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.
Are high pH readings and hi dissolved oxygen readings associated with the algae bloom seen on the Little Tallapoosa Arm of Lake Wedowee (upper lake)?
Over the past week we’ve seen a pretty good size algae bloom of “worms” and when the wind or waves get going it just breaks up into a yucky green translucency. We’ve had the algae in the past and it was no big deal, but this year we are getting very high PH readings — around 9 to 9.5 — around the algae. Jack Duncan says he doesn’t remember algae and high PH numbers together, but we’re not absolutely sure chemical testing was done when the algae was at its peak. Is the high PH “normal” with the algae? Is it the algae or maybe the nutrients feeding the algae that raise the PH? Or should we be looking for something different? The DO numbers were up in the 11 and 12 range at 22 centigrade water temp.)
The algal ‘noodles’ sound the same as what y’all saw on the Little Tallapoosa a year ago. Seems to me that last year’s bloom was the first time that a serious bloom was seen. We’d love to see a live sample if someone could scoop up some noodles, put them in a Ziplock freezer bag (heavy duty) and Fedex them to AWW.
The DO and pH shifts go hand-in-hand with algae blooms. We see these responses all the time in enriched catfish production ponds. When nutrients increase in non-flowing waters (ponds, lakes), algae populations increase greatly, the water becomes murky-green (depending on the species – may have floating mats or other forms), DO rises above ‘normal’ (pre-bloom) levels in the sunny afternoons (see attached equation), and afternoon pH levels increase – see equation again: note that CO2 dissolved in the water is converted to organic carbon (algae) and since dissolved CO2 is acidic (carbonic acid) when you effectively reduce its concentration, you make the water more basic (this is reversed at night, when only respiration occurs).