What Is A Perc Test? Understanding the Importance in Septic System Design

The old adage of a “perc test” was used to define if a particular piece of property could support a house with a septic system. Does the lot perc? Or how many bedrooms does this lot perc for? These were and still are questions people want answered prior to buying land where there is no public sewer. Today, an actual perc test may or may not be used to establish the suitability of a lot for a septic system.  

The Purpose Behind a Perc Test

A perc test, short for percolation test, is a method used to evaluate the infiltration rate of water into soil.  This test’s main objective is to determine the absorption rate that can be converted into a loading rate to size a drainfield. Basically, the faster the infiltration rate, the smaller the septic field is. 

 

Sizing of a conventional or alternative septic system relies heavily on the soil’s permeability. However, it isn’t the primary factor considered when determining if a lot is suitable for building with a septic system. Prior to evaluating the infiltration or “perc” rate, other factors must be considered. Most of these have evolved over the years based on septic system failures and contamination of drinking water wells, irrigation wells and streams.  

 

1. Protection of Groundwater: The primary purpose of a septic system is to treat the sewage from a dwelling or other occupied building to a level that significantly reduces the potential of contaminating groundwater and surface water. 

 

1. Soil Evaluation: The primary tool for determining if a lot “percs” is a detailed onsite soil evaluation. Soil maps are a general guide for septic suitability in broader areas, but a lot by lot evaluation is required to establish a suitable perc site. The depth, color and texture of the soils are the primary indicators of suitability. Soils that are shallower to restrictions like rock, hard pans or groundwater may mean an alternative system is required. Soils that are more red indicate water doesn’t stay in the soil long; where gray or a lighter color may indicate seasonal wetness or slower infiltration. Heavier textured (clayey) soils usually have slower permeability than lighter (sandy) soils. In general, if you have a large area of deep, well-drained soils, you can probably get a conventional drainfield. If any one of the other criteria above are limiting, you may need an alternative system

 

1. Setbacks: In addition to soil characteristics and perc rate, septic systems must meet setbacks to other features such as drinking wells, irrigation wells, streams, ponds, lakes, tidal areas, shellfish waters, property lines and other buildings. You may have a perfectly good site on the lot based on the soil evaluation and perc rate, but an existing well on a neighboring lot means your site can’t be permitted for a septic system. Variances to some of these setbacks can be sought, but encroachment on existing or already permitted wells (even if not drilled) are excluded from variances.  

Perc Rates Explained?

Septic systems in Virginia are designed based on the number of bedrooms. Regulations for a 3 bedroom house would be based on two full-time occupants per bedroom @ 75 gallons per person, per day or a total design flow of 450 gallons per day. 4 bedrooms would require 600 gallons per day and 2 bedrooms 300 gallons per day and so on. So, a drainfield area with clayey soils and a slower rate would be much larger than a drainfield area with sandier soils and a faster rate to serve the same 3 bedroom house. If the area of clayey soils is limited, then an alternative system may be needed as they usually require significantly less area than a conventional system.  

 

1. Estimated Rates: In most jurisdictions, estimated perc or infiltration rates are used to size a septic system. Evaluating the soil texture, structure and consistency combined with the licensed soil evaluators experience allow different soils to be classified as category I – IV. Each category has a range of perc rates assigned to it and as long as the final design loading rate corresponds to the soils classification, no actuarial perc test is required.  

 

1. Measured Rates: Measured perc or inflation rates may be run from time to time when a question arises or when the health department or soil evaluator wants a gut check on the estimated rate. New policies strongly recommend infiltration testing and measuring actual hydraulic conductivity when the soils on the site are heavy class III, class IV or when more than one limiting factors are present.  

 

1. Conducting the Perc Test: Most soil evaluators and health departments have moved away from the standard two-day perc test to a field measured hydraulic conductivity test.  These tests more accurately measure a soil’s permeability, require less water, and usually require less time. The devices used to conduct these tests are expensive to buy and must be maintained to ensure accurate results. There are very specific field test procedures that must be followed exactly and documented or the test results can be invalid.  

 

The outcome is usually expressed cm/day and can be converted into a Ksat – otherwise known as a saturated hydraulic conductivity measurement and a loading rate.  

What If the Test Fails?

Failure doesn’t necessarily mean an end to your construction plans. There are several options available:

 

1. Alternative Testing Sites: Especially for larger properties, it’s possible to test another site that might be more suitable.
2. Alternative Septic Systems: There’s an array of alternative septic systems designed for challenging sites. While these might be pricier and demand more maintenance, they can make it feasible to build on previously unapproved lots. Some also choose to use a smaller, more expensive alternative system even when a conventional site is available. Why would anyone do this you might ask.  Here are a few of the most common reasons. 
   1. Save money on clearing costs and save trees on wooded sites. 
   2. Allow for a more desirable house location
   3. Create additional space for a pool, detached garage or other structure
   4. Increase the bedroom capacity without increasing the size of the drainfield
   5. Discharge cleaner sewage into the drainfield

 

Wrapping It Up

 

A soil evaluation and perc test, while seemingly simple, is crucial in determining the feasibility and type of septic system you can install on a property. Always ensure you engage an experienced onsite soil evaluator licensed in Virginia. While most counties follow the state septic regulations, many counties in Northern Virginia have additional local codes that need to be considered and adhered to.   

 

Our family here at SES Mid Atlantic has years of expertise in conducting soil evaluations, infiltration testing and offering septic solutions tailored to our clients’ needs. Ensuring your land’s suitability for a septic system is our top priority, and we’re here to guide you through every step of the process.