He might be sampling from the cso, combined storm overflow, part of the sewer system. Newer parts of town generally have storm and sanitary sewer systems separated. There is some overlap in older parts of towns. As new lines are put in or upgraded, they are separated.
Hmm...
I don't know. I feel so clueless when it comes to what he's doing. Not like when he got his Bachelors and we strolled up and down the sides of the highways picking various plant samples and drying them. I learned a lot of what he was studying and understood it. (Agriculture, hort.)
This is just so far above my head that when he talks about it, mostly I just nod! LOL