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

Septic System Performance

Conventional septic systems can treat most of the organic substances found in residential wastewater. Treatment occurs in both the septic tank and the disposal field, particularly at the stone-soil interface. The soil between the disposal field and the seasonal groundwater table or restrictive subsurface layer provides the final treatment process, including the removal of many human pathogens. However, a conventional septic system requires additional separation distance to provide adequate treatment for nitrogen and phosphorus in areas near water bodies, or in areas where groundwater is used as a drinking water source. (Toxic chemicals and pharmaceuticals are not always treated adequately by a conventional septic system; however, dumping or flushing these substances should be strongly discouraged even for those who use public sewer service.) 

Pre-treatment systems can be added to overcome most or all of the negative impacts from a wide variety of wastewater contaminants.  Pre-treatment can also be used to reduce the size of the required disposal field, so that the state-mandated offsets from water bodies, wells, and property lines can be achieved.  

Conventional septic systems can be accommodated (in most instances) on 20,000 square foot lots without negative impact to water supplies, regardless of soil type. What led state regulators to reach this conclusion?  Several factors were considered: 

• The track record for septic systems built since the enactment of the 1974 code has been good, both for new systems on relatively large lots, and for replacement systems on smaller lots. The Division of Environmental Health (DEH) has received few reports of failures in the past decade, even for replacement systems on lots smaller than 20,000 square feet. A 1999 study by the DEP and the Maine Geological Survey found that 99.6% of all well samples on lots using septic systems met the drinking water standards for nitrates. The lots surveyed 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 1 acre (i.e., half were smaller and half were larger than 1 acre).  
• Professionalism among site evaluators, local plumbing inspectors, and installers of septic system has steadily improved.  Site evaluators have had to be state-licensed since the 1970s.  Local plumbing inspectors are state-certified following participation in a comprehensive training program. Many system installers submit to voluntary certification.
• Current standards for design and installation of systems ensure that most pollutants are removed in the leach field and will not contaminate local drinking water supplies.  A major advance in rural wastewater treatment is that modern septic systems are in fact designed to treat the effluent, not simply convey it underground.  In one study in Addison County, Vermont, a system was shown to have removed 99% to 100% of fecal coliform and 89% to 99% of phosphorus within three feet from its discharge from the field. There also are a number of advanced treatment systems and products that can pre-treat the wastewater in areas with poorly-drained soils or a natural hydrological barrier close to the surface that traps groundwater, as both of these conditions can create the potential for well contamination from leachate.  However, in most modern septic systems with the required separation of 12 to 24 inches between the bottom of the system and the restrictive layer, the risk of contaminating wells or nearby water bodies is quite low.
• The required separation distance (100 feet) between leach fields and drinking wells appears to be adequate for health and safety.   A 1999 DEP-Maine Geological Survey study concluded that “…the Maine Subsurface Wastewater Disposal Rules are adequately protecting residential wells from NO₃-N contamination caused by conventional septic systems.”  This separation distance (plus offsets from water bodies and property lines) can be met on 20,000 square foot lots in most instances. (DEH does not require reserved space for replacement systems, but if reserved space is required by local ordinance and the septic field is large, a lot size of greater than 20,000 square feet may be required.)  Maine’s Well Driller and Pump Installer Rules, enacted in 1994, have ensured that all new wells are placed more than 100 feet from septic systems.

 

Related Work Plan Components

• Climate Change and Infrastructure Resilience
• Modernizing Communications/Electric Utility Infrastructure

Workgroup Contacts

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

In Washington County: Judy East