Of these two forms, nitrate is usually the most important. Nitrate is an essential nutrient for growth of algae and other aquatic plants, and can be present at high levels due to a variety of sources. Nitrate is very difficult to measure directly. A common procedure is to first measure the level of nitrite and then reduce the nitrate to nitrite and measure the combined nitrite concentration. Subtracting the original nitrite level from the combined nitrite concentration will give you the nitrate level.
Some chemical indicators are specific to particular forms of pollution.
For instance, low dissolved oxygen often results from either the presence of raw sewage or acid mine drainage. Water density is another physical measurement that indirectly indicates the presence of chemicals. The salinity of a body of water is one of the main factors determining what organisms will be found there. All rights reserved.