Nelson Issue Brief: Nitrate contamination in drinking water and groundwater

How Dense Should Septic Systems Be?

Contact the authors

James LaGro Jr., Department of Planning and Landscape Architecture, UW-Madison,

Bradley T. Vowels, Department of Planning and Landscape Architecture, UW-Madison,

Septic systems collect, treat, and release wastewater into the groundwater. According to the U.S. EPA, failing septic systems are the second greatest threat to groundwater quality. Densely clustered systems can introduce nitrates, bacteria, and viruses into local water resources. We identified housing clusters in southeastern Wisconsin that may pose risks of groundwater contamination.

Wisconsin’s plumbing code allows rural houses to be served by private wells and on-site septic systems. Land-use policy now allows clustered housing development on rural sites that were once considered unsuitable for septic systems due to environmental constraints, but such clusters can create “hot spots” of groundwater contamination. At a density of two septic systems per acre, the estimated annual nitrate loading is equivalent to the nitrate leached from one acre of corn field. This contamination may cause private wells to exceed the EPA’s maximum contaminant level, but may go unnoticed since less than 10 percent of such wells are tested annually.

Diagram showing potential groundwater contamination risks to private wells

Conceptual diagram showing that housing spatial patterns may exacerbate groundwater contamination risks for private wells and surface waters located "down gradient" from unswered housing clusters. Not to scale. View a larger version of this graphic

Local land use regulations in Wisconsin typically require minimum lot sizes of at least a half-acre for new rural homes served by septic systems. But in Ozaukee County, 624 acres of residential subdivisions exceeded this per acre septic density in 2010. About 42 percent of this land is classified as having “high” groundwater vulnerability. About 200,000 septic systems in Wisconsin predate current regulations, and many of these systems have reached the end of their functional lives. As of 2015, 38 percent of septic systems in Ozaukee County were installed before permitting requirements were adopted in 1971. There is little information on the performance of these older systems.

Periodic well monitoring and septic system maintenance can help households protect their drinking water. For local governments, GIS analyses of existing septic systems and land suitability can ensure that future septic systems minimize risks to the environment and human health.

Learn more about nitrate contamination