Hey There! :)
I have this same question on an assignment I have to do and I was trying to figure it out as well and was gonna see if anybody else had this question answered. I'm sorry, I wish I could help. I'll get back to you if I have help and get it done. Good luck!
One of the biggest sources is called nonpoint sources pollution which occurs as a runoff. Hope this helps out
Answer:
There are 3 types of variables you need to know
Independent Variable — what you keep the same (during an experiment)
Dependent Variable — what you measure (during an experiment)
Control Variable — what you change (during an experiment)
