I'm only going to comment on the fact that on some occasions, horses DO drink toxic things, even if they have fresh water available. One reason that horses drink from muddy puddles instead of their trough is because - like someone else stated -they are lacking something. This horse could be needing something that he or she is not getting, and resorted to drinking the septic water.
For example I was using a bleach and water mixture a few months ago to scrub down the outside of our shed (it was mildewing because we'd been having a LOT of rain) and one of the kid's ponies bucked her off. I, ofcourse, ran to help her catch the pony and left my bucket of bleach water- assuming that the horses would smell it and be uninterested since it was a half and half solution...but when I came back ten minutes later, one of the ponies was drinking the water happily! He had to have his intestines pumped out to keep him from being burned internally, but he still drank it. I can't tell you WHY because I had just filled their buckets, but he did.
Horses are horses. Generally, yes, they ARE smart enough to know when not to drink something, but sometimes they arent. *shrug* not much you can do about it.
I do wonder why OPs horse is in that field at all, but I'm sure a lot of you are wondering why I'd leave a bucket of bleach out in a horse pasture! We all do stupid stuff sometimes. ^^