The Existing Problem
Wastewater from septic systems is known to account for 80% to 85% of human-caused impairment to groundwater, freshwater ponds, and estuaries. It is also known that the phosphorus and nitrogen discharges from these systems can impact ponds when:

• Septic systems are too close to and upgradient of (above) ponds
• The systems are closer to the groundwater level
• The systems are old or poorly managed

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​Phosphorus is the key ingredient that leads to cyanobacteria and algal blooms in nearby ponds, although nitrogen is also important. Phosphorus is in all urine and waste, and septic systems only remove a small amount of phosphorus. Once released from a septic system, soils adsorb phosphorus - but can only hold so much. Once the soil has adsorbed all it can, the phosphorus moves with the groundwater - not fast, but surely. If there is a pond nearby and down-gradient (below), the phosphorus will eventually reach the pond. Nitrogen, on the other hand, also is not substantially removed in septic systems, but it is not adsorbed by soil so travels readily into the groundwater and then towards ponds or the sea. 

There are several factors that need to be taken into consideration when determining how septic system nutrients may influence the health of nearby ponds:

• Age of the home in which the septic system is located
• Distance of the home to the pond
• Elevation of the leach field above the groundwater and pond
• Number of the home's residents and if the home is year-round or seasonally occupied
• The existing levels of iron and manganese in the soil
• ​How many septic systems are ​nearby ​

The diagram above shows how phosphorus travels through groundwater from a septic system that is located upgradient (above a pond).

The Study Rationale 
The Brewster Ponds Coalition is engaged in a study to determine how far nutrients, notably phosphorus, travel from septic system leach fields in groundwater towards ponds. Thus, the "Septic System Plume Mapping and Phosphorus Travel Study" will determine impacts of septic systems on ponds, where phosphorus is the key nutrient that leads to cyanobacteria and algal blooms. In particular, the study will help determine which septic systems might impact ponds, considering soil adsorption of nutrients, distance from ponds, leach field elevation above groundwater, and home occupancy.

The Study Process

The BPC study was started in the summer of 2023 by constructing monitoring wells on the properties of three homes on Elbow and Griffiths ponds, at the leach fields and down-gradient from them. Unfortunately, the properties selected did not work out as initially hoped. Surprisingly, the groundwater flow direction at Elbow Pond was opposite to what the United States Geological Survey (USGS) maps showed - in the wrong direction to be of use in the study. Also, the hydrology at Griffiths Pond turned out to be too complex for evaluation. 

​However, some useful data were obtained related to attenuation of nutrients due to the height of the leach fields above groundwater. More testing will be done at these properties to confirm the findings, but it appears that most of the phosphorus is held in soils when a leach field is high above the groundwater table. The BPC is continuing the study and has found two other properties near Lower Mill Pond where the hydrology and groundwater flow direction are clearer. The above map shows the locations. 

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Surveyor from the Horsley-Witten Group determining a monitoring well elevation. 

Anticipated Results
We have constructed a series of monitoring wells at the leach fields and downgradient (i.e., towards the pond) from these sites to evaluate the nutrient travel. Quarterly sampling of the wells throughout 2025 is planned to determine how far phosphorus travels in the groundwater. The study will also consider the effect of occupancy since one of the locations has full-time occupants and the other is seasonally occupied. 

Monitoring well on one of the properties. 

The groundwater levels will be checked to assure​ groundwater flow directions, and the water will be analyzed for a variety of parameters, including phosphorus, nitrogen, chlorides, pH, and dissolved oxygen. The project is being supervised by the Horsley Witten Group (environmental consultants) and well sampling is being done by Massachusetts Alternative Septic System Test Center (MASSTC) personnel. The Town of Brewster Water Planning Task Force, Board of Health, and Conservation Commission have expressed support for the study; people concerned about the impact of septic systems all want to see the results. 

Importance of the Results 
​The results could influence decisions about where new septic systems may be located and which septic systems might not need measures to protect ponds, such as upgrades or connections to sewers. The results are of interest to not only Brewster, but also to all towns on the Cape, as well as other areas where there are lakes and septic systems in sandy soil, since the impact of septic systems on ponds is a common concern.