Lead Solubility in Drinking Water: A Comparison of Experimental Lead Solubility and Geochemical Modeling Predictions

Abstract

Geochemical solubility modeling is a cost-effective method to estimate equilibrium lead (Pb) concentrations in drinking water under specific environmental conditions. Laboratory Pb-solubility studies (sometimes called coupon studies) are also economical and can generate comparative Pb solubility data for different water qualities. Both methods are widely used by utilities in screening corrosion control treatment, and both methods are assumed to provide insights on CCT for Pb without the influence of years of built-up scale. No research has compared the two methods to see if they give similar results for the same water. While these techniques have limitations and do not always represent Pb levels in service lines and premise plumbing, they are valuable for predicting Pb solubility trends under controlled conditions. In this study, Pb coupons immersed in chemically diverse waters provided experimental data on Pb solubility, which was then compared to predictions from two widely used geochemical models, MINEQL+ and LEADSOL. In tests without orthophosphate (PO₄³⁻), experimental Pb concentrations increased as pH decreased, consistent with model predictions. Between pH 7.5 and 8.5, Pb levels slightly declined as predicted by the model but were less dependent on dissolved inorganic carbon (DIC) than model predictions. However, at pH 8.5–10, Pb concentrations remained constant experimentally, whereas the model predicted significant reductions in Pb. Neither MINEQL+ nor LEADSOL models and experimental data were statistically the same using the built-in constants. Adjusting Log K_sp for hydrocerussite allowed the data and models to be statistically the same. In waters with PO₄³⁻, high DIC (50 mg/L as C) experimental results matched model predictions. At low DIC (3 mg/L as C), Pb concentrations varied less than modeled, and higher PO₄³⁻ doses were needed to reduce Pb levels compared to low-DIC waters. Overall, geochemical modeling and Pb solubility studies provide critical insights into Pb control strategies, and either or both methods can help screen the impact of possible water quality changes on Pb levels. Solubility testing is preferred, as site-specific solubility constants are generally unknown. Using the solubility models' adjustments to Log K_sp described in this paper is recommended if the site-specific Log K_sp is unknown.