considered potential landfill sites. If information is available in digitized format, geographical information systems (GIS) can be used to complete overlay analyses (Siddiqui 1994).
A small number of potential landfill sites are selected from the areas that remain after the exclusive criteria are applied. The final selection process uses nonexclusive criteria, such as hydrogeological conditions, hauling distance, site accessibility, and land use. This process can be done in one or two steps.
If digitized data exist for the entire region under investigation, planners can rank the remaining areas using GIS and an appropriate decision making model, such as the analytical hierarchy process (Siddiqui 1994; Erkut and Moran 1991). For example, USGS soil maps are available in digitized format and include depth to water table, depth to bedrock, soil type, and slope although information is available only down to 5 ft. Planners can also use USGS digitized maps to identify urban areas, rivers and streams, and land use.
If areas are ranked, planners use this information, along with nondigitized information and field inspection, to select a number of sites from the best areas. Otherwise, planners select a number of sites based only on nondigitized information and simple field assessments without the aid of area rankings.
Once a number of sites have been identified, planners should rank the sites in a scientifically justifiable manner using established decision making models such as the analytical hierarchy procedure, interaction matrices, or multiattribute utility models (Camp Dresser & McKee 1984; Morrison 1974; Anandalingham and Westfall 1988-89). This process identifies a small number of sites, usually less than four, to undergo detailed investigations regarding hy-drogeologic characteristics such as drainage patterns, geologic formations, groundwater depth, flow directions, and natural quality and construction characteristics of site soils. In addition, detailed information about existing land use, available utilities, access, political jurisdictions, and land cost is gathered (Walsh and O'Leary 1991a). Planners use this information to select the site for which regulatory approval will be sought.
Hydrogeologic information is crucial to the final site selection and has many uses. The main consideration is the proximity of groundwater, groundwater movement, and the potential for attenuation of leachate should it escape from the landfill. The proximity of groundwater is simply the depth to the groundwater table. Measuring the piezo-metric elevation of the water table in a number of wells on and around the potential site determines groundwater movement. The direction of flow is perpendicular to the lines of constant piezometric elevation. Groundwater movement is important (1) to assess the potential for landfill contamination to impact human health, for example if nearby drinking water wells are downgradient of the landfill site, and (2) to determine the placement of monitoring wells should a site be selected. Leachate attenuation is a function of mechanical filtration; precipitation and copre-cipitation; adsorption, dilution, and dispersion; microbial activity; and volatilization, most of which can be assessed via a subsurface investigation (O'Leary and Walsh 1991c).
The approval process can be demanding. Application writers should work closely with state permitting personnel, who can offer guidance on what is acceptable. To keep costs low, planners should start the siting process with the consideration of large areas based on limited and readily available information and end it with the selection of one site based on detailed information on a small number of sites.
At some point during the selection and permitting of a new landfill, planners hold at least one, and perhaps several, public meetings to ensure that public input is obtained concerning the selection of the landfill site. Generally all landfill sites inconvenience some portion of the local population, and thus most sites generate some public opposition. The siting process must be clear, logical, and equitable. The site selected must be the best available site. However, even if the best site is selected, equity considerations may necessitate offering compensation to residents near the site.
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