There are a lot of things that can go into the ground water. The key is whether what goes in will readily biodegrade and if not can it harm you or the environment. In the first large study to track the fate of a wide range of antibiotics given to dairy cows, University of California (UC) Davis scientists found that the drugs routinely end up on the ground and in manure lagoons, but are mostly broken down before they reach groundwater. Note that antibiotics are given to sick cows who are isolated from the regular milking herd until the antibiotic is absent from their system.