They've been testing streams and lakes for a long time
Scientists for a long time have been testing for chemicals and drugs that humans, um, process and release into streams, rivers, and lakes. This is a big issue in the inland US, where one town processes its sewage, releases it into a stream, and then the next town downstream draws from the same stream for its drinking water.
Most people don't realize it, but if their drinking water is taken from a lake or stream, they're literally drinking someone else's urine and wastewater. Modern sewage treatment plants are very good at taking out solids (garbage, feces, etc.) and killing bacteria, but they can do absolutely nothing with dissolved chemicals. In most inland cities, the runoff from streets following storms or snow melts also goes directly into streams or rivers without any treatment at all (which is why you shouldn't dump your used motor oil down the storm drain). What modern water systems rely on is that these chemicals are so diluted by the rest of the stream or lake water that it isn't an issue. Even so, excreted hormones are suspected of triggering earlier than normal puberty in children.
Of course, in Chicago the issue of the Chicago river dumping wastewater into Lake Michigan (where the city gets its drinking water) was so bad that they reversed the flow of the river so it dumped its water eventually into the Mississippi instead.
Anyway, given the issue with dissolved chemicals in inland waters, it's not so strange that scientists would wonder about the situation near coastal cities.
