is difficult to estimate the contributions of lakeshore septic systems to overall lake phosphorus loadings, but it is clear they sometimes deliver effluent directly to the lake. Think straight pipes or systems installed directly into shallow groundwater at the lake level. These must be replaced with well-designed, installed and maintained systems meeting proper lake setbacks.

Detergents and human waste account for the bulk of phosphorus found in wastewater. Over the past 30 years, lower-phosphorus detergents have become prevalent in the market and less detergent is needed to clean clothes. So the load to septic systems in the wastewater stream has been reduced. Soils in most cases are effective at tying up phosphorus and keeping it from moving to the lake.

TRAPPED IN SOILS

In the septic tank, phosphorus is changed to a soluble form that can be adsorbed onto surfaces of the mineral particles in the soil and form insoluble precipitates with iron and aluminum compounds. In most research studies, the amount of phosphorus removed by soils through these reactions exceeds predicted amounts. This is probably due to slow diffusion of phosphorus into soil particles.

Based on this research, it is estimated that the depth of penetration on fine textured soils (loams, silt loams, clay loams) would be four inches per year or less. The only soil type where there may be significant movement away from the system (up to 20 inches/year) is clean, inert sandy soils where the predominant sand particles are quartz. However, even under this worst-case situation movement over 20 years away from the system toward the lake, if the effluent plume is moving through, the sand would be about 30 feet. A lake setback or 50-75 feet would provide years of protection to the lake.

I recently saw a paper where researchers looked at 20-year-old filter bed system on a lake in Ontario, Canada. This paper seems to confirm the results indicated above. The total amount of phosphorus found within 6 feet of the system accounted for approximately all of the loading to the system over the 20 years. Most of the phosphorus recovered was in the form of insoluble precipitates coating the quartz sand grains.

PROPER SYSTEM IS KEY

In the shallow groundwater at a depth of 3-6 feet there was elevated levels of phosphorus at about 40% of the septic tank effluent level. However, there were no elevated levels beyond 15 feet from the system and continuous monitoring for a six-year period following indicated levels remained steady. This means levels appeared to be at a steady state and the precipitation reactions provide attenuation over a long period of time.

The sand used for the filter bed was granitic and free of calcium or magnesium carbonates so the pore water was slightly acidic, which probably enhanced removal capabilities. It does indicate or open the possibility from a design standpoint for mounds and sand filters of prescribing sands that will improve removal of phosphorus in environmentally sensitive locations.

Although the soil does not do a perfect job of treating septic tank effluent, proper siting, design and installation of soil treatment units are very effective in treatment and providing long-term protection of lake environments.  



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