You say your BIL does home construction. Does he ever have a backhoe or mini-excavator available after finishing up a job?
If so, then make your state dig locator call now, and then excavate a couple of "test pits", when the equipment becomes available.
Are there multiple possible locations for the new pond, or only one?
If there is only one location, then do a couple of tests across the proposed pond basin.
First, measure the top soil depth at each location. The pond contractor will need to scrape and stockpile the good topsoil. Spreading the good top soil on the back of the dam and across all of the disturbed ground around the perimeter of the pond will make it MUCH easier for you to quickly establish good groundcover plantings after the pond is finished.
Next, look at the layers as they come out of the ground. (Do NOT go into the trench!) Whenever I want to sample a layer, I do not reach the bucket to the far wall, instead I just do a scrape of the current layer at depth. Dump it on the ground and then get out of the cab and examine the contents of that bucket.
You need to watch out for sand layers because they will steal your pond water. You need to watch out for rocky debris and cobbles because you cannot compact that material to make a seal. You need to watch out for bedrock, because it will limit your pond depth.
You do need to find some good loam or clay-loam material to create the water seals for your pond. When this material is slightly moist, you should be able to roll it in your palms and make a "snowball". Don't worry if you feel some sand in your palms, if you can make a cohesive snowball, then that material should be just fine for a good pond builder. If you can roll it into "worms" then it has an even higher clay content.
Good pond builders have to build a sizeable "cushion" into their bids, because they never know what they will find once they scratch the surface of the earth. IMO, if you can mostly inform the builder what to expect, then he will be able to give you a better plan and bid.
For example, if you want to build the pond a certain way, and the builder identifies a few problems from your test pits, he can then tell you why your initial plan won't work, but here is the plan he can execute successfully that will be closest to your initial design.
ALL design changes that occur BEFORE heavy equipment is on the property are much cheaper than changes made during the middle of the project!
Have fun on your new pond project!
