It’s something that’s known and studied in the earth sciences, particularly environmental microbiology. For example on a sandy beach that has been contaminated by raw sewage the sand that has been exposed to sunlight the longest will have a faecal bacteria count lower than that which has been exposed the shortest period of time. A properly collected and processed set of samples can therefore be used to support a prosecution.

As an aside most bacterial contamination on cleaner beaches comes from dogs, so yeah, clear up after your pooches please people.