The electrical resistivity method has been used to delineate shallow zones of ground‐water contamination resulting from solid waste disposal sites. Application of this method to sites located in alluvial deposits in Iowa revealed that the degree of success of the method was directly related to the degree of contamination. A detailed investigation of the interrelationship between electrical resistivity, material variation, and water quality was conducted in the alluvial deposits of the Skunk River at Ames, Iowa. The lateral variation of materials at this site resulted in a large, natural scatter in the resistivity response. The decrease in resistivity due to contamination was not greater than this natural scatter and thus could not be detected. These results suggested a relationship between the scatter levels at any site and the minimum degree of contamination necessary to be detected. The minimum level of contamination which can be detected over the natural scatter is herein called the threshold value. The threshold value must be known before the resistivity method can be applied with confidence.
Cluster analysis of data from county soils reports and from aerial photographs produced three regions with distinctly different assemblages of glacial landforms. The northern region is composed of bogs, kames, terraces, and crossheated to circular feature ice-stagnation moraine. The central region is characterized by level, poorly drained soil and nonpatterned ground moraine. The marginal region consists of heated to cross-lineated ice stagnation features. These clusters are associated with differences in the bedrock topography. The success of this study in defining coherent, logical clusters of counties with similar landforms indicates that the landform data available in county soils reports can be employed in regional analysis even when using county-sized sample cells if the study does not require the establishment of definitive regional boundaries.
HE Des Moines drift sheet in north-central
The theoretical shape of a zone of contaminated ground water (henceforth called an enclave) can be predicted from a knowledge of the three‐dimensional, ground‐water flow pattern. The reasoning in LeGrand (1965) and Sendlein and Palmquist (1973) suggests that the horizontal shape of the enclave is a flame‐like plume extending from the source, parallel to the ground‐water flow lines, in a downflow direction. According to LeGrand, the ultimate size of the enclave will depend upon the relative rates of decay, diffusion, dilution and absorption of the contaminants. Similar reasoning suggests that, in vertical section, the boundaries of the enclave will follow the ground‐water flow lines between contamination source and ground‐water discharge site. This reasoning suggests that the three‐dimensional shape of an enclave is that of a tongue‐like lobe, the length, width and depth of which increases with increasing distance from and increasing height above the discharge site.
In Iowa, the hydrology of refuse sites emplaced in alluvium has been analyzed to determine the size and shape of the associated enclaves. The refuse sites range in size from 13 to 117 acres (5.25 to 47.3 hectares), in age from 9 to 40 years, and in topographic position from floodplain adjacent to river to terrace. The ground‐water quality and water‐table elevations were determined from multiple nests of piezometers (from depths of 15 to 45 feet) (4.7 to 13.6 meters) around the refuse sites. The surficial geology of the sites was established from both borehole and geophysical data.
The Iowa data suggests that the enclave in alluvium is plume‐shaped with the long axis parallel to the ground‐water flow lines and extending from the refuse site to the nearest stream. The SO4 enclave at the Ames site, for example, is over 7000 feet (2121 meters) long, 4500 feet (1362 meters) wide and extends to a maximum depth of 60 feet (18.2 meters). The highest SO4 concentrations are along the axis of the enclave at a depth of 30 feet (9.09 meters). The concentrations decrease with distance along the axis, laterally away from the axis and vertically away from the axis, such that the enclave is entirely surrounded by uncontaminated ground water except at the source. Analysis of the variation in water quality data with time indicates that most of the enclaves are not increasing in size but have achieved their maximum size and are in a steady‐state equilibrium condition.
The Iowa studies indicate that the size and shape of the contamination enclave resulting from refuse disposal sites can be predicted from the initial geohydrologic conditions and that it may become possible in the near future to estimate the concentrations within the enclave at any point in time and space. These possibilities open the way toward a strategy of minimizing potential contamination of aquifers through selective refuse site placement on the floodplain. The Iowa data indicates that floodplain sites may be desirable for disposal sites because of the predictability of enclave shape within al...
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