RobR: Water table is not an issue for me. Our house is at the high point in our neighborhood. A block away is a lake, 20+' downhill, which marks the water table. If I had to deal with a high water table, I'd want to check on the possibility of having the pond partially above grade. You've probably heard of empty swimming pools popping out of the ground when the water table rises in low lying areas. I'd sure not want to empty the pond and discover it starting to float like a boat. Maybe not a serious risk, but even a small percentage risk would deter me. I think in your area it is not unusual to have the water table rise & fall several feet from the end of the dry season to the end of the hurricane season.
There will be a wood "staircase" into the filter pit. Originally Carl had wanted to do poured concrete steps, but with the tight space, the stairs would have had to begun away from the pit and land at the bottom. That would have interfered with where I want fencing installed. So, had to compromise between twin preferences. Justin lost out on getting some more concrete work & John gets to try his hand at carpentry.
.... Wait until you see the simple little cover he is making for the vortex. There will be a cover for the filter pit, also fairly simple. If it had a real roof structure, lot line setbacks would be a problem.