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

Land Use Planning and Resource Protection

A watershed or community approach to land use planning can include a step-by-step process for choosing the appropriate options for onsite or community water supply and wastewater systems. Policy decisions on appropriate levels of wastewater treatment can be based upon desired land use density goals and the proximity of sensitive environmental receptors.

Detailed risk assessment processes and models for septic system contaminant transport have been developed in other areas of New England, particularly in Rhode Island and Massachusetts. In many cases, the ultimate fate of the effluent can be traced fairly accurately, including estimates of the amounts of wastewater constituents that will discharge into streams, lakes and ponds, and coastal waters. The University of Rhode Island Cooperative Extension Service developed a GIS-based analysis tool called MANAGE (Method for Assessment, Nutrient-loading And Geographic Evaluation) that identifies areas with environmental sensitivities and links them to desired performance standards. It is a planning-level tool that was developed for area-wide assessments such as the appropriate sizing of wellhead protection areas, but it requires local knowledge and input to refine and calibrate the results. Similar assessment tools and processes could be created for Maine communities.

In the U.S. Environmental Protection Agency’s Online Wastewater Treatment Systems Manual (2002) [NOTE: large file; 367 pages; takes a while to load], a “Probability of environmental impact” decision tree identifies various types of water resources and their likely sensitivity to impacts from septic systems under different developmet densities, with the outcomes listed as low, moderate or high potential for impact. A simplifed version of that decision tree (Table 2) incorporates suitable wastewater performance standards to protect each resource from the described threats. In particularly sensitive areas, it may be useful to specify treatment standards that must be met by all new septic system installations. Click here for Table 3 - septic system performance standards (based on corresponding wastewater treatment plant standards for primary, secondary, tertiary, and special wastewater treatment) along with the maximum allowable level of constituents for each level of performance. These performance standards could easily be adapted and cross-matched against the list of approved technologies for wastewater treatment in Maine.

The following paragraphs describe a planning process that is depicted graphically in the decision tree table. The results of this process are briefly discussed in Steps 4 (potential impacts) and 5 (treatment standards needed). All of the standards refer to the classification system shown in the Table 3 - septic system performance standards.

STEP 1: Evaluate Drinking Water Sources Relative to Land Use Densities

Infiltrating water recharges subsurface aquifers, which are often used in Maine as drinking water supplies. Bedrock aquifers are generally accessed using drilled wells, as are confined overburden aquifers that are overlain by finer grained, less permeable soils. Even if the land use density goals in the area are high, the potential for impacts from conventional systems on deep drilled wells is relatively low. (However, multiple drilled wells on small lots served by individual septic systems must be evaluated for their cumulative potential impacts due to drawdown effects.) Low-density areas over confined aquifers also typically have a low potential for impact. Thus, the wastewater treatment performance standard in these areas (from Table 3 - septic system performance standards.) would most likely be Treatment Standard 1 (TS1, roughly equivalent to primary treatment in a wastewater treatment plant).

Unconfined aquifers that have been identified as regionally important drinking water resources, particularly in densely developed areas or those with high-density development goals, have a high potential for impact from nearby conventional septic systems. Performance standards of TS4N and TS7 (particularly in Source Water Protection Areas) may be most appropriate for densely developed areas. There is still a moderate potential for impact in low-density areas, since there may not be adequate restrictive soil layers to filter contaminants from onsite septic systems. Overburden aquifers that are not being used for drinking water (and are not expected to be used for this purpose in the future, for example in areas served by a public water line), may experience a low to moderate impact from septic systems under high-density land use. Treatment standard levels of TS1 or TS2 may be appropriate. In low-density areas where overburden aquifers are not being tapped for drinking water, there is a low potential for impact, and TS1 appears adequate.

STEP 2: Evaluate Nearby Surface Waters Relative to Land Use Densities

Lakes, ponds, and slow-moving rivers can receive a moderate to high potential impact from nearby septic systems regardless of land use density, and they are very sensitive to nitrate and phosphorus loading. TS4N would be an appropriate treatment standard. Rivers and streams with higher flows can be moderately impacted in areas of high-density land use, also requiring TS4N standards. Fast-moving rivers in areas of low-density land use will quickly dilute and flush any contamination entering the surface water, so only the TS1 standard is needed.

Coastal waters can be divided into two types: bays and shellfish flats that are not well flushed, and areas facing the open sea with constant wave action. For high-density areas near sensitive shellfish flats and salt-marsh estuaries, septic system effluent can have a high impact, requiring performance standards of TS4N. Even low-density development with septic systems can contribute nitrogen to bay areas where less flushing action occurs, so these systems also can benefit from application of the TS4N standard. Nutrient and pathogen reduction might not be neccesary in a well-flushed, open-sea area with low-density use. Such areas might only need the lowest standard of TS1.

STEP 3: Align Wastewater Treatment Systems with Land Use Density Goals

Local town plans and zoning regulations typically identify areas where compact, village-style development or a thriving, walkable residential neighborhood can be placed. In general, rural residential developments with lot sizes of one acre or more are considered to be low density development, while more compact developments in growth areas would be considered high density development.

Identifying sites for clustered wastewater systems may be the preferred strategy in areas targeted for high density, as opposed to requiring and installing numerous high-performance onsite treatment systems. The cumulative impacts from many small systems are likely to be greater in an area of high density than the impacts from a single, larger system serving many lots. Clustered or community water supply systems serving small lots with onsite septic systems may also be another appropriate choice to achieve density goals, particularly where soils and groundwater recharge conditions are optimum and the selected well sites are located at least 300 feet from the nearest septic system. However, a hydrogeological analysis should be performed prior to construction, to ensure that wellhead drawdown will not siphon contaminated wastewater toward the well site.

Another option that protects the environment while encouraging higher density is to connect existing development along with new development into community wastewater disposal fields constructed for growth areas. Many growth centers are already partially developed, and large lot owners may be encouraged to subdivide if they are offered the opportunity to connect to a clustered offsite system. If any of the existing properties has an older or failing onsite system, bringing them into the clustered system may open up funding opportunities that will pay for the most or all of the entire construction project. The Town of Warren, Vermont was able to connect approximately 90 percent of its existing village properties to offsite community leach fields constructed on public property using an EPA demonstration grant and State Clean Water Revolving Fund money, and approximately 50 percent of the constructed design capacity is still available for future growth within the village

STEP 4: Identify Environmental Impacts of Existing Onsite and Clustered Systems

Based on the type of water resource and desired density goals, community planners can make a relative assessment of the potential impacts from existing onsite and clustered systems. Impacts can range from low to high, as discussed above. However, a treatment standard cannot be applied retroactively to existing properties, since existing septic systems are grandfathered under Maine law until they require replacement.

STEP 5: Specify Minimum Wastewater Treatment Performance Standards

With the help of the decision tree, community planners and officials can determine which (if any) of their community water resources face the greatest threats from septic contamination and may require special protection. If desirable, subsurface wastewater treatment standards can be directly tied to anticipated density by specifying required minimum treatment standards either by land use or by zone in the local zoning ordinance. Table 3 - septic system performance standards lists all of the standards, including the constituent limits for the different treatment levels.