QUESTION: Why do we worry about the phosphorus content in our septic system?

ANSWER: This is another frequent question I receive. Phosphorus is the limiting nutrient for algal growth in most freshwater lakes. These blooms can lead to toxic conditions for animals that drink or come in contact with water where these blue algae blooms occur. Further, the algal blooms can use up precious oxygen that fish and other aquatic organisms need to survive. This, combined with lower light penetration due to the algae in the water, can result in the loss of aquatic vegetation and lead to fish die-offs during these periods.

Obviously phosphorus is one of the nutrients contained in septic tank effluent, derived mostly from feces excreted into the system. Soaps and cleaning products used to contribute to these levels, but most no longer contain phosphorus. Most soils have the ability to fix or tie up large amounts of phosphorus through absorption, mineral precipitation and plant uptake. This results in very little movement away from the operating soil treatment unit – on the order of a few inches a year. One major exception to this would appear to be in areas of sandy soils where the primary weathered mineral forming the sand grains is quartz. This is based primarily on theoretical estimates of sites available to absorb the phosphorus and the minerals available to form precipitates.

ACIDIC CONDITIONS

A few months ago I ran across an article about a study conducted in Canada on a 20-year-old “filter bed,” looking specifically at phosphorus. The system was located on a lake in central Ontario, so with my ties to Minnesota and Wisconsin I found this intriguing. They took approximately 120 sediment samples and looked at the shallow groundwater below the system, which was the reason for the filter bed (or mound).

In sediment samples, they found that most of the phosphorus was tied up within 2 meters (a little over 6 feet) from the infiltration pipes. They also found that the phosphorus was not soluble, but tied up as mineral precipitates containing iron, aluminum and phosphorus coatings on the quartz sand grains. In the shallow groundwater, they did find elevated phosphorus levels in about 40 percent of the septic tank effluent levels in the plume, but there were no elevated levels beyond 5 meters (15-16 feet). Over six years of continuous monitoring, these values did not increase but appeared to stay steady. This means that the precipitates were not breaking down over time.

Sand used to construct the filter bed was transported to the site to solve the separation problem of shallow soils over bedrock – one of the problems we address through the use of sand mounds. The sand itself was granitic and free of carbonate buffering capacity, which means it was slightly acidic in nature. This means the effluent becomes more acidic as it moves through the sand. Under slightly acidic conditions, immobilization through precipitation reactions becomes more likely.

ENCOURAGING STUDY

This study affirms a long-held view that if our systems are installed and maintained properly, they will provide excellent treatment of septic tank effluent. Again, this means keeping systems shallow, utilizing as much of the soil profile as possible, and maintaining adequate separation distances to groundwater, bedrock and other limiting soil conditions. It also opens up the possibility that in areas where soils may not do as well with phosphorus removal, improvements can be made by specifying certain types of sand in mound design and installation.

This also has relevance for areas that have weathered granitic material at depth in the soil profile, a condition often found in mountainous areas in the West.

Typically these layers have been avoided or viewed as problematic even though the permeability of the material is very good. As usual, there is the need for additional study, but these results are encouraging and offer the opportunity to develop systems that will improve treatment.