Alum reacts with alkalinity through a series of reactions to form aluminum hydroxide at pH's between 4.5 to 8.5. This compound then cross-links with itself to form a sticky "polymer" which binds up the particles. That being said you need to significantly buffer the water if you're going to play with alum. Baking soda (sodium bicarbonate) will work well and you won't have to worry about pushing the pH too high. However (there's always one of those!) - adding alum will cause a pH swing to some extent even with adequate buffering. Also, there might be some concern about short-term toxicity to your fish. Long term the aluminum will precipitate out in you pond, in your biofilter, etc. -- just as it does in the water treatment plant. Feel free to contact me if you want/need more details or just continue to ask questions here on the forum.
---David

Thought I'd add the brief summary from a web article which discusses a little of what goes on with alum (as well as other inorganic coagulants often used). What is probably most relevant to your question is the following: "...1 mg/L of alum reacts with 0.45 mg/L of natural alkalinity...".

The easiest way to deal with this if you are going to continue using alum would be to add sufficient baking soda to keep your total alkalinity well above 100 mg/L (obviously depending on the amount of alum you intend to add). Hope this helps.
---David

Here is a link that might be useful: Brief discussion on alum reaction

What if I use lime? It'll add a teensy bit of alkalinity. Should I check the alkalinity of my well water to make sure there's some there? I've never checked it.

Which type of "lime" are you talking about? There's "quick lime" - CaO; "agricultural lime" - CaCO3; and "slaked lime" - Ca(OH)2. The calcium carbonate variety would somewhat raise alkalinity but is very slow to dissolve. The problems with the other two - yes they will raise alkalinity (and pH) BUT if you're off a smidgen on the dose you can push the pH up very high in a hurry. I would stay away from them. If you are going to add alkalinity I would continue to use baking soda. There is no real toxicity issue; there's no real problem about taking the pH too high. Yes, if you add massive amounts you will change the salinity of the water but I'm assuming you're not headed anywhere near those levels. I definitely recommend checking your alkalinity. Of all the water parameters you may wish to monitor in your pond, alkalinity, in my mind, head's the list...even more important than pH. The reason being if you've got the alkalinity adjusted where it needs to be then most everything else will pretty much follow. Oh you can tweak calcium levels, you can chase after dissolved organics and other things but with a low alkalinity you are boderline for a pH crash and wipe out of your fish. Usually with well water - specially deep wells - there is plenty of alkalinity present.
---David

I had a sneaking suspicion that the alkalinity would be really low- and sure enough, here's what I got-

Pond water: Alkalinity 11, pH 6.1

Well water: Alkalinity 13, pH 5.6

So I bet the alum would work really well if I was able to get that alkalinity up some. We always have to start adding lime here at the water plant when our alkalinities drop into the 40s.

Wow! That is low! I'm assuming you're talking about alkalinity in mg/L (as CaCO3 - one way it is usually expressed) and not "drops". Do you have any fish in your pond?? Unless you're going to keep some of the African varieties which prefer a slightly acid pH you'll probably find everything grows better if you're 7 or above. You will want the BIG box of Arm & Hammer! :-)) But given where you're starting from I'd add about 1/2 cup every 15 minutes or so until you get close to pH 8. Then you can dump it in as necessary to get the alkalinity where you want it. If you have no fish or frogs then just start adjusting away! You do NOT want to add alum to your water as is...you essentially have zero buffering capacity and even relatively small doses would significantly impact your pH. This assumes you wish to continue playing around with alum. At your present pH I would strongly expect some toxicity issues for any fish if you use alum.
---David

Yes I've got lots of fish. They've always done fine with the low pH.. and the alum.. but I only play around with alum a couple of times a year.

I'm going to try to get that pH up. It's going to be hard to keep it up though because it gets replenished with that acidic well water.

Thanks for your help.

And yes that's the alkalinity I was measuring.
The results didn't suprise me because I essentially have the same water coming into our other water treatment plant. Low pH, no alkalinity, no chlorides. Theirs is cleaner coming in though! Even though it's surface water.