Basic geologic, hydrologic and engineering criteria on which selection of sanitary landfill sites should be based include (1) thickness, excavation characteristics, permeability, solution-holding capacity, and reactivity of host and cover materials; (2) hydrologic properties including depth to water table, season variation in position of water table, and rate of liquid movement; and (3) nature of terrain in terms of slope, topography, and surface drainage. Evaluation of these features must be based initially on adequate mapping and inventory of surface and near-surface earth materials-both bedrock and surficial soils.
Solid wastes include a broad spectrum of residues of municipal and industrial activities. These waste solids include refuse from residential areas, sludges from water and wastewater treatment, plant trash, organic and inorganic chemicals, toxic materials, manures, oils, and other materials which are not discharged into surface waters or into the atmosphere.
Basic environmental mapping recently completed in an 18,000-square-mile area of the Texas Coastal Zone, an area embracing nearly one-third of the State's population and industry, shows the distribution of 130 substrate and landform map units. Map units are grouped into four main landfill suitability groups. Of these groups, only one is entirely suitable for landfill sites, two constitute very poor site areas, and one can be utilized only locally and with proper engineering design. Principal landfill suitability groups in the Texas Coastal Zone include (1) clays and muds with low permeability, high water-holding capacity, high compressibility, high to very high shrink-swell potential, low internal drainage, level to depressed surface relief, low shear strength, and high plasticity-such clays and muds provide secure landfill sites; (2) sands with high to very high permeability, low water-holding capacity, low compressibility, low shrink-swell potential, high internal drainage, high shear strength, and low plasticity-these sands are very poor landfill sites; (3) wetlands (marshes and swamps) with very low permeability, high water-holding capacity, high to very high compressibility, high shrink-swell potential, very poor internal drainage, depressed surface relief, and permanently high water table—these wetlands are poor landfill sites; and (4) clayey sands and silts with moderate to low permeability, moderate water-holding capacity, moderate compressibility, low to moderate shrink-swell potential, moderate internal drainage, and high shear strength—these sands and silts are marginal for solid-waste disposal but can be used locally with proper engineering design and control.
Of the approximately 100 in-ground solid-waste disposal sites currently in operation in the Texas Coastal Zone, only 20 percent are geologically and hydrologically secure sites. Thirty percent are in substrates that are insecure, potential or active pollution areas. The remaining 50 percent of the sites occur within highly marginal areas with at least some degree of active or potential pollution. Clearly, geologic and hydrologic criteria have not been used in the selection of most existing sites. The Texas Coastal Zone is not unique; in far too few landfill areas are adequate geologic and land-suitability maps available to the planner.
Within the Texas Coastal Zone and in most other areas, immediate economic considerations outweigh fundamental geologic and hydrologic suitability in site selection. For example, in the heavily populated and industrialized upper Texas Coastal Zone, the most secure and suitable substrates for solid-waste disposal also support the most valuable agricultural lands; by contrast, geologically and hydrologically insecure sites, such as wetlands and permeable sands, generally constitute less expensive land. Abandoned sand and gravel pits, for example, provide inexpensive, generally available, and ready-made sites, yet due to high substrate permeability, extensive pollution can result when these abandoned pits are used as solid-waste disposal sites.
As in most other areas, the bulk of solid waste in the Texas Coastal Zone is disposed of in open dumps; only a small part of the total solid waste is incinerated or disposed of in a sanitary landfill. Under present conditions, properly managed landfills provide the most adequate manner of waste disposal and are naturally far superior to open dumps. However, unless selection of the site is based on geologic and hydrologic criteria, sanitary engineers will be severely handicapped in their efforts to prevent unacceptable environmental pollution.