General Size Requirement

• The New Mexico Environment Department recommends assuming that a drain field will have two 150-foot laterals spaced 10 feet apart. This will allow more than adequate space for the drain field plus replacement laterals in the future. Most municipalities base size requirements partly on the number of bedrooms in the house rather than the number of residents. The number of square feet the trench bottom must have can vary between 125 and 400 square feet per bedroom with a total area set aside for the drain field ranging from 800 to 2500 square feet if the house has no more than two bedrooms.
  
  

Influencing Factors and Siting

• Soil type, expressed broadly through percolation rate, generally has a great influence on the required drain field size. Sites that have clayey soils require larger drain fields to account for the slow percolation rate while houses located on sandy or gravelly soils require a much smaller drain field to function properly. The drain field must also meet a number of restrictions that may vary depending on the municipality. There are commonly minimum setbacks from any structures, utilities and bodies of water that the drain field must have. The depth to the water table will also impact drain field placement and design.
  
  

Additional Size Considerations

• When selecting a site for the drain field or otherwise planning the system, bear in mind that drain fields have a limited functional lifespan, often lasting no longer than a few decades, depending on system installation and usage. Plan for this eventual failure and make the drain field area large enough to accommodate replacement trenches and pipes between existing pipes or next to the existing, active field.
  
  

Drainfield Alternatives

• Although conventional gravel and trench drain fields are inexpensive and simple, they may not be suitable in some situations. If there is not adequate space for a drain field and reserve drain field or there are soil or water concerns, drain field alternatives may be pursued. Chamber systems are more expensive than traditional systems but generally take up less space. A seepage pit, or dry well, is a pre-cast tank with holes placed in the ground and surrounded by gravel. Water exits the septic tank and flows to the seepage pit. Sand filters and aerobic treatment systems may also allow greater flexibility in system placement. Sand mound systems are used on sites with soil not suitable for a traditional drain field or where the water table high