Reports
Solid-waste disposal in the geohydrologic environment of Maryland
1972, Otton, E.G.
Report of Investigations 18
Abstract
In 1920 about 0.7 million tons of urban solid wastes were disposed of in Maryland. By 1970 the quantity of these wastes had increased to 3.2 million tons, and by 1980 it is predicted that disposal of about 4.5 million tons will be required. Solid wastes are disposed of by open dumping, sanitary land-filling, and incineration. The use of sanitary landfills and the abandonment of open dumping by Maryland towns is recommended by the Maryland Department of Health.
As Maryland is in a humid region having about 43 inches of precipitation annually, sufficient moisture passes through sanitary landfills to dissolve or otherwise carry along various metals, chemicals, bacteria, and other undesirable materials, collectively called “leachate.” The water may also trigger the formation of such gases as methane and carbon dioxide. Under certain conditions the leachate may enter surficial aquifers at or near the base of the landfill. Rates of leachate infiltration range widely depending on the nature of the earth materials and the available water. Infiltration rates in undisturbed soils have been reported to range from almost nil to about 5 inches per hour.
Chemical analyses of landfill leachate in other states indicate that abnormally high concentrations of total hardness, chloride, nitrate, sulfate, and iron are especially common. Bacteriologic analyses of coliforms in leachate show very high colony counts, values in the range of 1.1 x 10e6 to 10.7 x 10e6 per 100 milliliters having been reported from simulated landfills in West Virginia.
In this report, Maryland is divided into five terrane types on the basis of the hydrologic characteristics of the land as related to solid-waste disposal. Terrane I, shale, siltstone, and sandstone of the Appalachian province, appears to be moderately amenable to solid-waste disposal, with certain limitations. Degradation of the underlying ground water by leachate is least likely in areas underlain by shale or other impervious material, but leachate generated at such sites may enter nearby streams unless adequate precautions are taken. Some abandoned coal strip mines appear to be amenable to disposal of solid municipal wastes.
Terrane II, valleys underlain by limestone or marble, is somewhat less amenable than terrane I to solid-waste disposal because of the common development of solution channels and crevices in the underlying rocks. Thus, direct pollution of the underlying ground-water reservoirs is possible. Parts of terrane II, where the residuum or alluvium overlying the limestone is sufficiently thick or impervious to retard or inhibit vertical movement of pollutants, may be more amenable to solid-waste disposal than other parts.
Susceptibility of any terrane to disposal of solid wastes is, of course, also dependent upon the design of the landfill – the use of relatively impervious cover material, etc.
Terrane III, crystalline silicate rocks of the Piedmont, is suitable for sanitary landfills where a relatively thick zone of saprolite lies well above the water table. Where the saprolite is thin and the rocks are extensively fractured, the possibility of pollution of the ground water by leachate is increased. Terrane IV, upland Coastal Plain deposits, may be locally suitable for the protection of ground waters from leachate where the fill site is underlain by clayey strata well above the local water table. Local groundwater pollution is likely where sand and gravel underlies the surface at a fill site. Terrane V, low-lying Coastal Plain deposits, offers protection from leachate pollution of deep-lying artesian aquifers because of the thick and extensive impervious strata that normally underlie the land surface. However, ample leachate production is expected in filled swamps, rivers, and estuaries due to the high water table prevalent in these areas. Such leachate will, of course, be diluted by the native water.
Surface waters in all five terranes are subject to degradation by landfill leachate. Some protection can be afforded by designing the landfill to minimize the generation of leachate or by collecting and treating the leachate before permitting it to enter a surface watercourse.
