Hetch Hetchy Entrance Residential Area Percolation Test

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A percolation test is needed for data collection on the septic capacity at the Hetch Hetchy Entrance residential area. The data will be used to assess the septic system's capacity based upon current use. Two systems will be evaluated. One is located at house #2000 the other at the tent cabin area.

The percolation test is designed to determine the suitability of a site for a subsurface sewage disposal system (i.e. septic system). More specifically, a percolation test measures the ability of the soil to absorb liquid. Data is needed to assist with design of septic systems for compliance with state standards.

The percolation tests are designed to simulate conditions in a septic system. Standard industry practice generally consists of a hole 6-12 inches in diameter dug in the area of the proposed septic system. The depth of this hole varies depending on the soils encountered but it is generally not greater than 24 inches. The hole is initially filled with water (presoak) in an attempt to saturate the soil, allowed to drain away and than refilled with approximately 12 inches of water. The rate at which the water drops in the hole is measured at intervals over a period of time ranging from 30-60 minutes. The uniform slowest rate of drop of the water level over a measured time interval is converted to minutes per inch and used as a basis of design in determining the septic system size. For example, if the water dropped uniformly 1\4 inch every five minutes the rate would be 20 minutes per inch. The Health Code provides a simple table that determines the size of the system based on the measured perk rate and the number of bedrooms in the home. The greater the number of bedrooms and the slower the percolation rate, the larger the system required. Commercial systems are sized using the perk rate and projected estimates of water usage in gallons per day.

Percolation tests shall be scheduled with the park archeology office to facilitate archeological survey of the project area prior to test location selection. Locations of the testing shall be determined in consultation with the park archeology office, based on survey findings. In addition, a backhoe will be used to expose a few sections of the existing septic system so that an engineer may determine the condition of the existing facilities. The condition assessment holes shall be dug such that only previously disturbed soils are temporarily excavated. Archeological monitoring may be required during testing and condition assessment hole excavation.