April, 2001

Glacial Till Acts As Natural Landfill Liner

For decades Leggette, Brashears & Graham, Inc. (LBG) has provided consulting services related to the evaluation, assessment, monitoring and remediation of Municipal Solid Waste Landfills and other waste containment facilities. This expertise is utilized by corporations and governmental entities to provide cost-effective solutions to their solid waste challenges. One of those cost-effective solutions is the innovative conceptual design that uses in-situ materials as the bottom liner of a Subtitle-D compliant landfill.

The Sioux Falls (S.D.) Regional Sanitary Landfill, which covers over 440 acres, serves a population of approximately 175,000. It is the largest in the state, with waste receipts of approximately 150,000 tons annually. The South Dakota Board of Minerals and Environment granted the City of Sioux Falls a permit to operate and expand the landfill without a constructed bottom liner.

The natural liner concept, which was accepted unanimously by the board, will yield multi-million dollar savings for the city in engineering, construction, maintenance and monitoring costs. This cost savings, along with the maximization landfill space, will provide cost-effective future waste disposal space and allow for the continued economic development of the Sioux Falls area.

Before the permit application was submitted to the state, LBG hydrogeologists conducted a thorough and comprehensive hydrogeologic evaluation of the existing and expansion landfill areas, and reviewed data provided by the South Dakota Geological Survey.

The assessment's results indicate that a clay-rich glacial till underlies the site to a depth of up to 150 feet below grade. The Wall Lake Aquifer, the area's primary ground-water resource, is underneath the glacial till.

The glacial till is oxidized and fractured up to 20 feet below land surface with a transition zone extending to the unoxidized till at up to 45 feet below land surface. No significant sand layers or lenses were encountered within the landfill expansion area. Direct observation of the glacial till in trenches excavated for the existing portion of the landfill confirm that the unoxidized till is not fractured, and will not conduct fluids readily.

Slug-test and permeameter test data indicate that the unoxidized till has a hydraulic conductivity ranging from 10-7 to 10-9 centimeters per second, showing it to be relatively impermeable. These low hydraulic conductivity values indicate that no significant lateral ground-water movement is occurring within the unoxidized glacial till. Carbon-14 age dating of the ground water within the unoxidized till showed it to be 12,000 to 20,000 years old, further indicating that no significant vertical ground-water movement is occurring within the unoxidized glacial till, at least in the last 12,000 years.

The conceptual design for the expansion proposed four north-south trending trenches excavated to 50 feet below land surface. The trenches will be up to 585 feet wide and 2,440 feet long. Leachate will flow by gravity to the trenches' south end for collection and removal. The final height will be 100 feet above land surface with a total landfill volume of approximately 25.5 million cubic yards. At present disposal rates, the landfill expansion may last up to 60 years.

To be sure, this solution does not apply to every site. However, similar hydrogeologic conditions may exist elsewhere in the upper Midwest, including North Dakota, Nebraska, Minnesota, Iowa, Wisconsin and Michigan, and perhaps in Ohio, Indiana and Illinois. In some of the Western states, such as Wyoming, Nevada, Colorado and Montana, impermeable sediments also are found at the surface.

While this type of design requires a greater density of geological data than a conventional design, the costs may be significantly less than installing a constructed liner. In any case, a design that uses the site's hydrogeology to the benefit of the landfill owner can yield a highly cost-effective solution that is protective of human health and the environment. As with any technology, the site-specific characteristics are key to implementation and applicability. LBG can assist you in determining if your site can benefit from this technology, based on your site-specific conditions and regulatory climate.

For over 57 years, Leggette, Brashears & Graham, Inc. has provided state-of-the-art consulting services to our clients. The bulk of our work is repeat business from customers who have benefited from the value-added services we provide. If you would like more information on how LBG can add value to your ground-water or environmental projects, or if you believe that a Web site would benefit your project, please contact your local LBG office for assistance.