Should I expect any problems with a full-depth asphalt (total of 7 inches) when ground water is at 1.5 feet below grade? Would an untreated subbase be a better choice in this situation? [sig][/sig]
One alternative is to place a geofabric wrapped compacted rubble layer (minimum stone size of 4-6inches) of approximately 12 inches depth on the top edge of the ground water table and then a thicker pavement over it. This is usefull over relatively short lengths as it is permanent and prevents the GW from entering the pavement material. This has been used successfully to treat wet areas caused by underground springs.
Otherwise all you can hope to do is to permanently lower the GW table by ag drains to reduce the risk of water entering the pavement.
The above is all dependant on loading, other ground conditions, topography, etc.