Question

1. What are the important sources and sinks of DO in natural waters?

2. What is the difference between DO concentration and oxygen solubility?

3. What is the approximate range of oxygen solubility in fresh water in equilibrium with atmospheric O2 at 10-25°C? What is the trend of oxygen solubility when the water temperature increases?

Answer 1

Sources and sinks of natural water are water cycle, river and lakes, groundwater and harvested rainwater.

Dissolved oxygen level decreases as the water temperature increases. DO is the presence of free oxygen molecules in the water.The bonded oxygen molecule in water is in compound and does not count towards dissolved oxygen. Dissolved oxygen comes from the air or as a plant byproduct. Dissolved oxygen is also waste product of photosynthesis from phytoplankton, algae, and other aquatic plants.

The actual amount of dissolved oxygen depends on the water temperature, pressure and salinity. As the temperature increases, dissolved oxygen level decreases. Also when there is more saline water then the dissolved oxygen level again decreases. But when the pressure increases dissolved oxygen level also increases.

So the dissolved oxygen level is maximum at the bottom of the sea, because there temperature is less and pressure is more, so the level increases.

DO concentration is the concentration of dissolved oxygen in water in free state. Generally dissolved oxygen is denoted in terms of mg/L or as percent of air saturation.

100% air saturation = 8.915 mg/l or ppm of oxygen

Oxygen solublity is oxygen present in atmosphere. The air contains 20.9% of oxygen. DO concentration in water is comparitively less. The oxygen present in water depends upon the temperature. DO is used by fish to breathe.

In freshwater salinity is taken to be 0. The oxygen solublity in freshwater at 10°C is 11.3mg/l at 760mm Hg pressure.

At 15 °C it decreses to 10.1 mg/l, at 20 °C it is 9.1 mg/l and at 25 °C it finally decreases to 8.3 mg/l.

So as we can see as the temperature is increasing oxygen solublity is decreasing.