Field visits and data analysis of bacterial contamination of beaches identified a common set of potential sources for bacterial loading that were present at all or most beaches:
- Storm sewer outfalls are located directly next to or in the near vicinity of nearly all of the beaches. Storm sewers have the potential to collect fecal matter from watershed runoff and carry it directly to the lakes. Additionally, bacteria can persist in sediment or standing water held within the storm sewer system and is flushed into the lake during rain events.
- Waterfowl have been observed at many of the city beaches with geese being the primary culprit. Waterfowl and wildlife have been identified as the primary source of bacterial loading at beaches in various studies. The feces from waterfowl and wildlife can enter the lake through direct runoff or through the storm sewer system. Since geese and ducks are often observed on or very near the beaches, waterfowl may be affecting the beach through direct runoff.
- Starkweather Creek and Wingra Creek enter the lake near Olbrich Beach and Olin Beach, respectively. Streams can transport bacteria in the same manner as storm sewers. Additionally, bacteria can persist in the sediment within the stream and be carried into the lake during storm events. Multiple storm sewers drain into these creeks and may act as a conduit from the watershed to the streams.
Other potential sources observed during annual sanitary surveys include restroom facilities, which were present near all beaches. However, there is no data indicating whether any of the facilities are contributing to bacteria levels at any of the beaches. Since all facilities are connected to the sanitary sewer, the facilities would only be contributing to bacterial loading during overflows or if the sanitary sewer system needs repair. Dye tests may be performed to assess whether sanitary contamination is occurring.
While not directly a source of bacteria, the layout of the beaches and their surrounding areas may facilitate the movement of fecal matter from the watershed to the beach. Municipal parks with mowed lawn landscaping surround all surveyed beaches. At most of these beaches, the lawn is mowed to the shoreline. This kind of landscape offers little resistance to runoff, allowing higher volumes and velocities of runoff that can carry feces or other material with it. Additionally, lawn grass provides favorable habitat to geese and ducks, since it allows wide fields of view for the waterfowl to see potential predators. The combination of these factors creates a beach landscape with an abundant source for fecal matter in an area where it is likely to run off into the water.