That's a constant situation in south San Diego beach communities: Tijuana River outflow; rarely even commented on it's so common.
Don......
It's mainly in the winter after a good rain, but, can happen any time considering the pitiful state their sewer system is in. Our own storm water runoff contains lots of bacteria. After every rain, they suggest you stay out of the water until 3 days after the rain ends, or, sooner/later depending on what the tests show.