Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

J.R. Reid


Landfills are the primary means for land disposal of solid wastes, and as such, they represent potential sources of groundwater contamination. This potential for contamination is exemplified by the Hillsboro landfill, which was emplaced above the Hillsboro aquifer, within permeable surface materials, and under shallow water-table conditions. Burial of refuse in trenches 15-feet (4.57 m) deep ensure that at least portions of the refuse are below the water table, which varied from 5.4 to 13.2 feet (1.7 to 4.0 m) below the surface. Concern over these factors led to the present study.

Subsurface conditions were investigated by electrical, earth-resistivity surveying. Very little contrast was evident in the observed field data and high resistivity values were rare. Quantification of resistivity results and subsequent delineation of a contaminant plume based upon those results proved difficult for these reasons.

Water samples, and consequentially chemical concentration levels, were obtained from piezometers screened in silt and sand at various intervals within the zone of saturation. Although contaminant levels appear to be low, degradation of groundwater beneath the landfill is evident. Most notably, the concentrations of the trace metals arsenic, cadmium, selenium, lead, copper, chromium, iron, and manganese ranged from 3.5 to 20 times more than back ground levels.

The configurations of contaminant plumes for most chemical parameters are similar and believed to be the result of longitudinal and transverse dispersion within the saturated zone. These plumes indicate that the buried refuse within landfill trenches is the source of contamination. However, the plume shapes for lead and arsenic probably reflect isolated disposal or surface spills outside of the covered landfill trenches.

X-ray diffraction analyses revealed that, of the clay minerals present, smectite is dominant. Distribution of smectite in the Hillsboro sediments may be effective in attenuating trace metals and may contribute to apparent low levels of released contaminants.

Low precipitation and high evapotranspiration associated with drought conditions experienced during the study period suggest that leachate production was minimal. Leachate generation from percolation probably occurs only during years of above normal precipitation and, even then, only during periods of intense rainfall.

Based upon calculated average linear velocity of groundwater beneath the landfill, groundwater travel time from the northern portion of the buried refuse to the southern limits of the landfill site is nearly 28 years. Given this travel time, the 12-year residence time of the refuse may be inadequate to establish the magnitude of leachate generation.

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