Craig,
You got some homework to do, my friend. Read this site, lots of great info. And, be ready, there are several pond pros nearby who will chime in on your question.
Here's my quick two cents worth.
First, find out who built the dam. Ask them how it was engineered, and what type of soils are in the dam. Ask if the dam was cored, how deep and how wide, and backfilled with what kind of material.
If you can't get answers, hire an engineer to analyze the dam site. With a core sample or two or three, it won't take long to figure out if the dam was properly built. It is consulting money well spent.
The tile at the bottom...find out what it's made of, and how long its life should be. Then, find out if anti-seep collars were used during dam construction.
If the tile is good material, and it has anti-seep collar(s), then you can either put an ell with a standpipe, or a gate valve with long stem on the inside. Or, you can put a standpipe on the backside, in the dirt of your dam, where water can flow over the top, through riprap, and back into the creek.
If the tile is no good for an overflow pipe, concrete it, and be sure the emergency spillway will suffice.
Regarding chemicals, that certainly should be of concern, especially if there's no greenbelt between the lake and farmland.
Due diligence, my new friend, due diligence.

Teach a man to grow fish...
He can teach to catch fish...