A storm water run-off percolation rate test system includes a turbid water generator having a silt or sediment source providing silt into a mixing tank. A pipeline carries turbid water from the mixing tank to a test pit or test site. A flow meter measures the amount of turbid water moved into the test pit. Detectors detect a water level in the test pit. A controller automatically controls the turbid water generator and flow of turbid water into the test pit, and records data on the amount of turbid water provided into the test pit over time, to determine a percolation rate. In a method for determining a percolation rate, turbid water is created and used in the percolation rate testing, to more accurately simulate run-off.