The U.S. Air Defense Artillery Center and Fort Bliss Municipal
Solid Waste Landfill Facility (MSWLF) is located about 10 miles
northeast of downtown El Paso, Texas. The landfill is built on
the Hueco Bolson, a deposit that yields water to five public-supply
wells within 1.1 miles of the landfill boundary on all sides.
The bolson deposits consist of lenses and mixtures of sand, clay,
silt, gravel, and caliche. The unsaturated zone at the landfill
is about 300 feet thick. The Hydrologic Evaluation of Landfill
Performance (HELP) and the Multimedia Exposure Assessment
Model for Evaluating the Land Disposal of Wastes (MULTIMED)
computer models were used to simulate the time of first arrival
of landfill leachate at the water table.
Site-specific data were collected for model input. At five
sites on the landfill cover, hydraulic conductivity was
measured by an in situ method; in addition, laboratory values were
obtained for porosity, moisture content at field capacity, and
moisture content at wilting point. Twenty-seven sediment samples were
collected from two adjacent boreholes drilled near the
southwest corner of the landfill. Of these, 23 samples were assumed
to represent the unsaturated zone beneath the landfill. The core
samples were analyzed in the laboratory for various
characteristics required for the HELP and MULTIMED models: initial
moisture content, dry bulk density, porosity, saturated
hydraulic conductivity, moisture retention percentages at various
suction values, total organic carbon, and pH. Parameters were
calculated for the van Genuchten and Brooks-Corey equations that
relate hydraulic conductivity to saturation. A reported recharge
value of 0.008 inch per year was estimated on the basis of soil-
water chloride concentration.
The HELP model was implemented using input values that were based
mostly on site-specific data or assumed in a conservative manner.
Exceptions were the default values used for waste characteristics.
Flow through the landfill was assumed to be at steady state. The
HELP-estimated landfill leakage rate was 101.6 millimeters per
year, approximately 500 times the estimated recharge rate for the
area near the landfill.
The MULTIMED model was implemented using input values
that were based mainly on site-specific data and some
conservatively assumed values. Landfill leakage was assumed to
begin when the landfill was established and to continue at a
steady-state rate of 101.6 millimeters per year as estimated
by the HELP model. By using an assumed solute concentration in
the leachate of 1 milligram per liter and assuming no delay or
decay of solute, the solute serves as a tracer to indicate the first
arrival of landfill leachate. The simulated first arrival of
leachate at the water table was 204 to 210 years after the
establishment of the landfill.
