Environmental Health
Septic Systems
  Frequently Asked Questions  
  General Questions  l  Maintenance Questions  l  Drainfield Questions  

General Questions

How long do septic systems last?
Septic systems are designed to provide long-term, effective treatment of household waste when operated and maintained properly. However, most systems that fail prematurely are due to improper maintenance. Less serious problems are usually with plumbing (such as pipe blockages from tree roots growing into the pipe). Sometimes, the septic tank, although durable, can deteriorate or have other structural problems. The most serious problems are the result of a clogged drainfield. Unfortunately, this is the most expensive to repair. Once the absorption field is clogged, it must be replaced and can cost thousands of dollars. For more information, see Septic System Operation and Maintenance.

How can I learn more about my septic system?
We have many publications and brochures to download or can be mailed to you (Thurston County residents only, please). We also offers free septic workshops during the spring and fall.

Can my septic system contaminate my well? Or nearby streams and water bodies?
Yes, particularly if the effluent is not adequately treated, as in a failing system. Untreated effluent is a health hazard and can cause many human diseases. Once this untreated effluent enters the groundwater, you and your neighbor's wells can be contaminated. Also, if this sewage reaches nearby streams or water bodies, shellfish beds and recreational swimming areas may also be jeopardized.

Is there financial help available for failing systems or repairs?
Thurston County Environmental Health administers several low-interest loan and grant programs to help residents who live in shellfish protection areas or need financial assistance to maintain existing systems and repair failing septics. For a summary of the available programs, see Septic System Financial Assistance.

I'm buying a home, how do I know the septic system is in good working order?

Before a property with an on-site sewage system (septic system) is sold or transferred, Thurston County regulations require a Time of Transfer Evaluation Report issued by the Thurston County Health Department. The goal is to protect public health, to evaluate all septic systems, to identify and repair failing systems, and make records available to the public. This requirement went into effect September 1, 2010.

A septic system evaluation can be performed by a septic system professional. Thurston County also performs this service for a fee. The evaluation may contain a system record drawing, size and age of the system, maintenance history (if available), and condition of the system components, including the tank and drainfield.

Before I buy a building lot for a new home, what should I know?
Check with Thurston County Building Development Center, 360-786-5490, for setbacks and other requirements for septic tanks and drainfields. Determine if there is enough room for the septic system, replacement area, and drinking water well, if needed. Determine the soil characteristics, as the amount of sand, silt, and clay in the soil has an impact on the drainfield. Because there are many other considerations, including slope of the land, potential for flooding, in a wetlands area, nearness to water bodies, etc., seeking the help of a professional may be necessary.

I have a rental home, how do I educate my renters?
Water conservation and proper use of the septic system needs to be emphasized, particularly in homes near streams and water bodies. Thurston County has special posters and handouts for renters which you can request by calling 360-867-2626. You can also provide one of our brochures to your renters (see Publications and Brochures) or encourage them to attend a free workshop (offered during the spring and fall, see Septic System Workshops). Also, vacation rentals can pose special problems for septic systems, for more information, see Special Conditions.

Maintenance Questions

How often should I pump my septic tank?
How often you need to pump depends on the size of the tank, the number of people in the household, and the amount and type of solids. Generally, a septic tank should be pumped every 3 to 5 years, however, some alternative systems that are more complex may need pumping more frequently. If you are unsure if your tank needs pumping, have it inspected and get a recommendation for how many years you can go between pumping. Write this schedule down on a maintenance chart or where you keep your maintenance records and stick to it! For additional information, see Septic System Basics Maintenance Tips.

Can you tell if the tank needs pumping without digging it up?
Unless you have risers installed, you will need to dig up the ground above the septic tank to inspect it. Risers give easy access to the septic system without disturbing the soil above the tank. Also, by keeping maintenance records, you can have it pumped on a regular schedule, based on the previous years' rate of solids accumulation. Generally, a 1150-gallon septic tank used by a family of four with minimum garbage disposal use will need pumping every 3 years.

Who do I call to inspect or pump my septic tank?
You can find a list of county certified pumpers[PDF] and monitoring specialists [PDF] at our website or in your phone book, under "Septic Tanks & Systems - Cleaning & Service."

How much will it cost to pump my tank?
Pumping can cost from $250 and up. Because prices vary, get estimates from several pumpers and be sure to ask the number of gallons the estimate covers. NOTE: An average tank contains 1150 gallons.

How do I know where to find my septic tank?
Get a copy of your system's record drawing (previously referred to as "as-built"), a diagram showing where your system components are located and on file with the Thurston County Building Development Center (BDC). You can download the Request for Record Drawing/Permit Information [PDF] or contact the BDC at 360-786-5490. The BDC is located in Olympia at 2000 Lakeridge Dr. SW on the second floor of Building 1. When you call or visit, please have your eleven-digit tax parcel number ready. This is the number that appears on your county tax statements. (If you do not have your tax parcel number, contact the County Assessor's office.)

Another option to finding the septic tank is probing with a metal rod, following the pipeline from the house, until you find the tank.

How can I prevent a septic failure?
Maintenance, maintenance, maintenance! If your system has been properly designed, sited, and installed, the rest is up to you. Pump regularly, avoid excess water use, watch what you put down the drain and flush down the toilet. For additional information, see Septic System Operation and Maintenance.

I've been in my house for 18 years and never pumped my septic, why should I be concerned?
Over time sludge and scum build up in the septic tank and unless it is removed it will flow into the drainfield, clogging the soil pipes. Once a drainfield is clogged, it must be replaced, which is an expensive repair, costing anywhere from $2,000 to $10,000 or more. It is also quite possible that you could have a leak in the tank.

In either case, you risk contaminating ground and surface water resources, which could affect you or your neighbor's wells or nearby streams and other water bodies. And finally, you may eventually have a plumbing backup in your home. See Septic System Operation and Maintenance.

Effluent surfacing above drainfieldWhat happens when a septic system fails? How can I tell if my system is not functioning properly?
Usually when a septic system fails, the drainfield is not functioning properly. When a septic tank overflows, the effluent can pass to the drainfield, clogging up the pipes. This causes sinks and toilets to back up in the house. Other signs include: slow draining toilets and drains, an odor of sewage, wet area on or near the drainfield, or contaminated well water.

If your septic system should fail, contact Thurston County Environmental Health at 360-786-5490.

Why is water conservation important?
Septic tanks are mainly settling chambers. They allow time for solids and scum to separate out from wastewater, so clear liquid can safely go to the drainfield. Over time, the scum and sludge layers get thicker, leaving less space and time for the waste-water to settle before passing to the drainfield.

There are limits to the amount of water septic systems can treat. For every gallon entering the tank, one gallon is pushed out into the instances, too much water may back up into your house or overload the drainfield and surface in the yard. The problem is large volumes of water may not allow solids enough time to settle and may be carried out to the drainfield, ultimately clogging the pipes. For water saving tips, see Septic System Basics, Use Water Efficiently.

I've heard I shouldn't use a garbage disposal. Why is that?
Garbage disposals have a dramatic impact on pumping frequency. Food particles usually are not digested by the bacteria and accumulate as scum. If a large amount of water enters the tank, it can then push the food particles into the drainfield, causing clogging. If you must use a garbage disposal, you should get your tank pumped more frequently.

Should I be careful of what I pour down the drain?
Yes, many materials that are poured down the drain do not decompose and remain in the tank. In addition to minimal use of a garbage disposal (see question above), don't pour grease, fats, and oils down the drain or place coffee grounds and egg shells in the disposal or down the drain. Keep chemicals out of your system. See Septic System Basics, Minimize Solid Waste Disposal.

What should not be flushed down the toilet?
Flush only human waste and toilet paper down the toilet avoid flushing dental floss, cat litter (including "flushable" varieties), hair, Kleenex, cigarette butts, cotton swabs, feminine hygiene products, condoms, paper towels, static cling sheets, diapers, and disposable wipes.

Will additives help my system?
Adding a stimulator or an enhancer to a septic tank to help it function or "to restore bacterial balance" is not necessary. The naturally occurring bacteria needed for the septic system to work are already present in human feces. According to the U.S. Department of Health, none of these products eliminate the need for routine maintenance and pumping. For additional information, see Septic System Additives.

Do I need to get a permit to make repairs to my septic system?
Yes. You can download the permit application from Development Services (both a Master Application and Supplemental Application are needed) or contact the Thurston County Building Development Center (BDC), 360-786-5490. The BDC is located at 2000 Lakeridge Dr. SW, Olympia, on the second floor of Building 1.

Drainfield Questions

What is the replacement or reserve area?
This is an area that may be used for replacing or expanding the drainfield. It must meet the same criteria, such as acceptable soils, setbacks, etc., as a regular drainfield and should be protected in the same way. For additional information, see The Drainfield.

What can I plant over my drainfield and septic tank?
Grass is the ideal cover for drainfields. Grasses can be ornamental, mowed in a traditional lawn, or in an unmowed meadow. Or, you can try groundcovers and ferns (for specific plants, see Landscaping Your Drainfield). Select shallow rooted plants that require low-maintenance and low-water use.

For plantings over septic tanks, keep in mind, if you don't have risers installed, you will need to dig up the ground to access the tanks for inspection and pumping generally every 3 to 5 years.

How close can trees and shrubs be to the drainfield?
Trees and large shrubs should be kept at least 30 feet away from your drainfield. Be aware, though, that certain soils may require plantings be a greater distance away. If you wish to plant trees near a drainfield, consult an expert who can determine types of plants and distances, based on your soil. Trees and shrubs generally have extensive root systems that seek out and grow into wet areas, such as drainfields.

Can I plant a vegetable garden over my drainfield?
No. Growing vegetables over a drainfield is not recommended. Vegetables need watering, and excess water in the soil reduces its ability to treat wastewater. The deep roots of some vegetables may damage drainfield pipes. Bed preparation, such as rototilling or deep digging, can also damage pipes. Plus, there is the risk of contaminating food crops with sewage.

What about landscape plastic or fabric under mulch, can that be placed over the drainfield?
No. Plastic reduces the necessary air exchange in the drainfield soil. Even mulch or bark over the drainfield is not recommended, because it reduces air exchange and retains water.

Can I build a carport or camper pad over the drainfield? How about a tennis court or hot tub?
No, for two reasons. First, you should avoid driving over the drainfield; the pressure of vehicles and heavy equipment compact the soil and can damage pipes. Second, impermeable materials such as concrete and asphalt reduce evaporation and the supply of oxygen to the soil. Oxygen is critical to the proper breakdown of sewage by soil microorganisms.

How about putting my carport over the replacement area?
No. The designated drainfield replacement area (reserve area) should be left undeveloped and protected from compaction in case you must repair or replace your drainfield in the future.

Can cattle graze over the drainfield? How about just one horse?
Livestock should be kept off of drainfields. In the winter, livestock trample and muddy the soil; in the summer they compact it. Again, this is not good for the soil's ability to exchange oxygen. So, sorry, even one horse is not recommended.

Rainwater is directed onto my drainfield, is this a problem?
Yes. Downspouts and stormwater from surfaces such as driveways and patios should be diverted off the septic tank and drainfield. A small trench uphill from a drainfield can help direct water away.

How close to the drainfield can I install a sprinkler system?
Water lines should be at least 10 feet from all components of the septic system. Be sure all sprinkler lines are fitted with approved backflow prevention devices.

...and can I put a retaining wall and drains back there?
If planning to put drains (interceptor, French, curtain) or retaining walls within 30 feet of ANY PART of the septic system, check with the Thurston County Environmental Health Department (360-867-2673). Never cut through drainfields for drains, walls, or irrigation lines. French drains are notorious for carrying pollution from septic systems into water bodies or streets.

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Did you know...

10-20% of septic systems fail every year

Thurston County has about 45,000 septic systems

What you flush or pour down your drain can end up in local rivers, lakes and Puget Sound

Maintenance and monitoring is the key to a healthier, longer lasting septic system

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This page last updated: 08/28/20