1985-03-22 Letter, Condition of Septic System

Home Browse Search

Jan Gibson % Norwest Mortgage March 22, 1985 Page 2 Assuming a percolation rate range of 30-45 minutes per inch, typical of these soils, a trench drainfield system should hav c about 2 square feet of absorption area per gallon of wastewater per day, according to standard design criteria. For a new house today, the design rate of water use is 75 gallons per person per day, 2 people per bedroom. For this 4 bedroom house: 2 s.f./GPD x 2 people/Br x 4 Br x 75 GPD/persor. equals 1200 square feet drainfield in trenches no more than 3' wide. The system serving the existing house was not well documented when it was installed. Through probing the drainfield, we are able to discern 2 drainfield lines apparently 60' in length and 5' in width, for a total drainfield area of about 600 square feet. Note that in a "bed" configuration, i.e. with trenches exceeding 3' in width, the U. of M. Extension Service personnel are now recommending that the re- quired drainfield area be increased by 50% when beds are used, hence by most current thinking, this house should have 1200 x 1.5 or 1800 square feet of drainfield bed. It appears to have 1/3 that amount. What condition is this drainfield in? An absolute determination would require excavating the beds, in effect destroying them. We can make some assumptions, however. Since the amount of cover over the beds is 2-2 1/2', which is unusually deep by today's standards, and based on the assumed soil types described above and the topography of this specific site we can probably expect that the seasonal zones of saturation are very shallow and may surround the beds during wet seaons of the year. This seasonal saturation, depending on duration, may serve to allow the biomat inside the trenches to build up at an annual rate greater than its natural rate of degradation; hence the trenches tend to accumulate biomat (the characteristic black "sludge") and eventually drastically reduce the trench capability to accept effluent. We should probably assume, based on these saturated soils probably being present, that the existing system after 11 1/2 years is working at less than peak efficiency, especially during wet seasons of the year. What % of efficiency? There is no practica' method of determining this. Having read this far, you are undoubtedly wondering when I'm going to get to the point. My point is, it is impossible to make an accurate determination of the conditon of a system without making numerous assumptions and inferences, some of which may be correct and others not correct. For instance, if we assume that the system 1.60% of its "peak efficiency", and if we assume that it actuall; 'A)Fs have 600 square feet of drainfield bed area, and if we reduce the jomewhat because of the Extension Service recommendations, and if we assume a percolation rate of 30-45 minutes per inch which translates-f6—T-GPD acceptance