Like this?
https://maps.google.com/?ll=40.698519,-99.615719&spn=0.00937,0.021136&t=h&z=16Much of I80 follows the Platte ('flat') River. So the water table is close to the surface. The pits themselves might be left over from the freeway construction, especially ones close to interchanges where they needed extra soil to build up the overpass.
This river was too shallow to use for boat travel, but wagon trains followed it because it was a reliable water source.
http://www.nebraskafishing.com/article.aspx?ID=369fishing these borrow pits