So far the chloramine/chlorine seems like a possible answer though I would expect to have an equal number of large and small fish dead, not mostly large. Oxygen seems like the better answer.
Assuming no way for the sprayed area to be rained on and flood to a pond, in your experience how many feet is a safe distance to spray away from a pond?
Several variable involved. Many states do not have any regulations controlling this and those states that do have established buffer zones of from 100 to 250 feet away from any natural body of water.
There is this from Montana State University that explains the variables involved-
"Factors of Drift
Dave Smith, a Mississippi State University ag engineer, analyzed data from over 100 studies involving particle drift from ground sprayers. Of the 16 variables he considered, three were most important.
- Wind Speed. When the wind speed was doubled, there was almost a 70% increase in drift when the readings were taken 90 feet downwind from the sprayer. Spray when the wind speed is 10 mph wind or less.
- Boom height. When the boom height was increased from 18 to 36 inches the amount of drift increased 350% at 90 feet downwind.
- Distance downwind. If the distance downwind is doubled, the amount of drift decreases five-fold. If the distance downwind goes from 100 to 200 feet, you have only 20% as much drift at 200 feet as at 100 feet and if the distance goes to 400 feet, you only have 4% of the drift you had at 100 feet. Check wind direction and speed when starting to spray a field. You may want to start spraying one side of the field when the wind is lower. Also it may be necessary to only spray part of a field because of wind speed, wind direction and distance to susceptible vegetation. The rest of the field can be sprayed when conditions change.
Other important factors that must be considered in drift management are spray pressure, nozzle size, nozzle orientation, operating speed, air temperature, relative humidity, shields on sprayers and nozzles, application rate and instructions from the manufacturer of the spray product."
For the types of application usually encountered on a residential scale I have recommended at least a 20 foot buffer zone on a calm day and no spraying during a windy day within 100 feet.
