Acidity and alkalinity analyses are quantitatively and qualitatively related (Figure 1). These analyses are used in determining water quality, especially in areas of mining. However, other areas in which the earth has been disturbed (e.g., construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. Where these activities are known or suspected to be having an impact, it is customary to analyze site water for alkalinity—Standard Methods 2320 B (Alkalinity), Acidity—Standard Methods 2310 B (Acidity), and metal content using U.S. EPA Method 6010 (EPA 6010).
Figure 1 - Relationship of alkalinity versus acidity. (Data from Ore Knob and Barite Hills/Nevada Goldfield Superfund mine sites.)
U.S. EPA Region IV Science and Ecosystem Support Division (SESD) has determined the practical quantitation limit (PQL) for the acidity analysis to be 10.0 mg CaCO3/L, whereas the alkalinity results have a PQL of 1.0 mg CaCO3/L. This value establishes the minimum reporting limit (MRL) for these analyses. Analyzed samples with detections at or above these limits are reported with the value attained by the analysis. Samples below the PQL/MRL are reported as not detected at or above this limit.
Evaluation of SESD data
A study of SESD data, with respect to referenced research documenting the relationships between net alkalinity (defined in the literature, as explained in Eq. [1]), measured acidity (defined in the literature as net acidity), total acidity, pH, and the concentration of the metal species most likely to influence acidity results, indicated the availability of a screening technique to improve laboratory efficiency. Using alkalinity data with results of an ICP metals scan, both being more sensitive and requiring approximately 75% less time than the acidity analysis, reduces workload while continuing to provide quality results for both acidity and alkalinity.
The research shows that the following mathematical relationships have been established:
The effectiveness of this screening tool is best represented by the data included in Figure 2. It contains 88 separate data points collected and analyzed for acidity, alkalinity, and, in most cases, metals. Reportable values for acidity were obtained for 10 samples (11.4%); alkalinity analysis resulted in 82 reportable results (93.2%). Screening the samples as described below results in the same reportable data but eliminates most of the analysis with nonreportable acidity results.
Figure 2 - Analyzed versus theoretical acidity. (Data from Ore Knob and Barite Hills/Nevada Goldfield Superfund mine sites.)
Figure 3 demonstrates the efficacy of the calculated values for estimating the reportable acidity in extremely polluted samples. Since these samples had initial pH ranging from 2.33 to 3.83, alkalinity could not be determined because the pH was below the titration endpoint. Iron (432–7352 ppm), manganese (0.5–37 ppm), and aluminum (130–1897 ppm) concentrations resulted in acidity values that could not be obtained using standard electrometric methodology because the endpoint could not be reached with 0.02N or 0.1N NaOH. Instead, these samples were titrated with 1N NaOH; however, the calculated values are substantially equivalent to the measured acidity results.
Figure 3 - Highly acidic samples. (Data from Ore Knob and Barite Hills/Nevada Goldfield Superfund mine sites.)
Sample screening procedure
The screening process begins by performing the alkalinity analysis. This analysis requires identification of the initial pH, followed by titration of the samples with normalized H2SO4 to an endpoint of pH 4.5 for normal samples and pH 4.2 for low-level samples. When the resulting alkalinity is ≤10 mg CaCO3/L, regardless of initial pH, the sample is also analyzed for acidity by SM 2310 B. In the examples given in Figure 2, 34 of 88 samples (38.6%) would have been treated accordingly. As discussed in the literature, samples with an alkalinity >10 mg CaCO3/L and where the initial pH is 6 > pH > 7 can safely be determined to have no possibility for a reportable acidity value >10 mg CaCO3/L. This eliminates 26 samples (29.5%) from SM 2310 B analysis.
When a sample has an alkalinity value >10 mg CaCO3/L and is in the range of 6 ≤ pH ≤ 7, an aliquot is acidified with 1.5% HNO3 and 1.5% HCl and stored overnight. The following day, these samples are scanned for elemental constituents using ICP spectrometry method EPA 6010. Here, 28 of the samples (31.8%) would have been scanned.
Eq. (4) is used to combine the results from the scan with results from the alkalinity analysis to determine the expected measured (net) acidity. If the results of this calculation are ≥ –10 mg CaCO3/L, acidity analysis should be conducted according to SM 2310 B. Two samples (2.3%) fell into this category. All other cases (i.e., the other 26 samples) can safely be determined to have no possibility for a reportable acidity value.
This screening procedure has been implemented at U.S. EPA Region IV SESD laboratories and has reduced person-hours spent on acidity analysis by 46%, while continuing to provide the reliable, high-quality data necessary for Superfund project management.
References
- vanLoon, G.W.; Duffy, S.J. Environmental Chemistry: A Global Perspective. Oxford University Press: New York, 2005; pp 246–52.
- American Public Health Association (APHA), 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Assoc., Washington, DC.
- Kirby, C.S.; Cravotta III, C.A. Net alkalinity and net acidity 1: theoretical considerations. Appl. Geochem. 2005, 20, 1920–40.
- Kirby, C.S.; Cravotta III, C.A. Net alkalinity and net acidity 2: practical considerations. Appl. Geochem. 2005, 20, 1941–64.
Curtis Callahan, BS Environmental Chemistry and ACS Certified BS General Chemistry, is a Chemist with U.S. EPA Region IV, 980 College Station Rd., Athens, GA 30605, U.S.A.; tel.: 706-355-8806; e-mail: [email protected].