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ERRORS IN APPLYING LOW ION-STRENGTH ACTIVITY COEFFICIENT ALGORITHMS TO HIGHER IONIC-STRENGTH AQUATIC MEDIA
Citation:
Loux, N T. AND J W. Washington. ERRORS IN APPLYING LOW ION-STRENGTH ACTIVITY COEFFICIENT ALGORITHMS TO HIGHER IONIC-STRENGTH AQUATIC MEDIA. Presented at American Society for Testing and Materials 11th Symposium on Environmental Toxicology and Risk Assessment, Phoenix, AZ, April 2-4, 2001.
Impact/Purpose:
This research project sets out to design and conduct an assessment of the long-term ecological consequences of alternative management choices. As the first project to be done at this scale using predictive ecological endpoints, we will seek to identify the appropriate components of such an analysis. We will use experience gained in the conduct of this BASE analysis to identify key research and data needs for address, to estimate timing, resource needs, etc., for future analyses. We will extend this analysis beyond previous and ongoing studies in two ways: by incorporating biological endpoints, primarily properties of fish communities, and by introducing the concept of sustainability of ecological state under future scenarios contrasted with the present state of those same ecological resources. Requirements that are identified during the course of this study will permit the recommendation of specific capabilities that should be incorporated in a general modeling system currently under development to support BASE and other environmental assessments. Finally, the analysis is intended to be of value for establishing environmental management choices that will be beneficial and those that would be detrimental to the sustainability of ecological resources of the Albemarle-Pamlico Basin.
Description:
The toxicological and regulatory communities are currently exploring the use of free-ion-activity- models as a means of reducing uncertainties in current methods for assessing metals bioavailabi- lity from contaminated aquatic media. While most practitioners would support the desirability of Ns objective, several technical questions remain to be addressed in achieving this goal.
Mathemtical algorithms relating dissolved salt concentrations to ionic activity coefficients range from the purely theoretical Debye-Huckel Limiting Law (suitable to ionic salt concentrations less than 0.001 molar) to semiempirical "robust" relationships (the Davies and Extended Debye- Huckel expressions; rated up to an ionic strength of 0.5 molar) to the Pitzer relationships (applicable at ionic strengths up to supersaturated salt solutions). Given that enviroinnental aquatic systems may possess dissolved salt concentrations ranging from less than 0.01 molar (freshwater systems) to greater than 0.7 molar (marine systems), and that Pitzer relationships are not available for most trace ionic toxicants, the need for robust high ionic strength activity coefficient algorithms for trace toxicants is obvious.
This work will provide a quantitative estimate of the errors associated with applying the Davies and Extended Debye Huckel activity coefficient algorithms (both independently and in conjunction with the geochernical speciation model MINTEQA2) to aqueous solutions at ionic strengths below and above 0.5 molar/molal. A quantitative assessment of the errors will be made and the prognosis for developing interim improvements will be discussed.