The weight would be no problem but it really depends upon your local code & zone ordinances.

In my jurisdiction you may not put lawn chairs, picnic tables, childrens swingsets, wading pools or any thing other than grass on the septic tank field.

The concern is that the grass over the leach field is vital to the proper operation of the septic system and they feel that if you put lawn equipment in that area it would increase foot traffic and potentially create bald spots in the turf. With a mound system there is also the concern that the increased foot traffic would retard grass growth and there would then be a severe risk of soil erosion from the mound.

"The concern is that the grass over the leach field is vital to the proper operation of the septic system ..."

They work just fine in the desert with nothing over them but sand.

The water goes down, not up.

If only it were that simple designing septic systems would be easy. Where the water goes is totally dependant upon the local geology. In an arid desert region such as you suggest the soil does absorb the water, however desert plants tend to have a vast root network just slightly below the surface. In fact, a 20ft high suguaro cactus has a root system that covers 20 to 30 acres.

In many regions such as in the midwest they have 10" to 40" of topsoil over a thich hardpan yellow clay subsoil that is almost impervious to water. As the water leaches out of the leachbed it saturates the topsoil horizontally and slowly moves up to the surface by capilary action where it is either absorb by vegatation ground cover or is evaporated into the atmosphere. Anyone who lives in that area can tell you firsthand that the first indication of a problem with a leach line or sewer line will be a continual wetspot on the lawn which will progressively get worse until you end up with a swampy area filled with foul stinky standing water on the surface.

Now in regards to Fallingwaters original post, he/she is describing a "mound system". Years ago before you could start building on a lot the performed a "Perk Test" to determine how well the soil would absorb water. If your land did not meet the minimal standard on the perk test you simply could not build on that land. But in recent years the septic system technology has changed. In regions where you have a high water table and the land will not perk they now permit two solutions.

1. An aerobic septic tank, which is a normal septic tank that has a third chamber installed with an air pump and areator nozzles in the bottom of the third chamber. With this type of system the aerobic bacteria that would normally be found in the leach lines are in the third chamber and air is supplied via the pump and areators. With this type of tank the liquid discharged from the aereator chamber is pure water and may be discharged directly onto the surface watershed or into a nearby waterway. (In some jurisdictions they are required to have a chlorinator on the discharge line.)

2. A mound system is an above grade septic tank. In some installation. In some localities they require you to construct a retaining wall around an enclosure about 30' x 70' and the septic tank is installed sitting on the ground inside the retaining wall, then the enclosure is filled with sand up to the top of the wall (about 4') and the leach field is then laid on the sand. They then continue covering the leachbed until the sand is piled high in the enclosure with about a 45deg slop on all sides, and the sand is then covered with sod to prevent erosion.

In some areas the tank is partially embedded with the discharge line at grade level and the leach bed is then run on the surface and finally covered with 3' to 4' of sand and finally covered with sod.

The sod has a two fold purpose, it absorbs a substantial portion of the water and dissipates it to the atmosphere, while also preventing erosion of the sand covering.

In most areas where they permit mound systems they do not allow any activity on the mound that might be detrimental to the growth of the sod. That means no picnic tables, lawn chairs or childrens outdoor recreational equipment tht might increase foot traffic and cause bare footpaths in the sod.