What is the ruling on walking on dry dirt mixed with sewage water, and is the dirt ritually impure (najis) or pure (tahir)?

The default state of street dust and mud is purity, and no attention should be paid to doubts concerning it. However, if you are certain that the dust and mud in the street have become impure, then a small amount of it is excused due to the difficulty of avoiding it. But if the impurity is considerable, it is not excused, and one must avoid it or wash whatever it has touched.