Pollutants

Septic System Impact

The contribution of failing or inadequate septic systems to lake pollution, particularly through phosphorus and bacterial contamination.

What It Means for Your Lake

Septic systems (also called individual sewage treatment systems or ISTS) are underground wastewater treatment systems used by homes and businesses not connected to municipal sewer service. Around Minnesota and Wisconsin lakes, thousands of seasonal cabins and year-round homes rely on septic systems for wastewater disposal. When properly designed, installed, and maintained, modern septic systems effectively treat wastewater by settling solids in a tank and filtering effluent through a soil absorption field (drainfield). However, failing, outdated, or improperly sited septic systems can be a significant source of phosphorus and other pollutants to lakes. The Minnesota Pollution Control Agency estimates that approximately 10% to 30% of septic systems near lakes are nonconforming or failing, either too close to the lake, in unsuitable soils, or with undersized components. A single failing septic system can deliver 5 to 10 times more phosphorus to a lake per year than a properly functioning system. On lakes with dense cabin development, the cumulative impact of multiple substandard septic systems can be a major contributor to eutrophication. Phosphorus from septic effluent reaches lakes through groundwater flow, which may travel underground for hundreds of feet before entering the lake. Unlike surface runoff, septic-derived phosphorus is in dissolved reactive form, immediately available for algae uptake. Minnesota and Wisconsin have implemented progressively stricter septic system regulations over the past two decades. Minnesota requires inspections at time of property sale and mandates upgrade or replacement of nonconforming systems. Many lake improvement districts and watershed districts offer low-interest loans and cost-share programs to assist property owners with septic system upgrades, recognizing that this is one of the most cost-effective investments for improving lake water quality.

Frequently Asked Questions