How do you calculate percolation rate?

Answer 1

Percolation rate describes how long it takes for standing water in a hole to move through the bottom and the walls of that hole.

Basically, a hole with known dimensions is made, usually with a soil auger, post hole digger or shovel, and then a volume of water is added to the hole. The depth to the water surface is then measured. The surface of the water in the hole will drop as the water seeps into the soil. The depth to the water surface is measured repeatedly at set intervals (let's say these intervals are 10 minutes apart, for the sake of this question). Then, after each measurement, more water is added to the hole to fill it up to that original level, again.

Normally, when water is first added to the hole, the water will move through the soil fairly quickly. As the soil become saturated (all pores fill with water), the rate of water loss in the hole will slow down. When it reaches a steady rate of loss, that rate is taken as the percolation rate.

For example: After repeatedly measuring water levels, you find that the water level drops 3 cm every ten minutes. The percolation rate for that hole would be:

10min/3cm = 3.33 cm/min.

This percolation rate can only be realistically compared to percolation tests run with the same size hole, with the same depth of water.

Percolation tests for percolation rates are quick and dirty ways to estimate the soil's saturated hydraulic conductivity(Ksat). They are of limited value in comparison to actually running a Ksat measurement, which is directly comparable to other Ksat measurements, regardless of the size of the hole, how much water was put into it, and (hopefully) the scientist or technician running the test.

Some US states still use "perc tests" to determine the size and design of septic systems that they will allow a landowner to build on his/her property. One method, apparently used in Minnesota, can be found at the link.