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Home My WebLink About1985-03-22 Letter, Condition of Septic SystemCITY of ORONO Yost Office Box 66•Crystal Bay, Minnesota V)323•Municipal Offices On the North Shore of Lake Minnetonka March 22, 1985 Jan Gibson % Norwest Mortgage 3601 West 77th Street Suite 960 P.O. Box 35826 Bloomington, MN 55435 Re: Septic Syste - 65 Cygnet Place Dear Ms. Gibson: In reference to your request for an opinion on the adequacy of the existing septic system for the above referenced house, I am submitting the following brief report, including certain assumptions and facts which have a bearing on this opinion. Factors which affect the "adequacy" of a septic system, i.e. it's ability to treat and dispose of a given effluent load, include the following: 1) Design of the system 2) Construction practices used 3) Location of the system a) Topographically b) In relation to surrounding structures 4) Soil permeability and soil water conditions Relating to the system in question, I will deal mostly with the soil conditions. The City has no record of percolation tests Cr soil borings for this property, which tests may or may not have been r•ade prior to system installation. This testing for new septic systems was not required by the City of Orono until 1978. The system in question was installed in 1973. We must therefore rely on the Hennepin County Soil Survey and our knowledge of and experience with soils in the area. The Soil Survey identifies soil types only in general terms and normally does not indicate soil variations smaller than 2 acres in area, hence it is not generally used as a final authority in designing septic systems. However, it is the best information available in this case. The soils map indicates a Hamel loam - Kilkenny loam transitional area at this site. Hamel loam generally has seasonally saturated zones at 1-2' deep and is generally therefore not suited for a trench or bed type drainfield system, (also due to variable but generally slow percolation rates). The Kilkenny loam gererally has slower percolation rates but seasonally saturated zones below 5' ;r all seasons. A "mound" drainfield system is often requirea Kilkenny soils because of the slow permeability. BUILDING A ZONI\t, 473-7357 • ADMINISTRATION A I I%A\U 473 735H • pl HI U IAORKS 471 7354 ASSESSIN( 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 Jan Gibson % Norwest Mortgage March 22, 1985 Page 3 per 2 square feet of bottom area. . . . 0.6 x 600 s.f. x 1 x 1 GPD = 120 GPD 1.5 2 s.f. Under these assumptions, the system would be capable of handling 120 gallons per day. Now, I haven't even touched on the variability of water use among families with dissimilar habits. I know of families of 4 that use only 100 gallons per day total, or 25 GPD/per person. I also know of a family of 10 in Orono whose average water use rate for the period July 1983 to January 1985 was over 110 gallons per day per person. Which typical family is moving into 65 Cygnet P1,ce? How many gallons per day will they use? Is a 120 gallon per day system adequate for them? It apparently was for the last family in the house, since we have no record or evidence of past problems with the system, yet we did on the most recent inspection note that the system appearAd over - full; we can only surmise that this is due to any of a number of factors - totally saturated soils at the present time, a plugged tank outlet pipe, or maybe very low acceptance due to natural biomat build- up, or . . . .? I recommended that the tanks be pumped, which they now have been, which will give the drainfield a chance to recover a bit. I do not believe that this pumpout will have a long term effect on the continued functioning of the system, but should ensure that a new owner starts out with relatively solids -free tanks. The two tanks appear to be 1000-plus and 750 gallons in capacity respectively. It is impossible without digging them up and removing the covers to tell whether the baffle:, are still in place. It appears they do not have solid bottoms and are probably subject to some interference by seasonal saturation. Current codes for a new house would require sealed, precast 1-piece tanks rather than these site -built tanks which are not sealed. To summarize, they will likely continue to function but not as well as new tanks would. Within the City of Orono are about 1,050 septic systems. Roughly 80% of these were installed (or still use components that were installed) prior to 1978, when the code changed to require soil test- ing and up-to-date system design, construction and maintenance -tan- dards. However, the code does recognize "grandfathered" systems, and where an existing substandard system is still functional and does not create a healtr_ safety or welfare problem or an unsanitary condition or pollution hazard, we do not require upgrading of the system except to add the necessary tank inspection pipes. Jan Gibson % Norwest Mort -gage March 22, 1985 Page 4 I will try to briefly summarize for you: A) The existing tanks appear to be substandard but functional. They are of adequate capacity. We do not know whether the baffles are still in place. The baffles should be checked regularly (at each pumpout) as they are a critical part of the system. Since the tanks are not sealed, they may be affected by groundwater during wet seasons. B) The drainfield appears to be substandard but functional. If it was not functional, we would see evidence of sewage at the ground surface or backups would have occurred in the house. We have no way of knowing whether bac?ups have occurred in the house. C) We can surmise that the drainfield may be capable of accepting up to 300 gallons per day of wastewater under dry soil conditions, but may accept only 120 GPD or less under saturated conditons. Since the private well is not metered, we have no idea how much water the previous owners used. We cannot accurately surmise the actual amount of water future owners will use. D) The City has little evidence to suggest that this system is in a failing state, hence, unless it does fail by causing discernible sanitary or pollution problems, we will not require it to be upgraded. Orono does not have a hous-ng code requiring upgrading at the time of a sale. I realize that this treatise has been rather tedious, but it's intent is to illustrate that "adequate" may have many meanings. Based on current standards for a new house, the system is not adequate. But, based on standards in effect at the time of construction and based on the apparent functionality of the system, it is adequate enough that I could not justify through my ordinances requiring that it be upgraded. If a future owner produces amounts of wastewater exceeding the unknown capability of the system, we could expect that the system would become inadequate. Please contact me at 473-7357 if you have any questions. Sincerely, i Michael P. Gaffton, Assistant Zoning Administrator MPCA Inspector Cert. No. 00516