Exodus 7:24 - Ellicott's Commentary On The Whole Bible

All the Egyptians digged round about the river. — Wells may be sunk in any part of the alluvium, and will always yield water, which is, however, brackish and unpalatable. This water is, no doubt, derived by percolation from the river; but the percolation is a slow process, and blood would scarcely percolate far. The water obtained was probably in the ground before the miracle took place, and was not made subject to it.