City of Auburn Gets Needed Advice on Watershed Septic Systems
MAINE DEPARTMENT OF AGRICULTURE
Glenn E. Aho April 28, 2011
Dear Mr. Aho:
Thank you for your letter of April 13, 2011 asking for my professional opinion on several questions concerning the Lake Auburn Watershed Septic System Ordinance adopted by the City of Auburn. Specifically, section 60-952 (e) (1-5), Private Sewage Disposal Systems. As you are probably aware, I have been involved with this issue for a number of years, originally at the request of a consultant and subsequently at the request of a concerned citizen impacted by the ordinance. Please accept my apology for the time it has taken for this response and its length. This is a busy time of year for me and I have been in the field and/or participating in training workshops (5 of which are septic system workshops) every day since I received it. It is a bit lengthy because I felt is was important to provide you with some background information and an explanation of some complex issues.
Background: In case you or anyone you may share this letter with is interested, I would like to provide you with my professional history. I have been a licensed Site Evaluator for approximately 35 years and was once employed in the State Subsurface Wastewater Disposal (SSWWD) Program as the State Site Evaluator. I am now employed as the State Soil Scientist, a position I have held for over 23 years. In this capacity, I provide assistance to the Subsurface Wastewater Disposal Program, consultants, other State Agencies, towns and the general public. I developed a drainage key for use by Site Evaluators which was adopted by the SSWWD Program and incorporated into the January 18, 2011 Rules as well as requirements that provide for an exemption to additional permitting from DEP and LURC (NRPA and Shoreland Zoning). I am currently assisting the State Subsurface Wastewater Disposal Program in a series of 5 training workshops on the revised Rules where I do a presentation on the sections I wrote. I also am quite involved in reviewing projects for erosion/sediment control and stormwater management. In all of the work I do, I strive to maintain a neutral position when asked to comment. My focus is on the technical issues, not political positions. I keep in mind all of the interested parties concerns when rendering an opinion and strive to balance the need to protect the environment with landowner’s interest in using their property.
Question #1. Is the 36” minimum necessary?
To my knowledge, there is no technical/scientific support for any specific depth to limiting factor (ground water table, hardpan and/or bedrock) in the original soil as being necessary for the proper function of a septic system disposal field, provided that it is properly engineered. The Maine State Subsurface Wastewater Disposal Rules (SSWWDR) required 15” when originally developed in July of 1974 and now only requires 9”, outside of the shoreland zone. The only reason that 15” is still required within the shoreland zone is land use control (to limit development within the shoreland zone). After 37 years of the SSWWD Rules allowing the design of septic systems on soils with depths to limiting factor as low as 0”, we have learned that you can manufacture a suitable site that functions as well as one that nature provided. Of much greater importance to the proper function of a disposal field is how it is designed, installed and maintained. I believe the 36” depth to limiting factor in the soil was chosen to be on the safe or conservative side. It may also have been chosen to assure the entire site was well suited for all of the other development activities associated with a home or business. By assuring that only the best soils could be used for development within the watershed, there was less likelihood of that development impacting the water quality in Lake Auburn. It would also serve to restrict the amount of development that could occur within the watershed. In my opinion, conversion of land within the watershed from forest to urban development is a much more significant issue than the impact of septic systems designed and installed in accordance with the Maine State Subsurface Wastewater Disposal Rules. The relative impact of development on a waterbody however, can now be minimized by techniques such as Low Impact Development that did not exist when your ordinance was adopted.
Question #2. If yes, would a mound system be an adequate alternative for sites where there is less than 36” of soil?
Answer: No, as explained above I do not believe the 36” minimum is necessary.
Question #3. If not, what depth would be environmentally prudent?
Answer: As I explained above in my answer to question #1, I do not believe there is a minimum depth to limiting factor for the proper function of a subsurface wastewater disposal field. It is just a matter of proper engineering such as adding fill and/or lowering the ground water table to make up for what mother nature did not provide. Back in 1974, when Maine went from “Perk Tests” to the current system of Site Evaluation, no one knew what number to pick for a suitable soil depth to limiting factor because the concept was so new. They settled on 15”, primarily because the Soil Scientists they worked with felt it was a good place to start. It just so happens that 15” is the cutoff between soils classified as “somewhat poorly drained” and “moderately well drained”. Moderately well drained sounds a lot better than somewhat poorly drained and requires less engineering. I know this because I worked closely with those soil scientists until they retired and was told as much. In the 37 years that have gone by since then, we in the septic system regulation and design professions have learned a lot. Not just because of our own trial and error but because more states have adopted the site evaluation technique. There are also a number of officially sanctioned sites where a variety of types of septic systems have been installed and closely monitored for a number of years providing valuable data. As you might expect, many times we encountered existing homes and business with failing septic systems but with soil conditions that did not meet this 15” minimum standard. It would not be politically correct to tell those owners that they could no longer live or work in those locations due to poor soil conditions so we developed engineering techniques to overcome site and soil limitations. I am not aware of any ground and/or surface water quality problems as a result of allowing for the design and construction of disposal systems on soils with depths to limiting factor as shallow as 0”. As a matter of fact, I believe disposal systems designed and installed above ground, in fill, are superior to most of those designed and installed as per your ordinance. Let me explain:
There are four primary components to the renovation of waste water (or any water that contains contaminants). One of them is as the old saying goes “the solution to pollution is dilution”. This does occur but is not the mechanism we have the greatest degree of comfort with and certainly do not want to rely on as the primary waste water renovation mechanism. Cation exchange capacity or adsorption of contaminants is another of the mechanisms that serve to treat waste water. Cation exchange capacity is when positively charged particles in the waste water attach themselves to the surfaces of very small soil and organic matter particles. The smaller the soil particle, the more surface area it has and the better cation exchange capacity it has. Therefore, clay soil particles have the best cation exchange capacity and sand has the least. The greatest cation exchange capacity can be found in highly decomposed organic matter. These particles hold onto phosphorous, the primary agent that can cause eutrophication of lakes and ponds, and viruses, another drinking water supply concern. Bacteria are usually physically screened out in the small pores of fine textured soils. Nitrogen is an anion so it is not adsorbed by soil or organic matter particles because both are negatively charged. The third mechanism of waste water renovation is biological uptake of the contaminants. This is important for sites where long term renovation is required because all of the cation exchange sites can become occupied allowing for the eventual leaching of contaminants. If the waste water is discharged into the biologically active zone, where microbes and plant roots are present, the contaminants will be used by them, freeing up cation exchange sites for more contaminant removal. The biologically active zone will also take up the nitrogen discharged into the soil, before it can leach into the groundwater table. This happens most efficiently in fine textured soils where capillary pores can be found which will hold onto the contaminated water very tightly so that it can only move away very slowly. Capillary pores will also “wick” the water up, against gravity, providing more opportunity for uptake by plants and microbes. The fourth mechanism for waste water renovation is a biologically active zone called a “bio-mat” that is created by a disposal field after it has been in operation for a time. This bio-mat forms at the interface where the bottom of the disposal field intercepts the ground surface. It is comprised of particles that escape from the septic tank as well as the bodies of living and dead microorganisms that feed on the particles. It is an important treatment mechanism that is like a double edged sword. It is important to have this bio-mat for treatment but if it gets too thick, waste water will have so much difficulty moving through it that it may cause effluent to surface causing the septic system to fail. Bio-mats form in almost all soil types with the exception of the coarsest sands and gravels. These soils are so porous and aerobic that a bio-mat will not form.
Your ordinance requires a minimum depth to limiting factor in the original soils of 36”. In the State of Maine, that is a difficult depth to find. That is because our soils are considered to be relatively young, geologically speaking. Most of them were compacted by glaciers so they are very dense (hardpan soils) and have only weathered to a shallow depth (24” or less in many cases). For the most part, our only soils with a depth to limiting factor of 36” are what we call “outwash” or gravel/sand deposit soils. These soils provide the least amount of treatment or renovation of waste water of all the soil types. They depend more on dilution than any of the other renovation mechanisms unless a disposal field is designed on top of the ground. That is because these soil types usually have a finer textured “cap” on top of the relatively clean sands and gravels. Conventional thinking is that the fine material was wind blown from some other location after the glacier receded and the ground surface was bare. If a disposal field is installed so that the bottom of it is below the finer textured surface soil layer, there would be no biologically active zone or fine textured particles for cation exchange capacity. Some of these soils are so course that a bio-mat will not form. That means your ordinance requires, in many instances, disposal fields to be installed in sandy/gravelly soils and then they have to be installed below the biologically active zone where there is little or no cation exchange capacity and a bio-mat may not form.
Lake Auburn is a very important asset to the Town of Auburn. Not only is it your drinking water supply, it is also important for recreational activities. Anything that degrades the water quality of Lake Auburn will significantly impact the Town and will likely have costly repercussions. The town should therefore, be very concerned about any activity within the watershed that may degrade water quality. People who own land within the watershed, I suspect, share the towns concern but also want to retain reasonable rights to use their land for a variety of purposes such as urban development. That is a tricky balance and a big challenge for town planners that I do not envy. In my job as State Soil Scientist, I provide advice and consultation for a host of interested parties regarding land use issues. Following will be a few of my thoughts and suggestions that you may want to consider. You are also welcome to ask for whatever assistance I might be able to offer should you decide to revise your ordinance.
There are many activities that can take place within the watershed of a lake that have an impact upon it. One of them is septic systems. As I discussed at length above, I believe the Subsurface Wastewater Disposal Rules do a good job of protecting surface and groundwater from septic system effluent. That does not however, mean that a community shouldn’t expand upon those minimums, to build in a little assurance, such as requiring a greater separation distance from the bottom of a disposal field and the limiting factor. Your ordinance, in my opinion, is not so much designed to provide assurances that septic systems work better or provide more protection of the surface and/or groundwater as it is to limit the amount of development within the watershed. Limiting the amount of development within the Lake Auburn Watershed is not necessarily a bad thing. In fact I believe it should be a primary planning consideration but it makes more sense to me that it is done in a more direct way than to restrict septic system design and installation, particularly when that design potentially is counter productive. I believe the bigger water quality issue is the conversion of forest or field to urban development, not septic systems. Any conversion will have some impact but the degree of impact is directly related to how and where that development is designed. This is where Low Impact Development comes into play. If you are interested in limiting development within the Lake Auburn Watershed, one of the ways you could do so is to require minimum soil conditions for the area to be developed rather than just for the septic system. Septic systems can be engineered easily to function very effectively on very limiting soils. And, they take up a relatively small foot print. Stormwater management related to overall development is a bit of a trickier issue, particularly on poorly suited lots where numerous structural measures are needed that require continual maintenance in order to function properly. Development takes up a much larger foot print and involves a host of items that pose threats to the lake, from lawns (fertilizers and pesticides, pet waste), buildings, driveways (oil and grease, sand and salt), gardens (soil, fertilizers, pesticides), oil fill-up pipes etc. With proper planning, I believe it is possible to strike a balance between landowners and the protection of water quality in Lake Auburn.
Hopefully I have satisfactorily answered your questions about the Lake Auburn septic system ordinance. If not or if you have any further questions, please feel free to contact me at this office by e-mail or phone.
David P. Rocque
|Web Management by: DKP|