Chapter 4 Onsite Sewage Systems

Households and businesses that are not connected to the City’s wastewater system must treat and dispose of their wastewater on site. There are approximately 4,145 onsite sewage systems (OSS), also called septic systems, in Olympia and its UGA - about 1,900 in the City and 2,245 in the UGA. Figure 4.1 shows an example map of the distribution of OSS in a select area of the City’s sewer service area. Complete mapping of parcels served by OSS within each watershed basin (see Chapter 5) can be found in Appendix M.

Onsite sewage systems have historically been the most common method of sewage treatment in Thurston County. Many parcels served by OSS were not connected to public sewers after the area was annexed, even though sewer pipes were laid in the general vicinity.

In the 1950s, reports of failing OSS and pollution of Capitol Lake and Budd Inlet made it clear that significant sewer infrastructure improvements were needed in the Olympia area. In the 1970s, concerns about public health risks associated with OSS led the Thurston County Board of Health to require inspection and certification of OSS.

Currently, both Thurston County and the City of Olympia regulate the permitting and use of onsite systems within Olympia’s Sewer Service Area (see Section 4.5 below for more details). Property owners are responsible for maintaining individual OSS, and the City operates the only community onsite sewage system (COSS) within its Sewer Service Area.

This chapter reviews the types and functioning of onsite systems, the potential public health risks associated with the systems, proximity of OSS to Olympia’s sewer system, potential costs of conversion to public sewer and the current regulatory framework.

Challenges associated with OSS in the City and UGA are introduced and discussed in this chapter, and summarized in Chapter 8. Note that this approach is different from the discussion of other challenges facing the Wastewater Utility, where the challenges are introduced in an earlier chapter but discussed in detail in Chapter 8. Goals and Strategies related to OSS are presented in Chapter 9.

View Figure 4.1 Parcels served by individual Onsite Sewage Systems1

1Example map; see Appendix M for basin-specific maps of parcels served by OSS.

4.1 Types of Onsite Systems

There are two main types of onsite sewage systems, individual (OSS) and community (COSS). Normally, OSS only serve one dwelling, one duplex or one business. COSS treat wastewater flows greater than 600 gallons per day (gpd) or flows discharged from three or more dwellings. Under state law, a public jurisdiction must own and operate COSS. In cities and their UGAs, COSS are considered an interim form of wastewater service, to be used only until public sewers become available. As part of the 1992 intergovernmental agreement with Thurston County (see Chapter 2), the City owns and maintains all COSS within the City’s Sewer Service Area. Currently, there is only one COSS in Olympia’s sewer service area, built in 1994 and serving three single family residences.

Larger onsite sewage systems, or LOSS, a type of COSS treating flows greater than 3,500 gpd, were regulated by Thurston County until 2011, and are now regulated by the Washington State Department of Health. There are no LOSS in the City or its UGA.

An OSS typically consists of a buried 500-1500 gallon, two-compartment "septic" tank and a drainfield. The tank collects sewage (wastewater) from the residential structure(s), which is then separated into (1) solids that settle and are broken down biologically by naturally occurring bacteria, (2) liquid that flows out of the tank and into the drainfield, and (3) fats, oils and grease (FOG) that float on top of the liquid in the tank and get partially broken down. In a properly functioning OSS, the liquid wastewater either flows out of the tank by gravity, or is pumped to the drainfield, where it is evenly distributed in the drainfield.

As the wastewater percolates through the drainfield and underlying soil, further filtration of the wastewater occurs, as well as additional biological treatment before it reaches groundwater. The solids and FOG need to be pumped out of the tank on a regular basis, typically once every three to ten years based on use.

Figure 4.2 is a conceptual diagram of an individual onsite sewage system (OSS), and Figure 4.3 shows a community onsite system (COSS).

Proper functioning of onsite sewage systems depends on the soil’s ability to process and filter the effluent. With the large silt fraction of soils in the South Puget Sound region, less than one percent of Thurston County soils are ideal for onsite sewage treatment, and 87% of the land by area is inappropriate for OSS (LOTT, 1998). See the Geology and Soils section in Chapter 2.

View Figure 4.2 Individual Onsite Sewage System

View Figure 4.3 Community Onsite Sewage System

4.2 Public Health Risks of OSS in Urban Areas

Onsite sewage systems can be an effective and safe method of treating and disposing of treated wastewater when properly designed and installed, maintained regularly, and kept at moderate to low site densities. They are appropriate in rural areas, but were not intended for use in increasingly dense developed cities. They require a treatment and disposal area large enough to adequately break down and dilute effluent-borne contaminants.

The presence of over 4,000 OSS in the urbanizing area of Olympia and its UGA creates the potential risk to environmental and public health from groundwater, surface water and soil contamination. Figure 4.4 illustrates these risks.

View Figure 4.4 Potential Risk to Groundwater and Surface Water from Onsite Sewage Systems

Risk of OSS Failure

Industry research has indicated that the design life of onsite sewage systems is generally 25 years, meaning the potential for failure increases with time, even if the system is properly sited and consistently maintained. However, records show some systems last much longer.

Onsite systems that are not properly sited and maintained may threaten water quality and public health by releasing bacteria, viruses, nitrogen, phosphorous, heavy metals and chemicals from household products into the environment. "Failure" means the system threatens public health because it is not adequately treating sewage or is creating a potential for people to come in contact with sewage. Examples of failure include:

• Sewage on the surface of the ground.

• Sewage discharged directly to surface water or onto the ground.

• Sewage backing up into a structure because of slow absorption of effluent by the soil.

• Sewage leaking from a tank, pump chamber, holding tank or collection system.

• Inadequately treated effluent contaminating ground water or surface water (determined by dye tracing and/or fecal coliform count).

• Surface or ground water intrusion into a tank, pump chamber, holding tank, or collection system.

• Cesspools.

• Seepage pits where there is evidence of ground or surface water quality degradation.

Evidence of Contamination from OSS

Onsite systems, especially when used at urban densities, create threats to both groundwater and surface water. Nitrates are a common groundwater contaminant associated with OSS, while bacteria linked to OSS are often found in surface water.

Nitrate is increasingly observed in groundwater, including the City’s drinking water supply wells in Southeast Olympia. In some cases, the concentration of nitrate threatens the viability of both private and public drinking water supplies. Onsite systems have been identified as a significant contributor to the problem through detailed studies conducted in the 1990s and 2000s.

In addition, bacterial contamination from failing onsite systems is one of the principal causes of shellfish restrictions imposed on Puget Sound since 1980 (Grover 1996). Ongoing water quality monitoring confirms that streams and marine waters within Olympia have elevated levels of bacterial contamination.

Guidance on Siting of OSS

Research demonstrates that properly functioning onsite sewage systems can pollute ground and surface water if they are concentrated in too small a land area (DeFeo, 1991; Yates, 1985). In Olympia and its UGA, an estimated 41 percent of onsite systems are sited on lots less than the minimum recommended lot size of 12,500 square feet (WAC 246-272-20501; Article IV, Section 21). Similarly, ground and surface water quality impacts have been observed where the average density of OSS is more than four systems per acre, even in well-drained soils (Brown and Bicki 1987, 1991). The maximum density of OSS in Olympia’s sewer service area is approximately 4 systems per acre , in areas of the southeast UGA. More typical densities in areas with OSS are less than 2 systems per acre. As a comparison, all of Olympia and its UGA is zoned or planned for densities with residential lot sizes of approximately 5,000 square feet or about 8.7 lots per acre.

Additional guidance recommends that OSS should be adequately separated from drinking water wells. Analysis on virus mortality and migration suggests that OSS should be at least 400 feet apart to reduce virus concentrations below safe drinking water standards in the groundwater (Brown & Bicki 1997, 1991; LOTT 1998). Under current County regulations if a lot is served by a private well, the minimum lot size for an onsite sewage system is one acre (Article IV, Section 21). In addition, new onsite systems must be located at least 100 feet from a water supply source or other surface water and 200 feet from a public drinking water supply (WAC 246-272-09501; Article IV, Section 10).

Under State regulations, onsite systems cannot be installed within 100 feet of fresh or marine surface water (WAC 246-272A-0210). With waivers, Thurston County maintains authority to reduce the buffer distance to 50 feet. Under Olympia’s Critical Areas Ordinance, onsite sewage systems are not allowed in designated critical areas (e.g., wetlands and floodplains).

Table 4.1 summarizes the siting and characteristics of OSS in Olympia.

Table 4.1

Onsite Sewage System Characteristics (Olympia and its UGA)

Onsite Sewage System Characteristic

Approximate Total

% of Total1


Lots less than 12,500 sq. ft.




Lots with drinking water wells (100 ft. from well required by WAC, 400 ft. between OSS recommended)




Lots within 100 ft. of surface water




Lots within Olympia drinking water protection areas




Lots within Olympia portion of Henderson Inlet Watershed Protection Area



Assessment of Current Risks in Olympia

In response to increasing concern over the prevalence of OSS in the Lacey-Olympia-Tumwater area, Thurston County Environmental Health Program recently completed a planning-level analysis of existing OSS use and their environmental risks. The analysis used GIS technology to link the various densities of OSS in neighborhoods to screening criteria defining potential risks to both surface and ground water. This information provides a productive planning-level tool for considering jurisdictional needs for OSS policies and regulations, and the potential need to convert systems to the municipal sewer system.

The analysis documented the occurrence of individual onsite systems in the north Thurston County area. Areas with OSS were subsequently grouped into neighborhoods based on subdivision plats or lots that share similar characteristics. Commercial and multifamily OSS were converted to a single family residential equivalency unit. Onsite septic densities in the neighborhoods were calculated and grouped as follows: ? 1 OSS unit/neighborhood acre, 1-2 units/acre, 2 to 4 units/acre, and ? 4 units/acre. With this analysis the density of onsite systems is a key risk factor.

Given onsite densities, several natural resource parameters were used to refine the potential threat to both surface water and groundwater. The risk of surface water contamination from onsite systems increases with neighborhoods that are close to water bodies and that have soils that generate runoff rather than infiltrate. When combined with neighborhoods with relatively high densities of OSS, these geographic traits create a higher potential of contamination. Similarly, neighborhoods located within drinking water protection areas and with soils that readily infiltrate precipitation to groundwater generate relatively high risks to groundwater. Additional parameters and site specific information can also be used to supplement the basic evaluation.

The outcomes of the analysis suggest the following concerns for Olympia:

• OSS densities are typically relatively low. Isolated pockets of densities ranging from 2-4 units/acre are located in the extreme NE corner of Olympia (Sleater-Kinney Road) and SE Olympia. Within the City’s UGA, areas along Yelm Highway also have higher densities.

• Much of Olympia is potentially susceptible to surface water contamination due to relatively impermeable soils and the proximity of water bodies. However, when combined with the typical low density of OSS within Olympia, these natural traits does not create many high risk areas. The Sleater-Kinney Road area north of Martin Way stands out as the only relatively high risk area.

• From a groundwater perspective, the impermeable soils in Olympia do not facilitate a lot of infiltration and subsequent contamination. Additionally, few drinking water protection areas are located in Olympia. Several potentially problematic areas are located in SE Olympia along Yelm Highway.

The analysis highlights few neighborhoods with both OSS densities and natural traits with the potential to create regionally-significant water quality problems. However, the analysis does not suggest that failing individual onsite systems are not a problem. We are aware of individual OSS generating both surface and groundwater contamination.

The analysis does indicate that from a regional planning perspective the implications of onsite systems in Olympia may be modest. Maps depicting the outcomes of the analysis are provided in Appendix M. Chapters 8 and 9 further address OSS challenges and recommendations as well.

4.3 Proximity of OSS to Olympia Sewer System

Onsite systems are distributed throughout Olympia and its UGA. As surrounding homes and neighborhoods developed on public sewer, isolated or small pockets of systems have remained. Other areas such as portions of Northeast and Southeast Olympia include entire subdivisions served by onsite systems. Additionally, many undeveloped infill parcels remain in Olympia. At some point, most of these isolated parcels will develop and need sewer service.

In general, current City policies require a developing parcel or a failing existing OSS to connect to the public system if located within 300 feet of the sewer pipe. Existing lots greater than 1 acre are exempted from the requirement. Of the 4,145 onsite systems in Olympia and its UGA, over 1,200 are within 300 feet of public sewer. As shown in Table 4.2, an estimated 1,000 systems in the City and 1,900 in the UGA are further than 300 feet from sewer and could be connected if sewers were extended. The table also shows the distribution of onsite systems in relationship to existing sewers.

Table 4.2

Proximity of Onsite Sewage Systems to Public Sewer


Adjacent to Sewer Main

Within 200 feet

Between 200 and 300 feet

Over 300 feet


Within City Limits


















Table 4.3

Characteristics of Undeveloped Parcels Related to Onsite Sewage System Permitting1


Within 300 feet

Over 300 feet and <1 acre

Over 300 feet and > 1 acre


Within City Limits















1Not all undeveloped parcels are developable.

Many undeveloped parcels are within a feasible connection distance to the public system. Table 4.3 shows characteristics of undeveloped lots in relationship to existing sewers and permitting.

4.4 Potential Costs of Converting OSS to Public Sewer

For owners of onsite systems, the cost of connecting to City sewer can be substantial. Table 4.4 summarizes the potential costs of conversion and highlights the high degree of variability of construction costs.

Table 4.4

Typical Costs for Converting an OSS to Public Sewer


Range of Costs*

Construction Costs


Public Sewer Infrastructure (if not existing)

$15,000 - $25,000+


Side Sewer Construction to House (high end is for grinder pump or STEP connection)

$3,000 - $10,000


Septic Tank Abandonment

$800 - $1,200

Construction Subtotal =

$4,000 - $36,000+

2013 Applicable Fees and Permits


LOTT CDC (Capacity Development Charge)



City Wastewater GFC (General Facility Charge)

0** or $3,199


Permits for Sewer Connection

$147 - $1,200


Septic Tank Abandonment Permit (Thurston Co.)


Connection Fees Subtotal =

$5,076 - $9,328

Range of Total Costs to Convert

$9,000 - $45,000+

*    In 2013 dollars; rounded figures.

**    The City Wastewater GFC is waived per OMC 13.08.205(c) for properties with an existing onsite sewage system that connects to the sewer within two years of notice of sewer availability.

Through its Septic to Sewer Program, the City assists homeowners on an OSS to convert to public sewer. The program includes the following components:

• Public education and outreach

• General Facility Charge Waivers

• Neighborhood Sewer Extension Program

• Other services identified in the Strategies section of Chapter 9

The number of OSS conversions to public sewer has increased in recent years from an average of 6 conversions per year between 1992 and 2008 to an average of 23 conversions per year between 2009 and 2012. The increased rate of conversion corresponds with implementation of the City’s Septic to Sewer conversion program. More information on this program’s services is available on the City webpage.

4.5 Current Regulations

Privately owned individual onsite sewage systems (OSS) and community onsite sewage systems (COSS) are regulated by the Thurston County Board of Health. The County Environmental Health Division is responsible for reviewing permit applications for new onsite sewage systems and repair or expansion of existing systems. Its staff maintains onsite system records, and oversees the inspection of onsite systems before property ownership is transferred.

This section summarizes the regulatory framework for individual and community onsite systems, special regulations for the Henderson Watershed Protection Area and pending regulations on underground greywater irrigation systems.

Individual Onsite Sewage Systems (OSS)

The City has no responsibility for owning, maintaining or managing private individual OSS. However, the City does have the authority within its Sewer Service Area to determine if a new onsite system or repairs to an existing onsite system is allowable, or whether the proposed or existing building(s) is required to connect to the City’s sewer system. Therefore, Thurston County forwards all OSS repair or new construction applications for sites located in the City or its UGA to the City for review and approval or rejection. See Appendix O for a flowchart that guides City and County staff in determining whether or not a proposed OSS can be permitted within the City or its UGA.

City regulations for permitting new OSS are more restrictive than State and County regulations. Under current State and County regulations (WAC 246-272A-C and Article IV of the Thurston County Sanitary Code, respectively), new OSS are allowed under certain conditions, most importantly when the following conditions can be met: it can function properly, it is located in suitable soils at a safe distance from a water well, and no public gravity line is accessible. Under State and County standards, OSS served by a public water system must be located on lots of at least 12,500 sq. ft. (with a density of 3.5 lots per acre or less); the County code allows OSS on smaller lots of record (i.e. lots created before 1995) if they meet other criteria (WAC 246-272A-0210 and WAC 246-272A-0320). City permitting regulations restrict new OSS to parcels that are more than 300 feet from a municipal sewer system and for a historical parcel greater than one acre in size. Replacing existing OSS 300 feet from municipal sewer also can be permitted.

New OSS owners in the UGA must sign an annexation agreement and all new OSS in the City and UGA must be designed as interim and agree to connect within one year of being notified to do so.

The County Health Code requires owners of larger or more complex systems to have them certified and inspected every one to three years. High-risk OSS located in the Henderson Watershed have more stringent requirements (see below). A City-County Resolution (Olympia Ordinance 5861) also encouraged owners of onsite sewage systems to register with the Thurston County Operational Certificate Program. Olympia Water Resources cooperates with Thurston County in periodic educational activities to encourage proper maintenance by onsite sewage system owners.

Community Onsite Sewage Systems (COSS)

Community onsite sewage systems (COSS) are considered by the Department of Ecology to be public sewerage treatment facilities, requiring the City to assume ownership and maintenance responsibility. Under an October 1992 intergovernmental agreement with Thurston County, the cities of Olympia, Lacey and Tumwater own and operate COSS within their UGAs. Public ownership is meant to encourage development within the UGA in the interim before public sewers are extended, and to ensure consistent Wastewater Utility services to all customers as mandated by the Growth Management Act.

Olympia policy allows approval of a COSS only if topography or other constraints preclude connection to the public sewer, and if the cost of extending the sewer exceeds COSS installation and lifecycle costs by 50 percent. Before the City takes over ownership and maintenance of a COSS, the developer must pay all up-front connection fees to the City sewer system, including the CDC and GFC. Customers connected to a COSS must agree to pay the regular monthly sewer utility rate, and connect to the City’s sewer system within one year after sewer becomes available, including paying any connection fees not previously paid to the City at the time of connection to the COSS.

COSS are considered interim systems and must be designed for efficient conversion to sanitary sewer. COSS permits in the UGA require that property owners sign an agreement to support an annexation petition, to take effect when the area becomes contiguous to the City.

Currently, Olympia maintains one COSS, located on Devoe Road in the UGA.

Henderson Watershed Protection Area

In May 2004, a Thurston County citizen advisory committee recommended a program to enforce onsite sewage system maintenance in the Henderson Inlet watershed (see Figure 4.4), where fecal coliform bacteria from human waste are contributing to the pollution in streams and marine waters (Thurston County, 2002). Woodland and Woodard Creeks, which capture runoff from northeast Olympia, Lacey and Thurston County, are on Washington State’s 303(d) list of water quality impaired water bodies, a list maintained as a requirement of the federal Clean Water Act. The Olympia portion of these basins includes parcels with 762 onsite sewage systems, 444 within the City limits and 318 in Olympia’s UGA.

Based on the committee’s recommendations, Thurston County approved its first mandatory onsite sewage system operation and maintenance program to help restore water quality. The program requires that all high-risk onsite systems within the existing shellfish district be inspected on a regular basis and that owners maintain a current County Operational Certificate. See the most recent version of Article IV of the Sanitary Code for Thurston County for more information regarding this program.

View Figure 4.5 Henderson Watershed Protection Area

Greywater Subsurface Irrigation Systems

The Washington State legislature recognizes the need to conserve ground and surface water supplies, reduce the cost of treating wastewater and use sustainable building practices to conserve potable water. The legislature determined that the Department of Health shall adopt rules for greywater reuse that do not compromise public health or cause unacceptable environmental impact.

In 2006, enacted legislation required the Washington State Department of Health to adopt rules for subsurface greywater irrigation by December 31, 2010. The rule, chapter 246-274 WAC, establishes requirements that provide building owners with simple, cost-effective options for reusing greywater for subsurface irrigation. The chapter is intended to encourage water conservation and to protect public health and water quality.

- Quoted from the Preface of the Washington State Department of Health’s draft guidance document titled "Tier Two and Three Greywater Subsurface Irrigation Systems" (May 2011).

Tier 1 greywater systems are the simplest with up to 60 gallons per day of gravity flow. Tier 2 systems distribute up to 3,500 gallons per day, and typically rely upon pressurized flow. Allowable greywater sources for both Tiers 1 and 2 systems are bathroom sinks, showers, bathtubs and clothes washing machines. Tier 3 systems are similar to Tier 2, but typically use greywater from sources such as non-laundry utility sinks, kitchen sinks and dishwasher water.

The most likely scenario for implementing greywater reuse for subsurface irrigation is for property owners already connected to City sewer to divert some of their greywater, on a seasonal (when it is not raining or freezing) and occasional basis for watering plants.

According to 246-274 WAC, Thurston County has three years from July 31, 2011 to either adopt the new WAC by reference, or write and adopt local codes to address greywater re-use, consistent with the WAC.

Until Thurston County adopts code language addressing this, greywater reuse for subsurface irrigation is not allowed. However, residents can get an onsite greywater sewage system approved under 246-272A WAC, for example if they have a composting toilet and still need to treat/dispose of the greywater. Under current City and Thurston County regulations, residents would only be able to do this in locations where it is acceptable to site onsite sewage systems. This regulatory approach provides system redundancy.