Caltrans collects and tests water during the year, but especially during storms.
Scott McGowen is with Caltrans and says it's a federal requirement.
"With our stormwater permit, we are required to do monitoring at several locations up and down the state and it can tell you the actual data of the counts and how much of the different pollutant categories that are coming off the roadway."
McGowen says rain often washes brake dust and petroleum products off of cars and onto streets. The water then goes into storm drains and waterways.
Caltrans is asking people to wash their cars as sediment accumulates and to perform regular checks for fluid leaks.
Water samples routinely contains chromium, lead and copper as well as silt, nitrogen, phosphorous, trash and bacteria.