Pond is at least 1/4 of acre, around 2.5 feet deep on average so around 200,000 gallons, trees on east side of pond and a couple on south. Cow is 50 feet from nearest edge, water is 3 to 4 feet deep at this location. Cow was alive 5 days ago, around 1700 lbs. 

If we leave cow in pond how worried should I be about the quality water for cow drinking in a week? a month? 2 months? 

Any suggestions on how to get cow out of pond.