Based on feedback from numerous contractors as well as the inspector for Delaware Natural Resources and Environmental Control, I would suggest looking into Bentonite Mat. One source:  http://www.cetco.com/groups/Lining/products/GCL.asp

If you have a small basin, I think the poly liners would be quite expensive.  The travel to the site for the installation would cost more than the liner.

As Terry stated, something utilizing benetonite might be more affordable.  Whether it be a mat type system or you just incorporate bagged material into the top 9"=12" of soil within the basin.  If you use benetonite or other soil type liners, you will need to keep it moist by covering it with an adequate depth of other soils or maintain water in the basin.  If it dries and cracks the liner is useless.

What kind of soils do you have in the area?  If they are fat clays, they can be hauled in or used in place for the liner.  May check the permeability of the existing soils to determine if a liner is even needed.

I agree with semo.  I work in southcentral PA (Harrisburg area mostly).  Sinkholes are a huge concern here.  Still, native soil liners are almost always acceptable.  Budget?  A little (ok, a lot of) hauling and compaction.  Almost certain to beat a manufactured item, which will still involve a lot of preparatory earthwork.

Three stipulations: if you have USC soils CH, CL OR MH, ML available, and the client is willing to lab certify your specified compaction and w, and if the reviewer is willing to accept your specifications (three big ifs, I know).

Just to point out one thing on flexible liners (such as PVC) they can be prefabricated into the required size, there is a limit, but if so any local contractor can install it just by unrolling and pulling.  The cost of PVC or LLDPE is quite low and will be much cheaper to transport than bentonite, which many times costs more to transport than to buy.

Thanks for the input.  I talked to a few suppliers and got different advice from each.  The two most suggested solutions were the installation of a PVC liner and the installation of a Polypropylene liner.  The budget numbers for the PVC liner were about $0.50/sf and the polypropylene was about $1.50/sf.  Will probably spec out a 40 mil PVC liner.

The use of native soils or the addition benotimite are good ideas, but I think the local well head protection committee would feel more comfortable if they saw a physical barrier as opposed to just soil.