A CALIBRATION STUDY FOR THE COLBERT LANDFILL GROUNDWATER TREATMENT FACILITY, SPOKANE COUNTY, WASHINGTON

Douglas G. Gearhart

Abstract

The purpose of this study is to maximize contaminant removal using the ten existing extraction wells at the Colbert Landfill Groundwater Treatment Facility located 15 miles north northeast of Spokane, Washington. The contaminated area extends from approximately 1 mile north of the Colbert Landfill to about 1.5 miles south of Woolard Road, and from Hardesty Road, 1 mile east of the Landfill, to the Little Spokane River about 0.75 miles to the west.

The study area is situated in an alluvial valley that is surrounded by crystalline basement rock highlands to the east and west. Granite basement rock underlies the study area. During the Miocene Epoch basalt flows from the Grande Ronde Formation and the Priest River Member of the Wanapum Basalt Formation were emplaced in the study area. Concurrent to the basalt flows, the Latah Formation (sedimentary lake bed deposits) was deposited as the basalt blocked the flow of rivers and streams in the area, thus creating lakes and swamps. During the Pleistocene Epoch, repeated catastrophic floods deposited unconsolidated sands and gravels, as well as glaciolacustrine clays in the study area. Subsequent landslides have modified the geology of the study area.

Chlorinated organic solvents disposed of at the Colbert Landfill, between 1975 and 1980, contaminated the groundwater. Solvents were reportedly disposed of at an average rate of several hundred gallons per month, and primarily consisted of 1,1,1-trichloroethane and methylene chloride. Other organic solvents were detected in the groundwater, including trichloroethylene, tetrachloroethylene, 1,1-dichloroethylene, and 1,1-dichloroethane. These chlorinated organic solvents are referred to as the ‘Constituents of Concern.’

The Colbert Groundwater Treatment Facility consists of ten extraction wells and an air stripping tower. Five extraction wells are in the lower sand and gravel aquifer, four extraction wells are in the upper sand and gravel aquifer, and one extraction well is in a basalt aquifer. Groundwater samples were taken as these wells were run throughout the full range of pumping rates. This study shows that only three extraction wells, CP-W2, CP-W3, and CP-E1 need to be operated at their maximum flow rates, 260 gallons per minute, 260 gallons per minute, and 155 gallons per minute respectively, to maximize contaminant removal. Five extraction wells, CP-W1, CP-S1, CP-S4, CP-S5, and CP-S6 can be operated at their minimum flow rates, 70 gallons per minute, 45 gallons per minute, 15 gallons per minute, 15 gallons per minute, and 40 gallons per minute respectively, and still maintain containment and extraction of contaminants. The other two extraction wells, CP-E2 and CP-E3 can be operated at 50% pumping capacity, 2.0 gallons per minute and 120 gallons per minute respectively, while maintaining significant contaminant capture.