A site evaluation is an investigation of whether a property is capable of treating wastewater by physical, chemical and/or biological means and reducing or preventing the degradation of water quality, the risk to public health, and the degradation of environmental conditions.

Factors used to determine site capability include:  slope, parcel size, surface bodies of water, wells, escarpments or rock outcroppings, cuts and fills, unstable land forms, soil profiles, water table levels, useable area for initial and replacement disposal areas, encumbrances, sewerage availability and other observations as appropriate.

The purpose of the Onsite Program is to provide the homeowner with a sewage treatment and dispersal system that is as trouble free as possible and that protects residents from health problems relating to exposure to surfacing sewage and prevent contamination of the water supply.  The first step to approval for an onsite wastewater treatment system is the site evaluation.

How do I get started?

The development of an onsite system begins by studying the soil on the lot to be developed. Test holes should be dug in the area where the drainfield is planned and/or the area with the best soil. Test pits need to be at least 4 feet wide, 4 feet long and up to 5 feet deep.  You may stop digging the pit when you encounter a limiting layer, like hardpan or bedrock. The pit also needs to be sloped at one end to allow inspectors to get in and out of the pit.  The test holes need to be large enough for the inspector (sanitarian) to be able to get into the holes to examine the soil texture and the soil horizons. (Percolation tests are no longer performed.)

Once the test holes are ready you can apply for a site evaluation. Submit all of the following to the Community Development Department:

  • plot plans of your lot showing the lot dimensions and test hole locations,
  • the site evaluation application and
  • the appropriate fee

The sanitarian will examine the test holes and other variables and provide a report on, if the site is approved, the required drainfield type and dimensions along with a designated installation area.  If the site is denied, the report will provide the reasons for the denial.

The size of the drainfield is based on the soil texture and depth. Deschutes County typically has sandy loam soil. This soil type allows oxygen to enter the drainfield and also allows water to drain slowly enough to provide adequate treatment of the sewage before it enters groundwater. Soils with more clay require larger drainfields. Soils with a lot of gravel or sand drain so rapidly that special distribution techniques are required, like sand filters and pressure distribution drainfields.