Modeling Leachate Migration from a Landfill

MODELING LEACHATE MIGRATION FROM A LANDFILL

December, 2003

1 INTRODUCTION

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 Bogotб (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.

2 SITE CHARACTERIZATION - GIBRALTAR

The Gibraltar Landfill is located in Bogotб, 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 Bogotб 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

Parameter Unit Value

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 +6 mg/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 +6 51 to 39,450

123 to 50,238

559 to 60,511

64 to 13,206

6 to 12

315 to 56,550

0 to 5.5

0 to 0.16

1.2 to 2,384

0.019 to 2,09

0.01 to 1.07

0 to 0.01

Source: ???

2.1 HYDROGEOLOGY

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.

2.1.1 Stratigraphy

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

TABLE 2

STRATIGRAPHIC PROFILE

Layer Depht (m) Thickness (m)

Sand (Waste in the land fill area) 0.0 - 7.5 7.5

Oxidated Clay 7.5 - 10.5 3.0

Sand with lime 10.5 - 14.0 2.5

Grey Clay 14.0 and deeper more than 6.0

2.1.2 Hydraulic 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

Piezometer K (m/d) Head (masl)

1 0.003 2,537.88

2 0.150 2,542.66

3 0.150 2,543.08

4 0.400 2,543.48

5 0.210 2,543.86

6 0.500 2,543.67

7 0.080 2,542.52

8 0.150 2,539.48

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

2.1.3 Hydraulic Head Elevation (h)

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

2.2 HIDROLOGY

2.2.1 Bogotб River

The average water flow of the Bogotб 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 - BOGOTÐ' RIVER

Flow rate - Q

(mÑ-/s) Return Period

(years) Head -h

(m) Metres