Sulphur smell usually indicates the presence of anaerobic bacteria. Is the water stagnent?
Yes and how is that bacteria created????????
or is it because the soil goes spoiled.
They are always in the soil, especially soil that doesn't get enough air and has a lot of moisture. But even in healthy, well aireated soil anaerobic bacteria are still present. If the condition of the water and or soil gets to the point where the anaerobic bacteria are favored over aerobic bacteria, then they will flourish and it will start to smell. Note, they do not need soil to grow, dry soil will have none. They only need nutrient rich water with low oxygen levels. Anaerobic means 'without air', aerobic means 'with air'. While most anaerobic bacteria are killed by oxygen, which is to them a deadly poison, some can survive low levels of oxygen. Since in most cases oxygen levels are not the same throughout water or soil, a few of them manage to eke out an existance almost anywhere. They even survive in low oxygen areas of our bodies, such as the mouth, digestive tract, and other moist cavities not involved with the respitory system.
The interesting thing about anaerobic respiration is that in some cases tissues that normally use aerobic respiration can switch to anaerobic respiration. Muscle tissue, for instance. When involved in strenuous activity that requires short but intense bursts of energy or long extended output, muscle cells switch to anaerobic respiration because there's simply not enough oxygen coming to them over the short term. The more you exercise the more efficient the oxygen delivery system becomes, and the longer before the muscle cells have to switch to anaerobic respiration. That's how you get sore muscles, the buildup of lactic acid through anaerobic respiration. Now that I've gotten totally off subject, here's a statement from a FAQ on water pollution:
Why does water sometimes smell like rotten eggs?
When water is enriched with nutrients, eventually anaerobic bacteria, which do not need oxygen to practice their functions, will become highly active. These bacteria produce certain gasses during their activities. One of these gases is hydrogen sulphide. This compounds smells like rotten eggs. When water smells like rotten eggs we can conclude that there is hydrogen present, due to a shortage of oxygen in the specific water.