The listing agent wrote that there is a seasonal stream on the property. The next step to take if you are interested in the house would be to have the realty agent that will be representing you on your next purchase to ask the listing agent for an answer to this question. - To help frame the question, the questioner would have to know some details such as where along the stream did you observe the pollution? What kind of pollution is it? Trash? Smell, Oil? -If you were otherwise interested in the property you would want to know if the source of pollution was continuing or if it was a one time act of vandalism or negligence. You would want to get an estimate of the costs of cleanup, and can it be fully cleaned up? Is the damage remediable - Easy to pull out a tire or a shopping cart, much tougher if there are chemicals spilled.
The listing agent may or may not have answers readily available, she has a very good reputation in her county, and I am sure she will disclose what she knows and she will insist that her sellers disclose anything that they know. Even so, it is in a buyers best interest to conduct their own investigation.
The link below will take you to the El Dorado County Water Agency, the El Dorado county Resource Conservation District, and the El Dorado Irrigation District.