j09ln03, if your “distribution field” is an evapo-transpiration (ET) bed, then heavy foot traffic will negatively impact the ability of the wicking sand to function properly, and the bed will become a sodden mess. If however, you have a standard soil absorption system, with fairly permeable soils, then the foot traffic applied with an occasional volleyball game should have no effect on the proper performance of the leach field. Do not drive heavy motor vehicles over any type of final dispersal field, not even occasionally. This means “NEVER”.
Of course, if you have only one leach field, then it will fail at some point in time. Its service life may be many years, but soil absorption systems without the ability to “rest”, (alternating leach fields) will eventually fail, whether or not you have an occasional volleyball game above it. JWA