USGS Home
Contact USGS
Search USGS

Reporting Alkalinity and ANC

The U.S. Geological Survey (USGS) recommends that alkalinity and acid neutralizing capacity (ANC) be reported in milliequivalents per liter (or microequivalents per liter), if possible. Alternatively, report alkalinity and ANC in milligrams per liter as calcium carbonate.

USGS policy states that reported values of alkalinity and ANC should include all digits known with certainty, plus one digit that is uncertain. That last digit is called the Least Significant Digit (LSD). The same USGS policy also recommends that the analytical results be stored in the USGS database with one additional uncertain digit (LSD+1) because the extra digit may contain useful information when large datasets are evaluated. Published data, however, should be reported only to the LSD.

Using the typical levels of uncertainty associated with alkalinity and ANC titrations, and when following the methods recommended by the USGS National Field Manual (section 6.6.4), the following reporting guidance has been developed. If reporting alkalinity, ANC, carbonate, bicarbonate, and hydroxide in milligrams per liter:

• Report values less than 1,000 mg/L and greater than or equal to 100 mg/L to whole numbers.
• For values greater than or equal to 1,000 mg/L, report up to three significant figures.
• For concentrations less than 100 mg/L and greater than zero, report results to the nearest tenth.
• Do not report any carbonate, bicarbonate, or hydroxide concentration as 0.0 mg/L; instead, report such values as less than 0.1 mg/L.

Hydroxide, carbonate, and bicarbonate concentrations cannot be negative. Alkalinity and ANC, however, may be negative because they are a measure of the sample's H⁺ deficiency relative to a solution of carbon dioxide in water. A negative alkalinity or ANC simply indicates the presence of some amount of mineral acidity. Report negative values of alkalinity and ANC with the same number of significant figures you would use if the values were positive.

Report the average value for duplicate samples or the median when more than two replicate samples are used for quality control and the value falls within the appropriate quality-assurance criterion for variability.

Parameter Codes

For USGS database users, use the parameter and method codes in the tables below to assign values to alkalinity, ANC, carbonate, bicarbonate, and hydroxide. For more information, consult the USGS parameter and method code dictionaries.

Method and Parameter Codes for Alkalinity and ANC

Method codes have been created for field-derived alkalinity and ANC values to record the type of titrator used in the titration. These method codes also record the type of titration method (inflection point, Gran, or fixed endpoint). Use the following method codes for alkalinity and ANC:

Method and Parameter Codes for CO₃^2-, HCO₃^-, and OH^-

USGS method codes have been created to make a distinction between the available speciation methods for calculating carbonate and bicarbonate concentrations. They are:

• Simple Mass-Balance Speciation Method [deprecated]: hydroxide is not considered and speciation is determined from the titration equivalence points--this method is only suitable for samples with pH < 9.2 and is no longer used by USGS or the Alkalinity Calculator; see Office of Water Quality Technical Memorandum 2012.05 for more information.
• Advanced Speciation Method: speciation is calculated from sample pH and alkalinity or acid neutralizing capacity, using acid dissociation constants

Choose parameter and method codes from the following tables. Note that each table is for a different titration method.

Home | Version History | Methods | Speciation Criteria
Acid Lot Info | FAQ | Download