ECOSIM: The Athens groundwater case study

The Ano Liosia landfill is the major disposal site of Athens-Greece and it has been in operation since 1973. It currently receives the wastes from an urban population of nearly 4,000,000 inhabitants and extends to an area of 173 ha, of which 45 ha are covered by deposited wastes. The average quantity of wastes deposited daily is about 3500 tons. The daily per capita production of wastes is estimated at approximately 0.875 kg/capita/. The municipal waste is characterized by the presence of high quantities of organics (52%) which includes 20% paper, 9% plastics, 4% metals, 4% glass, 3% textiles, wood and leather, 4% inert, 4% other. It is estimated that until today 27 Mt of wastes have been deposited in Ano Liosia Landfill (14.2 Mt organics, 5.5 Mt paper, 2.5 Mt plastics, 1 Mt metals, 1 Mt glass, 0.8 Mt textiles, wood and leather, 1 Mt inert and 1 Mt other).

It is a moderately steep, semi-arid area, covered by wild vegetation consisting of low plants and grasses. The prevailing geological formations is limestone of the Triassic and Jurassic ages. The formations are characterized as quite permeable, with a percolation rate of 6-12 m/day. The bottom of the landfill is not lined and there is no provision for the collection of leachate. The wastes are compacted on site mechanically, deposited in layers of 200-350 cm and are covered by 50-150 cm of soil, sand gravel and clay materials. The depth of deposition in the center of landfill body exceeds 20 m.

The Problem

Leachates are generated as the result of water or other liquid passing through the waste. The natural humidity of the new waste may also release leachates when the successive layers of the landfill are compressed. The uncontrolled infiltration of leachate in the vadose zone and finally in the saturated zone is considered as the worst environmental impact of a landfill. The origin of these contaminated liquids can be attributed to many factors, including the water produced during the decomposition of the waste. In addition, when the rain reaches the surface of the landfill it can either penetrate through the waste or the ground cover or give rise to the level of a groundwater body. This depends on the nature of landfill (e.g. surface characteristics, type and quantity of vegetation, gradient of layers, etc.). The amount of the rain that penetrates into the landfill body depends on the water quantity retained by the plant roots if these are present, and also on the quantity which evaporates into the atmosphere through the complex process of evaporation.

Most of the waste entering a landfill is biodegradable, having a wide range of variability in its composition. As soon as it is unloaded it starts to decompose due to aerobic biological processes, and on a later stage anaerobic processes prevail. The leachate composition for a given landfill cannot be predicted from literature data since the parameters influencing its quality are not easy to justify. The composition of landfill leachate depends on many factors such as the fill material (organic, inorganic content, degradability), geological, climatic conditions and age of landfill. Leachate from the same source, as well as from different sources, is extremely variable.

The water that penetrates through the landfill body is heavily polluted by both organic and inorganic matter. The inorganic substances pass into the liquid phase through the following main mechanisms:

• At a first stage, the substances that are contained in the waste and soil material before any biochemical activity begins are diluted. These substances may be chlorides, sulphates and various alkaline salts. The solubility of the chlorides is increased as the biodegradation of the organic matter increases. This becomes more intense during the aerobic biodegradation. Chlorides half-time is about 60-64 years.

• At a later stage, with the initiation of biochemical reaction, the biodegradation of composite organic matter takes place. During this stage more simple organic as well as inorganic derivatives are produced, such as calcium, magnesium, sodium, potassium, chlorides, sulfates, phosphates, etc. These products due to their high solubility pass into the water phase.

The simulation study

In order to assess the pollution of the underground water, two general procedures can be followed. Experimental determination or estimation through mathematical modeling. In our case for the mathematical modeling chlorides were used for the estimation of the pollutants plume. Generally, chloride is conside red to be a conservative contaminant which is not affected either by the biochemical processes taking place in the landfill body or by the natural decontamination reactions in which the leachates are involved during their penetration in the vadose zone. Therefore using chlorides for simulation, the worst case scenario is being examined concerning the pollutants transfer.

Chlorides concentrations contained in landfill leachate can be affected by the landfill age (Table 1), the fill material (organic, inorganic content, degradability), geological and climatic conditions. In Table 2, the range in which the chloride concentration can fluctuate (as measured in various disposal sites all over the world).