The Water Quality Association defines alkalinity as "the quantitative capacity of water to neutralize an acid." In other words, it's a measurement of how much acid can be added to water without changing its pH.
Alkalinity in water is usually made up of bicarbonate (HC03), carbonate (CO3) and/or Hydroxide (OH), but phosphates and silicates can also play a role.
The related term, "total alkalinity," frequently called TA, is defined as "the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]."
Although alkalinity is related to pH, it isn't the same thing. High levels of alkalinity stabilize the pH, but water does not have to have a high pH to have a high level of alkalinity. As alkalinity is the capacity of water to neutralize its acidic content, alkalinity therefore measures how much acid can be added to a water body without changing the pH level significantly
Alkalinity is not regulated as a water contaminant, but it is often tested since it is a factor in amending and controlling pH.
Treatment of Alkalinity: In most cases, there is no reason to want to alter the water's alkalinity itself, but changing alkalinity is often involved in changing the pH. Neutralizing treatments with such items as Soda Ash or limestone (calcite) usually raise the alkalinity level as well as the pH.