Water can be polluted and unusable due to the presence of toxic chemicals, sediments, nutrient pollution, diseases and/or pathogens, as well as thermal pollution.

A secondary pollutant is any kind of pollutant that is not directly emitted form a pollution source. In this case, sulfur dioxide, carbon monoxide and nitric oxide are all emitted from the burning of fossil fuels, especially coal. Tropospheric ozone is formed in the reaction between volatile organic compounds and nitrous oxides and is a component in photochemical smog.

Point source pollution can be traced to a specific source. Factory waste is a good example. Nonpoint source pollution cannot be traced to a single source. Agricultural runoff and storm sewers are good examples.

The correct response ischlorofluorocarbons(CFCs)．These chemical compounds are known to deplete ozone from the atmosphere. During the 1960s and 1970s, refrigerators were manufactured with CFCs. This is what created the whole in the ozone which we have all heard about. The Environmental Protection Agency banned the use of CFCs and this is greatly improved the quality of ozone in the atmosphere.

The correct response iswetland habitat．Wetlands help filter out nutrients and actually help prevent eutrophication. Eutrophication is the process by which nutrients accumulate in a body of water. For example, big storm events cause nutrients to leach off the ground and into rivers and lakes via runoff. All of the other answer choices either increase the rate of runoff (i.e. urbanization) or increase the amount of nutrients leaching (i.e. fertilizers).

The correct response isoxygen．死区氧气完全耗尽。这can be due to algal blooms - which consume all the available oxygen in the system. Once all the oxygen is depleted, the algae die and fall to the bottom of the lake or ocean. Deadzones can be harmful for many aquatic organisms, especially fish and bottom feeders.

The correct response isthermal pollution．The heat island effect occurs in urban cities that have a lot of black top and impervious surfaces. Heat gets trapped in the cities because there aren't many trees or natural vegetation to absorb sunlight and respire. Studies have shown that temperatures in downtown regions are significantly higher than temperatures in the suburbs of major US cities. Thus, heat islands cause there to be an excess of heat and we call this thermal pollution.

You may have associated "caltrates" with calcium, which is a heavy metal pollutant and is not water soluble. Nitrates and phosphates are not inherently toxic, however, when present in large quantities, can throw off the balance of an ecosystem to disastrous consequences.

Acid mine drainage occurs when water seeps through a mine or a mine’s waste pile and results in a solution of sulfuric acid. Water is used to process ore and it can become contaminated with sulfuric acid, mercury and arsenic. The contaminated water can damage ecosystems and is particularly harmful to aquatic life. Some acid drainage can occur naturally when water interacts with rock but the degree of contamination is significantly different.

It is common to find some amount of fertilizers and pesticides in runoff water. It is difficult to determine the source of this pollution. Runoff may travel over many plots of lands including farms, golf courses, home gardens, etc. So there may even be multiple sources.