Read the following passage carefully and give the answer of following questions.

The problem of water pollution by pesticides can be understood only in context, as part of the whole to which it belongs – the pollution of the total environment of mankind. The pollution entering our waterways comes from many sources, radioactive wastes from reactors, laboratories and hospitals; fallout from nuclear explosions; domestic wastes from cities and towns; chemical wastes from factories. To these is a added a new kid of fallout - the chemical sprays applied to crop lands and gardens, forests and fields. Many of the chemical agents in this alarming melange initiate and augment the harmful effects of radiation, and within the groups of chemicals themselves there are sinister and little - understood interactions, transformations and summations of effect.

Ever since the chemists began to manufacture substances that nature never invented, the problem of water purification have become complex and the danger to users of water has increased. As we have seen, the production of these synthetic chemicals in large volume began in the 1940’s. It has now reached such proportion that an appalling deluge of chemical pollution is daily poured into the nation’s waterways. When inextricably mixed with domestic and other wastes discharged into the same water, these chemicals sometimes defy detection by the methods in ordinary use by purification plants. Most of them are so complex that they cannot be identified. In rivers, a really incredible variety of pollutants combine to produce deposits that sanitary engineers can only despairingly refer to as “gunk”.

Q:

The words  ‘gunk’ in the last line refers:

• 1
  to the waste products deposited by sanitary engineers
• 2
  to the debris found in rivers
• 3
  to unidentifiable chemical found in water.
• 4
  to the domestic water supplies.