Negative Impact of human activity on the environment (Sec. E 5.1 of CXC 2004 Syllabus)

1. Pollution by agriculture: fertilizers used in agricultural lands (e.g. rice fields) can run off in water ways. The fertilizers are usually high in phosphorous and nitrogen, that helps in growth of photosynthetic organisms. There, in the waterways, they cause algal bloom through a process called eutrification (a process where water bodies receive excess nutrients that stimulate excessive plant growth). The excess growth of algae may use up most of the dissolved oxygen in the water, harming other aquatic plants or animals living in the water. Since the algae grow mostly on the surface of the water, they would block sunlight from reaching the bottom of the pond/lake. Also, they may block drainage, or slow the rate that the water drains, causing floods. Pesticides may also runoff from farmlands and have detrimental effects on aquatic organisms. If humans consume the organisms (e.g. fish), then humans would be affected.

Image above is showing algal bloom

Source: http://en.wikipedia.org/wiki/Algal_bloom

2. Products of industries: Some industrial processes results in the release of harmful pollutants into the environment e.g. in gold mining, mercury is used. If mercury goes into water ways, aquatic animals will be affected, and humans depending on the aquatic resources. Some industries release heated water in rivers, harming organism there.

3. Improper sewage and garbage disposal: Raw, untreated sewage, released in rivers, or oceans would have similar effects to Eutrification. In addition, this would lead to increase incidence of pathogenic organisms to humans. Improper garbage disposal mainly lead to flooding. Plastic bottle especially ends up in drainage canals. During rainy seasons, the garbage blocks these drainages, increasing the impact of the flood. 

 (Image take from: http://jamaica-gleaner.com/gleaner/20121009/lead/lead4.html)