Chapter 5 Watershed Basin

In Chapters 3 and 4, we described Olympia’s wastewater system in terms of system components: gravity sewers, lift stations, STEP systems, grinder pump systems and onsite sewage systems. In this chapter, we begin looking at the system from a watershed perspective. By doing so, we can begin relating wastewater management to overall water resource management issues.

Surface and ground waters in the City’s Sewer Service Area drain naturally in different directions to various water bodies. Each of these water bodies has water quality characteristics and management needs that can be influenced by wastewater decisions. In many cases, these characteristics and needs relate directly to the wastewater challenges discussed in Chapter 8.

While the watershed basins are regional in nature and typically extend beyond the Olympia Sewer Service Area, this analysis is limited to the portion of the basin that is within the City’s sewer service area. Each watershed basin is briefly described in terms of receiving waters, existing infrastructure, projected development, wastewater flow, number of STEP and onsite sewage systems, water quality issues and specific challenges. Additionally, the maps in Appendix M show the location of gravity sewers, lift stations and force mains, STEP lines and tanks, and onsite sewage systems (OSS) within each basin.

The watershed basins are delineated in Figure 5.1. Each watershed basin contains a unique mix of wastewater infrastructure that interacts with the basin’s receiving waters. Table 5.1 summarizes the basin’s wastewater characteristics.

Table 5.1

Wastewater Characteristics of each Watershed Basin









Existing OSS within 300’ of Sewer








Total existing OSS








STEP Systems








Grinder Pumps








Lift Stations








Single-family Residential








Multi-family Residential








Commercial Customers








Total Sewer Customers








% Basin Undeveloped1








1Total undeveloped area of basin, not including parks and other public places, as a percentage of total basin area.

View Figure 5.1 Olympia’s Watershed Basins

5.1 Central Watershed Basin

The Central basin (Figure 5.2) encompasses the older developed areas of Olympia. It is dominated by the central business district; the Ellis, Mission and Indian Creek watersheds on the east and south side; and that part of the near west side of the City and its Urban Growth Area (UGA) that drains to Capitol Lake or Budd Inlet. Population and business density in the basin is high.

The precipitation, surface water and ground water within the Central basin discharge to Budd Inlet. Contaminants from many sources, potentially including wastewater, affect the Inlet’s water quality. Water quality concerns from a wastewater perspective include bacteria, nutrients, various contaminants of emerging concern, and potential reductions in dissolved oxygen. Most the City’s water quality monitoring data focus on Budd Inlet and its tributary waters. Budd Inlet is the focus of extensive technical analysis and regulation.

Much of this basin is already developed (86%) with future development largely limited to redevelopment and small new developments. Wastewater flows are not expected to increase appreciably. The anticipated increase in peak wastewater flows is only one percent through 2025. Nearly all of this projected increase is expected to come from conversion of onsite sewage systems (OSS) to gravity sewers, infill residential and commercial development/redevelopment. The existing wastewater collection system in the Central basin typically has adequate capacity and facilitates the connection of new development to public sewer.

View Figure 5.2 Central Watershed Basin

The main challenge with wastewater collection system in the Central basin is its age. Many pipes are well over 50 years in age. With age, the pipes become susceptible to structural deterioration, collapse, and increase infiltration and exfiltration. Infiltration occurs when groundwater enters the wastewater pipe through cracks. Similarly, wastewater can leave the pipe and enter soils and groundwater (exfiltration). Operation and maintenance needs in the basin are typically greater and more intricate than other basins.

The Central basin also contains the City’s highest percentage of combined wastewater/stormwater pipes. The combined system collects stormwater from streets and buildings and routes it to LOTT’s Budd Inlet treatment facility through wastewater pipes. The wastewater flow model developed in 2007 estimated that peak flows associated with storm events in the Central basin are 23 times higher than base flows. These high flows reflect the concentration of stormwater inflow through the combined sewer/stormwater pipes in the downtown core. They can tax the capacity of otherwise adequately sized wastewater pipes.

Key lift stations, including the large Water Street station, are essential to the operation of the Central basin wastewater system. Over 40% of the 33 lift stations in the City’s wastewater collection system are located in the Central basin. The service areas of the neighborhood lift stations (LS) are delineated in Figure 5.2.

Given its evolution over time, the Central basin’s sewer system is complex and sometimes challenging to analyze. The management of these pipe systems focuses on refining our understanding the system and maintaining its integrity as it ages. Pipe maintenance and upgrades in this basin are costly.

The basin has only 68 STEP systems, but high numbers of OSS. Like the sewer collection system pipes, many of the 1,197 OSS in the basin have reached or exceed their expected operational life. Approximately 44 percent of the OSS are within 300 feet of a public sewer system and could conceivably connect. However, pockets of OSS located more than several hundred feet from the sewer will remain financially challenged to connect.

The complex, aging public infrastructure combined with large numbers of OSS create the potential for water quality impacts to Budd Inlet. State efforts through Clean Water Act water quality studies are underway to improve water quality in Budd Inlet. From a water quality perspective, the Central basin is carefully managed to prevent spills, correct unintentional cross connections with stormwater systems, and ensure the structural integrity of the pipes.

5.2 Kettles Watershed Basin

The Kettles basin (Figure 5.3) is located southeast of the Central basin. It includes some of the initial residential neighborhoods that were developed as Olympia spread to the southeast in the 1950s-1970s. Development pressure remains strong in this basin. New development in the basin will include subdivisions, infill, redevelopment, and some light commercial.

Surface water in the Kettles basin drains to Ward and Hewitt Lakes and a number of other nearby kettles or depressions left by remnant ice from the retreating glaciers. These lakes and kettles infiltrate surface waters to groundwater. Some soils in this basin infiltrate well. Unidentified wastewater cross connections into stormwater infrastructure, leaking wastewater pipes and OSS can result in adverse impacts to groundwater quality in this basin.

The public sewer system in the Kettle basin is relatively contemporary, but is comprised of a fragmented mix of gravity pipes, lift stations, STEPs, grinder pumps, and OSS. The basin’s inconsistent topography resulted in this mix of wastewater technologies. The basin has a relatively large number of STEP systems (605) and OSS (833) for its total area. It is dominated by single family development.

The recent Yelm Highway road improvement project included extensive upgrades to City utilities. Wastewater pipes, pumps, and odor control facilities were incorporated into the road work. These wastewater improvements provide the basis for continued expansion of the public sewer systems in this basin as well as the adjacent Southeast basin.

View Figure 5.3 Kettles Watershed Basin

Ongoing new development in the basin prompts the need for carefully managed sewer extensions that facilitate the new development as well as existing developments. In concert with the Southeast basin, wastewater management in this basin requires understanding and coordination of pipe systems and networks.

5.3 Northeast Watershed Basin

The Northeast watershed basin (Figure 5.4) can be challenging from both wastewater and water resource management perspectives. Both topographical and development patterns make public sewer systems difficult to link into a regional system. Areas of relatively low development density and pockets of OSS hamper the orderly expansion of the sewer system.

The Northeast basin is within the Henderson Inlet Watershed Protection Area, a water quality and shellfish harvesting priority. The basin drains in a northerly direction to Woodard Creek and subsequently to Woodard Bay in Henderson Inlet. Historical bacterial contamination in Henderson Inlet has declined in recent years with the shellfish beds once again productive and commercially viable. Management of public and private wastewater systems is a key aspect of maintaining the Inlet’s shellfish industry.

Topographically, the basin slopes in various directions, depending on location relative to the nearest surface water. There are several lakes and ponds. Streams generally flow south to north. The main sewer lines in the basin flow north to south, along East Bay Drive, Lilly Road and Sleater Kinney Road. Secondary sewer pipes and lift stations collect and transport wastewater into these main lines. Flows subsequently travel west in the LOTT systems. Wastewater pipes in the basin range from older to contemporary systems. This basin includes many smaller basins that will likely require some method of pumped sewer service to connect to the existing collection system.

This basin is projected to experience a variable rate of development over the next 25 years, generally increasing from west to east. Development becomes increasingly residential, and less dense, in the northern portions of the basin. Light commercial development is scattered throughout, though there are concentrations in the Lilly Road and Sleater Kinney Road areas. The Lilly Road area also includes the Providence/St. Peters Hospital and Group Health Medical Center, along with ancillary medical practices in the immediate vicinity.

View Figure 5.4 Northeast Watershed Basin

The Northeast basin has 358 STEP systems and 759 onsite sewage systems. Many of the OSS are located in the Henderson Inlet Watershed Protection Area. In general, the OSS are located more than 300 feet from public sewer. Neighborhood lift stations are identified on Figure 5.4.

As development continues, the Northeast basin may struggle to extend public sewer systems. However, successful water resource management will focus on connection of new development to public sewer as well the conversion of OSS.

5.4 Southeast Watershed Basin

Like the Kettles basin, the Southeast basin will support appreciable development activity in Olympia over the next 20 years (Figure 5.5). Peak flows in this basin are expected to increase by 85 percent over that period. Planning for these flows is important to the orderly operation of the Utility.

View Figure 5.5 Southeast Watershed Basin

This basin, characterized by its flat topography, has been the focus of considerable STEP system development since the mid-1990s. Additionally, many of the older residences in this basin are served by OSS. There are 705 STEP systems and 1,010 OSS in the basin. OSS are typically distant from the gravity flow portion of the wastewater collection system.

Stormwater and surface water in the Southeast basin discharge to the Deschutes River and ultimately Budd Inlet. Water bodies include portions of Chambers Lake and Chamber Creek, which discharges into the Deschutes River. The river is a major contributor of flows and potentially contaminants to Budd Inlet. The basin’s topography requires several lift stations in order to serve the entire area with gravity sewers. Ongoing new development in the basin prompts the need for carefully managed sewer extensions coordinated with the Kettles basin. The LOTT Clean Water Alliance is planning to build a satellite treatment plant in southern Lacey off College Street (Chambers Prairie). In order to maximize flow diversion from the Budd Inlet Treatment Plant, the proposed satellite plant could draw from southern Lacey and the Southeast basin. Coordination between Olympia, Lacey and LOTT is critical as development continues in this basin.

5.5 Southwest Watershed Basin

The Southwest basin (Figure 5.6) includes older neighborhoods of West Olympia and most of the Westside commercial district. With both redevelopment and new development forecast for this basin, sewer flows in the basin will increase.

Surface water flows in the Southwest basin discharge to the Black Lake Ditch, Percival Creek, Capital Lake, and finally Budd Inlet. The Percival Creek system is the City’s largest stream and the most viable for salmon life cycle needs. Bacteria levels in the stream are typically low, potentially reflecting extensive sewer system and low number of OSS (41) in the basin.

View Figure 5.6 Southwest Watershed Basin

Older sewer systems dominate the residential neighborhoods of West Olympia. Conversely, the commercial and multifamily areas are typically served by newer pipe systems. The suitable topography of the basin supports extensive use of gravity pipe systems. The public sewer system in the basin is generally able to accommodate growth.

Future wastewater management will focus on maintaining the older residential wastewater collection infrastructure and ensuring the orderly extension of new sewer facilities.

5.6 Northwest Watershed Basin

The Northwest basin (Figure 5.7) has received considerable residential development in the past several decades. It is characterized by new development activity along Cooper Point Road and Mud Bay Road.

The surface and groundwater flows from the basin discharge to Green Cove Creek and other tributaries to Eld Inlet. The relatively high water quality of Eld Inlet warrants continued protection as urban scale development extends to the west of Olympia. In order to help protect its aquatic resources, the City has enacted special zoning and development requirements for the Green Cove basin.

View Figure 5.7 Northwest Watershed Basin

Sewer expansion in this area will be driven by development. Existing sewers in this basin feed into the LOTT-owned Grass Lake and Percival Creek interceptors, which flow to LOTT’s Capitol Lake Pump Station. Peak flow in this basin is expected to increase 59 percent by 2025. Development is likely to be dominated by residential subdivisions.

The basin includes 116 STEP systems, clustered along 11th Avenue NW, and 305 OSS, mainly located in the area surrounding Lake Louise and to the north along Cooper Point Road. Key challenges for this basin focus on providing sewer extensions to the low-lying areas.

Careful planning and implementation of sewer extensions is necessary for preserving the health of this basin.