Water pollution happens when chemicals or dangerous foreign substances are introduced to water, including chemicals, sewage, pesticides and fertilizers from agricultural runoff, or metals like lead or mercury. According to the EPA, 44 percent of assessed stream miles, 64 percent of lakes and 30 percent of bay and estuarine areas are not clean enough for fishing and swimming. The EPA also states that the most common contaminants in Canada are bacteria, mercury, phosphorus and nitrogen. These come from the most common sources of contaminants, which include agricultural runoffs, air depositions,and water diversions of streams.