What do I need to know about soil conditions? Do I need a percolation test?
The short answer to this is that a percolation test should be performed for every proposed new septic system, as well as a deeper observation pit to check for the presence of conditions below the proposed absorption bed that might make construction of a septic system impossible, or cause an early failure.
Not all soils in Guam are suitable for septic systems. Guam’s regulations set a minimum percolation rate of 1 inch in 60 minutes, below which a septic system may not be constructed. Percolation rates between that value and 1 inch in 30 minutes must be constructed using a trench system, as opposed to a bed (“leaching field”), which requires more space but makes better use of the slower-percolating soils to ensure the septic system provides a reasonable service life.
Southern Guam is particularly prone to low percolation-rate soils, due to its volcanic origins compared to the limestone plateau of Northern Guam, which in general is more forgiving, but there are also regions of volcanic geology within Northern Guam, as well as pockets of deep, clayey soils which are not as favorable for septic system construction. It is therefore strongly recommended to perform a percolation test prior to designing a septic system, no matter where in Guam you will be building.
In addition to the percolation test, which uses a small hole dug only to the depth of the proposed absorption system, a deeper observation pit should be dug for the purpose of identifying impermeable rock layers or signs of locally high water tables, both of which will prevent a septic system from functioning. Impermeable rock layers can sometimes be found near the surface in areas of volcanic geology, and will prevent the proper percolation of wastewater from the absorption system, often resulting in surfacing of wastewater down-hill of the septic system, and inability to accept wastewater during wet conditions.
The presence of groundwater less than 3 to 5 feet of the bottom of an absorption system will result in similar failures of the septic system. High water-table conditions typically occur at lower elevations, flood plans, or near streams and wetlands, but may occur anywhere and can be detected by an experienced soil evaluator through either the direct observation of standing water within the observation pit, or through discoloration of soils indicating occasional periods of prolonged saturation, which may not occur all the time, but will prevent a septic system from working during certain parts of the year.
Be cautious: It may be tempting to request additional percolation tests or soil evaluations if the results show that a site will not be suitable for a septic system. It is very possible that you may obtain a different result from a site located on a different portion of your property indicating a septic system may be feasible. It is natural for a prospective homeowner to want to reject what seems like bad news in favor of better results from a second test. But this can lead to disappointment or even misery, in the case that a septic system fails and the building plumbing cannot be used. Ensure that you work closely with your engineer or architect in such a situation, and be realistic about your expectations. The unfavorable test result was most likely a true representation of that portion of your property, and a more favorable test result obtained from another portion should be viewed with caution, and at most, the final design should be based on the average of all test results, not just the most favorable. Be especially cautious if there are also indications the soils may be saturated with water within 3 feet of the absorption trench bottom, or that the site receives stormwater runoff from nearby areas. These factors, on their own or especially in combination with a low measured percolation rate, should serve as a serious warning.
In Guam, the use of onsite wastewater disposal systems commonly referred to as ”septic systems,” is regulated through a number of laws and regulations. These laws and regulations cover specific requirements for the permitting, design and construction of septic systems in Guam. Guam EPA has developed this FAQ to help you understand the application of various requirements to the design, permitting, and construction of a septic system for a single family dwelling.
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