PUBLIC HEALTH ASSESSMENT
SEATTLE MUNICIPAL LANDFILL/KENT
KENT, KING COUNTY, WASHINGTON
The Seattle Municipal Landfill, better known as the Kent Highlands Landfill, is located in the City of Kent, approximately 14 miles south of the City of Seattle, Washington, at 23076 Military Road South. The landfill occupies about 60 acres of a 90 acre site located immediately north of State Route 516, about 0.3 mile east of the Interstate 5 Midway interchange. The site is fenced on three sides and has a security gate at the main entrance.
The landfill was established in a former ravine adjacent to the Green River. The ravine sloped downward generally from west to east toward the river. Midway Creek flows through the eastern perimeter. Surface water settling ponds, a leachate collection system, and gas collection system have been constructed. The volume of waste in the landfill is estimated at 8 million cubic yards which was placed between 1968 and 1986. Landfill gas was discovered in 1984 and, later, there were citizen complaints about odors. Washington State Department of Ecology conducted a site assessment, discovered organic contaminants in leachate samples, and recommended further monitoring of leachate and ground water. The U.S. Environmental Protection Agency reviewed the site, and because of ground water and surface water considerations, added the site to the National Priorities List in 1990.
Early gas removal was through the use of passive "Tiki" torches. Usually about 10 per cent of those torches were unlit, according to inspection records from the Seattle-King County Health District. Leachate was found to be entering the north surface water collection pond and all of the water from that pond was then pumped to a sewage treatment system along with the leachate from the collection system. That some settlement of the landfill cover material has occurred is affirmed by the gas torches which lean toward the east, or downgradient.
Many people use the area near the eastern boundary for recreation. The Green River attracts fishermen for steelhead and salmon. Joggers, bicyclers, and picnickers use the parks and pathways along the banks of the river and there is a golf course nearby which uses both sides of the river. The surrounding area is characterized by residences, commercial facilities, and vacant land. Community concerns have been few and related to odor.
Only one completed pathway exists, which is the use of Midway Creek by recreationists. However, worst case scenarios were evaluated and there did not appear to be a human health threat. Two potential pathways were analyzed, for landfill gas and ground water. Again the worst case scenarios did not reveal any imminent human health threat.
Kent Highlands Landfill is categorized as being no apparent public health hazard because, (1) none of the estimated exposures exceeded ATSDR Minimal Risk Levels, (2) data were available for all environmental media to which humans were exposed, and (3) there are no community-specific health outcome data to indicate that the site has had an adverse impact on human health.
The authors are recommending completion of the security fence, continuation of monitoring for ground water and cover stability and monitoring. The ATSDR Health Activities Recommendation Panel has reviewed this public health assessment for follow-up health actions. No actions are recommended at this time.
The DOH staff will review any additional data that becomes available for further follow-up
activities and provide health consultation as requested.
The Kent Highlands landfill is located in the City of Kent, approximately 14 miles south of the City of Seattle, at 23076 Military Road South (Figure 1). The landfill occupies about 60 acres of a 90 acre site located immediately north of State Route 516, about 0.3 mile east of the Interstate 5 Midway interchange (Figure 2). Frager Road, which parallels the Green River is the eastern boundary. The main access to the site is from its western side, via Military Road South. When necessary, the site can also be accessed from its eastern boundary via Frager Road.(5)
The landfill is located within a former ravine in a hilly area adjacent to the Green River Valley. The ravine, prior to filling, sloped generally from west to east toward the Green River. The ravine's side slopes rose steeply 120 to 150 feet above the floor of the ravine to the north, south, and west. Current site elevations vary from about 300 feet above mean sea level (MSL) near the western site boundary to about 40 feet above MSL near the Green River. The floor of the ravine was poorly drained and swampy, with a thick cover of brush and trees. A stream flowed through the ravine, fed by springs and runoff. This stream entered Midway Creek, which flows to the Green River. Peat was excavated and removed from the area where a toe buttress was built to help prevent soil slippage.
An extensive piping and ditching system carries surface water to ponds on the east portion of the site. Water from the south pond discharges to the Green River. Water from the north pond was discharged to a leachate pond where it was partially treated before being pumped to a sewage treatment plant in Renton. Presently, the north pond has been filled and can no longer discharge to surface water. All surface water is now piped to the south pond. The leachate pond is also a collection point for leachate, which is water from within the fill.(5)
Landfilling began in 1968, in the middle of the ravine and progressed for two years before the permits to fill the western end of the site were obtained. Filling then proceeded to the west and east of the original filling. It is estimated that the total volume of refuse and soil fill is 8 million cubic yards or more. The estimated areal extent of the refuse is shown in Figure 2.(5)
The Kent Highlands Landfill was operated by the City of Seattle under a lease from Kentview Properties, Inc., from June 1968 to December 31, 1986. It was administered during the 1980's under a "non-conforming permit" from the Seattle-King County Department of Public Health (SKCHD). Landfill operations included disposal of both putrescible and nonputrescible wastes from the City of Seattle's transfer stations and contract haulers, and from the City of Kent. In accordance with agreements between the cities of Seattle and Kent, the landfill did not accept refuse after December 31, 1986. During 1986, delivered waste quantities averaged over 1,000 tons per day. Wastes were unloaded in the active portion of the landfill, compacted, and covered with soil taken from a borrow area north of the site. Once a section of the landfill was filled to capacity, a layer of soil was placed over the refuse-and-soil fill, and portions of the area were seeded to help stabilize the soil and prevent erosion. Current on-site activities (as of 1994) include final grading, landfill gas collection, leachate system improvements, final cover construction, and storm water system construction as part of the final closure construction.
Subsurface gas was discovered in 1984, while monitoring wells were being constructed. Gas pipe risers which had torches to burn gas were installed. The gas control system has undergone several improvements and has now been completed. An ambient air sampling program was conducted during January through November 1989. No noticeable odors were reported during that time.(5)
In 1984, Washington State Department of Ecology (Ecology) conducted a Potential hazardous Waste Site Preliminary Assessment, which concluded there were no records of the landfill accepting any hazardous waste. The records did show low levels of organic contaminants were found in the leachate samples. Ecology then recommended further monitoring of the leachate and ground water. After the study by Ecology, the site was reviewed by the U.S. Environmental Protection Agency (EPA) for possible inclusion on the National Priorities List (NPL) of hazardous waste sites. Because of ground water and surface water considerations, the site was added to the NPL in August of 1990.(5)
Remedial investigations have been conducted to characterize the contamination and possible effects from the site.
On 3-18-91, Jack Morris from Washington State Department of Health (DOH) conducted a site visit. He was accompanied by the site manager for Ecology, the Project Officer for Seattle, and the Project Engineer for Seattle. On 6-21-91, Jack Morris met with the SKCHD staff to discuss any concerns that agency might have regarding the site. Direct observations and information gathered from these individuals are listed below:
- Several gas flares were noted. Two were large temporary extraction flares at the north and west areas. The others were small passive flares scattered throughout the landfill.
- A chain link fence secured the west, north, and east perimeters. Most of the south boundary was not fenced. Negotiations were being conducted between the city and Washington State Department of Transportation (DOT) regarding construction of a fence along State Route 516. The site has a main gate at the NW corner, where visitors must sign in.
- Commercial facilities such as auto repair shops were adjacent to the west property boundary. Three residences were noted close to the NW corner of the property on Bolger Rd.
- A pipe from the north drainage system in the area of the Toe Buttress had been ruptured and some material had spilled onto the ground.
- The north pond was equipped with a pump to move material to the leachate pond. It also had a gated overflow ditch which led to Midway Creek. The leachate pond material is pumped under the Green River to city sewer lines. The river crossing was through a permit issued by Ecology.
- Wildlife noted: Nesting Buteo Hawks, frogs, robins, herons, aquatic insects, and Mallard ducks.
- Several monitoring wells were noted throughout the site, and one ground water monitoring well was observed in a parking lot west of the site.
- Many people were noted jogging, bicycling, fishing, eating lunch, and golfing along the Green River next to the east boundary. The golf course is on both sides of the Green River.
- Soil movement eastward is evidenced by observing the uprights for wells and probes which have all tipped downward toward the east.
- SKCHD has made routine inspections of the site. Major concerns of that agency were historical problems with unlit gas flares, unstable slopes, and the ultimate use of the site. They have received very few complaints from the public about the site, and those received have been about odor.
Another site visit took place on June 2, 1994. There have been many changes to the site since the first visit. Present status is:
- The small passive "tiki torch" flares have been removed. There is one temporary passive flare on the new storm drain manhole at the toe of landfill.
- Discussions with the Washington State Department of Transportation regarding the fence have concluded. The fence is currently under construction.
- The ruptured pipe from the north drainage system has been replaced with the permanent storm drain pipe.
- The north pond no longer exists. It has been filled and covered.
- The stability of the fill and cover have been checked and DOH has been assured that the landfill is stable.
The City of Kent is approximately 14 miles south of Seattle, Washington, and has a population of approximately 34,860. In 1986 there were 12,700 people living/working within one mile of the site. The general area has relatively high ridges, where Interstate 5 has been constructed, and lowlands of the Green River. Grandview School and Grandview Park are approximately 2000 feet north of the landfill. Residential neighborhoods are located south of SR 516. The western boundary is adjacent to commercial businesses and vacant property located along Military Road South. The northern boundary is adjacent to vacant land that had been the site of a previous sand and gravel pit operation. The City of Seattle purchased most of the landfill site and land north of the site in 1990.(5)
The area around Kent/Highlands landfill has commercial, residential, and recreational properties. West of the site there are commercial facilities for automotive repair and RV sales. There are also private residences near the west side. North of the site there are residences, a park, and a school. South of the site are many single and multiple family residences. The east boundary is adjacent to Frager Rd., which follows the edge of the Green River. Along the river are jogging pathways, a golf course, fishing areas, a small park, and a plant nursery. East of the Green River are commercial facilities, residences, and light industry.
Natural Resource Use
A sand and gravel pit was in operation north of the site prior to the City of Seattle purchasing many acres of land to the north. Fishing is very popular in the Green River, since it receives substantial runs of anadromous fish, such as salmon and steelhead. Private wells are used for drinking and irrigation water. The closest private wells used for drinking water were found approximately 1,000 feet northwest (up-gradient) of the site boundary. The city uses the Green River for domestic supply, but the intake is many miles upstream from Kent Highlands Landfill.
There are three cancer registries in Washington State. One covers the northwest portion of
one is specifically for the Spokane area, and the third is a new registry which only started
data in January 1992. There is a statewide birth defect registry which has collected data since
DOH personnel could not locate information regarding health registries or specific documented
health reports for the populations in the Kent Highlands landfill area.
Comments from the community residents regarding Kent Highlands landfill have been minimal. The SKCHD has received very few complaints, and those which were received were about odor.
SKCHD, as mentioned in the site visit section, has been concerned about unstable slopes, and the early gas collection system, which is being replaced. That agency was also concerned in the past about problems with the drainage system for leachate. Most of the system has been upgraded or replaced recently.