What is the difference between hardness and alkalinity?
Alkalinity (the concentration of carbonates and bicarbonates in water) is a measure of the buffering capacity of water to resist change in pH. If the buffer is good, either acid or a base in the water will be taken up, and the water will be neutralized, by the carbonates and bicarbonates. A common source of the carbonates and bicarbonates in Alabama surface water is limestone (calcium carbonate). When limestone dissolves in water, carbonate and bicarbonate ions are released. These ions can take up free hydrogen ions (contributors to acidity) in low pH environments, or release hydrogen ions in high pH environments, keeping things more stable for aquatic life.
A simple way to think of this is that alkalinity is like the “Rolaid of the stream.” If we get acid indigestion from spicy foods, a Rolaid can “absorb” acid (or base) and get our digestive system back to normal. In Alabama, alkalinity can be 100-200+ mg/L in the northern part of the state, where limestone outcrops are common, and it can be 0-20 mg/L in the coastal plain, where limestone is scarce and coastal wetlands can contribute enough organic acids to completely use up the buffer system.
Hardness comes from the other part of the calcium carbonate molecule…the calcium. Some rocks have magnesium carbonate, and the magnesium and calcium ions that are released when these rocks dissolve typically make up 90%+ of water hardness in Alabama surface waters. Hard water makes it difficult to get suds from soap, and people in hard-water areas (again, especially in north Alabama) often install water softeners to trap some of the calcium carbonate before it forms a white mineral scale in tea pots, pipes, water heaters, etc.
It’s easy to get confused about the difference between alkalinity and hardness because, in Alabama, they typically come from the same source rock. However, there are other parts of the country where there is a lot of sodium carbonate that results in soft, yet alkaline water; or where there is calcium sulfate that results in hard, yet poorly buffered water. It all goes back to the geology of the area that strongly influences natural water quality. Then, people get involved and what they do to their land and what they dump down the drain has other influences on water that are commonly called pollution.