It can come as a surprise to some that an occasional source of industrial farm runoff can be found at dairy farms across the world: spilt milk.
Or dumped milk, rather. Dairy farms dump milk for any number of reasons: externally contaminated milk, milk produced by cows treated with anti-biotics, and accidental milk surpluses are all common. The quality of the milk is not always the issue, however. There have been many efforts from farming coalitions to get farmers to dump milk in order to raise milk prices artificially or get legislative attention.
Unfortunately, milk that is dumped incorrectly is extremely hazardous to ecosystems. Different organic runoff contaminants have varied biological oxygen demand (or BOD) within whatever body of water they runoff into. The higher the oxygen demand, the more animals will die from lack of oxygen and the more bacteria and other microbes that will take over the water. Milk is one of the farm runoff contaminants with the highest BOD, at 140,000 pppm (parts per million). Compare that to liquid sewage sludge's BOD at 20,000 ppm, or pig slurry at 30,000 BOD. Obviously milk is not as inocuous as it seems!
As an individual, the inner workings of the dairy industry can seem tough to influence. The good news is that many farmers are aware of the dangers of milk runoff, and most know of techniques that can be used to try and safely dump milk. Buying from local milk farmers and talking to your milk supplier at a farmer's market are great ways to ensure as a consumer you're getting milk that isn't getting dumped somewhere else.