Modeling Leachate Migration from a Landfill

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MODELING LEACHATE MIGRATION FROM A LANDFILL
December, 2003

1INTRODUCTION

A two-dimensional, steady state transport modeling using MODFLOW/CTRAN was carried out following the particle tracking calculations to simulate potential contamination for leachate migration from Gibraltar landfill.

Gibraltar landfill was a disposal waste site in Bogotá (Colombia) until 1988. The site presented problems related to contamination for leachate migration after its closure, since the operation of the site did not meet all the technical requirements for waste disposal.

A technical study was conducted in 1997 to propose alternatives for contamination control and posterior land use. The information gathered during that study is the base for the modeling presented in this report.

According with the results of modeling the flow velocity is very low with an average of 4 mm/d, therefore the leachate migration movements is slow. The advective transport of a contaminant from the landfill to the river and west drainage takes 90 years.

2SITE CHARACTERIZATION – GIBRALTAR

The Gibraltar Landfill is located in Bogotá, capital of Colombia (Figure 1), and has presented problems related to contamination for leachate migration since its closure in 1988.

A technical study was conducted in 1997 to propose alternatives for contamination control and posterior land use. The information gathered during that study was the base for the modeling presented in this report.

The Gibraltar site has an extension of 80 Ha. The layout of the site, base for the conceptual model, is presented in Figure 2. The limits of the site are the Bogotá River and two drainages.

The composition of the wastes disposed in Gibraltar was the typical for domestic residue. The characteristics of the leachate are presented in Table 1.

TABLE 1
TYPICAL COMPOSITION OF THE GIBRALAR LEACHATE - 1985
ParameterUnitValue
Biochemical Oxygen Demand (BOD)
Chemical Oxygen Demand
Totals Solids
Total Suspended Solids (TSS)
PH
Total Dissolved Solids (TDS)
Lead
Cadmium
Iron
Mercury
Chromium +3
Chromium +6mg/L O2
mg/L O2
mg/L
mg/L
Units
mg/L
mg/L Pb
mg/L Cd
mg/L Fe
mg/L Hg
mg/L C+3
mg/L Cr +651to39,450
123to50,238
559to60,511
64to13,206
6to12
315to56,550
0to5.5
0to0.16
1.2to2,384
0.019to2,09
0.01to1.07
0to0.01
Source: ???

2.1HYDROGEOLOGY

The hydrogeologic characterization of the site is based on the data gathered from excavations and eight piezometers built in the area with a maximum depth of 10.5 m.

2.1.1Stratigraphy

The general stratigrafic sequence of the site is presented in Table 2

TABLE 2
STRATIGRAPHIC PROFILE
LayerDepht (m)Thickness (m)
Sand (Waste in the land fill area)0.0 - 7.57.5
Oxidated Clay7.5 - 10.53.0
Sand with lime10.5 - 14.02.5
Grey Clay14.0 and deepermore than 6.0

2.1.2Hydraulic Conductivity (K)

Slug testing was conducted to estimate the hydraulic conductivity, the results are presented in Table 3.

TABLE 3
HYDRAULIC CONDUCTIVITY AND HEADS FOR PIEZOMETERS
PiezometerK (m/d)Head (masl)
10.0032,537.88
20.1502,542.66
30.1502,543.08
40.4002,543.48
50.2102,543.86
60.5002,543.67
70.0802,542.52
80.1502,539.48

The water table is located between 1.7 m and 2.5 m, with a maximum of 3.9 m.

2.1.3Hydraulic Head Elevation (h)

The heads measured in all the piezometers are included in Table 3.

2.2HIDROLOGY

2.2.1Bogotá River

The average water flow of the Bogotá River is 22.8 m3/s. The water levels of the river in the site were determined based on the flow rates, Manning and continuity equations, transversal sections, hydraulic gradient and river slope of 0.03 %. See Table 4.

TABLE 4
WATER LEVEL – BOGOTÁ RIVER
Flow rate - Q
(m³/s)Return Period
(years)Head -h
(m)Metres above the sea level
105.0
95.7
22.8100
50
0.53.92
3.67
2.02,541.25
2,541.0
2,539.3

2.2.2Precipitation

The...
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