This one felt impossible at first as i never had poop in the pool past the 1 star park. But I figured out a really fast way to get it.

Once you have your park built and ready for the last achievements in your achievement hunting journey, just remove all of the toilets and fire any cleaners and Unlock the vacuum.

Set up some vending machines near some pools and run 2-3 day cycles. You will have some naturally occurring pool dookies, but the speed happens when you can use the vacuum to quickly tap the poop that has accumulated by the vending machines while people are queuing. Tapping the vacuum aiming at the poop from a few feet away will make it travel in bunches, just guide them into a near by pool.

You can get around 5-10 dollops in each pool easily.