Abstract

The West Bountiful Landfill occupies approximately 150 acres on the eastern shore of the Great Salt Lake in Davis County, Utah. The landfill has received municipal waste since about 1962 from 6 nearby municipalities and Davis County. Since about July 1987, the landfill has been operated solely by the City of Bountiful.

To provide site-specific information regarding the potential for off-site movement of contaminated ground water, James M. Montgomery, Consulting Engineers, Inc. performed a Ground-Water-Quality Assessment study. To generate data necessary for this study, an extensive field investigation program was developed which consisted of drilling and sampling 7 perimeter soil borings to depths from 35 to 45 feet, constructing piezometers in each of these borings, and installing 8 new shallow monitoring wells. In addition, ground-water samples were collected from both the 8 new and 4 previously existing monitoring wells.

The geologic conditions which underlie the West Bountiful Landfill are similar to those found in many locations along the shores of the Great Salt Lake. The landfill is underlain by interbedded layers of fine-grained soil such as clay and silt with occasional fine-grained sand and silty sand layers which range from thin laminae to 3.5 feet thick. Evidence suggesting lateral continuity of at least some of the individual layers was found in an areally extensive clay marker bed which was identified beneath the landfill in all of the 8 borings drilled at the site.

Ground water beneath the West Bountiful Landfill occurs in fine sand and silty sand layers at various depths and in the refuse. The pattern of ground-water flow is generally from east to west in both the shallow zones, 8 to 12 feet below ground, and in deeper sand layers at depths of up to 40 feet below ground. Ground-water levels within the landfill refuse are elevated above the surrounding areas and create a ground-water mound. The difference between water levels in the ground-water mound and surrounding areas cause gradients to induce ground water to flow radially from the center of the landfill. Ground-water levels measured in piezometers indicate that the hydraulic head increases with depth which indicates that an upward vertical gradient underlies the landfill.

The findings of this Ground-Water-Quality Assessment indicate that subsurface conditions beneath the site make it generally suitable for a municipal waste landfill. The low permeability clay layers which underlie the site appear to isolate the refuse and leachate from deeper aquifers. In addition, upward hydraulic gradients also tend to prevent a downward migration of leachate. However, leachate appears to migrate from the refuse into the Great Salt Lake through shallow, water-bearing zones.