Polluted runoff originates in both the rural and urban areas of the Yahara Watershed. Studies of the Yahara Lakes show that urban sources of pollution are important to the quality of the lakes. Urban sources are from stormwater, construction sites, streambank erosion and hydrologic disturbances.
Urban stormwater, the water that runs off of paved surfaces into downstream waterbodies, brings different types of contaminants with it. These are sediment, nutrients, heavy metals and other toxic substances. Heavy metals include lead, copper, zinc, cadmium and chromium, and other toxic substances include PAHs, and pesticides. The pollutants of concern in the Yahara Watershed are nutrients and sediments.
Impervious surfaces such as streets, parking lots, driveways and roofs are a critical source of urban pollutant loads. Leaves carry a large nutrient load when they are washed downstream. Other things like grass clippings, pet waste and salt and sand used on city streets in the winter also contribute to the overall urban load.
The Lake Mendota Priority Watershed Project determined that construction site erosion was one of heaviest contributor to urban runoff in the mid-1990s. In fact, about 22% of the entire sediment load and 18% of the phosphorus load to Lake Mendota was from construction sites.
Since construction sites were recognized as a major pollutant source, a lot of effort has gone into correcting the problem. Much stricter regulations, inspections and enforcement have been used to cut the amount of sediment and nutrient pollutants coming from construction sites.
Fertilizers normally contain a mix of nutrients, including nitrogen, phosphorus and potassium. Rainwater runoff from lawns treated with fertilizers flows to storm drainage systems and into the lakes where the phosphorus from many sources causes excessive algae growth, and decreases water clarity, often turning lakes green. Decaying algae also depletes oxygen in the water, so that fish can no longer thrive.
In order to improve lake water quality by reducing phosphorus runoff, on April 15, 2004, the Dane County Board adopted Ch. 80 of the Dane County Code of Ordinances, "Establishing Regulations for Lawn Fertilizer Application and Sale (download pdf)." The County Executive subsequently signed the ordinance into law.
The ordinance went into effect in January 2005 in every town, village and city in Dane County. Beginning May 1, 2004, signs containing the ordinance requirements and the effects of phosphorus on Dane County waters must be prominently displayed where lawn fertilizers are sold.
The entire state of Wisconsin is now under a ban for using phosphorus fertilizers on already established lawns.