\u00a9 2020 wikiHow, Inc. All rights reserved. The second aspect of water pollution involves cleaning the waters that are already contaminated. When this process occurs in nature, it is called natural attenuation, and generally occurs at a slow rate. April 2001. The contaminants are captured above ground, where they can be safely treated and disposed of. December 2001. References June 2006. wikiHow, Inc. is the copyright holder of this image under U.S. and international copyright laws. Retention Time of Water in Various Water Bodies;Environment Canada and http://en.wikipedia.org/wiki/Lake_retention_time. This image may not be used by other entities without the express written consent of wikiHow, Inc.