I misspoke and I should have said wastewater system instead of plants, the wastewater system includes septic systems. If you think about the entire system, it is not a stretch at all. Wastewater treatment plants typically discharge into lakes and rivers, but septic systems discharge into the aquifer through the ground above it, and the aquifer feeds steams. The effluent discharged in the septic system leach fields percolates through the ground and pollutants are converted to harmless forms by time it reaches the aquifer, provided the system is not saturated, but many pharmaceuticals do not get broken down and end up in the aquifer unchanged. These can then get into small streams over time. Wastewater treatment plants operate basically the same, just at a much bigger scale, and they will require a special process to remove these contaminants, I’m not sure if that will be possible with septic systems. It’s possible it is a different source for the particular creek you are talking about, but it is a well known problem of septic systems causing pollution in streams, so it is a likely source.