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PREDICTING LEAD DISSOLUTION IN DRINKING WATER DISTRIBUTION SYSTEMS: EFFECT OF FLUORIDE ADDITIVES ON LEAD SOLUBILITY AND CORROSION
Citation:
SCHOCK, M. R., D. H. METZ, AND D. DIONYSIOUS. PREDICTING LEAD DISSOLUTION IN DRINKING WATER DISTRIBUTION SYSTEMS: EFFECT OF FLUORIDE ADDITIVES ON LEAD SOLUBILITY AND CORROSION. Presented at AWWA Conference, Cleveland, OH, October 10 - 13, 2006.
Impact/Purpose:
To inform the public
Description:
Many water systems have encountered difficulties in meeting the action levels established by the Lead and Copper Rule. Several chemical parameters contribute to the corrosion of lead plumbing and may influence the nature of the passivating films formed on distribution materials. Because, some of these constituents can be adjusted as part of a corrosion control program, it is critical to understand specific effects of these constituents on lead release, examining them in a systematic/quantitative manner. High lead concentrations occurred recently in Washington DC drinking water after a significant water quality change, causing renewed speculation about the impacts of several chemical variables, including fluoride. This speculation and the absence of definitive, well-controlled experimental data underscores the need for additional research. Dissolved lead species can be reduced by producing a drinking water that is non-aggressive and able to form a passivating film/scale on lead surfaces. Conversely, aggressive water with a high oxidation potential, while initially problematic, may rapidly form a rugged film of metal oxides on the metal surface that acts as an effective barrier against further metal dissolution. However, this layer may not be stable, if the oxidation potential is lowered. Additionally, mineralogy of the scale is important to metal protection. Scales that are not properly adhered or sufficiently crystallized can slough off, increasing lead concentrations and reducing protective layers on the pipe. The lead corrosion/passivation process is complex, making it difficult to predict the effects of various treatment regimes on aqueous lead concentration. Theoretical dissolution models have been of some value in making predictions. However, better predictive models must be developed utilizing a combination of well-established theoretical models and controlled exprimental data. This research is aimed at understanding the effects of significant water quality constituents on lead release. Chemical species of lead solids are determined and equilibrium constants are calculated and compared to literature values. Four-foot segments of lead pipes of typical diameters (0.5” to 0.75”) are being used with typical distribution system velocities (2 fps). Fifty-five-gallon recirculation systems deliver the carefully controlled feed water. Each test is run for at least 6 months in order to reach a stable equilibrium condition in the test system. Upon completion of the runs, lead pipes are removed and cut into sections and the layers of lead scale characterized. The results are compared to theoretical results using speciation models and then used to verify or refine thermodynamic constants for lead species.