Hi. Unfortunately your question is incomplete and more information would be needed before it can be answered. However, I will try to help you as best I can.
To answer this question, you need to look at a map, or text that shows where this water is flowing. The components of the environment where this water is fluid will be the source of contamination. For example, if water is flowing through a pasture region, that pasture will be the source of contamination. If that water flows through a compost area, that compost will be the source of contamination.
