SGPC maryada
most preferable into flowing water that empties into an large river/ocean
But can be disposed other ways
In punjab a preferred, common place is at Sri Kiratpur Sahib Gurdwara Patalpuri.
In USA its hard to get approval for legal eco friendly ash deposition into rivers *regulated by EPA*. but there is a seva undertaken by Sant Daljit Singh Rockford Illinois (near chicago), where he has legal approval and people from neighbouring areas are beginning to use his services.
Sangat may have better answers