Question

What is the problem associated with contamination of the waterways by water run off of agricultural fertilizers?

Answer 1

Nitrates (of most cations) are exceptionally soluble; and thus nitrates from agricultural runoff tend to accumulate in waterways.

Explanation:

But why should this be a problem? Well, a high concentration of nitrates in the waterways stimulates the growth of algae, which necessarily depletes the amount of oxygen gas available in the water. During the day, when the algae photosynthesizes, there is enough oxygen in the water to sustain micro and larger fauna.

However, during the night, photosynthesis stops (why?), but the algae still respire and have the same oxygen demand. So-called algal blooms can deplete nightly oxygen concentrations to such a level that other fauna in the waterways asphyxiate.

This phenomenon is known as `"eutrophication"` and is the leading cause of the impairment of many freshwater, and marine ecosystems world-wide.

See here for more details:

http://www.nature.com/scitable/knowledge/library/eutrophication-causes-consequences-and-controls-in-aquatic-102364466