s the significant factor in determining permeability. Large pores produce weak capillary forces, SO water moves rapidly down through sand in response to gravity. Clay has many pore spaces which can contain a large amount of water (around 60% by volume). However, the pore spaces are very small, SO the capillary force tends to pull water into these spaces like a blotter and hold it there. The force is strong enough to move water up against the pull of gravity. But, because the pore spaces are SO small, there is a great resistance to the flow of water, SO that movement-once the clay is saturated-i - is very slow. These proper- ties of clay are perfect for cleansing wastewater, but are the worst possible for moving wastewater away from the disposal site. Coarse sand, on the other hand, is excellent for moving the wastewater out of sight, but may provide little to no cleansing action. Percolation Whereas permeability is the capacity of soil to transmit water, percolation is the capacity to move water through a soil surface and into the soil. In determining how large to make a drain field, i.e., how much soil area we need to accept a specific volume of waste- water, we would like to know the soil permeability. But measuring ONSITE WASTEWATER DISPOSAL permeability is a complex process, SO we use a relatively simple test to measure percolation, and assume that the two are equal. In fact, the two rates are usually different, but standard design criteria pro- vide enough safety that the difference in rates is not the difference between a drain field that will work and one that won't. The percolation test, while controversial among scientists, is a commonly used method for sizing drain fields, and seems to work reasonably well if the results are used in combination with other information gained from the site inspection, particularly from the soil inspection. The percolation test described below is the basic falling head test of the U.S. Public Health Service Manual of Septic Tank Practice as modified by the U.S. Environment