Dissolved gases in water
Definition:
Dissolved gas is a situation which occurs when a gas becomes dissolved in another solute. People often think specifically of a gas-liquid mixture when they think of dissolved gases, but gases can also dissolve in other gases, as in the case of air. Gases may also be dissolved in solids, as in seen in the trace impurities which can be caused in metals when gases are introduced during the casting process.
In order for gas to dissolve in a liquid, it usually needs to be subjected to pressure and held under pressure to remain dissolved. If the pressure is reduced, the gas will start to precipitate out of the liquid. With solids, dissolved gases occur when there is pressure which traps the gas inside, and sometimes the gases precipitate out and form bubbles inside the solid; bread, for example, contains numerous bubbles formed by carbon dioxide released by yeast inside the bread.
Solubility of pure gases like
1. Ar – Argon 6. H2 - Hydrogen Gas
2. He – Helium 7. CH4 – Methane
3. N2 – Nitrogen 8. CO - Carbon Monoxide 4. NH3 – Ammonia 9. CO2 - Carbon Dioxide
5. O2 – Oxygen 10. Cl2 - Chlorine Gas
in water at one atmosphere (101.325 kPa) and different temperatures are indicated in the diagrams below.
Solubility of Argon - Ar - in Water
Solubility of Methane - CH 4 - in Water
Solubility of Carbon Monoxide - CO - in Water
Solubility of Carbon Dioxide - CO 2 - in Water
Solubility of Chlorine Gas - Cl 2 - in Water
Solubility of Hydrogen Gas - H 2 - in Water
Solubility of Helium - He - in Water
Solubility of Nitrogen - N 2 - in Water
Solubility of Ammonia - NH 3 - in Water
Solubility of Oxygen - O 2 - in Water
Dissolved Oxygen:
The dissolved oxygen (DO) is oxygen that is dissolved in water. How does O2 dissolve in water? The oxygen has to cross the air/water interface- often a slow process. Just how slow depends on whether the water is still or running, how much surface is in contact with the air, what's dissolved in the water, and whether any films (like soap, or broken bacteria or algae cell walls) are floating on the surface. Once the oxygen crosses the surface, it is caged by water molecules. The oxygen dissolves by diffusion from the surrounding air; aeration of water that has tumbled over falls and rapids; and as a waste product of photosynthesis. A simplified formula is given below:
Photosynthesis (in the presence of light and chlorophyll):
Carbon
dioxide + Wate
r ---> Oxygen + Carbon-rich foods
CO2 H2O O2 C6H12O6
Fish and aquatic animals cannot split oxygen from water (H2O) or other oxygen- containing compounds. Only green plants and some bacteria can do that through photosynthesis and similar processes. Virtually all the oxygen we breathe is manufactured by green plants. A total of three-fourths of the earth’s oxygen supply is produced by phytoplankton in the oceans.
The temperature effect:
If water is too warm, there may not be enough oxygen in it. When there are too many bacteria or aquatic animal in the area, they may overpopulate, using DO in great amounts.
Oxygen levels also can be reduced through over fertilization of water plants by run-off from farm fields containing phosphates and nitrates (the ingredients in fertilizers). Under these conditions, the numbers and size of water plants increase. Then, if the weather becomes cloudy for several days, respiring plants will use much of the available DO. When these plants die, they become food for bacteria, which in turn multiply and use large amounts of oxygen and this depleting all the oxygen.
How much DO an aquatic organism needs depends upon its species, its physical state, water temperature, pollutants present, and more. Consequently, it’s impossible to accurately predict minimum DO levels for specific fish and aquatic animals.
Environmental Impact:
Total dissolved gas concentrations in water should not exceed 110 percent.
Concentrations above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from "gas bubble disease";
however, this is a very rare occurrence. The bubbles or emboli block the flow of blood through blood vessels causing death. External bubbles (emphysema) can also occur and be seen on fins, on skin and on other tissue. Aquatic invertebrates are also affected by gas bubble disease but at levels higher than those lethal to fish.
How Dissolved Oxygen Affects Water Supplies:
A high DO level in a community water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes.
For this reason, industries use water with the least possible amount of dissolved oxygen
DO levels fluctuate seasonally and over a 24-hour period. They vary with water temperature and altitude. Cold water holds more oxygen than warm water and water holds less oxygen at higher altitudes. Thermal discharges, such as water used to cool machinery in a manufacturing plant or a power plant, raise the temperature of water and lower its oxygen content. Aquatic animals are most vulnerable to lowered DO levels in the early morning on hot summer days when stream flows are low, water temperatures are high, and aquatic plants have not been producing oxygen since sunset.
Biological Oxygen Demand (BOD):
Usually the BOD test measures the amount of oxygen consumed during 5 days in the dark in a sealed bottle with no air space at 20 °C.
The oxygen is typically consumed mostly by microorganisms as they decompose carbonaceous organic matter. However, some of the BOD may be due to microorganisms that can oxidize ammonium to nitrate to generate energy and some comes from the chemical oxidation of inorganic matter. Additional tests can separate these sources.
Nitrogen dissolved in water:
Nitrogen is a biological inert gas and the problem of excess N2 in water is that of super saturations. Often super saturations exist in waters pumped up from subsoil water and also in cases where rapid warring takes place. If air saturation increases over 110%, this super saturation can cause problems in several fishes, by causing “gas embolism” or the gas bubble disease. It is important to recognize that with increase in depth the contents of dissolved gases is increase and this has implications in using water pumped up from depths for aquaculture purposes.
Atmospheric nitrogen is fixed by heterocyst bearing blue green algae (Nostoc, Anabaena) and possibly also by other blue greens
Nitrogen (N) and water
Seawater contains approximately 0.5 ppm nitrogen (dissolved inorganic nitrogen compounds without N2). The amount is clearly lower at the surface, being
approximately 0.1 ppb. River water concentrations vary strongly, but are approximately 0.25 ppm in general.
Depending on water properties, various inorganic nitrogen compounds may be found. In aerobic waters nitrogen is mainly present as N2 and NO3-, and
depending on environmental conditions it may also occur as N2O, NH3, NH4+, HNO2, NO2- or HNO3.
Water in coastal areas mainly contains elementary nitrogen gas (N2). This can be no surprise, as air consists of 78% nitrogen, and water comes in contact with air regularly in coastal regions because of low water depth and active currents.
Does The Presence Of Nitrates Affect Water Quality?
Unlike temperature and dissolved oxygen, the presence of normal levels of nitrates usually does not have a direct effect on aquatic insects or fish. However, excess levels of nitrates in water can create conditions that make it difficult for aquatic insects or fish to survive.
Algae and other plants use nitrates as a source of food. If algae have an unlimited source of nitrates, their growth is unchecked. So, why is that a problem?
A bay or estuary that has the milky colour of pea soup is showing the result of high concentrations of algae. Large amounts of algae can cause extreme
fluctuations in dissolved oxygen. Photosynthesis by algae and other plants can generate oxygen during the day. However, at night, dissolved oxygen may decrease to very low levels as a result of large numbers of oxygen consuming bacteria feeding on dead or decaying algae and other plants.
Eutrophication:
Eutrophication “is the process by which a body of water acquires a high
concentration of nutrients, especially phosphates and nitrates”. These typically promote excessive growth of algae. As the algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen, causing the death of other organisms, such as fish.
Anoxia – Anoxic Event:
Anoxia is a lack of oxygen caused by excessive nutrients in waterways which triggers algae growth. When the plants die and decay, oxygen is stripped from the water, which then turns green or milky white and gives off a strong rotten egg odour. The lack of oxygen is often deadly for invertebrates, fish and shellfish.
How Do Nitrates Affect Human Health?
Nitrate concentrations are monitored in municipal water supplies and foods to prevent exposing people to the potential harmful effects of high levels of nitrates.
Nitrates are highly soluble, meaning that they easily dissolve in water. For many people in rural areas, the primary source of drinking water is well water, which may be contaminated with nitrates. Nitrates are colourless and odourless, so their presence cannot be determined without the use of special testing
equipment.
Nitrates can interfere with the ability of our red blood cells to carry oxygen.
Infants are more at risk of nitrate poisoning than older children or adults. Babies can turn “blue” when there is not enough oxygen being transported by their blood. This “blue baby syndrome” (technically known as methemoglobinemia) is a serious condition that can cause brain damage or death.
How Do Nitrates Affect The Health Of Aquatic Animals
? Fish and aquatic insects can be affected indirectly by increased nitrateconcentrations in the water.
Basically, any excess nitrate in the water is a source of fertilizer for aquatic plants and algae. In many cases, the amount of nitrate in the water is what limits how much plants and algae can grow. If there is an excess level of nitrates, plants and algae will grow excessively.
Excess plants in a body of water can create many problems. An excess in the growth of plants and algae create an unstable amount of dissolved oxygen.
During the day, there will be usually be high levels of dissolved oxygen, and at night the levels of oxygen can decrease dramatically.
This will create stressful conditions for fish. If they are stressed for a significant part of the day, they will not behave normally or reproduce. If the conditions persist for a long period of time, the stressed fish species may choose to leave that area or die off.
Excess plants and algae will also create conditions where organic matter accumulates. High densities of algae will create a condition where sunlight cannot reach very far into the water. Since plants and algae require some sunlight, plants and algae not receiving sunlight will die off. These dead plant materials will settle to the bottom of the water and bacteria that feed on decaying organic material will greatly increase in numbers. These bacteria will consume oxygen and, therefore, the level of dissolved oxygen in this water will fall to levels that are too low for many aquatic insects and fish to survive. Also, this can cause extreme changes in habitat.
