Acidity and Alkalinity in Mine Drainage: Theoretical Considerations

Abstract

Acidity, net acidity, and net alkalinity are widely used parameters for the characterization of mine drainage, but these terms are not well defined and are often misunderstood. Incorrect interpretation of acidity, alkalinity, and derivative terms can lead to inadequate treatment design or poor regulatory decisions. We briefly explain derivations of theoretical expressions of three types of alkalinities (caustic, phenolphthalein, and total) and acidities (mineral, CO 2 , and total). Theoretically defined total alkalinity is closely analogous to measured alkalinity and presents few practical interpretation problems. Theoretically defined "CO 2 -acidity" is closely related to most standard titration methods used for mine drainage with an endpoint pH of 8.3, but it presents numerous interpretation problems, and it is unfortunately named because CO 2 is intentionally driven off during titration of mine-drainage samples. Using the proton condition/massaction approach and employing graphs for visualization, we explore the concept of principal components and how to assign acidity contributions to solution species, including aqueous complexes, commonly found in mine drainage. We define a comprehensive theoretical definition of acidity in mine drainage on the basis of aqueous speciation at the sample pH and the capacity of these species to undergo hydrolysis to pH 8. We demonstrate that "net alkalinity" is a valid mathematical construction based on theoretical definitions of alkalinity and acidity. We demonstrate that, for most mine-drainage solutions, a useful net alkalinity value can be derived from: 1) alkalinity and acidity values based on aqueous speciation, 2) measured alkalinitycomputed acidity, or 3) taking the negative of the value obtained in a standard method "hot peroxide" acidity titration, provided that labs report negative values. We recommend the third approach; i.e., Net alkalinity = -Hot Acidity.Additional Key Words: calculated and measured acidity.

Effects of abandoned coal-mine drainage on streamflow and water quality in the Shamokin Creek Basin, Northumberland and Columbia Counties, Pennsylvania, 1999-2001

“…Before the basin-wide synoptic monitoring was initiated, precise locations of the AMD sources and previously sampled stream sites were determined. Published data on the locations of AMD and stream monitoring sites as reported by Gannett Fleming Corddry and Carpenter, Inc. (1972), Growitz and others (1985), Reed and others (1987), and Wood (1996) and unpublished data from SCRA (Kirby, 1995(Kirby, , 1998 were reviewed, compiled, and mapped. Then, during May and June 1999, an attempt was made to access each AMD site and document its location by use of a global positioning system (GPS).…”

“…Major changes have occurred in the flow and quality of the mine discharges and in the quality of Shamokin Creek in the decades since the Operation Scarlift report was completed in 1972 (Reed and others, 1987;Wood, 1996;Kirby, 1995Kirby, , 1998. Furthermore, passive-treatment systems that recently have been developed could be lower cost alternatives to remediate AMD compared to active treatment methods.…”

“…Although acidity was measured on refrigerated, tightly sealed samples by hot peroxide titration for the high base-flow samples (Greenberg and others, 1992) and by cold peroxide titration for the low base-flow samples, the interpretation of these measured acidities for AMD can be debated (Kirby, 2002;Kirby and Cravotta, 2004;Cravotta and Kirby, 2004). Therefore, acidity for all the samples was computed from pH and dissolved metals concentrations in milligrams per liter as Acidity computed (mg/L CaCO 3 ) = 50 .…”

“…Although space for treatment is limited, a vertical-flow wetland system such as that constructed at SR42 may be appropriate for treatment of SR15. The SCRA obtained approval for installation of a system in -2004(Carl Kirby, Bucknell University, written commun., 2003.…”

Effects of Abandoned Coal-Mine Drainage on Streamflow and Water Quality in the Mahanoy Creek Basin, Schuylkill, Columbia, and Northumberland Counties, Pennsylvania, 2001

“…The computed acidity avoids issues of different analytical methods, the lack of reporting negative values for the "hot" acidity, or an assumed value of zero for near-neutral pH samples; however, the computed acidity also involves assumptions regarding valence or speciation of the dissolved metals (Kirby and Cravotta, 2004;Cravotta and Kirby, 2004b). The net alkalinity was computed by subtracting the computed acidity from measured alkalinity as Net alkalinity (mg/L CaCO 3 ) = Alkalinity -Acidity computed .…”