Total Alkalinity (TALK)

Status and Trends

Alkalinity is a measure of the buffering capacity of water, or the capacity of bases to neutralize acids. Measuring alkalinity is important in determining a stream's ability to neutralize acidic pollution from acid mine drainage, rainfall or wastewater. Alkalinity refers to the ability of water to resist changes in pH. These buffering materials are primarily bicarbonate (HCO₃), and carbonate (CO₃), and occasionally hydroxide (OH^-), borates, silicates, phosphates, ammonium, sulfides, and organic ligands.

Waters with low alkalinity are very susceptible to changes in pH. Waters with high alkalinity are able to resist major shifts in pH. As increasing amounts of acid are added to a water body, the pH of the water decreases, and the buffering capacity of the water is consumed. If natural buffering materials are present, pH will drop slowly to around 6; then a rapid pH drop occurs as the bicarbonate buffering capacity (CO₃ and HCO₃) is used up. At pH 5.5, only very weak buffering ability remains, and the pH drops further with additional acid. A solution having a pH below 4.5 contains no alkalinity, because there are no CO₃ and HCO ions left.

Alkalinity not only helps regulate the pH of a water body, but also the metal content. Bicarbonate and carbonate ions in water can remove toxic metals (such as lead, arsenic, and cadmium) by precipitating the metals out of solution.

Median TALK ranged from 5.5 mg/L at Cherry Creek to 198 mg/L at Antietam Creek near Burnside Bridge. Median total alkalinity concentration was in the highest one-third of the benchmark data set at 39% of all stations. Stations with the highest concentrations were located in the Upper Potomac, Middle Potomac and Patapsco/Back River Basins. High alkalinity may be related to the carbonate geology as occurs in the valleys of the Upper Potomac Basin or limestone in urban concrete infrastructure. Stations with lowest median concentrations occurred at 18% of stations and were located in the western most portion of the Upper Potomac Basin, Ohio River drainage, the Upper Patuxent Basin and the Choptank. Intermediate TALK concentrations occurred at 42% of all stations.

TALK increased at 61% of all stations. There were no decreasing trends

TALK correlated positively with COND at 74% of stations. Alkalinity is a measure of the bicarbonate buffering capacity. The bicarbonate ion is associated with calcium and magnesium both of which contribute to specific conductance or COND. TALK correlated positively with pH at 42% of stations, principally the stations above Hancock, MD. TALK was inversely correlated with CHLA concentration at 30% of the stations below Hancock, MD. This relationship ma be spurious as the effect of increased photosynthesis would be expected to increase alkalinity by removing CO2 from solution thereby causing the two parameters to act in concert not inversely as they do here.

Total Alkalinity (TALK) data for sampled locations are summarized in these plots.

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This page was updated on 12/5/2005