THE DEPTH AND PLANAR DIMENSIONS OF YOUR PIT ARE SO SMALL THAT THAT YOU MIGHT CONSIDER FIRST SOME MINIMAL SOILS EXPLORATION )(AUGER DOWN TO 10') TO ASCERTAIN SOIL TYPE AND MOISTURE CONTENT IN MORE DETAIL ...IF THE SOILS ARE COMPETENT AND THE WATER CONTENT LOW, CONSIDER EXCAVATING TO SAY 3 ' WITH...
dont know much about it but entering term 'lithium-6' into the search line of www.google.com gets you to the following among other things:http://www.osti.gov/html/osti/opennet/document/press/pc23.html
You didnt indicate a size of pipe or distance to be traversed...a quick and dirty solution is to encase the pvc in steel pipe if it is just a relatively short crossing.
you might consider installing a barrier to seepage...possibly a membrane cutoff placed by a trencher being careful to assure a water tight seal at the interface of the underside of the spillway structure with the cutoff membrane.