I have a sand arena, built by one of my areas best arena companies, and it is set up to be watered twice a day.
The sprinklers are run at 2 AM and 2PM., using Rain Bird Falcon heads set up in a dual overlapping pattern… Each individual head runs by itself off of a 1 inch line at 70psi. There are 10 heads to cover a standard dressage arena (20x60 meters, or 66x198 feet). The AM cycle for the entire arena lasts 2 hours, the PM cycle 1 hour.
Sometimes that is not enough water to keep the dust down and the footing nice, and I am the only one riding on it. Sand arenas require lots of water, applied frequently.
My trainer leased a barn some years ago from a fellow who had bought a farm but was new to the life… BO agreed to set up an irrigated sand arena. When his first electric bill that included running his well pump to keep the arena properly irrigated came in, he went through the roof. My own electric bill increased over $100 per month after my new arena was finished.
Again, sand needs lots and lots of water to pack down, and it needs to stay moist, like the beach as the tide recedes.
Have you got any info about the specific type of sand that was used for your arena?