Ground‐Water Pollution Potential of a Landfill Above the Water Tablea
Authors: Apgar Michael A. , Langmuir Donald 
Published: July 2006

A study of the character and movement of landfill leachate through unsaturated soil was begun in 1967 at the State College (Pennsylvania) Regional Sanitary Landfill which has operated since 1962 employing the trench method of waste disposal. The landfill occupies a gently sloping underdrained valley with a water table more than 200 feet below land surface. Precipitation averages about 37 inches as rain per year. Residual sandy‐clay to sandy‐loam soils range from a few feet to greater than 70 feet in thickness on a sandy dolomite bedrock. Soil moisture samples were extracted at different depths from beneath two of the refuse cells using suction lysimeters. Water samples were also bailed from these cells and pumped from a water table well beneath the landfill. Monthly or less frequent analyses performed on water samples included Eh, pH, temperature, specific conductance, BOD, Cl, SO4, total alkalinity, NH3, NO2, NO3, PO4, Ca, Mg, Na, K, and total Fe. Soil samples from beneath the refuse were subjected to particle size and x‐ray analysis, and chemical analysis of soil pH, soluble salt content, exchangeable Ca, Mg, Na, and K, cation exchange capacity, and extractable P content.

The study showed that the quality and quantity of leachate beneath a landfill varies considerably with the topographic setting of landfill trenches or cells. Leachates 2 feet under an upslope cell which received only direct precipitation, had the following maximum values 3‐13 months after refuse burial: specific conductance 8445 μmhos, Cl 1890 mg/1, BOD 3300 mg/1, NH3‐N 540 mg/1, and total Fe 225 mg/1. Upon reaching a depth of 14.5 feet after about 21/2 years or more, maximum values of these species in the leachate had been reduced by 83%, 80%, >99%, >99%, and 98% respectively. In contrast, more water, including precontaminated surface and subsurface runoff from adjacent upslope cells; infiltrated a downslope cell, saturating the refuse. Even after moving downward in the soil to a depth of 36 feet in 7 years, the leachate beneath this cell had a conductance of 6600 μmhos, 600 mg/1 Cl, over 9000 mg/1 BOD, 40 mg/l NH3‐N, and 100 mg/1 total Fe. A practically continuous depletion of inorganic species in the refuse as indicated by the quality of leachate from both cells has occurred with time. However, concentrations of BOD and redox sensitive species such as Fe and NH3 in the leachate have fluctuated in response to changes in the moisture content and temperature of the refuse.

Leachate beneath instrumented cells is moving downward in the subsoil at the rate of 6‐11 ft/yr. Observed mechanisms of leachate renovation during this downward percolation, along with supporting evidence for each listed parenthetically, include: dilution and dispersion (decrease in Cl with depth); oxidation (Eh and pH measurements, decrease in BOD and Fe with depth); chemical precipitation (decrease in soil extractable P after leachate percolation); cation exchange (increase in percent base saturation of clays affected by leachate, and depletion of NH3 under reducing conditions—bacterial growth may also retard or remove NH3); and anion exchange (decrease in SO4 with depth). Removal of bacteria, suspended ferric oxyhydroxides and other particulates by soil filtration undoubtedly also occurs. Although renovation takes place, it is incapable of preventing highly contaminated leachate from moving to depths of 50 feet or more in soils beneath downslope cells. Ground‐water contamination has occurred, probably by leachate channelled down fractures in locally shallow bedrock, or by leachate contaminated runoff from heavy storms which has entered sinkholes or infiltrated along the valley bottom. The study shows that improper design of landfills emplaced above the water table in relatively permeable soils and bedrock such as are found in many carbonate‐rock terranes, can result in serious ground‐water pollution.