After seeing the pics, if it were me, I'd burn it where it is. Unless your pond has a really low PH (alkaline) I doubt it will hurt the pond. Also, from the size of the ditch you aren't likely to get a large inflow of ash at one time anyway because it doesn't appear to ever have a large flow of water. You could go to walmart or any store that sells swimming pool supplies and buy some water test strips to check the water PH. Who knows, your pond may be acidic anyway and then the ash would actually be good for the water.