Building 100x100 pond with a liner and wondering do we need filtration of some sort to keep it clean? We would put a fountain in the middle and use some chemicals but concerned with the liner there are no springs to constantly push fresh water through? Is a fountain and chemicals enough to keep it pleasant for swimming?

Are you looking for swimming pool clean or clean like a nice lake where folks water ski, swim, etc?

Lake clean for sure. Just want to make sure before we pull the trigger we will be able to maintain it and it won’t turn into a big muck pit etc because we will not have any springs coming in (liner) or river, creek, etc. will just be a great big hole in the ground with a liner.
Thanks!

IMO it is definitely possible to get lake type clean, especially if you are not trying to grow fish. I would think about a well designed aeration system to provide good oxygen content for aerobic bacteria to thrive (bio filter) and pond dye to limit sunlight penetration to reduce/suppress algae and vegetation. I would also consider a minnow species to control mosquitos. Once you're up and running, you can supplement control with appropriate chemical applications should need arise.

....Just my 1 cent

Edit: I would also cover the liner with 18 inches minimum of soil, sand, etc. to protect it and to provide substrate for the aerobic bacteria.

Great info thank you.

My pond has never had any chemicals put in it and no filter. It isn't clear but my kids get in there and swim all the time. We had a party last year and I bet there was 20 adults in there also.

I have a lined pond, two years old, one half the size of your proposed. The initial fill and top offs are provided from a water well. It has two bubble aerator outlets and a central lighted fountain. Mine is stocked heavily with fish, getting supplemental feed, with some now large catfish that stir up the sediment. The aerators also stir up sediment. I decided to install a pump and sand filter which does a lot of work...dumping pounds of brown "dirt" and green algae on each back flush. I also depend on bacteria enhancement...bottom sludge and circulating...which has proven effective at providing happy water. Finally, I tried liquid flocculant to clear suspended particles. That stuff is fantastic, clearing water in minutes (but adding to sediment). The water looks great for quite awhile, but the cycle of dirt, leaves, and fish waste continues seasonally. With a thought to swimming, aerators will give you substantial control over water temperature: Hot summer? Run the aerators during the cooler hours. Cold winter? Run the aerators on warmer days. Ha ha! That's exactly what I do for my fish.

Those are my good old days! We'd jump into just about anything water. Then a friend got stuck in a leg by an old trot line hook while we were all tubing down a river. He was sick within a couple of drunken hours and went through septicemia for days while pumped with anti-biotics. Now I recall getting "blood poisoning" after stepping on a piece of broken glass in a river. I do have to go wading deep in my pond on occasion.

With a liner it is much easier to avoid eutrophication...I can always vacuum it if needed. I dump yards of gravel in for spawning beds. I can plant plants in containers to limit spread or, if necessary, pull them out of the gravel base.

[spoiler][/spoiler]Thank you great info! The pond builder is saying we wouldn’t need a below water aerator. He said we will be fine with a fountain and pond chemicals since we don’t plan on stocking with fish etc. the sole purpose of this pond will be to swim and skate in the winter. Does that sound correct?
Thanks!

Just don't run your aerators in the winter...could thin the ice. Minnows (gambusia (mosquito fish) are good on mosquito larvae. A load of larger minnows such as golden shiners and several walleye could save you the fishing experience for all year. Ice fishing!

Bottom aeration is something that is relatively easy to add later if the fountain alone is not working out.

Thanks Dan! You think we would be ok without it to start or do you see problems?

It is all easy to install when and if you need it. If you do a pump and filter, line up the intake and outflow so they are opposed and you'll have a whole pond water current going round and round. Also, the end of the intake pipe must have a larger flow distributor to assure against some bad potential accidents.