We know water has loads of unique traits that make it a critical substance for the survival of life on Earth. However, when people put mass quantities of pollutants into the environment, water’s ability to dissolve these pollutants can actually make the damage to living organisms and the environment worse.
Wet acid deposition, such as acid rain, snow, sleet, or fog, is a good example of this problem. Acid precipitation forms when sulfur dioxide, SO2, and nitrogen oxides, NO and NO2, in the air dissolve in water and form acids. How do they get into the air in the first place? Sulfur naturally occurs in small amounts in fossil fuels, and when they are burned, it combines with oxygen in the air to form SO2. SO2 then undergoes some additional chemical reactions, forming SO3, which dissolves in water and forms sulfuric acid, H2SO4. Youch.
As for nitrogen oxides, nitrogen constitutes 78% of the gas in the atmosphere. During the process of combustion, like the burning of fossil fuels, nitrogen in the air combines with oxygen to form either NO or NO2. These molecules dissolve in water to form nitric and nitrous acids, HNO3 and HNO2, respectively. The result is that water droplets in the air are acidified, and when they fall to the ground as precipitation, they cause streams, lakes, marshes, and soils to become acidified as well. Sounds like bad news, and it is.
Most surface—as opposed to underground—bodies of water normally have a pH of between 6 and 8, which is pretty close to neutral. With acid deposition, however, the pH can decrease to much lower values. Little Echo Pond in Franklin, New York, has a pH of 4.2.6 Yikes! Low pH can adversely affect the development, health, and survival of fish and other aquatic organisms. Acid deposition can cause heavy metals, like aluminum, to leach out of soil and into surface water, which is an additional source of toxins for the organisms living there. Nutrients can also be lost, which can make it more difficult for plants to grow. Finally, acid precipitation can directly damage the leaves and roots of plants, making them weak and sickly.6
Coral reef decalcification is another example of how pollution and water’s special properties interact with each other. The amount of carbon dioxide, or CO2, in the atmosphere is in equilibrium with the amount in the water. As the CO2 rises above pre-industrial levels, the oceans are pressured into dissolving more and more of it. When CO2 dissolves in water, it forms carbonic acid (H2CO3), which decreases the pH of the water and acidifies it. Keep this in mind; we will refer back to this tidbit in one second.
Coral skeletons are made of a substance called calcium carbonate (CaCO3). Calcium carbonate is also a key component of seashells and eggshells. (She sells calcium carbonate by the seashore doesn't have the same ring to it, does it?) In order to make this substance, corals use calcium ions (Ca2+) and bicarbonate ions (HCO3-), both of which are floating around in the ocean water. The problem is that bicarbonate ions buffer carbonic acid, so when water becomes more acidic due to increased levels of CO2, more bicarbonate is needed to buffer the decrease in pH. That process leaves less for the corals to use, and in fact, bicarbonate is even stripped out of the corals so that calcium is released back into the water.7
Taking calcium away from corals means that they will not be as healthy and will not grow. Coral reefs are some of the most diverse ecosystems on the planet, and losing them would be a major blow to biodiversity, or the diversity of life. In addition, reefs help protect shorelines from violent storms. If reefs were to suffer substantial damage, it could have significant economic implications for humans.
We mentioned before that water’s ability to dissolve molecules is one of its special properties and one of the qualities that makes it critically important to life. In this case, the fact that it’s such a good solvent—of sulfur dioxide, nitrogen oxides, and carbon dioxide—leads to the formation of acid deposition and decalcification of corals. Too bad water can’t be a tad more selective in what it fraternizes with!