Jamestown Sanitary Landfill
The City of Jamestown retained Interstate Engineering to provide engineering services for an expansion of their municipal solid waste landfill. Based on EPA subtitle D guidelines, state regulations for municipal landfills require a hydrogeologic investigation for horizontal expansions. Based on the proposed expansion, a work plan including 22 borings, four monitoring wells, and one gas probe was prepared and submitted to the North Dakota Department of Health (Department). The proposed borings were positioned in a grid-like pattern to get a representative sample of soil conditions.
The Department approved the work plan, and Interstate Engineering was authorized by the City of Jamestown to invite bids from four qualified drilling companies. Midwest Testing (Terracon) was awarded the work. In October of 2014, Interstate Engineering staked the proposed boring locations. Midwest Testing advanced approximately 950 feet of borings; installed four monitoring wells and a gas probe. The gas probe was not technically part of the investigation. It was installed voluntarily as an early detection method for methane gas migration from the existing landfill towards the existing on-site maintenance building. The monitoring wells were installed as two nested pairs to sample and monitor groundwater quality in the Seven Mile Coulee and Midway Aquifers.
Aquifers are water-bearing layers of the earth. The three-dimensional location of the aquifer; and the ability of water to travel horizontally and vertically within the geology of the aquifer and overlying layers determines the suitability of the site for a landfill. The aquifers’ characteristics also help determine the design of the landfill. For example, in the case of the Midway aquifer, groundwater is approximately 70 feet below the existing ground surface. The overlying clay in the expansion area has an in-place hydraulic conductivity less than one by 10-7 cm/sec. If the bottom of the landfill is designed to be 30 feet below ground surface and a drop of liquid in the landfill is able to escape a hole in the liner, it will travel about three centimeters per year through the clay. Based on 40 feet of separation, it will take about 400 years for that drop of liquid to reach the Midway Aquifer. This isn’t the actual methodology used for landfill design, but it does illustrate the significance of the site’s hydrogeologic conditions.
A qualified hydrogeologist is required on-site during drilling to review site conditions; observe the samples recovered during drilling and make adjustments to the work plan as necessary. Interstate Engineering used Stephen Braun for this task.
The drilling activities used a 3.25-inch diameter hollow stem auger to advance the borehole into the ground. Soil samples were collected continuously with a 5-foot long split barrel sampler that collects a soil core sample ahead of the auger. Each core sample is examined and logged by the hydrogeologist to characterize the entire column of soil as accurately as possible. Observations of the material include soil type; structure; moisture; and color. Several Shelby tubes were also collected to evaluate the hydraulic conductivity and plasticity of the soil. If the soil is clay with the desired conductivity, it will work well both as an undisturbed barrier between the landfill and the groundwater; and as a borrow source for constructing the clay landfill liner that will envelop the waste. Plasticity is quantified by Atterberg Limits. Landfill liners need to be constructed with a plastic clay. The more plastic the clay, the less effect movement will have on its hydraulic conductivity. For example, a new liner that is highly plastic will be more likely to weather a freeze-thaw cycle without cracking (and leaking) than a low plasticity clay.
We prepared a hydrogeologic investigation report to summarize the findings of the investigation. The report was submitted to the Department of Health in July 2015. It includes figures like the one below to illustrate site conditions. Detailed design drawings and a permit modification report were also submitted to the Department of Health. It addresses numerous other facets of the design and permitting process. These include:
- Leachate Collection and Management Plan
- Soil Loss Equations
- Jurisdictional Determination of Wetlands
- Topsoil Suitability
- Liner and Final Cover Design
- Financial Assurance Plan
Although there is much more to landfill design than hydrogeology, the hydrogeologic investigation is a critical early milestone to successful completion of a landfill expansion.