IELTS Academic Reading Practice Test 12: Passage 1

For most of human history, getting rid of rubbish meant little more than throwing it onto a heap or into a pit on the edge of a settlement. Open dumps of this kind were often set alight to reduce their volume, and they attracted rats, flies and gulls. In the early twentieth century, engineers began to develop a more controlled approach. One of the first examples in the United States, opened in Fresno, California, in 1937, buried waste in trenches, pressed it down with machinery and covered it with soil every day. Modern landfill sites are based on the same principles, but they have become far more sophisticated, chiefly because of a growing understanding of the damage that buried waste can do to the surrounding environment.

The first task is to find a suitable location. Engineers look for ground through which liquids cannot easily pass, and they avoid areas that are liable to flooding or where groundwater lies close to the surface. Sites near airports are generally ruled out, because the birds that are drawn to landfills can be a danger to aircraft. Once a site has been chosen, it is divided into sections called cells, which are filled one after another over a period of years, so that only a small area of waste is exposed at any one time.

Although a completed landfill may look like nothing more than a grassy hill, a cross-section through one of its cells would reveal a series of carefully designed layers. At the surface, grass and other plants grow in a layer of topsoil. Only species with shallow roots are used, because deep roots could break through the layers beneath. Under the topsoil lies the cap, a covering of clay or plastic that is designed to keep out rainwater and so to limit the amount of liquid forming inside the landfill. Below the cap is the waste itself, built up in thin, tightly compressed layers, each separated from the next by a layer of soil. Beneath the waste is the lining, which prevents polluted liquids from escaping into the ground. Its uppermost part is a drainage layer of gravel containing a network of perforated pipes. This rests on a layer of fabric that protects the next layer down, a tough plastic sheet about two millimetres thick, from being punctured by sharp objects in the rubbish. At the very bottom, resting on the natural ground, is a thick layer of clay that has been compacted by heavy rollers.

The pipes in the drainage layer have a vital job. As rainwater trickles down through rubbish, it picks up substances from it, forming a liquid known as leachate, which can contain a mixture of harmful chemicals. If leachate were allowed to build up at the bottom of a cell, it might eventually force its way through the lining. Instead, because the pipes are laid on a gentle slope, it flows to a collection point, from which it is pumped out and treated, either at a plant on the site itself or at a nearby sewage works.

When a new cell is ready, lorries begin to deliver waste. Each vehicle is first weighed so that the operator can charge for the amount it brings. The rubbish is then tipped at the 'working face', where it is spread in thin layers and crushed by compactors, enormous machines whose steel wheels are fitted with teeth. Compaction is important because space is the most valuable resource a landfill has: the more tightly the waste is packed, the longer the site can remain open. At the end of each working day, the fresh waste is covered with soil to a depth of about 15 centimetres, which reduces smells, prevents litter from blowing away and makes the rubbish less accessible to birds and rats.

Inside the landfill there is very little oxygen, and organic material such as food and paper is broken down slowly by bacteria that can survive without it. When archaeologists in the United States dug into several landfills in the 1980s and 1990s, they found newspapers that could still be read decades after they had been thrown away. This slow decomposition produces landfill gas, which typically consists of roughly equal amounts of methane and carbon dioxide. Since methane is a far more powerful greenhouse gas than carbon dioxide, modern sites drill wells into the waste and draw the gas out through pipes. At many sites it is simply burnt off in a flare, but at larger ones it is used as fuel to generate electricity.

Even after the final cell has been capped, a landfill must be watched closely. Leachate and gas continue to be produced for decades, and the operator must regularly test water from monitoring wells around the site to check that the groundwater has not been contaminated. Because the waste continues to settle as it decomposes, the ground surface can sink unevenly, and closed landfills are therefore rarely used for buildings. Many, however, have been turned into parks, sports fields or nature reserves.