Strategies for Rural Development in Areas with Limited Public Infrastructure: Alternative Septic Systems

Lot Size, Site Evaluation & Improvements in Technology and Inspection

Some communities may not be in a position to implement alternative wastewater disposal systems that take advantage of existing sanitary districts, new community sanitary districts, or public water supplies.  Even so, the science, practice, and rules of subsurface wastewater disposal have advanced sufficiently over the last quarter-century that some of the widely held notions about appropriate lot sizes for onsite septic systems are no longer valid.  It is possible to comply with state subsurface wastewater disposal rules and environmental best management practices, while achieving economic development goals in designated growth areas using individual septic systems and wells on relatively small building sites.

It is important to reinforce that above the 20,000 square foot requirement in state law, there is no correlation between soils and the lot size required to install onsite septic systems.  Over a quarter century of experience under Maine’s 1974 plumbing code has found that it is proper system design, not lot size, that prevents contamination of wells and nearby water bodies.

A word of history:  In 1974, the Division of Environmental Health (DEH) (then known as the Division of Health Engineering, DHE) overhauled the rules for subsurface waste water disposal.  Among other things, the new rules replaced percolation (“perc”) tests with a much more detailed and scientific site evaluation procedure, linking the suitability of the disposal field soils with the type and size of the system required for effective performance. The new rules required all soil evaluations to be undertaken by a state-licensed site evaluator.  They established standards for the design of subsurface waste disposal fields, in place of the cesspools and dry wells used in older systems.  Finally, they continued to prescribe safe, conservative minimum distances between a septic system leach field, drinking water sources, and property lines. 

In 1974, DHE regulators believed that a combination of factors still warranted a heavy dose of caution to protect public health and safety.  These factors included the differing capabilities of various soil types to treat wastewater, the lack of long-term performance data for leach fields sized according to soil conditions, and the suspicion that many septic systems would be installed close to or even within the seasonal water table, due to the dearth of properly trained system installers and local plumbing inspectors in many parts of Maine.  Their caution took the form of a chart with recommended lot sizes, based on different soils profiles and conditions, that provided sufficient buffer space on each lot to cover all of these uncertainties.  It is important to stress that the lot sizes in the chart were not required; they were guidelines that were attached to the new plumbing code in an appendix. 

The recommended lot sizes ranged from the minimum of 20,000 square feet that remains in Maine’s Minimum Lot Size Law today, to 80,000 square feet – and in some soil conditions, the site was simply deemed infeasible for onsite septic disposal.   These guidelines were widely used as a reference by town officials and planners as they drafted comprehensive plans and zoning ordinances.  For simplicity and an extra measure of safety, many towns simply went straight to 40,000 or 80,000 square feet (or rounded up to one or two acres) as their minimum lot sizes, regardless of soil type.  They reasoned that if soils dictated the size of the disposal field, they must also dictate the dispersal area required for wastewater treatment in the soil beyond the fields;  therefore, increasing the dispersal area would provide an extra guarantee against failed systems and the resulting need for local government intervention.  In 1979, DEP  incorporated a similar chart, with minimum lot sizes ranging from 20,000 square feet to 80,000 square feet, into its rules governing soils standards under Chapter 376 of the Site Location Law.

Increasing the minimum lot size to protect water quality was not altogether unwarranted.  A number of studies across the U.S. had previously determined a relationship between relatively high densities of septic systems and degraded water quality, especially for nitrates.  The recommended minimum lot size to prevent contamination was generally found to be around 0.5 to 1.0 acre (20,000 to 40,000 square feet).  However, few if any of the studies took into account the design quality of the systems that were analyzed.  Many had been designed and installed with minimal state and local oversight. Few if any of the nationwide studies described systems that had been designed and installed under state wastewater disposal rules as diligent as Maine’s, which even in 1974 were among the most progressive and protective in the nation.

In 1995, and again in 2011, the Maine Subsurface Waste Water Disposal Rules were revised.  These revisions included:

However, the technical provisions governing system design established in 1974 did not significantly change.  DEH and soils scientists (including the State Soil Scientist) by 1995 had recognized that a properly designed septic system could function well and protect water quality, regardless of soil type, on a 20,000 square foot lot in most cases.  Therefore the appendix containing the chart of recommended lot sizes was dropped from the revised rules in 1995.  However, the chart lives on, both at the Maine Department of Environmental Protection (DEP) and in many local subdivision ordinances.

Why did DEH, the primary regulating agency for septic systems in Maine, decide to drop its previous soils-based recommendations for minimum lot size?  Several reasons: 

1.  Decades of experience with septic systems built after enactment of the 1974 code had proven the effectiveness of proper design, both with new systems on relatively large lots and with replacement systems on small lots.  Soils were being more accurately classified by licensed site evaluators.  The design of disposal fields, both in sizing and in construction standards, was far superior to pre-1974 systems.  DEH received few reports of failures, even for replacement systems on lots smaller than 20,000 square feet.  When a failure was reported, it could usually be attributed to faulty design, installation, or maintenance, rather than to flaws or omissions in the code.  A 1999 study by DEP and the Maine Geological Survey found that 99.6% of a sample of wells on lots using septic systems met drinking water standards for nitrates.  The lots were located in 18 different subdivisions, and sizes in the sample ranged from 0.3 acres to 33.8 acres, with a median size of about one acre (that is, half were smaller and half were larger than an acre).

2.  Professionalism among site evaluators, local plumbing inspectors, and installers of septic systems had steadily improved.  Site evaluators have had to be licensed since the 1970s.  Local plumbing inspectors have been trained and certified under the state’s Code Enforcement Certification Program since 1988.  Many installers of systems submit to voluntary certification.  There is still room for improvement, especially in the education of homeowners on proper use and maintenance of their septic systems; but errors due to “guesstimating” the proper disposal field size and inadequate training have been reduced dramatically.  Science and knowledge have increasingly replaced assumptions and crossed fingers, with positive results.

3.  Modern standards for design and installation of septic systems ensure that most pollutants are removed in the leach field, and that any pollutants remaining in the effluent will not reach nearby groundwater sources used as a drinking supply.  A major advance in rural wastewater treatment is that modern septic systems are in fact designed to treat the wastewater, not simply convey it underground. Former practice tolerated an “out of sight, out of mind” attitude.  Current practice is to treat the constituents of wastewater before they exit the system, and to capture any residual organics in suitable soils before they reach bedrock or the water table.  An example of this shift in attitude is the practice, which has now become standard, of building “at-grade” or “shallow” leach fields.  These infiltration fields are not sunk into the ground as in the past, but rather are built within or above the top 12 inches of the native soil.  In these top 12 inches, the action of microbes and plant roots are most effective.  In one study in Addison County, Vermont, such a system was shown to remove 99% to 100% of fecal coliform and 89% to 99% of phosphorus within three feet down-gradient of the field.  There also are a number of advanced proprietary treatment systems and products that can treat or pre-treat the wastewater, if the soils on a site are judged to be poorly suited to wastewater disposal or if suitable space for a disposal field is extremely limited. 

A large percentage of Maine’s soils have a natural hydrological soil barrier between the earth’s surface and any free-flowing sand and gravel layers (or bedrock) where groundwater flows for well water can be tapped. This barrier is usually a clayey soil or hard pan that drastically slows or prevents the gravity-induced flow of surface water farther down to sand and gravel layers or bedrock.  In earlier times these barrier layers were believed to be a contraindicator for subsurface waste disposal, since they can occur within a few feet of the surface of the ground and the seasonal high water table typically perches above them.  But when septic systems are designed to treat and not merely discharge waste, with a required separation of 12 to 24 inches between the bottom of the system and this restrictive layer, constituents of wastewater that might escape the primary treatment system – such as nitrates and toxic household chemicals that are thoughtlessly poured down the drain – are still unlikely to reach the source of the local drinking water supply, since they will be captured within the hydrological barrier layer.

4.  The required minimum separation distance (100 feet for typical residential systems, 300 feet for larger systems) between leach fields and onsite wells appears to be appropriately conservative.   The 1999 DEP-Maine Geological Survey study concluded that “…the Maine Subsurface Waste Water Disposal Rules are adequately protecting residential wells from NO3-N contamination caused by conventional septic systems.”   This separation distance (plus distances from property lines) can be easily met on 20,000 square foot lots in most instances.  Maine’s Well Driller and Pump Installer Rules, enacted in 1994, have ensured that all new wells are placed more than 100 feet from existing septic systems. 

In short, there is no longer any scientific argument for requiring larger lots to accommodate subsurface waste water disposal.  Municipalities may still wish to specify large lot sizes for other legitimate reasons – to ensure open space preservation in the designated “rural areas” of a comprehensive plan, for example – but providing additional space for leach fields is not among them. Within designated growth areas, lots will still need to be sized larger than those served by a public sewer system, because dispersal and separation distances between septic systems, wells, and property lines do require space.  But it would be highly unusual if all standards could not be met on a 20,000 (or, on an especially challenging site, 30,000) square foot lot.  Lots in this size range can help to create economically viable hamlets, villages, and walking-scale neighborhoods in designated growth areas.

Related Work Plan Components

Workgroup Contacts

In Aroostook County: Jay Kamm, Ken Murchison, Joella Theriault

In Washington County: Judy East